Passive Tomography to Image Stress Redistribution Prior to Failure on Berea Sandstone and Marcellus Shale for Caprock Integrity
Sadtler, Daniel Allan
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A recent concern is the cause and effect of global climate change. Many institutions give credit for these changes to the increased levels of greenhouse gases in the atmosphere, in particular the increase in the amount of carbon dioxide present. There is a growing interest in carbon capture and storage (CCS) as a means to reduce the global impact of CO2 on the climate as a greenhouse gas. Carbon capture is the process of removing CO2 from the atmosphere as well as preventing it from entering the atmosphere by means of exhaust. The captured carbon is stored underground in reservoirs. These reservoirs have the storage space to handle the volume of CO2 injected as well as a caprock layer preventing the injection fluid from returning to the surface. Additionally, CO2 can be used for enhanced oil recovery (EOR). To monitor the injection sites used for the CO2 storage or EOR process, the integrity of the caprock as well as the surrounding rock formations are the locations of interest. Knowing when a joint or a fracture is going to slip is necessary to prevent major failures within geologic strata. It is necessary to prevent these slips from occurring to retain the integrity of the caprock, which is keeping the fluid within the reservoirs. Passive acoustic emissions monitoring was used to determine how effectively failure locations could be located in three unique tests. Coupled with double difference tomography, the failure of a Berea Sandstone sample and Marcellus Shale sample were calculated to determine how well the stress redistribution within the sample could be mapped using the recorded data. For the main indenter tests two samples were tested, a piece of Berea Sandstone and a piece of Marcellus Shale. The secondary test was a transform shear test using sandstone, and the third test for caprock upheaval test attempted to recreate the failure of caprock due to injection pressure. For all tests, the samples were monitored using acoustic emissions software until failure or it was deduced that the test would not produce failure. The secondary tests did not progress through the data analysis as far as the indentation tests, however valuable information was gathered from these tests. The shear test demonstrated the iii effectiveness of the passive acoustic emissions monitoring system to record shear failure. This test provides confidence in this technology to record and located events that are not occurring in compression. The caprock upheaval tests were not successful in causing failure in the caprock, however during the testing the passive acoustic emissions monitoring system was able record and locate events that occurred within the sample around the boundary on the reservoir. At the reservoir boundaries there was evidence of fluid flowing through the reservoir, and the events align with these locations. This positive result shows that the monitoring system is able to locate events induced by fluid injection. The results of these tests provide confidence in the passive acoustic emissions monitoring system to record accurate data for the caprock integrity monitoring. The tomograms created from the recorded data accurately imaged the areas of interest within the rock samples. From these results, passive acoustic emissions monitoring systems coupled with double difference tomography has proven capable of monitoring homogeneous samples within a laboratory environment. With further testing, this technology could possibly be a viable option for monitoring carbon sequestration sites.
- Masters Theses