Laboratory Measurement of Hydraulic Fracture Conductivities in the Barnett Shale

Horizontal wells that intersect multistage transverse fractures created by low-viscosity fracturing fluid with low proppant loadings are the key to revitalizing production from the Mississippian Barnett shale in the Fort Worth basin in Texas. However, direct laboratory measurements of both naturaland induced-fracture conductivities under realistic experimental-design conditions are needed for reliable well-performance analysis and fracture-design optimization. In this work, a series of experiments was conducted to measure the conductivity of unpropped natural fractures, propped natural fractures, unpropped induced fractures, and propped induced fractures with a modified American Petroleum Institute (API) conductivity cell at room temperature. Fractures were induced along the natural bedding planes, preserving fracture-surface asperities. Natural-fracture infill was taken into consideration during conductivity measurements. Proppants of various sizes were placed manually between rough fracture surfaces at realistic concentrations. The two sides of the rough fractures either were aligned or were displaced with a 0.1-in. offset. After pressure testing on the system integrity, nitrogen was flowed through the proppant pack or unpropped fracture to measure the conductivity. Results from 88 experiments show that the conductivity of hydraulic fractures in shale can be measured accurately in a laboratory with appropriate experimental procedures and good control over experimental errors. It is proved that unpropped, aligned fractures can provide a conductive path after removal of free particles and debris because of the brittleness and lamination of shale. Moreover, poorly cemented natural fractures and unpropped displaced fractures can create conductivities of up to 0.5 md-ft at formation-closure stress, which is one to two orders of magnitude greater than the conductivity provided by cemented natural fractures and unpropped aligned fractures. This study shows that propped-fracture conductivity increases with larger proppant size and higher proppant concentration. Longer-term fracture-conductivity measurements indicate that, within 20 hours, the fracture conductivity could be reduced by as much as 20%.