The Use of CT Scanning in the Investigation of Acid Damage to Sandstone Core

Abstract

Abstract CT scanning has been used in acid flow studies in the past but has provided only qualitative information along the length of the core. A procedure for acid flow studies has been developed using a dual energy method on a CT scanner. This method determines the porosity change in volume elements in sequential axial slices along the length of the core. This paper describes the laboratory procedure used for the CT scanner to determine a treatment volume that is related to the volume of siderite in the formation. Introduction Acid flow studies have been proven beneficial to determine the effects of acid systems on formation rock. These techniques have improved over the years by using longer multitap cores that are 10-36" in length and by performing cation analysis of the effluent from the multi-ports. This has been successful in the determination of deep damage through acid precipitation products. Two drawbacks to multiport tests have been the lack of detailed knowledge of the acid flow path and change in the porosity distribution. This was apparent in an earlier CT scan test where CT images indicated that the acid flowed through high permeable layers of a sandstone core. Better knowledge of the flow path and porosity change could be used to accurately determine a mud acid treatment volume. Our goals in this experiment were 1) to determine the flow path, 2) obtain pre- and post-acid porosity distributions, 3) calculate an acid treatment volume based on the change in porosity and 4) determine if the acid created damage. To build from past techniques, we used a dual energy x-ray tomography approach to determine the change in porosity down the length of the core. This method determines the porosity change in 5 mm thick volume elements in sequential axial slices along the length of the core. The dual energy technique uses the ratio of x-ray absorption when images are acquired with two different incident x-ray energies. The ratio of x-ray absorption, when images were acquired with 80 and 140 keV x-ray beams, can be correlated to the average atomic number of the region of interest. When porosity is present, the effective atomic number of a volume can be changed by resaturation with a fluid of a different atomic number. Results of this test provided an acid treatment volume based on the depth of live acid penetration and the change in porosity. The test used core material from a sandstone formation that was predominantly quartz, siderite and glauconite. Previous flow tests indicated 5% HCl was required to remove formation damage. Field results indicated that too much acid could cause severe problems due to deconsolidation of the rock. The CT tests indicated that the optimum acid treatment volume is 11 gal/ft when the siderite volume of the formation is 5 wt%. P. 69