Measurement of realistic fracture conductivity in the Barnett shale

The Mississippian Barnett shale of the Fort Worth Basin is one of the most successfully developed shale gas plays in North America by applying multistage hydraulic fracturing stimulation techniques. The fracturing design involves pumping low viscosity fluid with low proppant concentrations at high pump rate, commonly known as “slick water fracturing”. Direct laboratory measurement of natural and induced fracture conductivity under realistic conditions is needed for reliable well performance analysis and fracturing design optimization.

During the course of this study a series of conductivity experiments was completed. The cementing material present on the surface of natural fractures was preserved during the initial unpropped conductivity tests. The induced fractures were artificially created by breaking the shale rock along the bedding plane to account for the effect of irregular fracture surfaces on conductivity. Proppants of various sizes were manually placed between rough fracture surfaces at realistic concentrations. The two sides of the induced fractures were cut in a way to represent either an aligned or a displaced fracture face with a 0.1 inch offset. The effect of proppant partial monolayer was also studied by placing proppants at ultra-low concentrations.

Results from the experiments show that unpropped induced fractures can provide a conductive path after removal of free particles and debris generated when cracking the rock. Poorly cemented natural fractures are effective flow paths. Unpropped fracture conductivity depends strongly on the degree of shear displacement, the presence of shale flakes and particles, and the amount of cementing material removed. The propped fracture conductivity is weakly dependent on fracture surface roughness at higher proppant concentrations. Moreover, propped fracture conductivity increases with larger proppant size and higher concentration in the testing range of this study. Results also show that proppant partial monolayers cannot survive higher closure stresses.