A New Recovery Technique for Heavy-Oil Reservoirs With Bottomwater

Abstract

Summary This paper proposes a new recovery mechanism for heavy-oil reservoirscontaining bottomwater zones. Many heavy-oil reservoirs are consideredunproductive because of the presence of a bottomwater zone. No known mechanismnow exists for producing oil from these reservoirs. Recently, horizontal wellshave been proposed as a unique solution for producing reservoirs that are thin, fractured, or contain a bottomwater producing reservoirs that are thin, fractured, or contain a bottomwater zone. For heavy-oil reservoirs, however, horizontal wells alone may not be sufficient. The electromagnetic heatingprocess has proved to be effective in in-situ heating of oil-bearingformations. Such heating, which is limited to small radii, in combination witha horizontal well offers an attractive process for recovering billions ofbarrels of oil unrecoverable by other means. The process of electromagneticheating proposed here is coupled with gas or water injection to create afavorable pressure gradient in the presence of a bottomwater zone. Scaled modelexperiments were performed to study the feasibility of the process of inert-gasinjection performed to study the feasibility of the process of inert-gasinjection along with electromagnetic heating. The variables studied wereoil/water-zone thickness ratio, oil viscosity, horizontal-well placement, inert-gas injection rate, and production-well temperature. A recovery as highas 77% oil in place (OIP) was obtained even in the presence of a bottomwaterzone as thick as the oil zone. This technique offers heavy-oil production at acost lower than that of any other thermal recovery scheme. production at a costlower than that of any other thermal recovery scheme. Introduction Many heavy-oil reservoirs contain an underlying high-water-saturation zone. If the oil viscosity is high, production from these reservoirs leads to veryhigh producing WOR's from the very early stages of production. Consequently, many of these reservoirs are considered unproductive. Several researchactivities have addressed the problem of heavy-oil reservoirs containingbottomwater zones. Islam and Farouq Ali explored the possibility of usingvarious additives (such as polymer, emulsion, foam, possibility of usingvarious additives (such as polymer, emulsion, foam, and silica gel) to improvethe efficiency of waterfloods in reservoirs containing bottomwater. While someof these additives were very effective for light-oil reservoirs, theirusefulness in heavy-oil reservoirs is questionable. Heavy-oil reservoirs haveextremely unfavorable mobility ratios, and mobilization of the heavy oil is achallenge in itself. Proctor et al. presented the results of a series ofsteamflood runs in heavy-oil reservoirs containing bottomwater. While it iswell known that the presence of bottomwater is detrimental to the sweepefficiency with any displacing agent, Proctor et al. found that, for veryviscous oil, a thin bottomwater zone actually improved recovery by acting as asteam-transporting channel. Sugianto and Butler reported experimental oilrecoveries as high as 48% OIP (above the horizontal well) by usingsteam-assisted gravity drainage in heavy-oil reservoirs containing bottomwater. Such systems may be quite expensive, however, and may not be applicable incases of very high oil viscosity where initial fluid communication is aproblem. One of the less expensive ways of creating a communication path is theelectromagnetic heating of the production well. This selective heating lowersthe viscosity of oil near the wellbore to an extent that the fluid mobilitynear the wellbore is several hundred times higher than that in the rest of thereservoir. Implementation of electromagnetic heating is relatively easy andcost-effective. In some cases, the use of horizontal wells is the mostappropriate technique for heating a heavy oil or tar sand. Electromagnetic oilrecovery along with gas or water injection offers an attractive recoverytechnique for heavy-oil reservoirs containing bottomwater zones. Gas injectionis one of the oldest oil recovery techniques and has recently gained wideapplication in heavy-oil recovery. Field results and laboratory studies showthat a high recovery is expected from gas injection at a low injectionpressure. However, no researcher has yet reported the use of inert-gasinjection in the presence of a bottomwater zone. This paper investigates thepossibility of using electromagnetic heating with gas injection to maximizeproduction from heavy-oil reservoirs containing bottomwater zones. Experimental Setup and Procedure Experiments were carried out in two sets of core holders. A series ofexperiments was conducted in a visual cell, and the rest of the experimentswere conducted in a scaled physical model. The visual cell, a typical 2Drepresentation of a porous medium, consisted of a rectangular box withdimensions of 30.5 × 30.5 × 0.6 cm. The cell had a thick glass front and aplexiglass back. These materials helped reduce heat loss through the sideplexiglass back. These materials helped reduce heat loss through the sidewalls. Fig. 1 is a schematic of the visual cell. Horizontal wells were placedat the top and bottom of the cell. A 0.3-cm-diameter heater was placed at thetop and bottom of the cell. A 0.3-cm-diameter heater was placed alongside thebottom horizontal well. The cell was packed with placed alongside the bottomhorizontal well. The cell was packed with unconsolidated sands and wassaturated with water by pulling vacuum in the system. An oilflood was thencarried out through the top horizontal well. Fluids were produced through thebottom well to establish an irreducible water saturation in the porous pack. Atthis point, the cell was ready for a complete experimental run. The heater wasactivated to establish a fixed temperature at the production well. The top wellwas connected to air pressure through a regulator. As the production well wasopened, air pressure through a regulator. As the production well was opened, air continually replaced the void in the top, creating a pressure gradient tomaximize production. To establish a bottomwater zone, sandpacks were preparedin a linear core and then laid down according to the thickness of prepared in alinear core and then laid down according to the thickness of the bottomwaterzone required. The scaled model was designed to represent a heavy-oil reservoirof Aberfeldy sand with a pay of 10 m and variable bottomwater-zone thickness. The dead-oil viscosity was 750 mPa.s at the reservoir temperature of 22C. Horizontal-well producers had a spacing of 60 m in the prototype. The scaledmodel was a high-pressure thermal model. The scaling criteria presented by Kimber et al. were used to scale the prototype (Approach 2). The followingdimensionless groups were satisfied by this approach: Only 30 m of the 300-m-long horizontal well was scaled. Experimentalevidence shows that the approximation of such a symmetrical element isjustified. Consequently, results were extrapolated for 300 m of horizontalwells. Also, the scaling groups listed above are valid for modeling aconstant-production-well-temperature case. For the case of constant heatingpower, the ratio of heating power, rather than temperature, appears as a power, the ratio of heating power, rather than temperature, appears as a scalinggroup. Fig. 2 is a schematic of the scaled physical model. Table 1 lists thecharacteristics of the model and the prototype. The model was provided with anoverburden pressure of 4200 kPa. The injection well was located at the topcorner of the model (except in one waterflood run). This configuration waschosen to maintain symmetry along a vertical line through the injectionwell.