Improving geological saline reservoir integrity through applied mineral carbonation engineering

Abstract

The most widely advocated method of carbon capture and storage involves the injection of CO2 into underground geological formations. Key to the development of this geological sequestration technology is the existence of suitable high-integrity geological sites for the safe, long-term storage of CO2. Unlike depleted oil and gas reservoirs which are historically proven to be well-defined, formations with saline brines may not have a similar proven sealing capacity. In the main, complex geochemical reactions occur in the supercritical CO2 / brine / host rock environment which can cause significant changes in the porosity, permeability and injectivity of the formation. Depending of the nature of the processes, the effects of the underground injection of CO2 may (1) yield increased storage capacity of the target horizon, or (2) lead to increased potential for leakage beyond the confining layers of the saline formation, or (3) impede the injection exercise as a whole. It is conceivable that accelerated, localized mineral carbonation could be induced at strategic places between the CO2 plume and fault zones or facies changes present in deep saline formations, in order to prevent the migration of CO2 outside the confined layers of the reservoir. The South African electricity producer, Eskom, generated 36.01 million tons of coal- combustion fly ash in 2010. About 5.6% were reused for the production of cement. The remaining 33.89 million tons were safely disposed of and managed on Eskom ash dumps and dams which are located adjacent to their corresponding power stations. South Africa has a long history regarding the development of new applications for this material and is very active in the development of ash technologies. Concurrently, the power industry is also a major carbon dioxide (CO2 ) emitter, with Eskom’s emissions approximating 225 million tons for 2010. In this study, the author introduces a theoretical concept whereby fly ash in a slurry form could be injected at strategic sites of deep saline formations. The purpose of this injection strategy is to prevent the migration of injected anthropogenic CO2 plumes beyond the confining layers of the formations, via induced in situ localized, accelerated mineral carbonation. The proposed application falls within the carbon capture and storage (CCS) initiative by geological sequestration and aims at improving the integrity of deep saline formations which may be at risk of leakage upon injection of CO2. The use of coal-combustion fly ash in industrial mineral carbonation and the research involving its applications in carbon capture and storage (CCS) has internationally gained increased attention. However, the work involving fly ash in industrial mineral carbonation has only focused on the sequestration of sub-critical CO2. This work demonstrates for the first time that fly ash can react with supercritical CO2 under varying pressure and temperature conditions. The experiments were conducted following an assumed geothermal gradient for deep saline reservoirs, as described by Viljoen, 2010, i.e. 44°C/80bar and 50°C/100bar. Ultra-pure water was used as a solvent. The duration of experiments ranged from 60 minutes to 7 days. Under these T/ P conditions, carbonates in the form of calcite (CaCO3 ) were only detected at completion of the 7 days experiment. Further investigation was undertaken at 90°C/90bar for 2 hours using synthetic brine as a solvent, in order to mimic the composition of deep saline formations. This work yielded both aragonite and calcite, which formed as sheets at the base and on the walls of the batch reactor. The carbonated sheet fragments were examined using scanning electron microscopy (SEM) and were found to have an approximate thickness of 16 μm. A thinner layer of white precipitate on the walls of the reactor was composed of aragonite and calcite and contained an amorphous phase of carbonate of ca. 1% by volume. The mineralogical composition of these carbonated sheets was confirmed using XRD, which demonstrated the presence of aragonite (23%), calcite (3%) and fly ash minerals (e.g. mullite, quartz). It also contained an XRD-amorphous phase of about 37%. These sheets were thus enriched in calcium and carbon but also other elements were found to be present (Al, Si, Na, Mg and Cl) as shown by SEM. It is, however, unclear whether these elements identified in the spectrum are part of the sheet or are rather indicative of an effect of analytical volume created by the SEM electron beam being larger than the thickness of the sheet. Small amounts of S were also detected. Fly ash particles as well as a small number of needle-shaped gypsum crystals were visibly embedded in the sheet (SEM). Copyright