A Potential Sealant for Controlling Leaks from Carbon Storage Reservoirs

Advisor

Sally M. Benson

Abstract

A possible mitigation strategy for leaks from CO₂ reservoirs is the introduction of a physical barrier in the form of injected sealant material. An organically crosslinked polymer (OCP) system was investigated as a potential sealant material for leaks from CO₂ reservoirs. The main components of the OCP system are a base polymer (copolymer of acrylamide and t-butyl acrylate, PAtBA) and a crosslinker (polyethyleneimine, PEI). The OCP system has been extensively used in the oil industry to address conformance issues related to fracture shutoff, gravel pack isolation, waterconing, casing leak repair, and high permeability streaks. The initial viscosity properties of the sealant were tested first for later use in modeling and core flood experiments. These tests were performed over a wide range of crosslinker concentrations and under temperatures relevant for barrier emplacement. Viscosity measurements demonstrate that gel times for this polymer system could be altered by changing the concentration of the crosslinker or by changing the gel setting temperature. Maximum gel times of 8.5 hours could be achieved with a crosslinker concentration of 1.0%. Temperature had a less pronounced effect on gel time, but an inverse correlation between gel time and temperature was observed. Sandstone core flood experiments were performed to determine changes in permeability when the polymer gel was placed within artificial fractures. Core flood tests were carried out at reservoir conditions of 50° C and 7 MPa. Permeability tests revealed a reduction of 100% in fracture permeability after sealant emplacement for two rock core samples (Core 1 and 2). Further reductions in overall core permeability were measured after CO₂ exposure. CT scans of these sealed rock cores were carried out last in order to visually assess the location of the sealant within the fracture. These scans revealed that the aperture and distribution of the sealant in the fracture varied along the length of the cores. The presence of the polymer within the fracture was evidenced by these scans. The fracture in Core 1 had an absence of sealant material within the middle of the core. Core 2 had a more uniform distribution of sealant material along its fracture length. The results from the experimental tests were used to create a basic injection scheme that modeled sealant injection via a horizontal well to a target fracture plane. The high initial viscosity was shown to create large differential pressures during injection. Formation penetration was limited by this sealant property and the short gel time. Despite these limitations, under ideal conditions, a physical barrier with an approximate radius of 5 m could be created using this polymer sealant system. These tests performed on the OCP system affirmed its potential as a candidate for controlling leaks from CO₂ reservoirs. Field scale trials of this sealant should be performed to gain insight beyond that of these modeling and laboratory tests.