Integrating borehole-breakout dimensions, strength criteria, and leak-off test results, to constrain the state of stress across the Chelungpu Fault, Taiwan

doi:10.1016/j.tecto.2009.05.016

Tectonophysics

Volume 482, Issues 1–4, 25 February 2010, Pages 65-72

https://doi.org/10.1016/j.tecto.2009.05.016 Get rights and content

Abstract

The paper describes the computation of the maximum horizontal stress (σ_H) magnitude in the vicinity of the Chelungpu Fault, Taiwan, host of the slip zone during the Chi-Chi earthquake (M_w 7.6; 1999). The scientific hole B intercepts the Chelungpu Fault at 1136 m. At the depths of logged breakouts (940–1310 m), the vertical stress (σ_v) as estimated from density logs increases linearly with depth from 22 to 31 MPa. A series of leak-off tests yielded two reliable shut-in pressures, 23.7 MPa at 1085 m and 29.8 MPa at 1279 m, which are lower than the estimated σ_v, albeit by only 2.1 and 0.6 MPa, respectively. In our analysis the shut-in pressures were considered to represent estimates of the least horizontal principal stresses (σ_h) at the respective depths, and consequently the test-induced fractures were assumed to have been vertical. Principal stress directions had been previously determined by others (105°–155° for the maximum horizontal stress, σ_H, except in the immediate vicinity of the Chelungpu Fault). The contribution of this paper is the estimation of the σ_H magnitude by considering that the state of stress at the points of intersection between breakout and borehole wall is in a state of limit equilibrium with the true triaxial strength criterion. The resulting σ_H in the range of logged breakouts increases with depth from 55 MPa at 940 m to 59 MPa at 1310 m. Thus, the estimated state of stress prevailing across the Chelungpu Fault is compatible with strike-slip, but marginally also with thrust faulting. However, the likelihood that the shut-in pressures actually represent σ_v magnitudes, and that the leak-off test-induced fractures were sub-horizontal, cannot be ignored. In that case the state of stress would clearly favor thrust faulting.

Introduction

Borehole breakouts identified on geophysical logs have been widely used as an important indicator of in situ stress direction. They provide indispensable data for the construction of the World Stress Map (Heidbach et al., 2008). In addition, laboratory tests have shown that the angular span of breakouts in cross sections of vertical boreholes is directly related to the magnitudes of the far-field principal stresses (Song and Haimson, 1997). However, the explicit relationship requires knowledge of the rock strength criterion, because of the reasonable assumption that the points of intersection between breakout and borehole wall cross section represent the boundary between stable rock on the outside of the breakout and failed rock on the inside. Equating the correct strength criterion to the state of stress at the points of intersection, one obtains a relationship in terms of the three in situ principal stresses. The vertical in situ principal stress (σ_v) is typically determined from the gravitational gradient. The least horizontal principal in situ stress (σ_h) can only be ascertained through independent measurements, and practically the only reliable ones in deep holes are leak-off or hydraulic fracturing (hydrofracturing) tests. Thus the only remaining unknown is the maximum horizontal in situ stress (σ_H), which can be computed by solving the strength criterion-state of stress equation at the point of breakout-borehole wall intersection.

At the University of Wisconsin we designed and fabricated an apparatus for determining the true triaxial strength of rectangular prismatic specimens subjected to three unequal principal stresses (Haimson and Chang, 2000). The first opportunity to use it for constraining σ_H came in conjunction with the stress measurements in the KTB scientific ultra deep well, Germany (Brudy et al., 1997, Haimson and Chang, 2002). We experimentally obtained a true triaxial strength criterion for the amphibolite, the dominant rock between 3000 and 7000 m depth in the KTB hole. Applying the criterion to the state of stress at the points of borehole-breakout intersections yielded estimates of the magnitude of σ_H between 3200 and 6800 m depth, which reaffirmed the assertion that the state of stress there is compatible with strike-slip faulting (Haimson and Chang, 2002).

The scientific holes drilled through the Chelungpu Fault in Taiwan offered a new opportunity to integrate logged borehole-breakout spans, rock strength criteria, and hydraulic fracturing (leak-off test) results in order to constrain the complete state of in situ stress following the destructive Chi-Chi earthquake. This forms the topic of the present paper.

Section snippets

The Taiwan Chelungpu Fault Drilling Project (TCDP)

Chelungpu Fault, Taiwan, is a major north–south striking, 90-km long fault, dipping 30° to the east (Fig. 1). The fault slips parallel to the bedding of the Pliocene Chinshui Formation. In 1999 the Chi-Chi earthquake (M_w 7.6) created a multi-kilometer surface rupture along the fault. Extensive studies of the fault and the earthquake were undertaken by the Taiwan Chelungpu Fault Drilling Project (TCDP). Under this project two boreholes were drilled during 2004–2005 (holes A and B) in

Known stress data

It is rational to assume that the state of stress in the vicinity of the two scientific holes A and B is one in which the vertical stress is a principal component, rendering the other two principal stresses horizontal. This is because the topography is gentle and not a factor at the great depths of 940–1310 m, there are no known igneous intrusions or salt domes, and the sedimentary layers are sub-horizontal, and thus not affecting the general verticality of the gravitational force from which

Unknown stress component

The only unknown stress component is the maximum in situ horizontal stress (σ_H). Computing σ_H from hydraulic fracturing or leak-off test results makes use of several assumptions that have recently been challenged by researchers (see for example Rutqvist et al., 2000, Ito et al., 2002). An alternative method of estimating σ_H had been introduced by Vernik and Zoback (1992). The method makes the reasonable assumption that the points of intersection between breakout and borehole wall cross section

True triaxial strength criterion of TCDP siltstone

We conducted true triaxial compressive tests simulating stress conditions at the borehole wall on core made available to us from 1251.3–1252.5 m in hole A, just below the fault zone at 1111 m. It is a siltstone belonging to the Pliocene Chinshui Formation, which is found between the depths of 1013 and 1313 m (Wu et al., 2007). Tests were conducted using the University of Wisconsin polyaxial loading system, which enables the application of three mutually orthogonal loads to rectangular prismatic

Estimation of the maximum horizontal in situ stress

We integrated the true triaxial strength criteria for the TCDP siltstone in the computation of the maximum horizontal principal stress magnitudes in the depth range of 940–1310 m of hole B, within which borehole breakouts were logged. An example of a Formation MicroImager (FMI) log showing clear electric images of breakouts is shown in Fig. 8 (Lin et al., 2007). The assumption made is that the state of stress at the breakout-borehole intersection (points B and B′ or their mirror image in Fig. 9

Discussion

The method of in situ stress estimation described here can only be carried out in boreholes that have developed breakouts. In addition, the method requires that hydraulic fracturing or leak-off tests be conducted, that borehole breakouts be logged (FMI, acoustic televiewer, or other techniques), and that the true triaxial strength of the rock be established. As such it is suitable mainly for major scientific deep-hole projects, like the KTB or the TCDP. Shallow boreholes of a few hundred meters

Conclusions

This paper reports on an alternative method for estimating the complete state of stress, and its use in the TCDP scientific holes. By integrating logged borehole-breakout dimensions with the true triaxial strength criterion of the Chinshui siltstone and leak-off test data, and assuming that the induced hydraulic fractures were vertical, the state of in situ stress in the vicinity of Chelungpu Fault was estimated as one favoring a type of faulting bordering between strike-slip and thrust.

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Instead of being directly measured, the upper limit value of the maximum horizontal in-situ stress can be determined by the stress polygon based on frictional limit theory (Moos and Zoback, 1990; Zoback et al., 2003; Zoback, 2007; Jaeger et al., 2009) and the improved stress polygon (Zhang and Zhang, 2017; Zhang et al., 2018a), while the lower limit can be obtained by the methods involving the shear failure and tensile fracture of a borehole. The shear failure method is related to the borehole breakout width (Zoback et al., 1985; Haimson and Herrick, 1986; Peška and Zoback, 1995; Barton et al., 1988; Moos and Zoback, 1990; Vernik and Zoback, 1992; Barton and Zoback, 1994; Haimson et al., 2010; Schmitt et al., 2012; Wu et al., 2012, 2019; Reis et al., 2013; Zakharova and Goldberg, 2014; Huffman and Saffer, 2016; Kim et al., 2017; Zhang and Zhang, 2017; Song and Chang, 2017, 2018; Fellgett et al., 2018; Lee and Ong, 2018; Zhang et al., 2018a, b; Valley and Evans, 2019; Massiot et al., 2019). The breakout width is found directly from the logging data, including the electronic resistivity image and ultrasonic borehole tele-viewer.
An accurate estimation of the magnitudes of three in-situ stresses can usefully improve the production performance of hydraulic fracturing and reduce the risk of wellbore instability. Considering two types of linear elastic stress around the non-perforated arbitrarily inclined borehole (plane strain and plane stress conditions), the theory of hydraulic fracturing, and the maximum tensile criterion, the generalized governing formulas are derived to explain the mechanism of hydraulically or drilling-induced longitudinal, oblique, and transverse tensile wall-fractures in arbitrarily inclined boreholes. These formulas provide useful implications for constraints on the magnitudes of in-situ stresses, which determine the initiation of different types of tensile wall-fractures under different faulting stress regimes. The present paper extends the initiating conditions of the common transverse tensile wall-fractures related to the works of Hossain et al. (2000) and Nelson et al. (2005). The prerequisite for the special transverse tensile wall-fractures that tensile fractures simultaneously initiated around a whole borehole wall (Huang et al., 2012) is generalized by introducing the concept of the turning locations. The formulas corresponding to the turning locations are provided under different faulting stress regimes, revealing that the work proposed by Huang et al. (2012) is only a particular case. It is found that the turning locations are related to the critical borehole inclination or azimuth angles. Overall, all the formulas in this study for longitudinal, oblique, and transverse tensile wall-fractures can be used to design hydraulic fracturing experiments and to determine the in-situ stress by using the information of drilling-induced tensile wall-fractures. Moreover, the governing formulas of the turning locations can also be used to optimize the borehole trajectory.
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Knowledge of the in situ stress state of rock mass is fundamental for engineering, geological and geophysical applications. In situ stress state determination requires in principle the evaluation of the three principal stresses and the related principal directions, but it is widely recognized in the literature that the maximum horizontal stress is the most difficult component to accurately estimate. In the context of borehole methods, this paper proposes a step-by-step analytical procedure to estimate some bounds to the maximum horizontal stress, starting from a geomechanical description of the rock and relying on information generally available in the engineering practice. The procedure is divided in substeps, each one requiring additional information about the mechanical properties of the rock and on the geometrical properties of the failed portion of rock: more information available implies a lower uncertainty on in situ stress estimate. Furthermore, since the proposed procedure is analytical, it allows a complete and very easy implementation in a spreadsheet. The aim of the work is thus to provide a rigourous but simple analytical tool that can be used in engineering practicte to estimate some bounds to the maximum horizontal in situ stress state. The approach is finally validated by means of both numerical simulations, performed with a sophisticated numerical tool, and experimental field data coming from the literature.
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After the Wenchuan Earthquake on May 12th, 2008, the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD) was initiated in order to investigate the structure of the fault zones and the mechanism of the earthquake. The WFSD contains four boreholes (WFSD-1, WFSD-2, WFSD-3 and WFSD-4) lying at the maximum displacement locations along the Yingxiu-Beichuan fault zone and the Guanxian-Anxian fault zone, and WFSD-2 is the second borehole and is still being drilled. Core samples, resistivity and acoustic image logging data were acquired from 50 to 1370 m. The natural fractures, borehole breakouts, drilling-induced fractures and drilling-enhanced natural fractures were identified from the cores and the image logs and were statistically analyzed. The strikes of the natural fractures systematically vary and can be sorted into four groups according to depth: (1) above 637 m, mainly striking ENE–WSW; (2) in the interval of 637–932.6 m, striking NNE–SSW; (3) in the interval of 932.6–1200 m, directed ENE–WSW then to WNW–ESE, while striking NE–SW from 1030 m to 1150 m; (4) from 1200 m to 1370 m, maintaining a strike of WNW–ESE. The natural fractures from 50 m to 637 m seem to be reverse faults which strike approximately parallelly to the main fault. Two sets of conjugate fractures around 1002.4 m indicating subvertical maximum principal paleo-stress direction may be a subordinate structure of the main fault caused by a local stress field, and it reveals the complex stress field of Yingxiu-Beichuan fault zone when the fractures formed. A total of 12 BOs, 2 sets of DIFs and one set of DEFs with an overall length of 30.4 m were interpreted from 960 m to 1370 m in WFSD-2. The average S_Hmax orientation interpreted for WFSD-2 (960–1370 m) is 120.7°–300.7°N (i.e. WNW–ESE) with the standard deviation of 9.2° and it is consistent with the stress status of Yingxiu-Beichuan fault zone which is one of the main fault zones in the 2008 Wenchuan Earthquake.
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Integrating borehole-breakout dimensions, strength criteria, and leak-off test results, to constrain the state of stress across the Chelungpu Fault, Taiwan

Abstract

Introduction

Section snippets

The Taiwan Chelungpu Fault Drilling Project (TCDP)

Known stress data

Unknown stress component

True triaxial strength criterion of TCDP siltstone

Estimation of the maximum horizontal in situ stress

Discussion

Conclusions

Int. J. Rock Mech. Min. Sci.

Tectonophysics

Int. J. Rock Mech. Min. Sci.

Int. J. Rock Mech. Min. Sci.

Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: implications for crustal strength

J. Geophys. Res.

Discussion on “Wellbore breakouts and insitu stress,” by M. Zoback et al.

J. Geophys. Res.

True triaxial strength of the KTB amphibolite under borehole wall conditions and its used to estimate the maximum horizontal in situ stress

J. Geophys. Res.

The Release 2008 of the World Stress Map