(451l) A Simple Technique to Use Decline Curve Equation Avoiding Violation of Its Assumption

Conventional Arpâ??s decline equation is widely used in oil and gas industry because of its simplicity. It offers an alternative to volumetric and material balance method and history matching with reservoir simulation for estimating original gas in place and gas reserves. Application of decline curve analysis techniques to gas reservoirs is most appropriate when more conventional volumetric or material balance methods are not accurate and or when sufficient data are not available to justify complex reservoir simulation. However, Arpâ??s equation is based on four important and widely violated assumptions such as well analysed is produced at constant bottom-hole pressure (BHP). If BHP changes, the character of the wellâ??s decline changes. The aims of this study are to use decline equation without violating its assumptions and to identify the well health issue.

Due to high demand of gas and the only natural resource in Bangladesh, most of gas fields under Petrobangla are producing at varying rates resulting the variation of BHPs. It gives an impression that decline equation can be used in gas fields in Bangladesh by violating its assumptions. This study finds a simple technique to use decline equation with obeying the assumption. The main two parameters of Arpâ??s equation are decline exponent â??bâ?? and initial decline rate â??D_iâ??. These parameters are calculated using production data by arranging the equation as a straight line. This indicates that if these parameters are calculated using production data which are obeying the assumption/s and decline equation is verified by history matching (actual production data with equation derived production) then the decline equation would more likely be realistic to use.

In this approach, first, well-head pressure (WHP) with time is plotted and observed the pressure trend, and selection of the time slot is made where pressures are steady or approximately stable. This time slot is chosen as reference. If necessary, elimination of out of range pressure data using any standard statistical method is made. Then, the production data within the referred time slot is used for the determination of â??bâ? and Di values. Finally, history is conducted to validate the final form of the derived equation.

This technique is successfully used in Kailastila gas field in Bangladesh and finds that expected production data is perfectly match the actual production data. The reserve estimated by decline curve analysis justified by flowing material balance reserve. The resulting equation is used to identify well health issue such as liquid load. The issue is verified by pressure analysis and exiting field data and information. It is found that liquid loading rates at well head are: 7.1 MMSCFD, 4.2 MMSCFD, 3.8 MMSCFD, 3.83 MMSCFD, and 6.82 MMSCFD for well no 1 to 4 and 6 respectively. Well no 5 did not analyse due to lack of data. In addition, economic life of all wells are estimated in exponential decline and compared with harmonic decline. Results show that economic life in harmonic decline is more realistic than that of harmonic. At the same time, gas-initial-in-place (GIP) and reserves of all wells are estimated in flowing material method at different abonnement pressures such as 1000, 800, 600, 400 and 200psi for future gas demand of the country.

Production enhancement study is done to meet the demand. It is found that total of 76.4MMSCFD addition production can be achieved by reducing 20 psi WHFP, increasing tubing size and eliminating skin by stimulation. A recommendation is made to: change tubing size by 1 inch well no 2-4 and well no1 and 5 by 0.5 inch, optimize choke and PCV setting, and add perforation or re-perforation or acidizing.