A Simplified Finite Element Algorithm and Coupled THM Problems in Stress-Sensitive Naturally Fractured Reservoirs During Injection/Production

ABSTRACT: Stress concentration in vicinity of a wellbore may suppress fracture network and restrict the fluid flux in naturally fractured reservoirs (NFR). Primarily reducing the absolute permeability in the fracture network, such an impact may be reach a significant level when productivity may not increase after a drawdown increase. To cope with such a possible reversal incremental fluid flow, a critical drawdown is identified, for the case when the permeability reduction effect may surpass those inremental contributions of a drawdown increase to the flow flux. A different strategy by increasing the external fluid pressure and maintaining a constant bottom hole pressure (BHP) should be applied, so that the near-wellbore permeability may remain unchanged. Both simulation and field applications are performed and more than 30% production rate increase can be expected depending on the in-situ stresses level against permeability changes for each well. Finite element method is used for a coupled THM problem and simplified finite element algorithm is applied with a dual-porosity mode in NFR 1. INTRODUCTION Then the permeability perpendicular to a confining stress (Principal stress direction or fluid flow directions) is conventionally defined as a function of the effective stress. Practically speaking the principal directions of fluid flow and stresses seldom coincide and in what follows, we propose to define changes of the principal permeability as a function of the normal stress to the principal flow direction, the former may be calculated by the principal stress tensor and the latter is characterized of the natural fractures and the geological bedding. Furthermore, if plasticity, sliding along a fracture or joint, and dilatancy occur, the shear component of the stress tensor should be also used. Thirdly we can determine if a production decline can be reversed by controlling the wellbore drawdown, i.e. if the drawdown and stress concentrations fall into the aforementioned sensitive range. Fourthly we may diagnose if a production decline in a specific field is caused by these stress sensitive permeability change. Finally we may find alternative ways to increase drawdown without bringing down the permeability at the wellbore at the same time. In other words, once we can correlate producing well drawdown to the induced stress changes, which are in the stress-permeability sensitive range, then we can adjust the producing drawdown to serve the purpose for production enhancement. Once these factors are identified, a dual-porosity model is built and finite element method is utilized for production calculations. It should be realized that without considering these factors, any attempt to increase production by lifting drawdown will be neutralized by the consequence of permeability reduction. Production declines in a carbonated reservoir northwestern China are observed and enhancing strategies are proposed.

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