Gravity Drainage To Horizontal Wells

Abstract

Abstract Pressure depletion and high water-oil ratios limit the recovery which can be obtained from many oil reservoirs using conventional methods, to disappointingly low values. The maintenance of pressure by water injection (waterflooding) or by gas injection can be relatively ineffective because of the tendency of the injected materials to finger through the oil. Although the density difference between oil and gas allows gravity segregation to improve the picture for downward f/ooding with attic gas injection, the tendency for the gas to finger down to the production well for practical production rates, limits the improvement with conventional wells. Horizontal wells can change the picture because of their much lower drawdown requirements. In this paper, the production of heavy oil reservoirs by gravity drainage with a gas cap advancing down toward horizontal wells is analyzed theoretically. It is assumed that reservoir pressure is maintained by the injection of gas at the top of the reservoir and that the production rate is controlled to just below the critical rate for gas coning. The theory indicates that, for such conditions, recovery rates of the order of several hundred B/day are possible and that relatively high recoveries can be achieved. By maintaining the pressure of the production well equal to that of a lower aquifer, it is possible to carry out the above operation without excessive water production. Introduction This paper discusses the production of oil by displacement created by the downward expansion of a gas cap. The displaced oil flows to parallel horizontal wells located near to the base of the reservoir. It is assumed that the gas pressure is maintained constant by injection and that the low viscosity of the gas, compared to that of the oil, maintains a constant pressure within the cap. The conditions are chosen so that gravity effects prevent the flow of free gas to the production wells; i.e. the process is carried. out below the critical coning velocity(l). The process is one of gravity drainage and can be expected to give high recovery(2,3). While such a process can be carried out using arrays of vertical wells, it is much more effective with horizontal ones, particularly if they are closely spaced. The effect of pattern size and shape for horizontal wells has been analyzed in recent papers(4,5) and it has been shown that for close spacings (but not so close as to be economically unfeasible) the productivity index for pseudo steady-state flow to horizontal wells can be as high as 50 to 100 times that for vertical wells drilled on the same aereal spacing. The practical significance of this, for the situation considered here, is that operation below the critical production rate may be feasible for horizontal wells in situations where it would be impractical with vertical ones. In such situations, the gas injection which is required for pressure maintenance is simply that to replace the produced oil on a volume basis plus a minor amount which is dissolved.