Novel Thermal Process for Recovery of Extremely Shallow Heavy Oil
- W. Terry Osterloh (ChevronTexaco E&P Technology Co.) | Jeff Jones (Berry Petroleum)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2003
- Document Type
- Journal Paper
- 127 - 134
- 2003. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.3.4 Scale, 2.2.2 Perforating, 1.6 Drilling Operations, 1.6.6 Directional Drilling, 5.5.8 History Matching, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation, 2.4.3 Sand/Solids Control, 5.4.6 Thermal Methods, 5.4 Enhanced Recovery, 5.4.10 Microbial Methods, 4.6 Natural Gas, 5.1.5 Geologic Modeling, 5.8.5 Oil Sand, Oil Shale, Bitumen
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In this paper, we propose a new process for the recovery of heavy oil from sands at depths of 100 ft or less and report its feasibility as determined by reservoir simulation. The process, called blanket heating, recovers oil by the traditional mechanisms of viscosity reduction and gravity drainage; however, it avoids the problem of handling large volumes of injected fluids and reduces the risk of surface emissions that would occur with typical steam enhanced oil recovery (EOR) methods.
Simulation results indicate that excellent, thermally efficient oil recovery is possible. Process sensitivity to many critical parameters was determined.
The entire Kern River field is produced by thermal recovery, using both cyclic steam stimulation and steamflood. High sand porosity (over 30%) and high vertical and horizontal permeability (2.5 to 5 darcies) enable steam-enhanced gravity drainage as the main recovery mechanism. Fig. 1 shows the viscosity/temperature relationship for the oil. Because gravity drainage is governed by
the temperature increase from 90°F to more than 212°F during steam injection results in relatively high production rates and a substantial reduction in oil viscosity.
The field contains many acres of oil sands at depths of 100 ft or less. Because these very shallow oil deposits outcrop at the surface in the immediate area, it is unlikely that they can be processed with conventional steamflood or cyclic steam stimulation without the very real danger of the injected fluids carrying pollution emissions to the surface. Unless a new recovery method is found, these reserves will be unrecoverable.
The area of interest in this work was the shallow oil sand on ChevronTexaco's Elwood property, Section 3, T29S, R28E. There may be similar, yet-to-be-described deposits in properties on the edge of the Kern River field, but there are at least 100 acres of the deposit in Elwood.
We propose a new method that might allow the shallow oil to be recovered economically by stimulating the native gravity drainage mechanism while eliminating the surface emissions that would occur if steam were injected directly into the formation. The method, called blanket heating, involves indirect heating of the formation - no fluid or gas is injected directly into the formation pore space. The heated reservoir fluids would flow downdip and be produced with inexpensive sump wells. The indirect heating would be accomplished by using horizontal directional drilling methods to place a horizontal grid of 2- or 3-in. internal diameter (I.D.) steel conduits along the base of the sand. Steam would then be circulated through the pipes as a heat-transfer fluid to conductively heat the surrounding formation.
While this method would avoid the drawback of handling large volumes of injected fluids, other important aspects were unknown. Could conductive heating transfer energy into the formation at a rate sufficient for economical oil recovery? What density of conduits would be needed? Should the conduits be laid on strike or parallel to dip? Would the chance of surface emissions be reduced? The shallow Elwood oil sand is overlaid by an air sand that outcrops nearby to the surface. Whether a seal exists between the oil sand and the surface was not clearly understood. What effect would the presence or absence of flow barriers have on the blanket- recovery process? Thermal reservoir simulation was used to obtain first-pass answers to these questions and to help design the blanket-heating process.
Simulation Model and Parameters
A geological model was not available for the Elwood area, so a simple model that captured the important geological aspects of the area was used. Commercially available simulation code was used.
The simulation grid was designed to model an area that, with 4° of dip, encompassed depths from the surface to approximately 100 ft. A 5/8-acre square pattern was chosen that contained depths from the surface to 110 ft. For all but two cases, half of the 5/8-acre model was used; the half-strip was cut parallel to dip.
The model contained an oil sand and an air sand, with the air/oil contact located at 50 ft. The initial pressure in the model was 14.7 psia at the surface, with a gas and liquid head below.
To simulate the flow of fluids between the model and the atmosphere, 16 influx wells and 16 vent wells were evenly distributed and perforated in the top layer of the model. The influx wells would inject nitrogen at 95°F when the pressure at the perforation fell below 14.68 psia, and the vent wells would produce liquid and/or gas when the pressure at the perforation rose above 14.72 psia.
Conduits and Steam Generation
The type of horizontal steel conduits that would be used with the blanket-heating process are drilled regularly by contractors to provide utilities such as natural gas, water, and electricity. Using micro-tunneling methods, they claim to be able to accurately place horizontal conduits for hundreds of feet and precisely control depth, areal spacing, and exit points.
An advantage of confining the steam in conduits is that temperature can be controlled by simply adjusting backpressure at the pipe outlet. Temperature difference is the driving force behind oil-sand heating rate, and these pipes can be heated to 600°F with a conventional, dedicated steam generator. Thermal efficiency of the process would be high relative to other steam EOR processes because heat is either being transferred directly to oil sand or returned to the boiler to be reheated and returned to the grid.
The steam generator would have to be dedicated because it would be operated continuously at 1,500 psig as a closed heat-transfer- fluid (water/steam) heater.
Production wells were perforated in the bottom 20 ft of the oil sand. Preliminary simulation results indicated that this configuration was optimal. The production wells were "pumped off"; that is, the bottomhole pressure (BHP) target at the topmost perforation was set to 14.7 psia, and there was no head gradient below this perforation. For most cases, the production wells were located in inverted 5-spot positions. A few other well configurations were examined to determine the effect of well location and density.
Model Grid Configuration
For most cases, the model contained no barriers to the vertical flow of fluids. A few cases were run with complete and partial barriers to vertical flow. The exact characteristics of these barriers are described later.
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