EFFECT OF GAS-OIL-RATIO ON OIL PRODUCTION
Maximum production from an oil well can be achieved through proper selection of tubing size. The selection of optimum tubing size must be evaluated when completing a well in any type of reservoir especially solution gas drive reservoir since there is likelihood of producing more gas as the reservoir pressure declines. The most widely used methods such as Tarner, Muskat and Tracy methods for predicting the performance of a solution gas drive reservoir were discussed and used to estimate the behaviour of producing GOR. A comparison was made between the results from each method. System analysis approach was adopted for this study. The future IPR curves were determined by a combination of Vogel and Fetkovich correlation. Beggs and Brill multiphase spreadsheet was used to produce the TPR curves by estimating flowing bottomhole pressure for several tubing size using the predicted GOR produced and a range of flowrates. The effect of water production was also considered in this study. The results showed that for IPR5 as GOR increased from 1052 to 1453 scf/stb, oil production rate for 2 7/8-in increased by 17.6% and 3.3% for a further increase in GOR at 2610 scf/stb. At a GOR of 2610 scf/stb oil production decreased by 3.17% at water-cut of 5% and 9.5% at water-cut of 25%. All things being equal, the percentage reduction in production reduces as GOR increases from 2610 to 5635 scf/stb for all the tubing sizes used.
Production optimization identifies the opportunities to increase production and reduce operating costs. The overall goal is to achieve the optimum profitability from the well. To achieve and maintain this, it is essential to evaluate and monitor different sections of the production system including, the wellbore sandface, reservoir, produced fluids, production equipment on surface and downhole. Several methods are being used for production optimization. The most common and widely used method is the system analysis approach commonly known as nodal analysis.
Optimization of the wellbore is considered mainly during well completion stages. Tubing joints vary in length from 18 to 35 feet although the average tubing joint is approximately 30 feet. Tubing is available in a range of outer diameter sizes. The most common sizes are 2 3/8-in, 2 7/8-in, 3 1/2-in and 4 1/2-in. The API defines tubing as pipe from 1-in to 4 1/2-in OD. Larger diameter tubulars (4 1/2-in to 20-in) are being termed casing. (Schlumberger, 2001)