Summary 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 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 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%.

Multi-stage hydraulic fracturing is the key to the success of many shale gas and shale oil reservoirs. The main objective of hydraulic fracturing in shale is to create fracture networks with sufficient fracture conductivity. Due to the variation in shale mineralogical and mechanical properties, fracture conductivity damage mechanisms in shale formations are complex. Standard fracture conductivity measurement procedures developed for fractures with high proppant concentration had to be modified to measure the conductivity in fractures with low proppant concentration. Water-based fracturing fluids can interact with the clay minerals in shale and eventually impact shale fracture conductivity. All these challenges require more experimental studies to improve our understanding of realistic fracture conductivity in shale formations.The aims of this work were to design an experimental framework to measure fracture conductivity created by low concentration proppants and to investigate the mechanisms of conductivity damage by water. We first presented the laboratory procedures and experimental design that can accurately measure fracture conductivity of shale fractures at low concentrations of proppants. Then we measured the undamaged shale fracture conductivity by dry nitrogen. Water with similar flowback water compositions was injected to simulate the damage process followed by secondary gas flow to measure the recovered fracture conductivity after the water damage.This study shows that the developed laboratory procedures can be utilized to reproducibly measure shale fracture conductivity by both gas and liquid. The conductivity measurement of propped fractures by small size proppants at low concentrations requires strict control on gas flow bypassing the fracture both parallel and perpendicular to the fracture length direction. Shale fracture surface softening is identified as the dominant cause for the significant conductivity reduction after water flow.

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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%.