Models of Thermal EOR in Fractured Reservoirs

Abstract

AbstractModels for hot-water or steam injection into a fractured shale or diatomite reservoir are developed from existing analytic models of energy transport and counter-current imbibition.Radial convective heat flow through a horizontal fracture system is modeled with conductive heat flow into the low permeability matrix. The flow geometry approximates hot fluid injection into a five-spot pattern. Recovery mechanisms accounted for in the models include capillary imbibition and thermal expansion. Temperature dependence of viscosity and interfacial tension are included in the imbibition estimate. Laboratory data is needed to quantify the magnitude of the imbibition mechanism, which is usually the primary contributor to oil recovery. Data representative of either the Belridge Diatomite or the Antelope Shale, two giant fractured oil reservoirs, are used for the model forecasts.The steamflood model has been partially validated against a large-scale project in the Belridge Diatomite. Using public-domain information, a reasonable match was obtained between the model and field project over a four-year injection period. Comparison to conventional thermal simulation was also performed and indicated reasonable agreement with the steamflood analytical model.The models have been used to determine the key factors determining the success of thermal recovery in fractured, low-permeability reservoirs. Steam injection is shown to be superior to hot water injection in heating the matrix. Key factors enhancing recovery include: reduced fracture spacing, increased matrix permeability and increased injection temperature. Model results indicate that steamflood recoveries in excess of 40% of the original oil-in-place (OOIP) may be achieved by injection in both diatomite and shales containing light oils. Application to diatomites containing heavy oil also shows good performance. Successful application in diatomite reservoirs is forecast to be possible in the current low oil price environment. Economic application in fractured shales, would require a higher oil price because of the higher well costs and lower oil content relative to diatomite projects.Because of the significant volumes of remaining oil in place in both the diatomite and shale reservoirs, application of thermal enhanced oil recovery (EOR) to these resources represents the logical next step in steamflood development.