Application of the Modified Borehole Wall Stress Relief Method in Deep Vertical Boreholes

The in situ stress conditions of reservoirs are the fundamental parameters for determining the arrangements of horizontal boreholes and assessing borehole stability, especially for the exploration and exploitation of unconventional oil and gas. The borehole wall stress relief method (BWSRM) is a new technique that can directly measure three-dimensional (3D) stress without any assumptions regarding the stress field. In deep vertical borehole situations, a new experiment is proposed to prevent electronic components from becoming damp while simultaneously maintaining borehole stability. The effect of drilling fluid pressure on the strain measurement of the stress relief processes is studied. The finite-element model (FEM) simulates the stress relief process and verifies the theoretical solutions under different testing depths. The results show that complete stress relief is impossible under deep vertical borehole conditions, and the wall drilling process changes the stress conditions of the measured points from modified plane strain to triaxial compression. Therefore, the data measured by the strain gauges cannot be directly imported into the original relationships between the measured strains and far-field stress. The comparisons between the original theoretical solutions and FEM results indicate that if the confining pressure is ignored, then the maximum error between the measured and real in situ stress components is over 35%. After correcting the estimated strain data using the proposed methods, the stress tensors obtained from theoretical equations are consistent with the natural stress tensors.