Dynamic Permeability Models in Dual-Porosity System for Unconventional Reservoirs: Case Studies and Sensitivity Analysis

Abstract

Abstract Dynamic Permeability is one of the most important key features for unconventional reservoir. The production of unconventional reservoirs, such as shale oil and gas, tight oil and gas, coal bed methane etc., heavily depends on the effective permeability of fracture. However, its permeability changes nonlinearly due to dual-porosity system, which affects the accuracy of unconventional reservoir simulation. This paper investigates two commonly accepted dynamic permeability models in details, S&D model and G&W model. Furthermore, these models are compared and applied in field cases. This study can help us understand the physics behind dynamic permeability in dual porosity system, and facilitate applying dynamic permeability model from core scale to field scale. Firstly, unconventional oil and gas recovery process in dual porosity system is analyzed to determine the key petrophysical controlling factors. In particular, the relation between permeability and fracture porosity is analyzed and discussed in details. Then the most popular two models, S&D model and G&W model, are compared and analyzed. The advantages and disadvantages of those models are discussed, including gas desorption effect, matrix shrinkage effect, micropososity change effect. In addition, the models are applied to several field cases, and the results of these two models are discussed and compared with each other. Finally, the models are applied to pressure transient analysis, to test their feasibility in unconventional reservoir simulation. By analyzing the model, the important role of fracture porosity in permeability prediction is addressed. The results of cases study validate the accuracy of G&W model, which can give a better prediction of dynamic permeability, and perfectly describe rebounding pressure phenomenon as pore pressure decreases. For different models, the sensitivities of several parameters in geomechanical equations and Langmuir isothermal equation are analyzed by Monte Carlo method, and these key sensitive parameters have impact on the simulation results for unconventional reservoirs. From geomechanical perspective, the effects of fracture porosity on permeability change are investigated and discussed in details, which is critical in dual porosity system. The field cases and relevant sensitivity analyses are essential in understanding the physics behind the problem. By comparison and applications of the models, it is concluded that dynamic permeability models can be applied to unconventional reservoir simulation.