Unconventional gas reservoirs have complex storage and transport properties that are difficult to characterize and are dynamic. The large internal surface area of nano-scale organic pores gives them the capability to store adsorbed gas along with free gas, with the amount of free gas storage changing as a function of adsorption/desorption. Further, diffusion and slippage mechanisms compete with compression and possibly matrix shrinkage effects to alter absolute permeability of the organic matter pores. An additional factor controlling permeability change, which has not been previously considered, is the change of effective radius of organic matter pores as a function of adsorption/desorption. A statistical study is required to fully explore the overall effect of the above mentioned parameters on unconventional gas well production performance, along with other properties such as total carbon content, pore connectivity configuration, adsorption capacity, pore size distribution, and natural fracture intensity.
In this study we investigate two sets of matrix pore sizes thought to represent a reasonable range observed in unconventional gas reservoirs. Pore size affects the interaction between the gas molecules and pore walls and results in a distribution of adsorbed phase thickness. Therefore, the adsorbed layer modifies the effective hydraulic radius for flow of gas which in turn alters the effective permeability to the gas phase in organic matter pores. This complexity can be captured by combining the simplified local density model and apparent permeability approaches. The contribution of compression and matrix shrinkage to fracture permeability change with pressure is also investigated and compared to changes in apparent permeability in the matrix caused by diffusion and slippage mechanisms.
A commercial reservoir simulator is used to study the effects of pore size and pore size alteration, and consequent permeability changes, on unconventional gas well performance. A screening statistical method is used to quantify the relative importance of each factor.
The results of this study will help the engineers evaluate the relative importance of all of the permeability-altering processes that can affect unconventional gas well performance, and make appropriate simplifying assumptions. This will enable them to prioritize history matching parameters for real production data analyses and forecasting, which in turn decreases the time required for a complete field simulation study.