A substantial future potential for gas production lies in tight gas reservoirs with porosity of 5-8% and permeability less than 0.1 mD (micro-Darcy range). To construct a commercial well in these reservoirs, a well must contact as much as possible a large drainage area of the reservoir. Therefore, wells drilled in these challenging reservoirs must be hydraulically fractured to yield commercial gas production rate. The number of hydraulic fractures and their conductivities are vital parameters to ensure production increase and production sustainability during the life of the well. This paper focuses on the proppant fracture conductivity and the optimum number of fractures required along a horizontal well in a tight gas sand (TGS) reservoir.
The flow system in TGS reservoirs include matrix and natural/induced hydraulic fractures that contribute to the gas transport in a very complex manner. Laboratory tests were performed to determine the stress dependant permeability as a function of stress. A tensile natural fracture was simulated by splitting a whole core by failing it under tension using a Brazilian test procedure. The tensile fractured core was then propped with a small mesh proppants and the permeability of the propped fracture was determined. The experimental results of stress dependant permeabilities provided a basis for optimum fracture conductivity based on the flow properties of the porous components of a given reservoir (matrix, natural fractures, and propped fractures).
The results from the experimental phase were employed in a reservoir simulation study to determine the optimum fractures along a horizontal well required to convert a marginal well into an economic well. A series of reservoir simulation runs were performed given a set of well and reservoir characteristics. Common factors include reservoir properties, PVT, drainage area, well geometry, and created fracture geometry. The controlling factors include number of fractures, reservoir permeability, skin damage, and fracture conductivity. The number and optimum fracture conductivity required for appreciable production increase in tight gas reservoirs is demonstrated. A comparison between stress dependant and the stress independent permeabilities indicates the importance of including the effect of stress in any reservoir simulation of tight gas reservoirs.