Experimental Study of CO2-Sensitive Chemicals for Enhanced Sealing of Leakage Pathways in CO2 Geological Storage Process

Many demonstration projects conducted in recent years have proved that CO2 capture and geological storage (CCS) can be an effective technology to reduce the emission of the greenhouse gases. The prospective storage sites under investigation mainly include deep saline aquifers, depleted oil and gas reservoirs and unminable coal seams, which should have huge storage capacity to sequestrate CO2 safely and permanently. Technically, the CCS method is feasible and can be acceptable to the public, but the storage safety and the economics of the CCS projects are of the main concerns. There are mainly three types of leakage pathways, including the failure of injection or other wells in the storage sites, undiscovered faults or fractures and seepages of cap rocks, and newly formed fractures during CO2 injection process. For CO2 storage in depleted oil and gas reservoirs, poor well integrity can be the main reason for CO2 leaks, in which CO2 can be leaked through cracks generated in cement ring or at the interface between casing and cement to upper formations. It is proposed that chemical method can be used to enhance sealing in failed wellbore or to block the leakage in near wellbore formations, since chemicals (solution) can be readily injected into wellbore or to where they are needed to form sealing packs as measures of precaution or emergency remediation. In this study, an innovative method for blocking CO2 leakage is proposed, which is based on CO2-sensitive chemicals to formulate selective agents and can associate with other common technology such as cementing to enhance the sealing of leakage pathways. The CO2-sensitive chemicals are referred to that, when they encounter the leaked CO2 in the formation or in wellbore, reactions will be triggered by CO2 to form gel or solid structure inside the leakage pathways, because CO2 is an active chemical that can take part in reaction or generate acidic environment when it dissolves into the water. The selection and design of the chemicals are based on two mechanisms: (1) certain polymers and resins will take part in crosslink reactions when CO2 appears in their aqueous solution, resulting in gelation or solidification. (2) CO2 can react with certain compounds in aqueous solution, in which precipitation will occur or solid can be produced to form blocking substance. In the paper, the CO2-sensitivity of some selected chemicals have been demonstrated in terms of generation of gels and blocking structures through visualization experiments and gel strength measurements. The chemicals studied include polyacrylamide, sodium aluminate, and phenolic resin. Their blocking capability was evaluated via core and sandpack flooding experiments, and the effects of important influencing factors on the chemical reaction and gelation prevailed at reservoir conditions are investigated, including the formulation and concentration of the chemicals, temperature, pressure and the salinity of formation water. The experimental results show that polyacrylamide and CO2 can react via a cross-linking mechanism, in which gelation with a class H visual strength has been observed at 70 degrees C and 1MPa. Good blocking performance was also achieved at sandpack flooding conditions. The solution of sodium aluminate can react with CO2 and generate solid Al(OH)(3) under high pressure and temperature conditions, which can significantly reduce the permeability of sandpacks or form a blockage for the moving CO2. The reaction product of CO2-cured resol phenol-formaldehyde resin exhibited good blocking strength. These results can provide guidelines for the design and deployment of CO2 sensitive chemicals for blocking or enhancing the sealing of CO2 leakage pathways in geological formation and wellbores. (C) 2014 The Authors. Published by Elsevier Ltd.