Evaluation of the Steam-Injection Process in Light-Oil Reservoirs
- Kaveh Dehghani (Chevron Petroleum Technology Co.) | Robert Ehrlich (Chevron Petroleum Technology Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2001
- Document Type
- Journal Paper
- 395 - 405
- 2001. Society of Petroleum Engineers
- 5.1.5 Geologic Modeling, 5.4.10 Microbial Methods, 5.4.1 Waterflooding, 6.5.2 Water use, produced water discharge and disposal, 5.1.3 Sedimentology, 5.5.8 History Matching, 5.2.1 Phase Behavior and PVT Measurements, 5.1.2 Faults and Fracture Characterisation, 5.1.1 Exploration, Development, Structural Geology, 5.8.7 Carbonate Reservoir, 4.3.4 Scale, 5.1 Reservoir Characterisation, 5.4.6 Thermal Methods, 5.3.4 Reduction of Residual Oil Saturation, 4.1.5 Processing Equipment, 5.8.5 Oil Sand, Oil Shale, Bitumen, 1.2.3 Rock properties, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 4.6 Natural Gas, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc)
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Feasibility of steam injection for three light-oil reservoirs in different geologic settings has been evaluated. The settings studied were a waterflooded deltaic sandstone, a waterflooded vuggy dolomite, and a deltaic sandstone structural trap with a gas cap. Optimization of steam injection to take advantage of individual reservoir characteristics is demonstrated.
Results show that light-oil steamfloods can be designed to take advantage of post-secondary oil-saturation distribution. The resulting project may be carried out in a considerably different fashion from that of conventional heavy-oil steamfloods.
We also re-evaluated an unsuccessful light-oil steamflood (LOSF) project carried out in the past. The re-evaluation correctly predicted failure because of early steam breakthrough. The results show that by considering details of geology and displacement process physics, the recent advances in reservoir characterization and modeling tools enable us to predict the performance of these projects more accurately.
Steamflooding in shallow heavy-oil sands is a mature, successful technology with large commercial projects in the U.S., Canada, Indonesia, and Venezuela. Light-oil reservoirs have had fewer steamflood applications even where depth and other factors are favorable because of the generally lower post-secondary oil in place.1,2
Unlike heavy-oil steamflood projects, there are no large-scale commercial light-oil projects to use as analogs for design purposes. There are, however, a number of field trials in the literature, which have been reported both as successes and as failures.
A review of the reported cases reveals some common reasons for success or failure. The major successful LOSF field cases are reported in Tables 1 and 2.3-10 Using steam as a heating agent in heterogeneous or extensively fractured reservoirs has given positive results (e.g., Teapot Dome field, Wyoming, U.S.A., and Lacq Supérieur field, France).9,10 Also, injecting steam into thick reservoirs with gas caps has resulted in expanding the gas cap and accelerating the gravity-drainage process (e.g., Shiells Canyon field, California, U.S.A., and Smackover field, Arkansas, U.S.A.).5,7 When steam is used for drive only, field trials have been successful in reservoirs with favorable geology (e.g., Schoonebeek field, The Netherlands, and Brea field, California, U.S.A.).3,4
The major unsuccessful11-15 LOSF field trials are shown in Table 3. One common characteristic of unsuccessful field trials has been steam channeling through thief zones (e.g., East Coalinga, California, U.S.A.; Triumph field, Pennsylvania, U.S.A.; El Dorado field, Kansas, U.S.A.; and Buena Vista field, California, U.S.A.).11-15 Scaling was another reason given for project failure (e.g., Elk Hills, California, U.S.A.).15
The overall screening criteria for the light-oil steamflood are shown in Table 4.1 All the above reservoirs met these screening criteria. Although these criteria are useful preliminary guidelines, the failed projects show that each reservoir should be examined individually.
When reservoir geology and oil gravity are considered, the major recovery mechanisms in light-oil steamfloods can change significantly, as compared with conventional heavy-oil steamfloods. Viscosity reduction may increase recovery by accelerating the gravity-drainage process in thick columns for light crudes of 20 to 25°API. The beneficial effect of steam specific to lighter oils is distillation of light hydrocarbons, which results in very low residual oil saturation where steam displacement occurs, as well as enhanced solution gas drive and accelerated depletion from zones that are heated but not displaced. Because of greater initial fluid mobility in light-oil reservoirs, high steam-injection rates can be used. The light-oil steamflood projects can benefit from wider well spacing than conventional heavy-oil steamfloods.
All of these can result in effective projects in a variety of geologic settings, despite lower oil in place than would be considered acceptable for heavy oils.
The objective of this work was to examine the feasibility of steam-injection processes that are optimized to take maximum advantage of specific reservoir geologic settings. This study was conducted for three Chevron reservoirs:
A waterflooded deltaic sandstone reservoir with channel and bar deposits.
A waterflooded heterogeneous vuggy carbonate reservoir.
A structural trap deltaic sandstone with a gas cap.
To help validate the evaluation process, Buena Vista Hills,14,16 a failed light-oil steam field trial reported by Chevron, was also re-evaluated.
The first reservoir considered for steamflood evaluation is Minas in Central Sumatra. The reservoir is in early Miocene sandstones in the Sihapas formation at an average depth of 2,000 ft subsea with a maximum vertical oil column of 480 ft. Average porosity is 26%. The oil is 36°API with an average initial bubblepoint pressure of 235 psig. Current reservoir pressure is approximately 350 psig. Reservoir temperature is 207°F. The original oil in place (OOIP) estimate is 9 billion bbl.17,18
Initial development was on 214-acre spacing. Initial production was by aquifer drive that was augmented by peripheral water injection beginning in 1970. Starting in 1978, infilling reduced spacing to 71 acres. In the early 1990's, phased pattern waterflood development was implemented by use of 71-acre inverted seven-spot patterns. This development is approximately 70% complete and is targeted only in the thickest parts of the field. Ultimate recovery following completion is estimated to be 51% of OOIP.
The Sihapas group of interbedded sandstones and shales was deposited as part of a large delta complex. Principal sand bodies are channel deposits and subtidal bar deposits. The channel deposits have an erosive base overlain by coarse sand and gravel and become finer-grained and shalier upward. The bar sands have a gradational base and become cleaner and coarser-grained upward. This is shown in Fig. 1.
Both gravity and permeability contrasts will cause water to underrun in the channel sands, creating high-oil-saturation bypassed zones. Because of the natural tendency of steam to override, oil from these zones should be swept in a steamflood. It was felt that areas of the field with the greatest total thickness of these channel sand tops had the best steamflood potential.
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