There has been a great deal of interest in mapping regional trends in stress orientation and their relationships to crustal deformation processes over the last quarter century. Less emphasis has been placed on explaining local variations in the stress field. However, it is variations in the local stress field which are most critical to characterizing hydrocarbon reservoirs. Fracture orientation, well stability, well orientation, and permeability anisotropy are all strongly affected by variations in the local stress field.Using stress orientation data from a number of fields in different tectonic environments, we have tried to determine some of the tectonic and geological controls on variations in in situ stress orientation. We find that nearness to faults, fault structure, and magnitude of tectonic stress play primary roles in determining whether the regional stress field will be perturbed in a given reservoir and whether small-scale variations in the stress field can be expected.We find that high tectonic stress environments (large horizontal differential stress) lead to a much more consistent local stress field than more tectonically quiescent areas. Faults can play a large role in rotating the local stress field. The smaller the difference between the maximum and minimum horizontal stress magnitude (i.e. lower tectonic stress), the larger a fault's zone of influence is. Large-scale faulting with segmentation of the reservoir into discrete fault blocks can lead to significant stress orientation variations across the reservoir even in areas of large differential stress.
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AbstractThe "critical state" concept of elastoplastic constitutive behavior has been utilized extensively to model the stressstrain response of many geomaterials that undergo plastic yielding. This paper uses a critical state model to investigate the deformation and deformation-induced permeability changes observed in laboratory experiments conducted on siliciclastic and carbonate petroleum-reservoir rocks. We find that the shape of the yield surface has significant impact on post-yield fluid flow response. Differences in behavior between the rock types tested can be explained qualitatively through consideration of differences in the micromechanics of deformation. We find that permeability changes with both stress and strain in general follow a similar constitutive model as deformation. However, we observe experimental behavior not fully accounted for by the critical state concept, due to mechanical complexity on the microstructural level.
We studied the correlation between near‐surface in situ stress and the preferred orientation of microcracks at two quarries in the Milford granite and one quarry in the Conway granite, New Hampshire. The orientation of in situ stress was determined by overcoring doorstoppers and from the strike of induced borehole fractures produced by a pressurized packer. The preferred orientation of vertical microcracks was determined using ultrasonic measurements to determine P wave velocity Vp on core and in situ and from thin section analyses. In situ Vp anisotropy was determined from interborehole travel time data. At all three sites, directions of maximum compressive stress σ1 and induced borehole fractures are aligned with the preferred orientation of open microcracks determined from core Vp and thin section data. An analysis of the microcracking sequence within each pluton suggests (1) that the quarry grain resulted from cooling‐induced thermal stresses and (2) a method of distinguishing the paleostress axes at the time of cooling from contemporary stress.
Two dimensional acoustic images of sedimentary rocks were generated in order to characterize centimeter‐scale heterogeneities and microstratigraphy. The images are based on dense sampling of the compressional phase velocity at frequencies near 1 MHz. The acoustic observations are compared to visual and x‐ray images of the same samples. The results are intrepreted in terms of microscopic compositional variations. Sedimentary rocks are more variable on the scale of a centimeter than we expected. The homogeneous Berea sandstone exhibits a ±3% random variation in velocity. Heterogeneous rocks show 20 and 30% velocity variations. In the particular case where porosity is the sole compositional inhomogeneity, the acoustic and radiographic images agree. In general however, the acoustic images yield different information from the visual and radiographic images.
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