Laboratory Measurement of Hydraulic Fracture Conductivities 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 in horizontal wells. 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 both natural and induced fracture conductivity under realistic experimental conditions with the Barnett shale samples is needed for reliable well performance analysis and fracturing design optimization. During the course of this study a series of static conductivity experiments was completed. The goal was to measure the conductivity of propped and unpropped natural and induced fractures using a modified API conductivity cell at room temperature. The cementing material present on the surface of the natural fractures was preserved during the initial unpropped conductivity tests and removed for subsequent propped fracture conductivity measurements. The induced fractures were artificially created by breaking the shale rock along the bedding plane to account for the effect of the irregular fracture surface 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 concentration. The 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. The aligned induced fractures have conductivities one order of magnitude lower compared to displaced induced fractures when unpropped. Poorly cemented natural fractures are effective flow paths. Unpropped fracture conductivity depends strongly on the degree of shear displacement, the presence of free debris and particles during fracture generation, and the amount of cementing material removed. The propped fracture conductivity is weakly dependent on fracture surface roughness at higher proppant concentrations because the proppant pack is the dominant contributor to fracture conductivity. Moreover, propped fracture conductivity increases with larger proppant size and higher areal concentration in the testing range of this study. Results also show that proppant partial monolayers cannot survive higher closure stress. Therefore, proppant packs with multiple layers of proppant are more beneficial than a partial monolayer by maintaining the conductivity at elevated closure stresses.