A Nonlinear Combined-Flowing Water Breakthrough Simulation Model in Naturally Fractured Gas Reservoirs
Authors: Zhang LieHui , Li Yun , Ho Peter , Zhou H.B. , Hu Yong , Gao XiaoCun 
Published: December 2007

Abstract
In the fractured water drive reservoirs of China, because of the complex geologic conditions, almost all the active water invasions appear to be water breakthrough along fractures, especially along macrofractures. These seal the path of gas flowing, and make the remaining gas distributed in the pores into water blockage gas, and lead to gas and water interactive distribution in the fractured gas reservoir. These complicated fractured systems usually generate some abnormal flowing phenomena such as the crestal well produces water while the downdip well in the same gas reservoir produces gas, or the same gas well produces water intermittently. It is very difficult to explain these phenomena using existing fracture models because of simply handling macrofractures and not considering nonlinear flowing in the macrofractures and the low permeability matrix. Therefore, a nonlinear combined-flowing multimedia simulation model is successfully developed in this paper by introducing the equations of macrofractures and considering nonlinear flowing in the macrofractures and the matrix. Next, we applied this model to actual fractured bottom water gas fields. And we have completed sensitivity studies of production gas by water drainage in fractured gas reservoirs and calculated the effect of different water drainage intensity and ways on actual gas production using this model. This model has been extensively used to predict the production performance in various fractured gas fields and proven to be reliable.

Introduction
Simulation of multiphase flow in heterogeneous two porosity reservoirs such as naturally fractured system is a difficult problem from both a reservoir description and a numerical standpoint. The reservoir is represented by two collocated continuums, i.e. a fracture continuum and a matrix continuum. The fracture continuum has high permeability and low storage volume while the matrix continuum has low permeability and high storage volume.
Simulation of naturally fractured reservoirs has received much interest since the extension of Warren and Root's1 model to multiphase systems by Kazemi et al2–4. Further developments were made by Iffly et al5., Yamamoto et al6., Kleppe and Morse7, Rossen8, and Thomas et al9., and others. These models include laboratory investigations of oil and gas recovery from individual matrix blocks and simulation of single- and multiphase flow in fractured reservoirs. Most of the multiphase models account emphatically for the viscous, gravity and capillary forces, or several of them. In these models, usually the matrix blocks are assumed isolated block surrounded by a continuous fracture medium. The matrix blocks act as sources or sinks to the fractures, which are the main flow channel. This kind is called dual-porosity model. The other is dual-permeability model, also considering the matrix to matrix flow.
In recent years, the accurate modelling of matrix-fracture transfer has been the most important aspect of the simulation of naturally fractured reservoirs. Numerous papers have appeared in the literature11–15 discussing various approaches for improving the handling of the transfer in black-oil simulation where the interaction between capillary and gravity forces plays an important role. The main deficiency of the original dual-porosity was the treatment of the gravity terms in the exchange terms. Several approaches have been proposed to account for gravity effects to enhance the dual-porosity concept. A summary of the various approaches can be found in Gilman and Kazemi3 and Fung and Collins10–12. It has received more and more attention.