Experimental evaluation of temporary plugging performance and mechanism analysis of degradable preformed particle gel based on NMR technology

Abstract

Utilizing chemical temporary plugging systems to further increase the stimulated reservoir volume (SRV) and improve oil recovery (IOR) has become a hot topic in the current petroleum industry. Pre-formed gel particles (PPG) possess excellent swelling properties and can form high-strength plugging in high-permeability channels. In addition, degradable pre-formed gel particles (DPPG) further exhibit self-degradation characteristics and excellent temporary plugging and fluid diverting performance. In our previous studies, we examined the temporary plugging mechanisms of DPPG prepared with different molecular weight (MW) of crosslinking agents (PEGDA) through bottle tests and core displacement experiments. However, the microscopic swelling mechanism and temporary plugging mechanism in pore throats of reservoir cores are still unclear. In this paper, comprehensive research was conducted on the microscopic temporary plugging mechanism of DPPG prepared with different MW crosslinking agents using both and method. Results showed that DPPG with different compositions had different micro-swelling performance. NMR-T₂ analysis for bottle test showed that DPPG-M1 (with the smallest MW of crosslinking agent) gradually decreased in nuclear signal during the water absorption and swelling process, and had the largest swelling volume and swelling rate. On the other hand, DPPG-M5 (with the largest MW of crosslinking agent) exhibited a slow diffusion process of surface water towards the nucleus, resulting in the smallest swelling volume and swelling rate. However, its complete degradation time was longer. Moreover, the NMR T₂ (i.e., spin-spin relaxation time) results of the cores in the dynamic temporary plugging experiment showed that the decrease of the MW of the crosslinking agent enhanced the plugging performance, but with less damage to the formation. The MRI images confirmed that DPPG-M5 had better injection capability in the core and caused greater damage to the core. The research results of this study showed that NMR technology can reveal the temporary plugging mechanism of DPPG from a micro-level perspective.