A Practical Procedure to Predict Cresting Behavior in Horizontal Wells

摘要

Water and gas cresting in horizontal wells are important phenomena in reservoirs that have an aquifer and/or a gas-cap. In practical situations, many reservoirs are produced under supercritical rates and a breakthrough of the displacing phase becomes inevitable. At the beginning of a reservoir simulation study, it is desirable to make an estimate of the breakthrough time and the post-breakthrough behavior. Grid sensitivity runs are also required to obtain the appropriate grid block sizes. An accurate representation of cresting behavior requires a very fine grid, which is not always practical. In this work a procedure was developed to obtain accurate breakthrough times using just coarse grid simulations. The flow equations were written in dimensionles sform and important parameters affecting multiphase flow were identified. Simple correlations for a quick estimate of breakthrough time, maximum oil rate and post-breakthrough behavior were derived based on an appropriate set of dimensionless variables and an extensive number of simulation runs. The effects of gridblock size and grid pattern were investigated in detail. Effects of rate, mobility ratio, well drainage area, well height, and end-points and shapes of relative permeability curves were also included. A procedure to derive pseudofunctions either using numerical correlations or coarse grid simulations is also presented. These pseudofunctions can be used to improve the performance of coarse grid simulations. An optimum grid pattern to start a reservoir simulation study is proposed. Application to a real field example shows that the correlations and procedures derived are reliable and accurate, and can be used for quick estimates before starting a reservoir simulation study.