Abstract
Producing gas oil ratio trends in hydraulically-fractured shale oil wells are different than the trends in conventional oil wells. One peculiar behavior is that the ratio stays flat for an extended period of production time, even when the bottomhole flowing pressure is below the bubble point Pb. Several factors may contribute to this trend but the dominant mechanism is not known. The objective of this study is to use a simulation model of reservoir flow in a dynamic permeability field, to show that the in-situ mechanical stresses, namely the fracture closure stresses, could play a significant role on the trend. We considered that the matrix/fracture permeability values reduce as a function of in-situ stress during production, and we show that the reduction due to fracture closure stress is a major control in GOR. Firstly, the reservoir flow simulation study shows that the production with finite conductivity fractures have GOR that is more affected by the closure stress than in the infinite conductivity case. In the former case, below Pb the GOR trend is a complete flatness caused by the stress in the system, while in the latter case a small amount of increase is observed despite the stress in the system. In the finite case, the stress dependence in the fracture has a much greater impact on the GOR than in the matrix. In infinite conductivity case, the stress dependency is similarly important in both the matrix and the fracture permeability. The study considers four cases: In Case 1 the permeability of both the matrix and the fractures are held constant, the static case; Case 2, the permeability of both the matrix and the fractures change, i.e. the dynamic case; Case 3, the fractures permeability changes, but the matrix permeability is kept constant; Case 4, the fractures permeability is kept constant, and the matrix permeability is changes. The difference between cases 1 and 2 is the change in GOR caused by the stress-dependence of permeability field, while Cases 3 and 4 show the relative contributions of the fractures and the matrix. The literature has previously addressed GOR trends due to several factors: the nano-confinement effects on the fluid behavior, production, flow regimes change, formation compressibility, and general stress dependence on the permeability. This article, however, shows that the unpredictable nature of the GOR is mainly due to the hydraulic fractures closure stress. This observation gives a lever of control to the operators. Knowing the differences in GOR with finite and infinite conductivity fractures will challenge the assumptions the operators are making of their wells.
