Two-Phase Flow Effect on Carbonate Matrix Acidizing

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Cheng, Haoran
matrix acidizing; two phase flow; CO₂; numerical simulation
thesis / dissertation description
Matrix acidizing treatments are commonly used to improve the productivity of oil and gas wells in carbonate reservoirs. For a given volume of acid injection, the optimal injection rate can achieve the deepest wormhole penetration. Normally, the optimal condition is obtained through laboratory core flooding experiments with acid fluid injection into water saturated cores. However, in field treatments, acid fluid injected into oil and gas saturated formations creates a two-phase flow region. Two-phase flow effect on optimal treatment condition needs to be evaluated. The research is conducted through experimental investigation and numerical modeling. In order to investigate two-phase flow effect in matrix acidizing, a systematic experimental study that covers a variety of back pressures, temperatures and injection rates is conducted. Computerized Tomography (CT) scan images are taken for each core sample after acid injection to evaluate the structures of the wormholes. With the experimental study, the effect of evolved CO₂ on wormhole propagation is examined. The test results show that at low injection rate, CO₂ present as a gaseous phase lowers wormhole propagation efficiency dramatically, and enlarged wormhole diameter is observed. This work verifies that two-phase flow affects the wormhole propagation. A numerical model is developed to simulate two-phase flow effect on wormhole propagation. The model uses 3-D numerical simulation with two-scale continuum model and finite volume method to characterize matrix acidizing process. The model can capture both gravity effect and two-phase flow effect on wormhole propagation. From this numerical model, we are able to estimate the properties that cannot be measured from lab experiment. With the numerical model, we study the effect of several key factors, such as injection rate, viscosity, and residue non-wetting phase saturation on wormhole efficiency. The results show that wormhole propagation in two-phase flow region is quite different to single-phase flow. The presence of an immiscible phase, such as gas or oil, before acid injection, can increase the wormholing efficiency. The higher the oil viscosity, the lower volume of acid needed for wormhole penetrating the core. A higher initial saturation of oil, also reduces the breakthrough pore volume. High residue oil saturation has a positive effect on wormholing efficiency. It is important to investigate the effects and calibrate the lab results before using them in oil and gas reservoirs.