Study of Wellbore Breakdown under Fluid Injection in Transversely Isotropic Poroelastic Formations

Abstract

Summary Fluid injection into the formation through a completed wellbore, as an important process for the oil and gas industry, is commonly seen in well tests and hydraulic fracturing operations. The problem of wellbore breakdown during fluid injection is revisited in this work by a thorough and comprehensive consideration of some essential issues involved in this subject. First, fluid injection may not necessarily start simultaneously with the completion of wellbore drilling, as is usually assumed in the reported analytical solutions. Second, there exists a common limitation that the borehole must orient in a direction normal to the isotropic (bedding) planes in the previous analytical studies addressing the wellbore problem drilled in a transversely isotropic formation. Last, but not least, the implicit boundary condition of radial stress, which is usually ignored but essential to accurately evaluate the breakdown pressure on such fluid injection, also adds to the difficulty of solving the problem. This paper presents a novel numerical scheme enhanced by user subroutines/functions that can work with mainstream simulators to promote the in-depth study of wellbore breakdown under fluid injection with various model settings. On validation, the effects of the idle time t0 after drilling and the material anisotropy on the poromechanical responses of the wellbore are extensively studied for the most commonly encountered horizontal wells. The idle time has slight effects immediately after fluid injection starts; however, the influences become generally negligible after a short period of time of fluid injection. It is also revealed that the anisotropy in rock properties has a more significant influence on the stress responses for horizontal wells than the previously studied case of vertical wells. Another important finding is that tensile fracture may not always initiate vertically from the top/bottom of the horizontal borehole wall because the maximum tensile stress occurs elsewhere, although vertical in-situ stress is the maximum principle stress.