Reservoir Characterization and Determination of Residual Oil Saturation Using Single-Well- Chemical-Tracer Test - Significance of Geochemistry, Rock Dissolution, and Surface-Charge Alteration
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It is generally recognized that the determination of residual oil saturation (Sor) plays a significant role in production forecasting and the selection and management of enhanced oil recovery (EOR) methods. Failure to accurately determine the saturation state will lead to incorrect recovery estimates and a poor understanding of EOR efficacy. The Single Well Chemical Tracer (SWCT) method is a field method to determine oil or water saturation in one well EOR pilots so that it brings EOR methods from the laboratory to the field enabling an efficient and accurate evaluation of the oil mobilization. In this thesis, the procedure and importance of the numerical interpretation of field SWCT test data have been introduced. This thesis also provides a combination of log inject log (LIL) and SWCT test methods to improve the accuracy of Sor measurement and is a novel approach to determine current oil saturation. In addition, a comprehensive framework has been developed to understand the extent to which different parameters may affect the SWCT test profiles. This could help us to manage and design tests more efficiently under different reservoir conditions In addition to the SWCT test design and field test data interpretation, the significance of geochemistry on the SWCT tests has also been investigated. Since hydrolysis of an ester as the main part of the method leads to the formation of acid as well as alcohol, the equilibrium state of the reservoir is displaced and thus changes the pH. The significance of the effect of the water chemistry changes on the SWCT test has been investigated by coupling a multiphase flow simulator to the geochemistry package PHREEQC. It has also been found that reservoirs containing a negligible calcite mineral concentration, weak buffer capacity and soluble hydrocarbon components at high temperature provide weak resistance against the pH variation during the test. The water chemistry changes and the drop in pH might affect two mechanisms: 1 Calcite mineral dissolution: The water chemistry could dissolve rock minerals and affect the wettability state. On the one hand, calcite dissolution has been identified as a key mechanism for dampening the pH variation during SWCT tests by improving the buffer capacity of water. On the other hand, calcite dissolution has been also recognized during the last decade to be an underlying mechanism for liberation of the adsorbed oil from the surface by modified salinity water injection. The study of the potential of the calcite dissolution on the oil liberation from the rock surfaces during the test found that calcite dissolution is more substantial during shut in time than transient time. This potential could be more significant at higher temperatures although the initial buffer capacity also could have an effect. This work also found that the pH of the reservoir reaches its lowest level at the beginning of the shut in time and the pH of the reservoir is not changed significantly afterwards. With longer shut in times, the additional ester hydrolysis and acid product are neutralized by the calcite dissolution and the buffer capacity of water. 2 Surface charge alteration: The water chemistry could affect the charge of the rock surface and the rock wettability. Determination of how the displacement of the equilibrium state and pH drop during the test could affect the surface charge of the rocks has been investigated using the surface complexation models (SCM). The results reveal a pH drop during the SWCT tests whilst the calcium concentration is held constant in the water. This yields a less positive/more negative surface charge so that wettability of carbonate rock might be altered to a less oil wetting state. More careful study indicates another mechanism. Calcite dissolution during the test leads to an increase in calcium concentration in the water and a SWCT test in carbonates is accompanied by a rise in the calcium concentration whilst the pH drops which yields an increase in the surface charge of carbonate rocks. Therefore, the pH drop does not directly affect the surface charge of carbonate rock during a SWCT test and the calcium concentration rise from calcite dissolution could control surface charge more significantly.