Comprehensive evaluation of chemical breakers for multistage network ultra-high strength gel

Abstract

Polymer gels have been accepted as a useful tool to address many sealing operations such as drilling and completion, well stimulation, wellbore integrity, water and gas shutoff, etc. Previously, we developed an ultra-high strength gel (USGel) for medium to ultra-low temperature reservoirs. However, the removal of USGel is a difficult problem for most temporary plugging operations. This paper first provides new insights into the mechanism of USGel, where multistage network structure and physical entanglement are the main reasons for USGel possessing ultra-high strength. Then the effects of acid breakers, encapsulated breakers, and oxidation breakers (including H2O2, Na2S2O8, Ca(ClO)2, H2O2 + NaOH, Na2S2O8 + NaOH, and Ca(ClO)2 + NaOH) were evaluated. The effects of component concentration and temperature on the breaking solution were studied, and the corrosion performance, physical simulation and formation damage tests of the breaking solution were carried out. The final formulation of 2%–4% NaOH + 4.5%–6% H2O2 breaking solution was determined, which can make USGel completely turn into water at 35–105 °C. The combinations of “acid + breaking solution”, “acid + encapsulated breaker” and “encapsulated breaker + breaking solution” were evaluated for breaking effect. The acid gradually reduced the volume of USGel, which increased the contact area between breaking solution and USGel, and the effect of “4% acid + breaking solution” was 23 times higher than that of breaking solution alone at 35 °C. However, the acid significantly reduced the strength of USGel. This paper provides new insights into the breaking of high-strength gels with complex network structures.