Back-pressure Control of Flowing Wells

Abstract

The energy stored in the compressed natural gas absorbed in or otherwiseassociated with the oil in reservoir sands is usually the most important factorin oil recovery. It is recognized that hydrostatic pressure and gravity may aidmaterially in the drainage of oil, but in the early life of the majority offields the principal source of propulsive energy is the expansive force of thegas associated with the oil. Since gas is admittedly the most important factorin the production of oil, its control so as to produce the maximum amount ofoil with the minimum amount of gas will be reflected in increased oil recoveryfrom reservoir sands. Controlling the gas means regulating its expansion sothat the velocity of flow of oil through the channels in the reservoir sands isneither so great that frictional resistance becomes excessive nor so slow thatthe gas, because it can travel more rapidly than oil through the irregularpassageways in the sands, will by-pass and reach the well without doing usefulwork in bringing oil from the sands. Oil and gas flow from the reservoir sands to the well because of differencesin pressure. The rate of flow is dependent upon the differential pressuresexisting in the sands and in the well. By altering the amount of back-pressuremaintained on the well this differential pressure can be regulated and changedfrom a maximum (well flowing "wide open") to a minimum when there willbe no flow. Oil and gas flow through the irregular channels in reservoir sandsat different rates mainly because of the fact that one is a liquid and theother a gas. Therefore, because oil and gas have different characteristics offlow, application of back-pressure changes the rates of flow with relation toone another so that wide variations in gas-oil ratios are possible. Obviously, the formation gas energy will be utilized most efficiently when a unit of gasdelivers the maximum amount of oil to the well.