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INTRODUCTION ABSTRACT: The fracture of rock is influenced by the development of an intrinsic process zone in the form of a localized region of microcracking. This zone has a fundamental importance for defining the system behavior in terms of the post-peak instability and in terms of material strength such that size effects appear. This paper illustrates the change in the load-displacement response and presents evidence of the process- zone development with varying specimen size. It is demonstrated that fracture problems do not require geometric or material nonlineaddty to produce instability. The size of a structure that fails by fracture is an important factor. Further more, experiments suggest that an intrinsic zone develops in rock as a material characteristic. Because of this intrinsic length, two competing factors define the nominal strength---the positive contribution of the process zone and the depleting aspects of the undamaged volume, that is, the size. Experiments on rock may exhibit behavior quite different for specimens of various size. This leads to the problems of interpreting test results, generalizing their significance, and identifying materials properties. For example, in rock-like mateddais, the nominal strength and the global response of a given geometry and load configuration are dependent on size (Bazant 1993). Due to the existence of a localized zone of microcracking, a specimen composed of rock may fail at a stress quite differenthan that shown by another specimen size. Conversely, for elastic- brittle and elasto-plastic materials, experiments and stress analysis indicate no dependence on scale: specimens of different sizes fail at the same maximum stress and in the same manner. An explanation for size effect was first offered by Weibull 0939)using a statistical argument. He showed that the strength of a material is a function of the volume of the specimen through the application of the weakest link concept. As explained by Bazant & Xi 0990, however, the Weibull theory may be inadequate because it ignores the stress distributions due to localized damage (the intrinsic process zone) prior to maximum stress. Extensive evidence is available from direct or indirect tensile tests indicating that the apparent or nominal strength is size dependent (Gluckiich & Cohen 1967; Hardy et al. 1973; Swan 1980; Bazant & Kazemi 1990). For uniaxial compression, Millard et al. (1955) and Evans & Pommeroy (1958) experimentally observed a strong correlation between size and strength, and suggested a relation in the form [Equation available in full paper](1) where o? is the nominal strength, k and n are constants, and a is the length of the cubic specimen, a characteristic size of the structure. Millard et al. noted that n on the order of 0.5 is expected if existing crack lengths are proportional to the sides of the cubic specimens. It is evident from experimental observations of rock (Zietlow & Labuz 1998) that an analysis of the structural behavior and in particular, an evaluation of the nominal strength requires a knowledge of the evolution of microcracking as a function of applied loads. Among the methods used to examine development of microcracks within a test specimen is the acoustic emission (AE) technique (for instance, Shah & Labuz 1995), which is based on the recording of transient elastic waves resulting from the sudden release of energy due to microcracking. The locations of AE close to peak load can define the region of localized damage prior to a visible fracture.
ABSTRACT: It is well known that depleting a hydrocarbon reservoir can redistribute the in-situ stresses sufficiently to reactivate and induce slip in nearby faults, which otherwise usually evolve over geologic periods. In this paper, we develop a method to calculate the redistributed stresses around a depleted, heterogeneous reservoir with non-uniform pressure distribution. Using this method, we analyze the stability of nearby faults and show that different reservoir depletion strategies can affect the fault stability differently. The reservoir is modeled as a thin poroelastic inclusion in an elaStic matrix while the fault gouge obeys Mohr- Coulomb failure criterion. The developed method allows considering the reservoirs and faults to be of arbitrary shapes and matedHal properties while the pressure inside the reservoir does not have to be uniform. After the fault is reactivated, it is modeled as a shear (mode II) fracture. Displacement discontinuity between the sides of this fracture gives an estimate of the fault slip magnitude. INTRODUCTION Geological discontinuities such as faults are inherent in most petroleum formations. It is well known that changing fluid state in a hydrocarbon reservoir can redistribute in-situ stresses sufficiently to reactivate and induce slip in nearby faults, which otherwise usually evolve over geologic periods. This work centers on analyzing the stability of faults due to reservoir depletion. From the engineering view point, the consequences of fault reactivation can range from shearing of the boreholes (drilled through the fault zone), to dynamic release of the stored elastic energy and induced seismicity, to drastic change of the formation permeability and the production-depletion strategies. From the scientific viewpoint, the addressed phenomenon represents a clear and robust demonstration of the importance? of poroelastic effect (in this context first suggested by Geertsma, 1966). Indeed, the stability of the fault is not affected at all if the poroelasticity of the reservoir material is not taken into account. Fault reactivation due to stress redistribution caused by natural resource recovery is not unusual and represents a typical example of human-induced seismicity. In mining operations, it is well known that man-made underground openings (cavities) redistribnte remote stresses in their vicinities affecting the stability of near-by faults and triggering seismic events (e.g., see Ortlepp, 1997). Similarly, in the petroleum industry, fracture reactivation is sometimes attributed to small-scale failure associated with drilling boreholes (e.g., Mokhel et al., 1996; Thiercelin and Atkinson, 1996). In both cases, open cavities probably represent the extreme case of real situation maximizing the stress disturbance. However, filled cavities (inclusions) can also redistribute stresses sufficiently for the fault to slip. This is of importance for hydrocarbon recovery when the solid matedHal is not actually removed (mined), but the initial state of stress is disturbed by the withdrawal of subsurface fluids (e.g., Geertsma, 1966; Segall, 1989; Addis et al., 1998; Rudnicki, 1999). Probably the largest seismic events triggered by gas extraction are three major earthquakes (with magnitude M> 7) reported near the Gazli (Uzbekistan) gas field in 1976-1984 (e.g., Simpson and Leith, 1985). The largest registered earthquake triggered by the oil withdrawal is probably the 1983 Coalinga (California) earthquake with M=6.5 associated with production from the Anticline Ridge oil fields (McGarr, 1991). Detailed description of these and many other examples of extraction- induced seismicity can be found in the reviews of Yerkes and Castle (1976) and Nicholson and Wess
- Europe (1.00)
- North America > United States > Texas (0.46)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
INTRODUCTION ABSTRACT: The paper applies a new method based on block theory, stress analysis and linear programming to study the stability of a statically indeterminate 2-d block under arbitrary loads. The stability of keyblocks subjected to a range of directions of a fictitious loading or support force is analyzed using linear programming. Based on this optimization procedure, a keyblock can be classified as stable, unstable or potentially unstable. The range of loading force where a keyblock may be potentially unstable (uncertainty in stability) is affected by the indeterminacy of the force system and the angle of applied load. For a wide range of applied load angles, the uncertainty is small. Hence, the technique presented here would provide useful results for design. Block theory is a useful analysis tool for investigating the stability of a rock mass (Goodman & Shi 1985). The method of block theory is based on a geometric analysis of blocks formed by the intersection of a particular set of half-spaces. First, all infinite and non- removable blocks are eliminated by kinematic analysis. Second, block theory uses kinetic analysis to differentiate between stable and unstable blocks without regard to surface metions arising from in-situ stresses. Finally, based on the physical and mechanical properties of the joint planes, the stability of keyblocks is assessed. If the keyblock is unstable, the required support force is calculated, and the support scheme is designed accordingly (Hatzor & Goodman 1993, Warburton 1993, Windsor 1997). The standard block theory approach, however, can lead to excessively over-conservative design because it ignores the stabilizing effect of surface forces arising from field stresses (Mauldon et al. 1997a,b). Roof blocks bounded by steeply inclined joints in the crown of a tunnel, for example, may be stable because of frictional shear stresses on the joint plane(s). Block theory, however, would predict such roof blocks to be unstable under gravitational loading. Not only does block theory lead to over- conservative design, but when the action of additional loading or support forces is considered, the problem becomes statically indeterminate. Current approaches to handling the indeterminacy include relaxation techniques (Brady & Brown 1993) and numerical methodsuch as DDA (Shi 1988, Yeung 1991, Lin et al. 1996), and UDEC (Cundall & Hart 1993). Geologic uncertainties in tunneling have been discussed by Einstein et al. (1996). Using linear programming, limits on the stability of 2-d keyblocks under the combined influence of field stresses, self weight, and additional loading or support forces can be evaluated. The paper uses this optimization technique to examine the effects of applying loading forces at various angles relative to the free face of a keyblock in the crown of a tunnel (see Table 3 and Fig. 4).
INTRODUCTION ABSTRACT: An efficient approach to the management of wellbore instability in shales is presented. It takes into consideration the factors which determine the degree of complexities required in developing mud weight program to provide the required effective mud support with time. A range of design and analysis tools required for wellbore stability analysis is described systematically. The application of some of the tools is demonstrated through two field case studies to develop strategies to control shale instability in wells drilled in the North West Shelf of Australia. The tools provide a practical means of opfimising the approach in developing the solution, including drilling fluid design (weight, type and chemistry), to manage shale instability efficiently. Wellbore instability, experienced mainly in shale sections, may be induced by either in-situ stresses that are high relative to the strength of the formations (stress-induced) or physico-cbemical interactions of the drilling fluid with the shale or a combination of both (Tan & Willoughby 1993, Mody & Hale 1993, van Oort et a1.1995, Last et al. 1995, Tan et al. 1998). The dominant stability mechanism(s) is dependent on a wide range of factors including type of shale, in-situ stress environment, thermal gradient and drilling fluid system. Hence, for efficient management of shale instability, the significance of the factors should be assessed, and the most efficient approach adopted on a field by field basis. This paper describes an efficient approach to the management of shale instability. It takes into consideration the factors which determine the degree of complexity required in developing a mud weight program to provide the required effective mud support with time. A range of design and analysis tools required for the stability analysis is described systematically. The application of some of the tools is demonstrated through two field case studies to develop strategies to control shale instability in wells drilled in the North West Shelf of Australia.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)