Heat Flow During Steam Injection Into a Fractured Carbonate Reservoir

Abstract

Summary Steam injection into fractured carbonates has been examined in the laboratory and with a fully implicit reservoir simulator. These studies showed that significant oil can be recovered from matrix elements when hot water or steam is injected into fractures. Heat is rapidly replenished to fractures and heat conduction into the matrix plays a major role in matrix heating with a resulting increase in the rate of oil release. At steam temperatures carbonates decompose to produce a significant amount of CO2 which further enhances the recovery rate. Carbonate disks were flooded in the laboratory with hot water and steam. Oil recovery from competent as well as fractured disks increased as injection temperature increased. Oil recovery from fractured disks approached 30% 0IP at 200°F even though pressure differentials were small. In other experiments, CO2 con-centration in effluent gas was determined. One mole of CO2 was produced for each mole of H2O injected at steam temperatures. Reservoir simulations were accomplished using two kinds of porosity/permeability grid blocks. Fracture grid blocks were continuous from injection to production wells and accounted for less than 0.5% of the total reservoir volume. Oil recovery and water/oil ratios were dependent on matrix grid block size, steam Injection rate, initial saturations and distributions, and CO2 generation. Oil recovery increased with in creasing steam injection rates, however, water/oil ratios were high. These results show that there is an optimum steam injection rate for a specified reservoir volume and matrix grid block size. The rate of heat conduction from fracture to matrix is the controlling factor.