- Gel Damage; Fracture Conductivity
The key to producing gas from tight gas reservoirs is to create a long, highly conductive flow path, via the placement of a hydraulic fracture, to stimulate flow from the reservoir to the wellbore. Viscous fluid is used to transport proppant into the fracture. However, these same viscous fluids need to break to a thin fluid after the treatment is over so that the fracture fluid can be cleaned up. In shallower, lower temperature (less than 250oF) reservoirs, the choice of a fracture fluid is very critical to the success of the treatment. Current hydraulic fracturing methods in unconventional tight gas reservoirs have been developed largely through ad-hoc application of low-cost water fracs, with little optimization of the process. It seems clear that some of the standard tests and models are missing some of the physics of the fracturing process in low-permeability environments. A series of the extensive laboratory “dynamic fracture conductivity” tests have been conducted. Dynamic fracture conductivity is created when proppant slurry is pumped into a hydraulic fracture in low permeability rock. Unlike conventional fracture conductivity tests in which proppant is loaded into the fracture artificially, we pump proppant/ fracturing fluid slurries into a fracture cell, dynamically placing the proppant just as it occurs in the field. Test results indicate that increasing gel concentration decreases retained fracture conductivity for a constant gas flow rate and decreasing gas flow rate decreases retained fracture conductivity. Without breaker, the damaging effect of viscous hydraulic fracturing fluids on the conductivity of proppant packs is significant at temperature of 150oF. Static conductivity testing results in higher retained fracture conductivity when compared to dynamic conductivity testing.