Abstract

Underground gasification of crude oil is a new, improved oil recovery technique for application in light oil reservoirs. Inert gas is generated inside a downhole gasification (DHG) unit by gasifiying part of the produced oil. A string of DHG units ispositioned 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 other suitable gasdisplacement 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 surface steamreforming plant. Catalytic steam reforming of the pseudo-light naphtha fraction, containing reservoir gas, achievedconversions 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. The economics of the process are very attractive, if energy is recovered from the hot gas stream. The produced gases are stored-up in the reservoir, so that there are no gaseous emissions. The hydrogen-product can also be recovered later.

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 et al.(1) proposed using an electrical heater to heat part of the oil formation, so that the oil was partially vaporized, 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 effect was probably only marginal, because there is no permanent overall reduction in the crude oil viscosity. Nielsen (2) proposed a method of heating shale oil layers by pumpinghot gases in the reservoir matrix, in order to pyrolise the oil, or kerogen material. The vaporized fractions were then collected at the surface. The new, Underground Gasification, or DHGtechnique, first proposed by Davidson and Yule (3), differs from all other previous attempts, in one major respect. The heat is provided electrically inside a DHG unit, so that there is no direct contact with the reservoir matrix. The DHG technique is also designed mainly for improving oil recovery from light oilreservoirs, ca. 30 ° API, and is not applicable to heavy oil and shale oils. However, it may be possible to apply it to some medium heavy oils, which can be displaced at economic rates and which have a significant light naphtha content. Thereappears to be very broad scope for applying the new technique in light oil reservoirs that have previously been water flooded, or otherwise depleted to a low oil residual.

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