Sensitivity Analysis of Hydraulically Fractured Shale Gas Reservoirs
Xu Zhang, C. Mike Du, 2011. "Sensitivity Analysis of Hydraulically Fractured Shale Gas Reservoirs", Uncertainty Analysis and Reservoir Modeling: Developing and Managing Assets in an Uncertain World, Y. Zee Ma, Paul R. La Pointe
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This study presents a reservoir simulation model to analyze the impact of reservoir and hydraulic fracture parameters on gas production from a shale gas reservoir. The model was constructed as a multiporosity system with matrix subgrids to account for transient gas flow from the matrix to the fractures. The extended Langmuir isotherm was applied to control the desorption process of multiple components during the production. Primary hydraulic fractures perpendicular to the horizontal wellbore were modeled explicitly with thin grid cells that preserved the conductivity. The hydraulically induced fracture network around the horizontal well was characterized by the matrix-fracture coupling factor and permeability of the fracture system.
The linear experimental design technique of the Plackett-Burman type was used to screen influential parameters including porosity and permeability of the reservoir matrix and fractures, matrix-fracture σ factor, matrix subdivisions, primary hydraulic fracture half-length, height, spacing, and conductivity, rock compaction, non-Darcy flow coefficient, and gas content. A quadratic response surface model was constructed with three-level uncertainty parameters selected from the initial linear screening process. The model was verified by confirmation runs. Sensitivity studies provided important insights into the impact of reservoir and fracture parameters on shale gas production forecasts, which can be critical for fracture treatment design and production scheme optimization.
During the past few years, natural gas production from shale gas reservoirs has been a key source of energy in North America and will likely become an increasingly important component of the world's energy supply. A shale gas reservoir is characterized by organic-rich sediments with extremely low matrix permeabilities in the order of nanodarcys, and clusters of mineral-filled natural fractures that are not open to flow. Shale gas storage capacity is represented by the gas adsorbed onto the organic material and free gas in the pore space of the shale rocks. Horizontal drilling and hydraulic fracturing have proven to be the most effective technologies to stimulate the reservoirs and open natural fractures for economic production.