The state of stress in a rock mass is known to be important data for all geomechanical applications, from wellbore stability to reservoir stimulation. There is, however, no single method that allows one to access the full stress tensor. In this paper, we present a case study in a carbonate/shale sequence in which the full stress tensor was determined as a function of depth with a combination of microhydraulic fracturing and wellbore image analysis applied to vertical, deviated, and horizontal wellbores.
Vertical wells are the best starting point for the determination of the complete stress tensor. In this setting, openhole hydraulic fracturing is the most robust method to determine the magnitude of the minimum in situ stress. The use of a wireline formation tester and a special methodology allows one to determine the precision of the measurement. Image analysis completes this technique by providing the orientation of the minimum stress. Once this information is known, the combination of hydraulic fracturing and image analysis in deviated and horizontal wells allows the determination of the complete state of stress.
Analysis of the position of breakouts in deviated wellbores provides both the ordering of the stresses and a shape factor related to their relative magnitudes. In this study, the vertical stress proved to be the intermediate stress in most of the sequence. In deviated wells, the hydraulic testing of preexisting fractures selected on images also allows the determination of the vertical stress. In our example, vertical stress was found to differ from the integration of the rock density depending on the point in the sequence. Using packers to fracture the formation and analyzing the orientation of the resulting fracture also yielded an estimate of the complete stress tensor.
In horizontal wells drilled parallel to the direction of a principal stress, the combination of hydraulic fracturing and packer fracturing allows the measurement of the two stresses acting perpendicular to the wellbore. In the case study, in which the well was drilled parallel to the minimum stress, the technique also provided the minimum stress magnitude. In such a setting, image analysis is crucial to confirm the orientation of the created fractures and thus which stress is measured.
This case study demonstrates that these two methods are very complementary. In particular, their combination provides access to the maximum horizontal stress, which is notoriously difficult to estimate.