Rise Of Interfering Steam Chambers

Abstract

Abstract An approximate expression has been developed which predicts the rate at which asteam chamber will rise in a reservoir. It is assumed that the steam chamberrises at a constant rate which is determined by the ability of steam to flowinto the advancing steam fingers and for the oil to drain down from aroundthem. The predicted velocities are proportional to the reservoir permeability and arestrongly dependent upon the steam temperature and oil viscosity. Calculated curves predict steam chamber rise rates which varyfrom 0.004 m/dayfor Athabasca bitumen heated by steam at 100 °C, to 0.19 m/day for Lloydminstercrude with steam at 300 °C. An interesting feature of the theory is that fingers of different sizes rise ata rate which is independent of their size, i.e. small fingers rise at the samerate as large ones. The size of the fingers is inversely proportional to thepermeability. Using reasonable assumed relative permeabilities, the theory predictsvelocities which are in line with those reported by Closmann and Smith for afield experiment in Athabasca. Introduction In the thermal recovery of bitumen and heavy oils using steam, it frequentlyhappens that a steam saturated zone, the steam chamber, rises through thereservoir with oil draining counter currently. The rate at which this upwarddisplacement can occur is of considerable interest because it affects theperformance of many steam recovery projects. This paper contains a theoretical analysis of the process which predicts therate at which displacement occurs. Mechanism The nature of the flows occurring during steam chamber rise is depicted in Figure 1. The rectangular boundary in this figure is assumed to rise at aconstant velocity through the reservoir. Flowing downward through the top of the boundary is the cold-saturatedreservoir material together with its fluid contents; the velocity is the risevelocity of the chamber. Steam flows upward through the lower boundary. Thisprovides the heat to raise the reservoir and its contents to steamtemperature. Heated materials leave through the lower boundary as a number of identifiablestreams. The reservoir rock and residual oil leave at the velocity of rise ofthe chamber, The flowing hot oil and condensate leave at higher velocitiesbecause they have a downward velocity relative to the hot rock and residualoil. Also, the entering steam moves at a higher velocity than the chamber inorder to pass through the lower boundary. At the very top of the chamber steam fingers move into the relatively coldreservoir. Heat is transferred from these fingers to the reservoir material byconduction and the oil drains downward around and between the fingers as itbecomes mobile. At this point, substantial temperature gradients exist in themobile oil and the average oil temperature is below that of the steam. This isimportant as the rate at which the process can proceed is determined by therelative mobility of the oil to that of the steam. Gravity is moving the oil downward and allowing the steam to rise.