Modeling transient flow in fractured shale reservoirs

Kılıçaslan, Ufuk
Oil and gas production from shale reservoirs has been popular in North America for more than two decades. Commercial production from these extremely low permeability reservoirs is only achieved by multi-stage fractured horizontal wells. However, production performance of these wells is quite different than wells drilled in conventional reservoirs. Main distinct behavior seen in these wells is very long period of transient flow due to tightness of such reservoirs. This thesis questions validity of existing dual-porosity reservoir simulation technique for fractured shale reservoirs. In this respect, analytical solutions of pressure diffusion are presented for constant fracture pressure, constant rate and constant fracture pressure followed by linearly declining fracture pressure boundary conditions. According to these solutions, time-dependent shape factors are derived for 3D rectangular anisotropic matrix. Obtained shape factors and proposed simplifications are verified against fine scale single-porosity numerical models. Key finding from this study is that matrix – fracture transfer function (shape factor) is not constant, but rather decreases with time until reaching to a constant value. Therefore, dual-porosity simulation of fractured shale reservoirs using constant shape factor does not capture actual physics of matrix to fracture flow and yields inaccurate performance prediction. Also, proposed simplifications either as an empirical function or reduced form are robust in modeling this phenomenon. In addition to those, common features of decline curve analysis are tested for fractured shale reservoirs. Time dependency of b-parameter used in hyperbolic decline curve analysis is assessed for different reservoir properties by sensitivity analysis. Proposed empirical functions are used to obtain b-parameter for these cases and results are compared with actual ones.
Citation Formats
U. Kılıçaslan, “Modeling transient flow in fractured shale reservoirs,” Ph.D. - Doctoral Program, Middle East Technical University, 2020.