Study on migration and plugging performance of polymer gel in fractured cores using nuclear magnetic resonance technology

Abstract

In situ cross-linked polymer gels (ISCPGs) are widely used for conformance control and enhanced oil recovery (EOR) in heterogeneous petroleum reservoirs. However, the mechanisms behind their migration and plugging in fractured cores remain unclear. In this study, changes in the injection pressure difference before and after the treatment of HPAM/phenolic resin gel was monitored by core displacement experiment. The gel was injected with two different gelation states, i.e., gelant and pre-crosslinked gel. Meanwhile, we analyzed the nuclear magnetic resonance (NMR) T2 signal and imaging of the cores that were treated with the polymer gel system during various injection stages. Results indicated that NMR technology can effectively analyze the migration and plugging performance of polymer gels in fractured cores at a microscopic level, both before and after gelation. The gelant effectively plugged the fracture when the injection rate was low, but a great amount of the gelant entered the matrix and damaged the matrix. In contrast, it formed a plugging layer at the inlet face and also caused damage at high injection rates. For conformance treatment of pre-crosslinked gels, the gel did not easily enter the matrix. When the injection rate of pre-crosslinked gels increased, its plugging efficiency for the fractured cores enhanced. Therefore, engineers should optimize gel state, injection rate, and injection pressure according to the specific reservoir conditions before oilfield operations. This study can help researchers understand the microscopic migration and plugging mechanism of polymer gels in cores at the microscopic level. It will also help researchers to pay more attention to the NMR technique to further understand its application in petroleum reservoir conformance improvement.