Abstract
The importance of the differential pressure at the point of injection in continuous-flow gas-lift design is discussed. The role played by differential pressure in the selection of optimal flow in gas lift is also explained. It is shown that good wells with high productivity have continued increase in production as the differential pressure decreases. Weaker wells with low productivity, however, are less sensitive to the change in differential pressure. Also, a concept of error envelope surrounding the point of gas injection is presented. Suitable valve spacing in this error envelope is shown to offset any errors in locating the depth of injection caused by errors in the multiphase flow correlations or in the well productivity. The maximum valve spacing within the error envelope is shown to be directly proportional to the differential pressure. The smaller this differential pressure, the smaller the valve spacing.
Introduction
The theory behind continuous-flow gas-lift design is quite simple. It allows injection of gas in the production string to aerate the producing fluids which in turn lowers the bottomhole flowing pressure (BHFP). Any reduction in BHFP causes the reservoir to respond with increased flow rate. Consequently, once the piping system is fixed, the extent of reduction in the BHFP depends on two parameters-the amount of gas injected and the depth of injection. Although the increased volume of gas injected should yield higher production, there is an upper limit to the volume of gas injected. This upper limit can be an economic limit of gas injection beyond which the cost of gas injection supersedes the price of extra oil produced as discussed by Kanu et al. The economic limit is beyond the scope of this discussion. There is a physical limit of gas injection too, which results in the reversal of the tubing gradients caused by the increased irreversible pressure losses in the tubing. Consequently, a sensitivity analysis on the volume of gas injected should always be carried out before any decision is made regarding this parameter. The second parameter that significantly affects the efficiency of continuous-flow gas-lift design is the depth of injection. The maximum depth of injection achievable in a gas-lift design is function of surface injection pressure and rate, if all other variables remain constant. Once the surface injection pressure is fixed, the depth of injection can be controlled by altering the differential pressure at the point of infection. The lower this differential pressure, the lower the point of injection will be before bottomhole injection starts (see Fig. 6). However, the computed depths of injection may be inaccurate because of errors associated with the use of pressure gradient correlations. As a result, an error envelope surrounding the point of injection is created to define the upper and lower limit of the point of injection caused by calculation errors resulting from pressure loss correlations or well productivity. Considerations such as declining productivity with depletion can also be accounted for in the selection of error envelopes. Judgments based on the closeness of valve spacings, valve interference, and costs must be exercised in making the final selection of the differential pressure at the point of gas injection.
SPEJ
P. 885^
A Mathematical Model of Intermittent Gas Lift in Elevation-Production Operation with Line-Pack and Line-Drafting Phenomena in a Gas Line