New approaches to dual porosity modeling of waterflooding in naturally fractured reservoirs

Naturally fractured reservoirs as natural stores and conductors for fluid resources occur worldwide. Complexities of fractured reservoirs make modeling of this type of reservoir very difficult. While details of individual fractures in a reservoir would be difficult to explicitly model and are normally not even available, numerical models are generally based on conceptual abstractions, such as the dual porosity model. The objectives of this research are two-fold: (1) to conduct theoretical studies of countercurrent imbibition process, and (2) to develop an accurate and computationally efficient approach to dual porosity modeling.; A numerical simulator capable of modeling fluid flow in both matrixes and individual fractures has been developed. Imbibition characteristics are identified by analyzing the results of detailed single matrix block simulation. Based on the identified characteristics, the nonlinear diffusion equations in 1D and 2D spaces are analytically solved by using a relative flow concept. The solution provides an approach to integrate various petrophysical parameters into an equivalent coefficient, which greatly simplifies the analyses of countercurrent imbibition. For 3D problems, a bond number is used to determine the relative contribution of gravity vs. capillary pressure. The results in this part of the study can easily be incorporated into a single porosity model to simulate dual porosity behavior.; A new dual porosity simulator (UTDUAL) has been developed for modeling waterflooding in fractured reservoirs. Equations for the fracture system are the same as for single porosity models, except for source/sink terms accounting for matrix/fracture transfer flow for each gridblock. Finite-difference equations are solved implicitly. UTDUAL treats the equations for matrix blocks with three different options: (1) an IMPES method, (2) diffusion methods, and (3) using results from single matrix blocks studies. For the first two options, the matrix blocks are discretized into subgrids to model transient flow. The resulting equations are mathematically decoupled from the fracture equations. The advantage of this decoupling is that it can be used to modify a single porosity simulator to a dual porosity one with little effort. The primary variables are still solved implicitly.; UTDUAL is verified against a single porosity model, with analytical transient solutions of a dual porosity model, and a commercial simulator (VIP-DUAL). These include the Buckley-Leverrett waterflooding problem, Warren and Root's analytical solution, and a new solution for transient flow in matrix blocks developed for this study. Results of UTDUAL are also compared with fine-grid simulation of an ideal fractured reservoir, in which both the fracture network and matrix blocks are discretized. Excellent agreement is achieved for a variety of reservoir properties. UTDUAL is also used to model published laboratory experiments and to conduct field-scale simulation studies.