New Insights Into Carbonate Matrix Stimulation From High-Resolution 3D Images of Wormholes Obtained in Radial Acidizing Experiments

Abstract

Abstract Usually, carbonate acidizing is studied by conducting linear coreflooding experiments in which a hydrochloric acid (HCl) solution is injected into small cylindrical core plugs, such as 1.5×6 in. (diameter×length) plugs. Results from these conventional experiments have provided extensive information on acidizing and wormholing mechanisms, but under conditions of essentially 1D flow along the axis of the plug. Meanwhile, field acidizing scenarios dictate radial (divergent) acid flow conditions around the openhole wellbore, and these conditions influence wormholing aspects such as initiation, branching, and death, in a way that cannot be simulated in linear coreflooding. In this study, we analyzed the results of large-scale radial acid injection experiments to advance understanding and characterization of wormholes created during matrix acidizing treatment. We analyzed the results of high-energy X-ray CT scans for three 20×16×16 in. block samples of Indiana limestone from our recent radial acidizing experiments with HCl. CT scans were done for complete samples, and 3D images of internal dissolution patterns were built. Each 3D image consists of submillimeter-size voxels to capture fine details of individual wormholes. The obtained 3D images of acidized blocks allow us to observe from various angles the details of the wormholes obtained in the three experiments, which demonstrated wide pore-volume-to- breakthrough (PVBT) range with three different injection rates. Also, variation of the CT number reveals the presence of depositional layers in all block samples, and the obtained CT images were used to interpret the effect that rock layers might have on wormhole penetration at different injection rates. Further, the built voxel models were used to calculate the radial distributions of dissolved rock volume, which otherwise are not obvious for such complex branched structures as wormholes. We observed how varied experimental conditions affected those distributions. The radial distributions were found very similar across the samples, which suggested the way to detect the effective wormhole penetration depth. Radial acidizing experiments represent more closely the real conditions of wellbore acidizing; however, only a few radial acidizing experiments have been published to date. In this study, we enriched the results of our recent radial acidizing tests with high-resolution 3D CT images of the created wormholes which are quite rare considering the size of the acidized blocks. In the absence of field techniques to observe the wormholes downhole, CT images of the radially acidized laboratory samples provide valuable information about the appearance of wormholes, as well as quantitative data to validate models of wormhole growth and detection. Particularly, analysis of the obtained images showed how wormhole density responds to changing experimental conditions.