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Accepted Manuscript
In-situ stress orientations in the Xiagou tight oil reservoir of Qingxi Oilfield, Jiuxi Basin,
northwestern China
Wei Ju, Zhaoliang Li, Weifeng Sun, Haoran Xu
PII:
S0264-8172(18)30344-1
DOI:
10.1016/j.marpetgeo.2018.08.020
Reference:
JMPG 3460
To appear in:
Marine and Petroleum Geology
Received Date: 22 June 2018
Revised Date:
12 August 2018
Accepted Date: 15 August 2018
Please cite this article as: Ju, W., Li, Z., Sun, W., Xu, H., In-situ stress orientations in the Xiagou tight oil
reservoir of Qingxi Oilfield, Jiuxi Basin, northwestern China, Marine and Petroleum Geology (2018), doi:
10.1016/j.marpetgeo.2018.08.020.
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In-situ stress orientations in the Xiagou tight oil reservoir of Qingxi Oilfield,
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Jiuxi Basin, northwestern China
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Wei Ju a, b, c, *, Zhaoliang Li d, Weifeng Sun a, c, Haoran Xu c
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a. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China
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University of Mining and Technology, Xuzhou 221008, China;
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b. Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong
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University of Science and Technology, Qingdao 266590, China;
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c. School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China;
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d. China Aero Geophysical Survey and Remote Sensing Center for Land and Resources, Beijing 100083, China;
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* Corresponding author. E-mail: wju@cumt.edu.cn.
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Abstract: Knowledge of the present-day in-situ stress orientation is important for borehole stability,
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fluid flow in fractured reservoirs, and hydraulic fracture stimulation, etc. The Qingxi Oilfield of
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Jiuxi Basin, northwestern China, is an old oilfield; however, within which there were no systematic
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investigations into the present-day in-situ stress orientation prior this study. In the present study, the
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orientation of horizontal maximum stress (SHmax) in the Qingxi Oilfield was interpreted and analyzed
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based on 149 borehole breakouts (BOs) and 131 drilling induced fractures (DIFs) from electrical
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borehole imaging logs in 23 wells. Our interpretations revealed a prevailing ~NE-SW-trending
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(14.94°N~52.74°N) SHmax orientation in the Xiagou tight oil reservoir of Qingxi Oilfield. However,
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the SHmax orientation indicated various tendencies both laterally and with burial depth within single
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wells, which were influenced by well-developed faults, natural fractures and bedding planes. The
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presence of these structures caused great contrasts of rock mechanical properties, influencing stress
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orientation variations. In addition, the effects of present-day in-situ stress orientation on natural
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fractures, hydraulic fracture stimulation and borehole stability were discussed. In the Qingxi Oilfield,
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~NE-SW-trending natural fractures indicated favorable contributions to subsurface fluid flow.
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Hydraulic fractures would propagate vertically following ~NE-SW-trending. Wells were more likely
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to experience borehole instability issues if they were deviated towards ~NE-SW-trending. The
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results can provide geological references for subsequent tight oil production in the Qingxi Oilfield of
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Jiuxi Basin.
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Keywords: in-situ stress orientation; drilling induced tensile fracture; borehole breakout; Xiagou
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tight oil reservoir; Qingxi Oilfield; natural fracture
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1. Introduction
In-situ stress refers to the internal stress within the Earth's crust, and is closely related to
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gravitational and tectonic stresses (Bell, 1996; Kang et al., 2010). Generally, the in-situ stress state
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can be described by the orientations and magnitudes of three orthogonal principal stress (Engelder,
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1993; Tingay et al., 2009; Rajabi et al., 2017; Ju and Wang, 2018). As the stress-generating
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acceleration of gravity is directed downwards and the Earth’s surface is considered to be a free
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boundary, one principal stress acts vertically within sedimentary basins. Therefore, the present-day
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three-dimensional state of crustal stress in the earth is typically simplified to four components, the
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magnitudes of horizontal maximum stress (SHmax), horizontal minimum stress (Shmin) and vertical
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stress (Sv), and the SHmax orientation (Bell, 1996; Tingay et al., 2009; Rajabi et al., 2010, 2017; Ju et
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al., 2017; Ju and Wang, 2018).
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Among these four components, determination of the SHmax orientation has received extensive
attention. The scientific importance of the SHmax orientation is highlighted by the findings in the
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World Stress Map (WSM) Project, which is a global compilation of information on the present-day
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stress field maintained since 2009. The current new WSM database release 2016 contains 42,870
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data records within the upper 40 km of the Earth’s crust, which is almost twice the amount with
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respect to the previous WSM database release 2008 (Heidbach et al., 2010, 2016). However, recently,
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more studies have analyzed the significance of basin- to field-scale stress field. Knowledge of stress
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orientations at these scales can provide a better understanding of the exploration and development of
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hydrocarbon resources (Bell, 2006; Tingay et al., 2010; Kingdon et al., 2016; Rajabi et al., 2016; Ju
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et al., 2017; Ju and Wang, 2018), borehole stability evaluation (Hillis and Williams, 1993; Zoback et
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al., 2003; Tingay et al., 2009; Ju et al., 2017), optimization of hydraulic fracture stimulation (Fuchs
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and Muller, 2001; Tingay et al., 2009), fluid flow in fractured reservoirs (Fuchs and Muller, 2001;
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Rajabi et al., 2010), etc.
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In the Qingxi Oilfield, tight oil production typically involves hydraulic fracturing, and the
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present-day in-situ stress orientation is a critical factor influencing this operation. Within the oilfield,
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despite hydrocarbon exploration and development have been conducted for many years, detailed and
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systematic information regarding the SHmax orientation is still unclear and unavailable. Hence, efforts
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to develop tight oil reservoirs in the Qingxi Oilfield would benefit from an improved understanding
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of the present-day in-situ stress orientation. In this study, based on the analysis of borehole breakouts
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(BOs) and drilling induced fractures (DIFs) interpreted from imaging logs, systematic investigations
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into the SHmax orientation in the Xiagou tight oil reservoir of Qingxi Oilfield was carried out for the
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first time. The results can provide geological references for hydraulic fracturing design, borehole
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stability evaluation, tight oil exploration and production, etc., in the Qingxi Oilfield.
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2. Geologic settings
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The Jiuxi Basin, admired as the cradle of China’s oil industry, is located in the west of the Hexi
Corridor, northwestern China, with an area of approximately 2,700 km2. It is bounded by the
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northern Qilian thrust belt to the south, Kuantai and Hei mountains to the north. From west to east,
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the Jiuxi Basin is divided into five first-order tectonic units: the Qingxi Depression, Yabei Uplift,
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South Uplift, Shida Depression and Jiaxi Uplift (Fig.1a; Wang and Coward, 1993; Wang et al., 2005).
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Among them, the Qingxi Depression is regarded as the most important petroliferous area in the Jiuxi
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Basin with the estimated petroleum amount more than (2.58~4.00)×108 tons (Chen et al., 2001). In
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addition, it is identified that hydrocarbons in the Jiuxi Basin are mainly derived from the Qingxi
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Depression through long-distance migrations (Fig.1a; Wang and Coward, 1993; Chen et al., 2001;
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Wang et al., 2005; Guo et al., 2018).
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Figure 1 here
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The Qingxi Oilfield is located in the Qingxi Depression of Jiuxi Basin with an exploration area
of approximately 80 km2 (Fig.1a; Ma et al., 2002). Based on seismic interpretations, faults are
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extensively developed in this oilfield with various sizes and levels (Fig.2). The Lower Cretaceous
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Xiagou tight oil reservoir within this oilfield has a great sedimentary thickness (Fig.1b) and is of
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significant interest for tight oil exploration and production (Guo et al., 2018). The most favorable
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reservoir rocks in the Xiagou tight oil reservoir of Qingxi Oilfield are a suite of politic dolostones,
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dolomitic mudstones and dolostones, which are formed in a deep lacustrine environment (Yang et al.,
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2004; Ju and Sun, 2016a).
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Figure 2 here
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The Xiagou tight oil reservoir is naturally fractured (Ju and Sun, 2016b), viability of tight oil
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production is partially dependent on natural fracture permeability. Furthermore, the ability of these
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natural fractures to transmit fluid is partially in-situ stress dependent, with natural fractures that are
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critically stressed being more likely to be effective fluid conduits than those that are not (Zoback,
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2007; Brooke-Barnett et al., 2015).
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3. Methodology
The SHmax orientation plays an important role in different aspects of geosciences (Bell, 1996),
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which can be derived from interpretations of borehole stress induced failures (Zoback et al., 2003),
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paleomagnetic analysis (Yin et al., 2017), earthquake focal mechanism inversion (Sperner et al.,
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2003; Heidbach et al., 2010, 2016), etc. Geophysical borehole studies have indicated that bores alter
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their geometries with time, tending to form rock failures (Plumb and Hickman, 1985; Cuss et al.,
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2003), e.g., BOs and DIFs (Fig.3). Borehole imaging logs can provide high resolution images of the
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borehole wall based on either ultrasonic velocity or resistivity. Resistivity contrasts from borehole
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imaging logs show features that are indicative of stress orientation (Williams et al., 2015; Kingdon et
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al., 2016). Hence, interpretation of these stress-induced rock failures from borehole imaging logs
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becomes the most common approach to determine the SHmax orientation underground in drilling wells
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(Bell, 1996; Zoback, 2007; Rajabi et al., 2010; Kingdon et al., 2016).
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Figure 3 here
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BOs are rock failures on opposite sides of the borehole wall due to removal of materials during
boring, which occur in those parts of a borehole where the circumferential stress exceeds rock
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compressive strength (Bell and Gough, 1979; Zoback et al., 2003). In borehole imaging logs, BOs
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are observed and interpreted as poorly resolved parallel and enlarged conductive zones on opposite
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sides of the borehole. The long axes of BOs in vertical wells are oriented approximately parallel to
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the Shmin orientation (Plumb and Hickman, 1985; Hillis and Reynolds, 2003; Zoback et al., 2003;
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Rajabi et al., 2016, 2017; Figs.3 and 4).
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Figure 4 here
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DIFs, however, form where the circumferential stress is less than rock tensile strength in the
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borehole wall (Aadnoy and Bell, 1998; Zoback et al., 2003; Figs.3 and 5). In borehole imaging logs,
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commonly, DIFs appear in two different manners. i) DIFs-1. They are symmetrically aligned two
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vertical/subvertical fractures parallel to the borehole axis on the opposite sides of the borehole wall
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(Figs.5a and 4b). ii) DIFs-2. They are en-echelon fractures around the borehole, exhibiting traces
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180° apart at the borehole surface and inclined relative to the borehole axis (Figs.5c and 4d). In
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general, DIFs occur perpendicular to the orientation of BOs, and indicate the SHmax orientation
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(Aadnoy and Bell, 1998; Brudy and Zoback, 1999; Hillis and Reynolds, 2003; Zoback et al., 2003;
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Brooke-Barnett et al., 2015; Williams et al., 2015; Rajabi et al., 2016, 2017).
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Figure 5 here
In addition, in order to investigate the reliability of individual stress indicator, a quality ranking
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scheme is recommended and generally applied with ranking criteria based on those developed by the
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WSM (Heidbach et al., 2010; Table 1), which allows for the comparison among different techniques
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for determining the SHmax orientation (Zoback, 1992; Heidbach et al., 2010; Williams et al., 2015). In
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the WSM ranking scheme, the combined length, number of BOs or DIFs observed, and standard
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deviation are taken into account, and the quality varies between A (the highest reliability) and E (the
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lowest reliability) (Table 1).
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Table 1 here
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4. The present-day in-situ stress orientation
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In the present study, approximate 1.6 kilometers of electrical borehole imaging logs from 23
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wells were analyzed for interpretation of BOs and DIFs in the Qingxi Oilfield. The locations and
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details of studied wells are summarized in Fig.2, Tables 2 and 3. This is a region where little prior
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information on the SHmax orientation was available in the WSM database. Using analysis criteria for
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detection of BOs and DIFs adapted from those recommended by the WSM Project (Sperner et al.,
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2003), wells with unsuitable data were eliminated. Totally, in this analysis, 149 BOs with an overall
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length of 298.4 m and 131 DIFs with a combined length of 293.6 m were interpreted from imaging
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logs for determining the SHmax orientation in the Xiagou tight oil reservoir of Qingxi Oilfield (Tables
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2 and 3).
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Table 2 here
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Table 3 here
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In order to clearly interpret the results, all the SHmax orientations derived from both BOs and
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DIFs were summarized (Fig.6) and plotted on a map of the Qingxi Oilfield (Fig.7). It is apparent that
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the stress indicators varied between 14.94°N and 52.74°N with an average 40.76°N and showed a
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dominant ~NE-SW-trending SHmax orientation, which was consistent with the finite several stress
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orientations around the study area in the WSM (Heidbach et al., 2010, 2016).
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Figure 6 here
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Figure 7 here
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The analysis of the SHmax orientation derived from 23 wells indicated localized perturbations in
different regions (Fig.7). Wells were in low (e.g., Well U4) and high (e.g., Well Q47) intersection
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angles between the SHmax orientation and N-S-trending. Previous studies indicated that different local
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structural patterns, variable sedimentary facies and lithological compositions, and material strength
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were significant controls for the SHmax orientation in many sedimentary basins worldwide (Bell, 1996;
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Cuss et al., 2003; Brooke-Barnett et al., 2015; Rajabi et al., 2017). Decoupling of the stress field
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from regional trends adjacent to faults may lead to part from fault compartmentalization (Williams et
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al., 2015), resulting in significant SHmax orientation variations. In addition, previous studies (e.g., Bell,
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1996) analyzed the effects of stiff and weak materials on the variations in SHmax orientation. For
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weak materials, the SHmax trajectory tends to perpendicular to stiff materials within them (Fig.8a);
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whereas, it parallels with weak materials (Fig.8b). Well-developed faults in the Qingxi Oilfield are
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typically considered as weakness zones, which acted as an important role in affecting SHmax
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orientation variations.
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Figure 8 here
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In the Qingxi Oilfield, all the interpreted stress indicators were classified as being of category
B-D quality based on the WSM ranking system (Heidbach et al., 2010; Tables 2 and 3). Table 2
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shows that 4 of the 15 calculated SHmax orientations derived from BOs are in category A-C quality.
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Table 3 shows that 5 of the 19 calculated SHmax orientations derived from DIFs are in category A-C
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quality. These data are regarded as reliable indicators of SHmax by the WSM project (Sperner et al.,
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2003; Heidbach et al., 2010). As to all remained category D indicators, they are generally limited by
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the total cumulative length of BOs or DIFs (less than 20 m). To sum up, the results indicated that the
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investigated 23 wells showed relatively reliable SHmax orientations with overall low standard
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deviation (less than 17.45°; Tables 2 and 3).
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The Qingxi Oilfield was divided into 210 grids to further evaluate the variations of stress
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orientation in details. In this study, the average SHmax orientation in each grid was predicted based on
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the Kriging interpolation method (Fig.9). The results of this statistical analysis indicated that the
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predicted average SHmax orientations were generally NE-SW-trending, except that in the northeastern
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regions of the study area, the stress orientations rotated to NNE-SSW-trending.
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Figure 9 here
In the Qingxi Oilfield, in a search area centered of each grid with a search radius 1500 m, the
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reliability (λ) was defined and calculated to understand the predicted results.
=
×
(1)
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where λ is the reliability, q is the quality, in this study, 0.75 for B-, 0.50 for C-, 0.25 for D-quality
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data, d is the distance between grid center and the location of measured data. In this study, the
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calculated λ ranged between 0 and 0.011, and regions with more measured data indicated higher
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reliability (Fig.9).
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In addition, our interpretation results also showed variations of the SHmax orientation with burial
depth within a single well in the Qingxi Oilfield (Fig.10). Detailed analysis of imaging logs
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indicated that local rotations of BOs appeared near natural fractures and bedding planes, both of
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which were considered as weakness planes representing contrasts of rock mechanical properties.
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Moreover, previous studies (e.g., Faulkner et al., 2006) also indicated that faults and natural fractures
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may affect the mechanical properties of rocks to a different degree.
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Figure 10 here
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In the Qingxi Oilfield, different SHmax orientations in the lower part of Fig.10a and Fig.10b
indicated that they were related to the presence of natural fractures; whereas, the changes of SHmax
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orientations shown in the upper part of Fig.10a and Fig.10c were mainly resulted from the effects of
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bedding planes (Fig.10).
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5. Discussions
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5.1 Implications for natural fractures
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The Xiagou tight oil reservoir in the Qingxi Oilfield is naturally fractured (Ju and Sun, 2016b).
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Knowledge of the in-situ stress orientation is important for understanding fluid flow in naturally
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fractured reservoirs (Finkbeiner et al., 1997; Kingdon et al., 2016).
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In the Qingxi Oilfield, characteristics of natural fractures were interpreted and analyzed based
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on borehole imaging logs (Fig.11). Generally, in the Qingxi Oilfield, mainly two dominant trends of
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natural fractures were observed in wells striking in ~NE-SW-trending (set A; average 52.36°N) and
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~NW-SE-trending (set B; average 316.23°N) (Fig.12). In this study, no detailed production data
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were available, hence, the productivity of each fracture set was unable to evaluate. However, under
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the dominant present-day ~NE-SW-trending SHmax orientation (average 40.76°N; Fig.6 and Fig.7),
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the interpreted fracture set A is oriented parallel (or subparallel) to the SHmax orientation, these natural
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fractures tend to be open and conductive; hence they have favorable contributions to fluid flow and
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further tight oil development.
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Figure 11 here
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Figure 12 here
In this study, to quantitatively investigate the effect of present-day in-situ stress field on natural
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fractures, the Coulomb failure mechanism and selected natural fractures with various orientations
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from Well G7 (Fig.11; depth: 4178~4266 m) were utilized for analysis (Fig.13). Critically stressed
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natural fractures are estimated as the distance (pore pressure increase, ∆P) of each possible plane
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from the fracture envelope. A standard frictional coefficient value 0.6 was set to the cohesionless
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pre-existing natural fractures. The Biot’s coefficient was 1.0. In addition, based on study results from
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Zhang (2009), in Well G7, i) the SHmax orientation was NE-SW-trending with an average 32°N; ii) the
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gradients of Sv, SHmax, Shmin and pore pressure were 27.05 MPa/km, 23.03 MPa/km, 19.21 MPa/km
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and 11.66 MPa/km, respectively.
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Figure 13 here
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Under the present-day in-situ stress state, all investigated natural fractures in Well G7 of Qingxi
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Oilfield were not critically stressed (Fig.14a). However, with pore pressure increased (commonly the
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injection process), NE-SW-trending fractures with high dip angles first became critically stressed,
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then NE-SW-trending fractures with intermediate dip angles, and last NE-SW-trending fractures with
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low dip angles (Fig.14b-e). NW-SE-trending fractures (fracture B) were hard to be critically stressed
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during the entire process (Fig.14). Obviously, in the Xiagou tight oil reservoir of Qingxi Oilfield,
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fracture set A (~NE-SW-trending fractures) under the present-day in-situ stress state exhibited
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excellent fluid flow characteristics.
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Figure 14 here
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Indeed, this concept, sometimes is referred to as “structural permeability” (Sibson, 1996). In
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addition, for naturally fractured reservoirs, drilling wells intersecting with more conductive fractures
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will definitely increase the production (Rajabi et al., 2010). Hence, in the Xiagou tight oil reservoir
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of Qingxi Oilfield, horizontal wells drilled approximately towards the ~NW-SE-trending would be
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expected to encounter great numbers of critically stressed natural fractures.
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5.2 Implications for hydraulic fracture and borehole stability
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A critical factor in hydraulic fracture stimulations is the present-day in-situ stress orientation.
Generally, hydraulic fractures will propagate along the path of least resistance and create width in a
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direction that requires the least forces (Kingdon et al., 2016). Therefore, hydraulic tensile fractures
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will propagate parallel to the SHmax orientation (Brudy and Zoback, 1999; Brooke-Barnett et al.,
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2015). In addition, the present-day in-situ stress orientation also influenced borehole stability
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evaluation. Generally, borehole instability is an adverse condition formed when an open hole dose
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not maintain its shape and size. Mechanical collapse of boreholes due to excessive breakouts can
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caused a series issues, e.g., hole cleaning difficulties, etc. (Hillis and Williams, 1993; Zoback, 2007).
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Therefore, prior to drilling, understanding the in-situ stress orientation is imperative to guard against
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borehole instability. Reducing borehole instability can be achieved by minimizing shear failure of
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the borehole wall by reduction of the maximum circumferential stress (Zoback, 2007; Rajabi et al.,
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2016).
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Based on measured in-situ stress magnitude data from Zhang (2009), in the Xiagou tight oil
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reservoir of Qingxi Oilfield, the present-day in-situ stress is under the Sv>SHmax>Shmin condition
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(normal faulting stress regime according to Anderson’s classification, 1951; Fig.15); therefore,
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hydraulic fractures will propagate vertically following ~NE-SW-trending (Fig.15). Wells are more
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likely to experience borehole instability issues if they are deviated toward ~NE-SW-trending due to
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the relatively high Sv-Shmin differential stress magnitude. Knowledge of the present-day in-situ stress
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orientation can be used to identify the most favorable well designs to minimize potential borehole
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instability.
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Figure 15 here
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6. Conclusions
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Knowledge of the present-day in-situ stress orientation is important for petroleum exploration
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and development in fractured reservoirs. In this study, the orientation of horizontal maximum stress
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(SHmax) was determined and analyzed in the Xiagou tight oil reservoir of Qingxi Oilfield, Jiuxi Basin.
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The SHmax orientation from interpretations of borehole breakouts (BOs) and drilling induced fractures
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(DIFs) demonstrated a predominant NE-SW-trending (average 40.76°N) over the oilfield. In addition,
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variations in the SHmax orientation were observed both laterally and with depth within a single well.
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Well-developed faults, natural fractures and bedding planes were responsible for the changes of
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SHmax orientation within the Xiagou tight oil reservoir of Qingxi Oilfield.
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In addition, the SHmax orientation has great implications on natural fracture status, hydraulic
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fracture stimulation and borehole stability evaluation. In the Qingxi Oilfield, ~NE-SW-trending
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natural fractures were more easily to become critically stressed, indicating favorable contributions to
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fluid flow and further tight oil production. During hydraulic fracture stimulation, hydraulic fractures
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in the Xiagou tight oil reservoir would propagate vertically following ~NE-SW-trending. Wells were
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more likely to experience borehole instability issues due to the relatively high differential stress
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magnitude if they were deviated towards ~NE-SW-trending.
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Acknowledgements
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We would like to express our gratitude towards the anonymous reviewers for offering
constructive suggestions and comments which improved this manuscript in many aspects. This work
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was financially supported by the National Natural Science Foundation of China (41702130), China
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Postdoctoral Science Foundation (2017T100419, 2015M581891), and Postdoctoral Science
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Foundation of Jiangsu Province (1501059A).
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Captions for figures and tables
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Fig.1 (a) Geological map showing tectonic units and oilfields in the Jiuxi Basin, and (b)
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Stratigraphic cross section AB in the Qingxi Depression (after Wang et al., 2005)
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In Fig.1b, N+Q: the Neogene and Quaternary, N1+2s: the Lower and Middle Neogene Shulehe
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Formation, E3b: the Paleogene Baiyanghe Formation, K1: the Lower Cretaceous, and S: the Silurian.
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Fig.2 (a) Structural map of the top Lower Cretaceous Xiagou tight oil reservoir in the Qingxi
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Oilfield, and (b) Stratigraphic cross section AB in the Qingxi Oilfield
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In Fig.2b, N+Q: the Neogene and Quaternary, E3b: the Paleogene Baiyanghe Formation, K1z: the
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Zhonggou Formation, K1g2+3, K1g1 and K1g0: the upper, middle and lower part of Lower Cretaceous
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Xiagou Formation, respectively, K1c: the Chijinpu Formation, AnK: Formations before the
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Cretaceous, C-P: the Carboniferous and Permian, and S: the Silurian.
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Fig.3 Orientations and typical examples of BOs and DIFs with respect to the circumferential stress
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around the borehole (after Hillis and Reynolds, 2003)
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In this study, it is assumed that positive values are compressive stresses and negative values are
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tensile stresses.
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Fig.4 BOs interpreted from borehole imaging logs in the Qingxi Oilfield
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The BOs are identified as a pair of poorly resolved conductive zones observed on opposite sides of
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the borehole. The BOs herein are oriented approximately NW-SE-trending and thus indicate a
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present-day SHmax orientation of approximately NE-SW-trending.
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Fig.5 DIFs interpreted from borehole imaging logs in the Qingxi Oilfield
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DIFs in a and b are observable as vertical or subvertical fractures running parallel to the borehole
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axis; whereas, in c and d, they are en-echelon fractures inclined relative to the borehole axis. All the
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DIFs herein are oriented approximately NE-SW-trending and thus indicate a present-day SHmax
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orientation of approximately NE-SW-trending.
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Fig.6 The overall SHmax orientation in the Qingxi Oilfield derived from 280 interpreted DIFs and
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BOs
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Fig.7 Variation of the SHmax orientation in different wells over the Qingxi Oilfield derived from DIFs
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and BOs
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Fig.8 Schematic plan view for understanding the rotation of SHmax orientation in the vicinity of
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different geological structures (after Bell, 1996)
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Solid red lines indicate the SHmax orientation, which will swing perpendicular to mechanically stiff
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structures (a) and parallel to mechanically weak structures (b).
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Fig.9 The predicted average SHmax orientation within each grid over the Qingxi Oilfield
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Fig.10 Typical examples showing BOs rotations observed in borehole imaging logs in the Qingxi
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Oilfield
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BOs are outlined in blue rectangles. Rose diagrams illustrate the present-day SHmax orientation.
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Obviously, the BOs scale and angle of rotations are variable and different from each case with burial
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depth in a single well.
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Fig.11 Characteristics of natural fractures interpreted from borehole imaging logs within Well G7
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FMI: Formation Micro Imager
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Fig.12 Rose diagrams showing the strikes of natural fractures within the Xiagou tight oil reservoir of
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Qingxi Oilfield
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Fig.13 Example of determining critically stressed natural fractures (after Mildren et al., 2002)
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All possible fracture planes are plotted as a point within the shaded region of the three-dimensional
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Mohr circle. Critically stressed natural fractures are estimated as the distance (pore pressure increase,
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∆P) of each possible plane from the fracture envelope. SHmax: horizontal maximum stress, Shmin:
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horizontal minimum stress, and Sv: vertical stress.
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Fig.14 Lower hemisphere stereonet plots and three-dimensional Mohr circles showing critically
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stressed natural fractures in the Xiagou tight oil reservoir of Well G7, Qingxi Oilfield
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Black dots in stereonet plots are normal lines of natural fracture planes, and red dots are critically
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stressed ones. Red dots in Mohr circles are critically stressed natural fractures. SG indicates specific
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gravity, g/cm3. ∆P is the pore pressure increase. (a) ∆P=0 MPa, (b) ∆P=10 MPa, (c) ∆P=20 MPa, (d)
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∆P=30 MPa, and (e) ∆P=40 MPa.
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Fig.15 Schematic illustration showing the Anderson’s tectonic classification (after Brooke-Barnett et
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al., 2015)
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(a) SHmax>Shmin>Sv, reverse faulting stress regime, (b) SHmax>Sv>Shmin, strike-slip faulting stress
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regime, and (c) Sv>SHmax>Shmin, normal faulting stress regime. The pink plane represents the
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orientation of a propagated hydraulic fracture in the associated stress regime.
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Table 1 Quality ranking scheme for the SHmax orientation derived from DIFs and BOs (after
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Heidbach et al., 2010)
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Table 2 Summary of the SHmax orientation derived from BOs in the Qingxi Oilfield
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Table 3 Summary of the SHmax orientation derived from DIFs in the Qingxi Oilfield
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C quality
≥ 6 distinct breakouts and
≥ 4 distinct breakouts and
< 4 distinct breakouts or
≥ 40 m combined length
≥ 20 m combined length
< 20 m combined length
in a single well with a
in a single well with a
in a single well with a
standard deviation ≤ 20°.
standard deviation ≤ 25°.
standard deviation ≤ 40°.
≥ 10 distinct fractures and
≥ 6 distinct fractures and
≥ 4 distinct fractures and
< 4 distinct fractures or <
≥ 100 m combined length
≥ 40 m combined length
≥ 20 m combined length
20 m combined length in
in a single well with a
in a single well with a
in a single well with a
a single well with a
standard deviation ≤ 12°.
standard deviation ≤ 20°.
standard deviation ≤ 25°.
standard deviation ≤ 40°.
and ≥ 100 m combined
BOs
length in a single well
with a standard deviation
DIFs
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≥ 10 distinct breakouts
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Table 1 Quality ranking scheme for the SHmax orientation derived from BOs and DIFs (after Heidbach et al., 2010)
E quality
Wells without reliable
breakouts or the standard
deviation > 40°.
Wells without fracture
zones or the standard
deviation > 40°.
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Table 2 Summary of the SHmax orientation derived from BOs in the Qingxi Oilfield
Well
Number
Data interval (m, bgl)
Total length (m)
U4
3
4068.6~4500.5
7.6
U9
26
4121.6~4464.3
68.3
G4
8
4034.2~4285.0
11.8
G7
8
3868.2~4461.6
12.9
G8
11
3383.4~4241.1
G15
7
G101
Average SHmax azimuth (°N)
Quality rank
14.94
5.24
D
43.65
14.36
B
32.16
9.53
D
32.03
8.65
D
14.6
38.66
6.76
D
4327.8~4422.1
11.4
27.75
10.05
D
33
3991.7~4660.1
58.7
51.29
11.55
B
G105
4
4797.4~4835.8
5.4
40.93
2.61
D
G110
3
4626.2~4824.4
8.0
37.36
17.45
D
Q2
3
4144.5~4313.5
10.8
42.15
6.40
D
Q4
13
3874.4~4095.4
32.2
32.76
10.70
C
Q7
2
3810.5~4257.9
5.6
37.76
1.34
D
Q14
6
4762.9~4891.7
9.9
37.96
10.54
D
Q36
17
4653.9~5688.2
34.4
37.36
11.11
C
Q47
5
3926.2~3990.9
6.8
52.74
3.99
D
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Table 3 Summary of the SHmax orientation derived from DIFs in the Qingxi Oilfield
Well
Number
Data interval (m, bgl)
Total length (m)
U9
3
4219.8~4453.5
7.5
G4
10
4046.3~4632.8
17.3
G6
4
4392.8~4404.2
10.7
G8
4
4093.1~4244.7
4.5
G101
9
4035.0~4600.4
G104
5
4537.8~4587.4
G105
6
G110
Average SHmax azimuth (°N)
Quality rank
49.02
12.36
D
46.74
12.85
D
41.90
6.22
D
38.59
4.52
D
24.1
52.57
8.26
C
5.8
35.42
16.59
D
4789.0~4905.8
28.1
34.81
5.46
C
4
4821.8~4846.4
4.9
47.42
3.97
D
Q1
2
4542.6~4547.6
4.0
49.77
2.88
D
Q2
7
4213.8~4402.5
14.1
52.34
11.98
D
Q4
22
3994.2~4097.4
52.3
34.00
12.95
B
Q6
4
4646.1~4695.8
5.3
41.84
4.63
D
Q7
4
3883.8~3955.6
5.9
41.44
13.23
D
Q14
16
4901.6~5125.0
28.1
39.98
9.37
C
Q30
4
4310.8~4338.6
10.9
41.52
7.95
D
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Standard deviation (°)
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4391.2~4411.8
13.8
26.44
3.77
D
Q36
15
4836.1~5740.2
39.6
45.32
14.20
C
Q37
5
4543.1~4824.9
11.2
26.99
11.83
D
Q39
2
4662.7~4625.8
5.5
47.30
3.97
D
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Highlights
* The BOs and DIFs were interpreted from imaging logs in the Qingxi Oilfield.
* The SHmax orientation in the Xiagou tight oil reservoir was systematically studied.
* Effect of present-day in-situ stress state on natural fractures was analyzed.
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* Implications of in-situ stress orientation on hydraulic fractures were discussed.
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