From Wellbore Instability and Grain Mixing to Injectivity Reduction

Abstract

Wellbore instability has been registered and expected as one of the risks for successful delivery of horizontal injectors with open-hole stand-alone screens in depleted reservoirs. Special drill-in fluids and breaker fluids were designed to ensure stability of these wells prior to screen installations. Borehole collapse was expected in most cases after the wellbore pressure dropped to hydrostatic at the end of the drilling process. Unlike open-hole producers, the loose particles (or grains) trapped in the annulus of the open-hole injectors will likely undergo constant mixing and resorting during each injection and shut-in cycles. In this study, we examined the impacts of grain mixing on the reduction of porosity and permeability in the annulus using a simple binary mixing model. Laboratory experiments based on this hypothesis have also been conducted. Our preliminary results suggest that if the fines are 10 to 20 times smaller than the larger particles, the presence of a few percent in volume fraction of fine particles can reduce permeability by an order of magnitude. The predicted reductions are consistent with the observed equivalent skin increases at some of our injectors during the first few years of operations. After multiple stimulations to remove fines and residual drilling additives, we successfully re-established injectivities to the initial states.