In many oil fields the oil bearing formation comprises a gas cap zone and an oil bearing zone. Many of these fields produce a mixture of oil and gas with the gas-to-oil ratio (GOR) increasing as the field ages. This is a result of many factors well known to those skilled in the art.
The produced mixture of oil and gas is typically separated into an oil portion and a gas portion at the surface. The gas portion may be marketed as a natural gas product, reinjected into the gas cap to maintain pressure therein, or the like. Further, many such fields are located in parts of the world where it is difficult to economically transport the gas to market; therefore, the reinjection of the gas into the gas cap preserves the availability of the gas as a resource in the future as well as maintaining pressure in the gas cap.
Such wells may produce gas/oil mixtures having a GOR of over 25,000 and comprise less than 1% liquids. Typically, however, a GOR from 2,500 to 4,000 is more than sufficient to carry the oil to the surface as a gas/oil mixture.
Normally the oil is dispersed as finely divided droplets or as a mist in the gas so produced. Typically, in fields having such wells, gathering lines gather the gas/oil mixture into common lines and pass the mixture to production facilities or the like where crude oil and condensate is separated from the gas and transported as crude oil. Natural gas liquids are then recovered from the gas and optionally combined with the crude oil and condensate. Optionally, a miscible solvent which comprises carbon dioxide, nitrogen, and a mixture of hydrocarbons containing from one to about five carbon atoms may be recovered from the gas stream and used for enhanced oil recovery or the like. The remaining gas stream is then passed to a compressor where it is compressed for reinjection. The compressed gas is reinjected through injection wells, an annular section of a production well, or the like back into the gas cap.
It can be appreciated that surface facilities required for separating and returning sufficient gas to the gas cap to maintain oil production must be of substantial capacity. Clearly the substantial size of the surface equipment required to process the mixture of oil and gas, and the capacity limitations on the ability to handle the produced gas, may become a limiting factor on the quantity of oil which can be produced from the formation.
To reduce the size of the surface equipment required to process the mixture of oil and gas, U.S. Pat. No. 5,431,228 entitled "Down Hole Gas-Liquid Separator for Wells" issued Jul. 11, 1995 to Weingarten et al and assigned to Atlantic Richfield Company, discloses that an auger separator can be used downhole to separate a gas and liquid stream for separate recovery at the surface. A gaseous portion of the stream is recovered through an annular space in the well, and liquids are recovered through a production tubing.
In SPE 30637 entitled "New Design for Compact Liquid-Gas Partial Separation: Down Hole and Surface Installations for Artificial Lift Applications" by Weingarten et al, it is disclosed that auger separators as disclosed in U.S. Pat. No. 5,431,228 can be used for downhole and surface installations for gas/liquid separation. While such separations are particularly useful as discussed for artificial or gas lift applications and the like, all of the gas and liquid is still recovered at the surface for processing as disclosed. Accordingly, the surface equipment for processing gas may still impose a significant limitation on the quantities of oil which can be produced from a subterranean formation which produces oil as a mixture of gas and liquids.
To reduce the quantity of gas recovered at the surface, and hence to reduce the GOR of production fluids, co-pending patent application, Ser. No. 757,857 entitled "A Method for Increasing Oil Production from An Oil Well Producing A Mixture of Oil and Gas" filed Nov. 27, 1996, by John R. Wolflick, James L. Cawvey, Jerry L. Brady, John R. Whitworth, and David D. Hearn, and assigned to Atlantic Richfield Company, discloses using a rotating compressor mounted downhole in combination with an auger separator to separate and compress the gas downhole and inject it into the gas cap without using surface equipment.
A continuing search has been directed to the development of methods which can reliably reduce the GOR of oil and gas mixtures produced from subterranean formations, and thereby reduce the required capacity of surface equipment to process the produced oil and gas mixtures.