Matrix acidizing is a technique used for stimulating carbonate formations. Fluids used for this purpose are routinely characterized in the laboratory by means of core flood testing. Though several key insights can be obtained using the laboratory core flood technique, simulating exact downhole environments and extrapolation of the results obtained to field-scale has proven challenging. This paper addresses two specific topics related to acid wormholing—upscaling laboratory results to field scenarios and the interaction of reaction products in high pressure reservoirs.
To study the upscaling of acid wormholing experiments in field scenarios, a radial core flood testing apparatus was set up to better mimic actual wellbore acidizing flow conditions. The effects of radial flow path and completion type (i.e. openhole versus cased hole) as well as the applicability of the upscaling wormhole propagation model were studied. Furthermore, CT scans of related core samples were performed to characterize wormhole patterns generated by acid dissolution. In addition, tests were performed at high pressures and flow rates to study the effects of the interaction of reaction products on the wormholing process at typical high bottomhole pressure values.
Results from radial core flood techniques showed significant differences in terms of pore volume to breakthrough (PVBT) based on well completion type. Further, a current upscaling algorithm for wormhole propagation modeling was verified by comparison to corresponding experimental data, thus demonstrating the suitability of described linear core flow testing methods for fluid characterization and data modeling. The wormhole propagation model (based on the use of fitting coefficient) is well-justified by the probability of increased fluid loss from the wormhole in the case of deeper radial penetration, which overall reduces the efficiency of wormholing. CT scan results revealed nonuniform radial distribution of wormhole propagation, thus shedding light on challenges associated with achieving 360° stimulation around the wellbore. Experiments at higher pressures showed that, at higher flow rates investigated (or beyond optimum) wormhole propagation has a higher than one third dependence on interstitial velocity, signifying that high pressure reservoirs require greater volumes of acid for proper stimulation. Experimental observations are presented to correlate the results obtained thus far and help resolve controversies between various publications.
Presented studies highlight the effects of higher pressure in acid wormholing and signify the need for proper volume accounting in terms of job design. A simple radial core flood technique has been developed in this work, and has an advantage in terms of diversion studies by providing a means of arranging high and low permeability core samples in the same order as formation layers and accounting for fluid entrance position.