Effects of transient borehole deformation on rock stress and rock properties analysis
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Date
2021-06-02
Authors
Han, Hong Xue
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Understanding the in-situ stresses and rock mechanical properties is necessary to ensure successful drilling, quality wellbore completion, and reservoir performance analysis. However, the conventional determination methods normally require a substantial financial investment, including prolonged rig standby time, and results might only be available for limited formations in a few wells in an oilfield. In this research, a novel, economical, and rapid methodology is presented to estimate the in-situ stresses and rock mechanical parameters from borehole deformation data, which are determined from four-arm caliper logs. Nevertheless, three significant challenges exist in the development of the new approach.
The first challenge is the conventional application of circular-borehole-based linear elastic analytical solutions to the estimation of the in-situ stresses in shale formations, which often deform in a time-dependent manner so that the borehole becomes progressively more non-circular after initial elastic deformation. In order to address this issue, a three-dimensional poro-visco-elastic simulation approach is developed using the Finite Element Method (FEM) to analyze the time-dependent borehole deformation and assess its influence on the inversion process for in-situ stress estimation.
The second challenge is the dilemma of using the default bit size as the original borehole size to calculate borehole deformations. To address this dilemma, an original borehole size is estimated that is different from the bit size for the quantification of the borehole deformations. The influence of the original borehole size on the borehole deformation response is investigated in this research.
The third challenge is the non-uniqueness of solutions due to the limitations of the available known parameters. This problem is addressed by applying optimizations of the in-situ stresses inversion using the normalized weighted-sum multi-objective function. Continuous in-situ stresses profiles are generated for practical applications using this approach.
The methodology has been successfully demonstrated in the determination of in-situ stresses and the rock mechanical parameters in cases studies in North America and China. The simulation results for these case studies indicate first that the time-dependent borehole deformation is mainly influenced by the visco-elastic properties of the rock; second, that pore pressure diffusion effects have a negligible influence on the time-dependent borehole deformations; third, for visco-elastic rocks, from a geomechanics point of view, borehole breakouts will be enhanced by rock creep effects; and last, drilling induced (tensile) fractures will not likely happen during the period of the time-dependent borehole deformations.