The Rise of Interfering Solvent Chambers: Solvent Analog Model Of Steam-Assisted Gravity Drainage

Abstract

Abstract In a previous paper the authors described, both experimentally and theoretically, the sideways spreading of a solvent region as bitumen is leached in situ and the heavier solution drains by gravity towards a production well below. In this paper, new experimental and theoretical studies towards leaching of bitumen by a lighter solvent are described. The mechanism differs markedly from that for the sideways motion of the leaching surface but is closely analogous to that observed for rising steam chambers in steam-assisted gravity drainage. An experimental technique was developed that utilizes a vertical Hele-Shaw cell. The experimental work led to the observation that a boundary layer, delimited by two shock fronts which are characterized by sudden changes in concentration, is formed around a rising solvent finger. The boundary layer mechanism is relevant to miscible fooding in general and it is believed that this is the first time the phenomenon has been observed and described theoretically. A solvent chamber theory was developed which predicts, without arbitrary parameters, the drainage rate, solvent concentration at both shock fronts, the shape of the solvent fingers and the width of the boundary layer. A comparison between experimental and predicted values shows a degree of agreement which is encouraging. Introduction The remaining conventional oil reserves in Canada are estimated at 0.8 billion m3 and the rate of new onshore discoveries is declining. As large oil discoveries have become rarer, conventional oil reservoirs are becoming increasingly marginal. Furthermore, there are large exploration costs and the development of offshore and Arctic fields requires huge investments. In contrast, the enormous bitumen deposits in Alberta are well known. Most of these untapped oil resources lie under 0 to 520 m of overburden and hold more petroleum than the conventional oil fields of the Persian Gulf. They are contained in sand and carbon sedimentary formations of the Athabasca, Cold Lake, Peace River and Wabasca regions as well as in the so called Carbon are Trianglel. The total oil-in-place for these deposits has been estimated to be more than 400 billion m3 (2.7 trillion barrels)(1). About 12 billion m3 of the tar sand deposits are at sufficient shallow depth (0 to 46 m) to be potentially mineable. The deeper deposits can only be produced by in situ methodsl. The major obstacle to the production of bitumen by in situ recovery methods is the high viscosity of the bitumen in its native state; this is, typically, 100000 mPa.s for Cold Lake and 1 000 000 mPa.s for Athabasca bitumen deposits. Such high values of in situ viscosities preclude primary or secondary recovery techniques and require the direct implementation of tertiary (or EOR) methods, in particular the thermal processes (cyclic steam injection, steam drive, hot water drive and in situ combustion) and the miscible gas processes (hydrocarbon miscible and carbon dioxide miscible flooding). More than 80% of EOR production is by thermal methods. Steam-based thermal recovery processes, particularly the cyclic steam technique, are the most widely used.