Behavior and Control of Natural Water-drive Reservoirs
- George R. Elliott (Phillips Petroleum Co)
- Document ID
- Society of Petroleum Engineers
- Transactions of the AIME
- Publication Date
- December 1946
- Document Type
- Journal Paper
- 201 - 218
- 1946. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 2.4.3 Sand/Solids Control, 5.1.2 Faults and Fracture Characterisation, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.2.1 Phase Behavior and PVT Measurements, 4.6 Natural Gas, 4.1.5 Processing Equipment, 1.6 Drilling Operations, 5.4.2 Gas Injection Methods, 1.2.3 Rock properties, 5.2 Reservoir Fluid Dynamics, 6.5.2 Water use, produced water discharge and disposal
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Methods are presented for measuring and comparing "degree" of waterdrive, and for observing the control that rate of withdrawal exerts overdecline of reservoir pressure.
Degree of water drive is studied from pressure-production curves of 10reservoirs; the method used to present these comparatively is to convertcumulative oil production from barrels to percentage of ultimate. Degree ofwater drive is also represented by a "water-encroachment factor,"representing barrels of water influx per pound-month per acre-foot of oilreservoir.
Pressure-production curves for the water-drive reservoirs after a period ofdeclining pressure show a diminishing angle of slope, in some cases to thepoint where pressure ceases to decline or where it even increases; the shapesof these curves are in marked contrast to the dissolved-gas type of curve,which shows a general trend toward zero pressure at ultimate oilrecovery.
Yearly rate of oil production in percentage of ultimate oil recovery iscorrelated with pressure decline. The correct rate of production of oil withits associated gas and water sustains pressure in reservoirs having a highwaterencroachment factor. For reservoirs having a low water-encroachmentfactor, it is necessary to inject the produced water and even inject largevolumes of-water from an outside source.
The discussion of the actual reservoirs is preceded by a general review and isfollowed by a substantial bibliography.
It is now generally accepted that water drive is capable of yielding 60 to 80per cent of the original oil in place in most reservoirs, as compared with 20to 40 per cent for dissolved-gas drive. There is no need to question theimportance of controlling a reservoir in order to take advantage of such anefficient mechanism.
What is a water-drive reservoir? Oil reservoirs usually combine two or more ofthe four basic mechanisms: dissolved-gas drive, gas-cap drive, water drive,gravity drainage. A water-drive reservoir is one in which water advances totake the place of the oil and gas produced. When the oil withdrawal rate issufficiently low, the water will tend to maintain pressure. The maximumeffectiveness is attained when the water follows the oil withdrawal so closelythat little or no decline in pressure occurs.
Oil in its underground reservoir is associated with water. This water issignificant in the production of the oil according to the source of supply andthe permeability of the surrounding formation. As oil is produced and thepressure is lowered, water can move in to replace the oil. With adequate supplyand sufficient permeability, actual movement takes place on an extensivescale.
The first movement of water takes place by expansion of the water and itsassociated fluids. While this expansion is of little importance in some fields,in others it is sufficient to supply all of the energy required for theproduction of oil.
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