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_Journal of Structural Geology 32 (2010) 1061-1069_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Stress determination in active thrust belts: An alternative leak-off pressure interpretation Brent A. Couzens-Schultz*, Alvin W. Chan Shell International E&P, Inc., P.O. Box 481, Houston, TX 77001, United States Article info Article history: Received 12 May 2009; Received in revised form 17 June 2010; Accepted 20 June 2010; Available online 25 June 2010 Keywords: Stress; Stress determination; Thrust compression; Wellbore stability; Leak-off test Abstract In thrust belts, fluid flow through critically stressed fractures will occur at pressures less than the overburden stress, which is the minimum stress. We propose that low leak-off pressures obtained in active thrust belts may result from this mechanism, leading workers to infer that apparent minimum stresses are 30-60% less than the overburden stress in some compressional settings. Traditionally, leakoff pressure data have been used to constrain the magnitude of minimum stress, assuming that the rock is dilating against the minimum stress during a leak-off test. In our new interpretation, we constrain the stress state by assuming that the leak-off test causes shear failure along pre-existing weaknesses rather than tensile opening. While this mechanism has been discussed in a small number of borehole stability and hydraulic fracture papers, it has not been directly applied to leak-off tests. We considered this interpretation because we observed that some leak-off tests imply an apparent contradiction between the stress states from the standard interpretation of leak-off tests versus the stress state inferred from geologic and geophysical evidence in tectonically active thrust belts. We present two examples, one in an onshore fold-thrust belt and one in a deepwater fold-thrust belt. Our new interpretation of stresses based on shear failure resolves the contradiction and also provides a constraint on the maximum horizontal stress in the fold-thrust belts. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Leak-off tests are commonly used to interpret the minimum stress magnitude (Baumgartner and Zoback, 1989; De Bree and Walters, 1989; Sarda et al., 1992; Addis et al., 1998; Yamamoto, 2003; Zoback et al., 2003). The test is a routine procedure used to determine the pressure at which the exposed formation will fracture (the fracture pressure). It is performed with drilling mud, which is also the material that is circulated through the borehole during drilling. The mud is composed of a mixture of water or oil, clays, weighting materials and other chemicals that are used to control its properties including viscosity and density. Drilling mud prevents destabilization of the wellbore walls and is used to counteract the pressure of fluids inside the rock so that they cannot enter the wellbore. When casing is set, a leak-off test (LOT) is performed to determine the fracture pressure at the base of the casing and thus, the upper limit to the mudweight for further drilling of the borehole before additional casing will need to be set. A LOT is a pumping pressure test, similar to a fracture test. After the casing is cemented in place, a few meters of open hole is drilled out below the casing. During the test, the well is shut-in and pressurized by drilling mud delivered through the drill pipe from a cementing pump set on the drill rig floor (Fig. 1a). The pressure in the open hole is the sum of the weight of the drilling fluid column and the pumping pressure. During pumping, pressure is measured at the surface and sometimes by a gauge that is placed on the bottom of the hole. Over time, or volume pumped, the mud pressure builds linearly as the mud column compresses and the casing and rock around the borehole expand elastically. When fluids begin to enter the surrounding rock from the borehole, or "leak-off," the pressure build-up will deviate from this linear trend (Fig. 1b). The point where this deviation occurs is known as the leak-off pressure (LOP). The LOP then dictates the greatest mudweight that can be used to drill the next section of open borehole. Most commonly, casing is set and a LOT is performed in a low permeability mudrock. In these cases, the leak-off pressure is assumed to reflect either the opening of existing fractures in the rock or the initiation of a new tensile fracture. Therefore, at leakoff, the mud pressure in the open borehole may represent the minimum stress in a fractured or weak formation, or the minimum stress plus the tensile failure strength of an intact borehole, which is controlled by local stresses around the borehole (Fig. 2a). Guidelines for interpreting LOT data often conclude that leak-off pressures in mudrocks can be used as reasonable estimate of the least principal stress (Baumgartner and Zoback, 1989; De Bree and Walters, 1989; Sarda et al., 1992; Addis et al., 1998; White et al., 2002; Yamamoto, 2003; Zoback et al., 2003). However, issues do exist with testing procedures, equipment, and interpretation that lead to uncertainties (Kunze and Steiger, 1991; Enever et al., 1996; Gj?nnes et al., 1998; Raaen et al., 2006). To overcome these uncertainties, the LOT can be run further as an extended LOT (Fig. 1b) to determine a fracture closure pressure (FCP, Fig. 1b; Gaarenstroom et al., 1993), which is measured after pumping is stopped and drilling fluids are no longer propping open any existing or created fractures. The test can also be run multiple times (e.g., Yamamoto, 2003), and a consistent fracture closure pressure gives greater confidence in the minimum stress interpretation. However, due to the test duration and associated cost, simpler tests that run to only leak-off pressure are most common in the oil and gas industry. The data discussed in this paper includes some tests taken only to leak-off pressure and some extended tests taken through one cycle to fracture closure pressure (Fig. 1b). In a deltaic basin on a passive margin, where both horizontal stresses are less than the overburden, it is assumed that during a LOT, any fracture that is generated will be approximately vertical and normal to the minimum horizontal stress and the LOP will reflect the minimum stress magnitude. In an active thrust belt setting, where horizontal tectonic compressive stresses are expected, the minimum stress is close to vertical. Therefore, we assume that any fracture generated during a LOT will be subhorizontal and the LOP should be near overburden. In a compressive system, it is possible that overburden is not exactly the minimum stress because the principal stresses in thrust systems can be rotated near active thrust faults or detachments (Hafner, 1951; Last and McLean, 1996). If the stresses are rotated, then the minimum stress will be less than the overburden pressure. To obtain an upper bound for how much less the minimum stresses could be, we examined the difference between the minimum principal stress and the overburden stress assuming the stress field is rotated 30°-45°. The difference between the maximum and minimum principal stresses is a function of the frictional properties (e.g., Jaeger and Cook, 1979). If we assume that the rocks fail according to Byerlee’s (1978) law and that the minimum stress is vertical, a simple Mohr Circle analysis shows that in a hydropressured compressional system the maximum stress can be two to three times greater than the minimum stress. Using a maximum to minimum stress ratio of two to three, simple stress ellipse geometry shows that the difference between the minimum principal stress and the vertical stress is about 10°-25°. Therefore, including rotated compressive stress fields, we might expect all leak-off pressures in compressive settings to be between 90% and 100% of the overburden pressure. However, drilling experiences reveal that the leak-off pressure in thrust belts can be much less than 90% of overburden, in some cases up to 60% less. This experience has led to interpretations of normal fault and strike-slip stress regimes in these thrust systems (e.g., Last and McLean, 1996; Tutuncu et al., 2006; Lin et al., 2007). One explanation is that the current stress state in a fold and thrust belt has evolved to strike-slip conditions due to a stress drop. 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