How the In-Situ Combustion Process Works in a Fractured System: 2D Core- and Block-Scale Simulation

Simulation of an in-situ combustion (ISC) process was performed for a fractured system at core and matrix-block scales. The aim of this work was: (1) To predict the ISC extinction/propagation condition(s), (2) understand the mechanism of oil recovery, and (3) provide some guidelines for ISC upscaling for a fractured system. The study was based on a fine-grid, single-porosity, multiphase, and multicomponent simulation using a thermal reservoir simulator. First, the simulator was validated for ID combustion using the corresponding analytical solutions. 2D combustion was validated using experimental results available in the literature. It was found that the grid size should not be larger than the combustion-zone thickness in order for the results to be independent of grid size. ISC in the fractured system was strongly dependent on the oxygen diffusion coefficient, while the matrix permeability played an important role in oil production. The effect of each production mechanism was studied separately whenever it was possible. Oil production is governed mainly by oil drainage because of gravity force, which is enhanced by viscosity reduction; possible pressure-gradient generation in the ISC process seems to have a minor effect. The nature (oil-production rate, saturations distribution, shape of the combustion front) of ISC at core scale was different from that in a single block with surrounding fracture. The important characteristics of different zones (i.e., combustion, coke, and oil zones) at block scale were studied, and some preliminary guidelines for upscaling are presented.