Due to critical production-demand situation, proper pressure survey can not be conducted on a regular basis in the gas fields of Bangladesh. Shutting in the wells for pressure survey is also very expensive for the gas production companies. Flowing well method provides an opportunity for updating the reserves without interrupting the production. In recent years, pressures of different sands of Bakhrabad Gas Field have been declining sharply and it is believed that the field would be exhausted very shortly. Therefore, Bakhrabad Gas Field is a perfect candidate for applying this new method to estimate the gas reserve and verify it by the actual reservoir performance. Flowing well method of estimating the gas reserve is based on the assumptions that the wells have produced long enough to reach the pseudosteady state condition and the produced gas is relatively dry so that there is no liquid buildup in the well. In this study, actual flowing pressures of the four producing wells of the J Sand of Bakhrabad Gas Field have been analyzed in a similar fashion as the average reservoir pressure in a conventional material balance study. Both wellbore and wellhead flowing pressures have been used to find the reserve. Analytical justifications have been provided in support of this method. Limitations are also explained when applied to a particular gas reservoir. The flowing well method does not require shutting in the wells and conducting a pressure survey. The results of this study have been compared with those of conventional material balance and simulation studies. The results have also been compared with those of other studies previously conducted on Bakhrabad Gas Field. These comparisons show that the gas in place values obtained from flowing well method compare well with those of other studies.
The estimation of gas reserves is a fundamental aspect of reservoir engineering and is conducted throughout the life of a reservoir. Correct estimation of reserve is important for the development of a gas field, design of production facilities, and furnishing gas contracts. Traditionally, reserves are calculated by three methods: volumetric, material balance, and production decline. The volumetric and the material balance methods yield the total gas in place estimates whereas the production decline method estimates the recoverable gas.
The accuracy of the reserve calculation by the volumetric method is dependent upon the accuracy of data available, especially the seismic and log data. In a fluvio-deltaic sequence, as in Bangladesh, the likelihood of large errors in estimating rock volumes is very high. Limited exploration and drilling activities suggest that most of the reservoirs of the country are stratigraphic as opposed to structural in nature. As a result, there may be gross errors in estimating the original gas in place by volumetric method. On the other hand, the accuracy of reserves calculated by the material balance is dependent upon the production and pressure data. The material balance method is not sensitive to the reservoir geometry and its accuracy increases with time as more pressure and production data are available.