GRAVITY DRAINAGE TO HORIZONTAL WELLS

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 flooding 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.