Effect of faults and rock physical properties on in situ stress within highly heterogeneous carbonate reservoirs

摘要

This study characterizes the reservoir-scale in situ stress state within a carbonate oil and gas field in the southeast United Arab Emirates (UAE) to further our understanding of the effects of faults and rock property heterogeneity on variations in stress. The in situ stress state was constrained using image-logged wellbore wall stress indicators (i.e., borehole breakouts and drilling-induced tensile fractures) observed within several vertical boreholes penetrating layered carbonate rock formations. The orientation and magnitude of the in situ stress state vary significantly with both location and depth. The azimuth of the maximum horizontal principal stress (S-Hmax) has diverse orientations that vary significantly with depth and span a similar to 60 degrees range from NW to NNE. The magnitudes of horizontal principal stresses also vary significantly, meaning that no unique stress-depth relations can be defined. Despite considerable fluctuation in stress magnitudes with depth, the stress state in the reservoir is generally in a strike-slip faulting regime, which is consistent with that prevailing throughout onshore UAE. These widespread variations in stress orientation and magnitude appear to be linked to a combination of geomechanical processes associated with the presence of a set of reservoir-scale faults and the highly heterogeneous mechanical properties of the carbonate reservoir. Our analysis indicates that the S-Hmax azimuth rotates proximal to faults from the prevailing far-field stress orientation toward the strike of the faults, but the angle of this azimuthal rotation is dependent on the Young's modulus characteristics of individual formations. The gradient of the horizontal principal stress tends to increase with increasing Young's modulus values, indicating that stiffer formations transfer higher magnitudes of stress. The observed stress variations also agree well with the results of three-dimensional finite element modelling. Our study demonstrates that the strong mechanical heterogeneity of carbonate reservoirs controls the in situ stress state in a complex but explainable way.