In this study, we present regional in situ stress results analyzed based on geophysical logs and formation micro imager log data from two vertical boreholes in a carbonate oil and gas field to demonstrate how rock mechanical properties control in situ stress tensors. The carbonate reservoir exhibits highly heterogeneous rock properties depending on lithology. Horizontal principal stress magnitudes constrained by using borehole breakout width and the presence of drilling-induced tensile fractures show a wide variation in stress gradient. We note that the maximum horizontal principal stress gradient increases approximately linearly with Young's modulus of layer, indicating that the heterogeneity in rock mechanical properties controls in situ stress magnitudes severely. Thus, a stiffer layer conveys a higher horizontal stress. Our results show that the influence of rock mechanical properties on in situ stress should be considered in field development.
During the last few decades, the substantial increase of in situ stress data has made a great advance in understanding the stress field of Earth's crust. In general, crustal stress field shows partial variation at different scales of observation. At local scales, a deviation from regional pattern may be observed implicating the presence of other factors simultaneously affects crustal stress field. So far, local scale stress perturbations are believed to be the result of the presence of natural discontinuities (such as faults, fractures or bedding planes) and the mechanical heterogeneities in a rock formation. For example, there is ample evidence that the slip on faults and fractures induces stress release enough to perturb the stress state adjacent to them (Barton and Zoback, 1994; Shamir et al., 1992, Sahara et al., 2014, Rajabi et al., 2015, Lin et al., 2010). The influence of rock mechanical properties on stress state is especially pronounced in the heterogeneous rock formations such as carbonates, which has also been well documented in several previous studies (Teufel, 1991; Bruno and Winterstein, 1994; Wileveau et al., 2007). Understanding these perturbation mechanisms in local stress field is important for hydrocarbon reservoirs management.
In the present study, we had an opportunity to characterize the state of stress in a carbonate oil and gas field, onshore United Arab Emirates, where the reservoir is inherently heterogeneous in term of both geological structure and rock properties. The interbedding deposition characteristics of the reservoir and non-reservoir sections represent cyclical sedimentation, in which transgressions result in deposition of dense layers and regressions form the clean porous reservoir zones. It is worth to note that such depositional feature causes the great heterogeneity in physical and mechanical properties within reservoir. Our estimation of rock mechanical properties show that Young's modulus significantly varies in a bimodal pattern approximately between 20 and 60 GPa. From the geomechanical point of view, carbonate formations in the study area can be separated into two main distinctive mechanical units, i.e., stiff and soft layers based on Young's modulus (Fig. 1).
