A Multiple-Continuum Approach for Modeling Multiphase Flowin Naturally Fractured Vuggy Petroleum Reservoirs

摘要

The existence of vugs or cavities in naturally fracturedrnreservoirs has long been observed. Even though these vugsrncan be largely attributed to reserves of oil, natural gas, andrngroundwater, few investigations of vuggy fractured reservoirsrnhave been conducted. In this paper, a new multiple-continuumrnconceptual model is developed, based on geological data andrnobservations of core samples from carbonate formations inrnChina, to investigate multiphase flow behavior in such vuggyrnfractured reservoirs. The conceptual model has beenrnimplemented into a three-dimensional, three-phase reservoirrnsimulator with a generalized multiple-continuum modelingrnapproach. The conceptual model considers vuggy fracturedrnrock as a triple- or multiple-continuum medium, consisting ofrn(1) highly permeable fractures, (2) low-permeability rockrnmatrix, and (3) various-sized vugs. The matrix system mayrncontain a large number of small or isolated cavities (ofrncentimeters or millimeters in diameter), whereas vugs arernlarger cavities, with sizes from centimeters to meters inrndiameter, indirectly connected to fractures through smallrnfractures or microfractures. Similar to the conventionalrndouble-porosity model, the fracture continuum is responsiblernfor the occurrence of global flow, while vuggy and matrixrncontinua, providing storage space, are locally connected torneach other (and interacting with globally connectingrnfractures). For practical application of the multi-continuumrnconcept in reservoir simulation, we propose a novel upscalingrnmethod for computing equivalent gridblock permeabilities ofrncoarse blocks containing large isolated vugs, in which thernlocal problems consisting of Darcy and Stokes flows arernsolved. In addition, we describe an efficient boundaryrncondition for accurate computation of upscaled permeabilities.rnIn the numerical implementation, a control-volume, integralrnfinite-difference method is used for spatial discretization, anda first-order finite-difference scheme is adapted for temporalrndiscretization of governing flow equations in each continuum.rnThe resulting discrete nonlinear equations are solved fullyrnimplicitly by Newton iteration. The numerical scheme isrnverified and applied to simulate water-oil flow through thernfractured vuggy reservoirs of the Tahe Oil Field in China.