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Stress dependent compaction in tight reservoirs and its impact on long-term production
Cui, Qi
Cui, Qi
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2016
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2017-07-06
Abstract
Stress dependent compaction in low permeability reservoirs and its impact on long time reservoir recovery were investigated in this work. The research was inspired by the observation of steep production decline in many shale wells. The objectives of this research are in the following aspects: 1) To measure pore pressure and confining stress dependent tight rock permeability, which is rarely documented in the literature; 2) To identify permeability decline characteristics at various effective stress conditions of different types of tight rocks; 3) To generate pressure dependent permeability decline models for numerical simulation study; 4) To evaluate the impact of stress dependent compaction on tight reservoir recovery by integrating permeability decline models into traditional reservoir simulation; 5) Last but not least, to provide practical suggestions for production enhancement in shale wells; more importantly, to inspire more research and field pilots in this area to further validate the viability of the potential EOR/IOR approaches. This work consists of two parts: laboratory sample measurements and numerical simulations. In the first part, two core measurement assemblies were utilized to calibrate stress dependent tight sample permeability. Pressure transmission test method was employed and three different types of tight samples were used for the experiments. Multiple combinations of pore pressure and confining stress were applied to each sample to cover different stages during reservoir depletion. The interpretation of measured permeability decline as well as the change in sample downstream equilibrium time for all three samples confirms the stress dependent compaction phenomenon. The data also suggest that the tight rock permeability change does not follow monotonic trend in a specific reservoir, as the algorithm changes from linear to power law increase with the increase of applied pore pressure. The critical confining stress to pore pressure window was identified for different samples which indicate the starting point of permeability decline signature changes. Using the data of pressure transmission tests, the Biot coefficient was determined using trial and error technique such that the effective stress can be obtained. The effective stress dependent permeability model was then built for the numerical simulation. Coupled permeability decline numerical simulation was conducted after the experimental work. Multiple simulation scenarios were run to study the production response due to compaction. The following aspects were investigated with the non-injection, horizontal producer only model. First of all, the compaction model was compared with the traditional non-compaction model. It was found that the non-compaction model would overestimate the 20 years cumulative oil recovery by more than 20%. The overestimation with non-compaction models is proportional to the stress sensitivity of the reservoir rocks. Secondly, different well constrain scenarios with the compaction model were investigated. Multiple realizations were run for each compaction model with different bottom-hole pressure. Unlike the speculation that the stress dependent compaction would dominate long-term production, the results showed that pressure drawdown is still the key production mechanism in stress sensitive tight reservoirs and the hydrocarbon recovery would not be enhanced by bottom-hole pressure maintenance. Therefore, another possible approach for improved recovery was investigated with numerical simulation. The potential IOR method is thought to be related to pressure maintenance, as severe permeability decline was observed from the laboratory measurements when the pore pressure was depleted. Gas injection simulations were run for four compaction models with different injection rates. The results indicate that approximately 20% - 60% incremental oil would be produced comparing to the non-injection models as long as the desired injectivity can be achieved. Nevertheless, the injectivity could be a critical issue in extremely low permeability reservoirs. In such cases, essentially no incremental oil would be produced with low volume of injected gas. Although the preliminary results of experiment based gas injection simulation showed EOR potentials in tight reservoirs, it is recommended that further field scale tests to be conducted in order to validate the laboratory and simulation works. Since improved recovery in unconventional tight reservoirs is a huge project and each research approach reveals only a small portion of the big picture, continuous efforts in different aspects and integrated research strategies are keys to success.
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