Numerical Study of Reservoir Geomechanical Pressuremeter Testing under Anisotropic In-situ Stresses

• Author / Creator

  Liu, Lang

• The anisotropy of horizontal stresses in the saturated medium presents tremendous difficulties to the determination of in-situ rock properties especially when the formation is poorly overconsolidated. The application of reservoir geomechanical pressuremeter designed to pressurize the borehole at considerable depth has been numerically assessed and has shown considerable promise for capturing the key in-situ geomechanical parameters. This research has two primary objectives: (1) to investigate the mechanical behavior of borehole during the sequence of drilling, relaxation, expansion and contraction in the testing pockets and (2) to develop an interpretation methodology to invert for horizontal anisotropy and other in-situ parameters from the downhole pressuremeter testing in soft rock. The formation drainage condition was examined considering factors such as pressuremeter loading rate and formation permeability by evaluating both the excess pore pressure generation and void ratio variation within the formation. The finding shows that the cavity expansion in most soft sedimentary formations is very likely occurring under partially drained conditions and the in-situ permeability is only minimally affected by loading. Yielding, if any, which is triggered by drilling and developed in the relaxation, can soften the materials and thus lead to a remarkable rise for both pore pressure and radial displacement if the borehole is subsequently expanded. The possible stress regimes for the deep formations were analyzed based on the yielding criterion of Cam Clay model. The short term and long term borehole deformation and damages induced by drilling were both investigated. The time-dependent convergences were correlated with the initial horizontal stress anisotropies, which makes the inversion possible by matching the convergence rate in the chart of characteristic curve. The influences of initial stress anisotropy and drilling-induced modulus heterogeneity on the resultant stress profile after expansion were discussed. The possible shear failure regions under different loading regimes were compared. It is necessary to extend the study into the relatively high permeability formation that exhibits the behavior of partial drainage to the outer boundary. The variation trend of excess pore pressure under such an environment was analyzed. The pressuremeter membrane was calibrated first and its constitutive parameters were obtained as the input for the FEM model. An entire cycle of loading and unloading was tracked under varying stress anisotropies. The simulation provides an insight into the soil-structure interaction between probe membrane and cavity wall, with the emphasis of the pore pressure response at borehole surface that differs over the in-situ stresses and stiffness of the mediums. Two indices - critical expansion pressure and critical excess pore pressure were introduced to interpret the in-situ properties. By understanding the possible responses at each step of downhole instrumentation, a new interpretation method was suggested to better evaluate the in-situ stresses.

• Subjects / Keywords

  • In-situ Stress Anisotropy
  • Pressuremeter Testing
  • FEM

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R32B8VQ54

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.