Abstract
Simulations with a fit-for-purpose fracture-cleanup model have previously explored the impact of matrix relative-permeability quality on cleanup and productivity in tight-gas sands. Gas-water relative-permeability curves that have a crossover in the range of 0.1% of rel-perm have a region of "rel-perm jail." Previous simulations demonstrated that water invasion in such situations can cause cleanup problems. However, a low rel-perm quality should also act as a fluid-loss-control mechanism during fracturing, thereby preventing deep invasion in the first place. The fracture-cleanup model was upgraded to mimic fracturing treatments to explore the impact of rel-perm quality on fluid efficiency, fluid invasion, and cleanup. The model explored certain aspects of pressure-dependant leakoff (PDL) that might bypass rel-perm jail restrictions.
The causes and prevention of poor-performing fracturing treatments in tight-gas sands have plagued the industry, with many mechanisms suggested and various solutions attempted as cures. Verifying the mechanisms and solutions often requires a trial- and-error approach because of the physical and chemical complexities involved. This model has a unique capability of exploring physics of flow and chemical physics of overall complexity that cannot be effectively studied in the laboratory. The upgraded model was used to study a new mechanism by which PDL might induce fracture-face damage.
Fracturing and PDL were successfully modeled using the method of pressure-dependant porosity and permeability for the fracture and matrix. Details of the method are presented. Simulations show that in the absence of PDL, poor quality rel-perms provide effective fluid-loss control and should prevent deep-matrix invasion. Under certain circumstances, PDL can bypass the fluid-loss-control mechanism causing rel-perm-induced matrix damage once the fracture has closed. Fortunately, there is currently no strong evidence that this damage mechanism is common.
