TEMPERATURE INDUCED FRACTURE RECONSOLIDATION OF DIATOMACEOUS ROCK DURING FORCED WATER IMBIBITION

Diatomite reservoirs in California hold about 12 billion barrels oil in place. This resource is high porosity, low permeability, and oil quality is variable. Steam injection is technically feasible to unlock such oil. Our previous laboratory work demonstrated the tendency of fractures to reconsolidate or heal during thermal operations. We conducted a series of forced water imbibition experiments to study the role of pH, temperature, and salinity on silica dissolution of outcrop diatomite core. Then, several fractured cores were prepared to study the mechanism of fracture reconsolidation. Fractures were oriented lateral to and normal to flow. Fractured cores were subject to different brine formulations, temperatures, and confining pressures. Results suggest that temperature and pH impact silica dissolution of diatomite, in agreement with the literature. At elevated temperature, significant silica dissolution occurs under basic or acidic in‑situ conditions. Wormholes form during many pore volumes of injection of hot alkaline fluid if basic pH is maintained within rock. As a result, permeability is enhanced and porosity increases. The presence of steam hinders silica dissolution because less aqueous phase is available to carry ions. Fracture healing and rock reconsolidation were observed when fluid was injected at elevated temperature. Tests suggest that both silica dissolution and confining stress are necessary for fracture reconsolidation. Fractures that are not closed by the confining stress do not tend to heal. The proposed mechanism for this process has three steps: (i) aqueous silicate production by silica dissolution, (ii) silicate gelation within the pore space and fracture, and (iii) stress closure of fractures to ensure that deposited silica cements the fracture closed. Given sufficient heating and liquid injection, fracture reconsolidation happens. Under laboratory conditions, fracture healing is possible. The resulting rock is relatively dense and strong. Results suggest conditions that might be applicable to the field.