Cyclic Gas Injection and Withdrawal in Underground Gas Storage: Reservoir Stress State and Wellbore Stability

This book addresses the issues related to the operation of underground gas storage facilities (UGSF). A geomechanical approach has been developed to investigate the deformation processes in an UGSF reservoir during the cyclic gas injection and withdrawal. It has been established that the stress state of the reservoir as a whole, changing with cyclic changes in reservoir, has a significant impact on the mechanical processes in the bottom hole zone (BHZ). Calculations of the stresses occurring in BHZ have been carried out for both uniform and non-uniform natural stress fields. The orientation of horizontal wells relative to the principal axes of the original natural stress field has been considered. Physical modeling of deformation and filtration processes in BHZ of UGSF wells during gas injection and withdrawal has been conducted using a unique Triaxial Independent Loading Test System (TILTS). In these experiments, real three-dimensional stress states were created in the rock specimens arising in BHZ as result of change pressure in the well and reservoir pressure. A significant distinguishing feature of the developed geomechanical approach is the consideration of the dependence of reservoir rocks filtration characteristics on the stresses. This dependence is determined experimentally during physical modeling on the TILTS simulating real deformation processes in the UGSF wells during gas injection/withdrawal. A methodology has been developed to assess the possibility and method of enhancing the permeability of reservoir rocks by creating stresses of required type and level in the BHZ, particularly through the effective application of the DUR method. The results of experimental studies on the deformation and strength properties of reservoir rocks of specific UGSFs are presented. During experiments using the "hollow cylinder" scheme on specimens of weakly cemented sandstones, a non-typical strength anisotropy was discovered. This type of anisotropy can cause directed in two opposite directions breakouts on walls of well drilled in the reservoir with a uniform natural stress field. The potential manifestation of this discovered type of strength anisotropy in the rocks must be considered when designing and operating wells to prevent failure and sand production, as well as during the analysis of well measurements. The developed geomechanical approach and the physical and mathematical modeling of deformation and filtration processes in BHZ based on it allow for predictive recommendations to ensure the optimal operation of UGSFs, the stability of wellbores, maximum allowable pressure drawdowns, well flow rates, and the prevention of sand production at the studied UGSF. The book will be of interest to students, postgraduate students, scientists and specialists in the oil and gas industry.