Experimental Study of a Novel In Situ Gasification Technique for Improved Oil Recovery From Light Oil Reservoirs

Abstract

Abstract In situ gasification of crude oil using downhole gasification (DHG) is a new, improved oil recovery technique for application in light oil reservoirs. Inert gas is generated inside a DHG unit by gasifying part of the produced oil. A string of DHG units is positioned along a horizontal producer well, or on a vertical well. The gases, mainly hydrogen and CO2, are then directed into a gas cap formation. Incremental oil recovery is achieved via gravity stabilized gas injection (GSGI) or another suitable gas displacement drive method. There is essentially no net heat release into the reservoir if efficient heat recovery is practiced. A series of high pressure, continuous flow experiments was performed in a small-scale gasifier unit, using n-pentane as feedstock, to represent the light naphtha fraction from a crude oil. The experiments were operated at pressures up to 100 bar, compared to the 20 to 30 bar normally used in a surface steam reforming plant. Catalytic steam reforming of the pseudo-light naphtha fraction, containing reservoir gas, achieved conversions of 70 to 80% at a gasifier exit temperature of 720 °C. The produced gas contained up to 50% hydrogen, with the remainder comprising carbon oxides. The new IOR technique has wide potential application in depleted light oil reservoirs where the pressure is less than about 200 bar, although it may be possible to increase this limit. The economics of the DHG process appear to be attractive, if energy is recovered from the hot gas stream. The produced gases are stored-up in the reservoir and any emissions are therefore limited to those which are released from the crude oil when it is produced. If the reservoir pressure is low, then the amount of gas produced with the oil will be relatively small. The hydrogen product storedup in the reservoir is a valuable resource, which could be recovered later for use in the future "hydrogen economy." Introduction Previous attempts to increase oil recovery from (mainly) heavy oil reservoirs by introducing a source of heat have focused on heating up part of the reservoir. Thus, Niles(1) proposed using an electrical heater to heat part of the oil formation so that the oil was partially vapourized in order to increase the reservoir pressure. It was also claimed that the oil viscosity was reduced, thereby increasing the flow of the oil. This was probably only a marginal effect because there is no permanent overall reduction in the crude oil viscosity. Nielsen(2) proposed a method of heating shale oil layers by pumping hot gases into the reservoir matrix to pyrolyse the oil or kerogen material. The vapourized fractions were then collected at the surface. The new underground gasification, or DHG technique, first proposed by Davidson and Yule(3), differs from all other previous attempts in one major respect. Heat is provided electrically inside a DHG unit so that there is no direct contact with the reservoir matrix.