| dc.description.abstract | Matrix acidizing is an effective stimulation technique for carbonate reservoirs and it has been practiced for years in the industry. By injecting acid below the formation fracturing pressure, highly permeable paths called “wormholes” are created to bypass the near wellbore damage and penetrate the formation as deep as possible to improve flow conditions. For various types of carbonate formation, it is important to design the volume of acid needed and the optimal acid injection rate to achieve minimum acid consumption. Besides, acid type, acid concentration, core size, mineralogy and petrophysical properties of the carbonate rocks affect the optimal conditions for matrix acidizing. This research focuses on the characterization of carbonate formation at multiple scales to investigate how the petrophysical parameters affect matrix acidizing. The study covers three different scales: micro scale, core scale, and log scale.
For micro-scale study, three types of rock samples (Indiana Limestone, Desert Pink, and Travertine) was selected and micro-Computer Tomography (micro-CT) technique was adopted to capture the microscopic heterogeneity in the pore structure. Image processing was performed and important petrophysical parameters quantified, including pore size distribution, pore connectivity, and the surface-area-to-volume ratio of the rock. The quantified parameters were used to correlate to the optimal conditions obtained by physical experiments and rock permeability. A concept named equivalent pore radius was defined. This study determined that this parameter, equivalent pore radius, is tightly related to the permeability of rock and can be used to improve the optimal conditions prediction model for matrix acidizing.
For the core-scale study, the optimal conditions for one type of the Travertine, a highly heterogeneous carbonate rock, is measured with core flooding test in the laboratory. The optimal conditions for various rock types under different experimental conditions are collected and sets of curves for optimal conditions are generated. The results of this study indicate that the optimal conditions for most carbonate rocks lie in a narrow range, which is useful for guiding matrix acidizing design.
Finally, the characterization for carbonate formation at log scale mainly focuses on the most important petrophysical properties (porosity and permeability). The methods for porosity estimation, lithology estimation, and permeability estimation are discussed. The depth-by-depth porosity profile, permeability profile, and lithology are integrated with a horizontal well acid stimulation software (HWAS), which helps customize matrix acidizing design. Field application based on true formation properties are demonstrated. | en |
