Horizontal wells intersecting multistage transverse fractures created by low viscosity fracturing fluid with low proppant loadings are the key to revitalizing production from the Mississippian Barnett shale of the Fort Worth Basin in Texas. However, direct laboratory measurements of both natural and 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 were conducted to measure the conductivity of unpropped natural fractures, propped natural fractures, unpropped induced fractures and propped induced fractures using a modified API conductivity cell at room temperature. Fractures were induced along the natural bedding planes preserving fracture surface asperities. Natural fracture infill was kept for initial conductivity measurements and then removed for subsequent propped fracture conductivity tests. Proppants of various sizes were manually 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. After pressure testing on the system integrity, nitrogen was flown through the proppant pack or unpropped fracture to measure the conductivity.
Results from the 88 experiments show that the conductivity of hydraulic fractures in shale can be accurately measured in a laboratory with appropriate experimental procedures and good control on experimental errors. It is proved that unpropped induced fractures can provide a conductive path after removal of free particles and debris due to the brittleness and lamination of shale. Moreover, poorly cemented natural fractures and unpropped displaced fractures can create conductivities up to 0.5 md-ft at formation closure stress, which is one order of magnitude higher 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 also show that within 20 hours the fracture conductivity could be reduced by as much as 20%.