A New Formulation for Numerical Simulation of Compaction, Sensitivity Studies for Steam Injection

Abstract

A thermal simulator was modified in order to study the effect of different formulations of the compaction phenomenon in modelling steam injection processes. The modified model can simulate total or partial irreversibility of this phenomenon, as well as the reversible formulation commonly used. Options to recalculate block permeabilities, transmissibilities, and thermal conductivities according to changes in porosity and thickness due to compaction are porosity and thickness due to compaction are included. Results from a series of simulations using data of hypothetical reservoirs are presented; these show a strong dependence of the quantities injected and produced, with the chosen formulation for compaction. It is found that the results are more sensitive to changes in the pore volume, than to changes in permeability and transmissibility. This improved representation of the compaction phenomenon in the simulator, proved to be of phenomenon in the simulator, proved to be of great help to understand injection and production mechanisms in steam injection processes. It also widens the possibilities of using reservoir simulators for history match purposes, and also enables the user to take more into-account the compressibility data obtained from laboratory experiments. Introduction Both theoretical and field studies have shown the importance of the compaction phenomenon as an additional mechanism of production of oil. The influence of compaction on isothermal processes was studied by Finol and Farouq Alil, while its influence in both cyclic and continuous steam injection was reported by Rattia and Farouq Ali and by Ertequin and Farouq Ali. In recent years, advances in laboratory techniques together with new theories have contributed to a better understanding of the compaction phenomenon, thus helping to develop more appropriate mathematical formulations. Most of the efforts to include rock compaction in numerical reservoir simulators, have been oriented toward the use of a single value of a compaction coefficient or compressibility of the porous media. Using this approach it is found that the predicted behavior of injection and of production processes are very sensitive to the chosen value of the compressibility. Furthermore, most of the available simulators consider the process to be completely reversible, i.e. when the pressure increases there is an expansion that obeys the same formulation that appears during compaction. Laboratory experiments have shown that this assumptions do not conform with reality, in fact the compaction coefficient depends at least with the pressure and also with the rock type, which can vary with the block depth. Also, some authors have shown that the coefficient of compaction also varies with the direction of the change of the block pressure, the coefficient being smaller in general during expansion, creating in effect an irreversibility of the compaction process. The purpose of the work is to study the influence of the compaction mechanism on the prediction of production performance of steam injection processes, and also to study the sensitivity processes, and also to study the sensitivity of this results to change in some variables that are influenced by the compaction such as porosity, permeability, transmissibilities, and thermal permeability, transmissibilities, and thermal conductivity. The study was conducted using a thermal numerical simulator with the necessary modifications to model the phenomenon, and the input data used for the numerical examples correspond to typical values of heavy oil reservoirs of Venezuela. DESCRIPTION OF THE MODEL The numerical simulator used as a starting point for this study is a 3-Dimensional, implicit, finite-difference model designed to simulate hot water and steam injection processes. p. 139

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