Propped Fracture Conductivity in Shales

Abstract

The successful development of the major shale gas plays in North America hinges upon the creation of complicated fracture networks by pumping low viscosity fracturing fluid with low proppant concentrations at high flow rate. Direct laboratory measurement of hydraulic fracture conductivity created in the networks is needed for reliable well performance analysis and fracture design optimization. A series of experiments were conducted under realistic hydraulic fracturing conditions to measure the conductivity using a modified API conductivity cell. Natural fractures were preserved and fracture infill was kept for initial conductivity measurement. Fractures were also induced along the natural bedding planes to obtain fracture surface asperities. Proppants of various sizes were placed between rough fracture surfaces at realistic concentrations. The two sides of the rough fractures were either aligned or displaced with a 0.1 inch offset. Results show that the hydraulic fracture conductivity of shale samples with rough surfaces can be accurately measured in a laboratory with appropriate experimental procedures and good control on experimental errors. The unpropped offset fracture can create conductivity as much as poorly cemented natural fracture, while the conductivity of unpropped matched fracture is minor. The presence of proppants can elevate the fracture conductivity by 2 to 3 orders of magnitude. Propped fracture conductivity increases with larger proppant size and higher proppant concentration. This study also indicates that within 20 hours propped fracture conductivity can be reduced by as much as 24% as shown in the longer term fracture conductivity measurements.