Well/reservoir evaluation by using pressure transient and material balance analysis of a gas well in Bangladesh

Abstract

To reach a decision as how best to produce a given reservoir it is essential to know its deliverability, properties, size and initial gas in place (GIIP). Estimating reservoir properties has long been a challenge. Traditionally pressure survey or well testing is conducted to estimate the reservoir properties, which is expensive; also production loss is associated with pressure survey. The equations used for well test analysis are derived from the constant terminal rate solution of the radial diffusivity equation. Well testing data is obtained from a relatively short period of time with a controlled environment. If properly done, the data quality is good and results obtained from this test can be reliable. This technique is used to estimate Skin Factor, Formation Permeability, Reservoir Drainage Area, Average reservoir pressure, distance to faults, Connectivity among well etc. If well testing is done only occasionally, developing a good understanding of the reservoir from well testing alone is often difficult. Conventional Decline Curve Analysis normally used to estimate original gas in place and gas reserves. The development of modern Decline Curve Analysis began in 1944. This technique used to analyze and interpret production data and pressure data from wells using Type Curves. This technique also can estimate skin, permeability and gas in place. But most of the time it is impossible to maintain controlled condition to collect undisturbed data of decline curve analysis for a long period of time like short period of time of well testing data. In this study for a well evaluation real cases was analyzed using both well testing and decline analysis. First Skin and Permeability are determined with the help of pseudo pressure versus Horner time. Also non-Darcy flow coefficient is determined from deliverability test equation. This Skin, Permeability and non-Darcy flow coefficient is a reference point to model a reservoir. Classical material balance and its output GIIP is also another reference point to model a reservoir for modern Decline Curve analysis method The GIIP estimated from classical and flowing material balance methods are 600 BCF and 580 BCF respectively. The same is estimated to be 629 BCF by Fetkovich, 630 BCF by Blassingame and 473 BCF by Arp’s method. Except for Arp’s the rest of the methods provided reasonably close results. The skin factor estimated from well testing analysis is in good agreement with Decline analysis result. Skin, s from Horner plot is 21.15, from type curve is 20, Fetkovich type curve is 19, and Blassingame is also 19. Rate dependent skin is also detected during well test analysis, which was characterized by non-Darcy flow coefficient of 0.456 [MMscf/D]-1. Skin value is quite high, the major contribution is due to formation damage (sd=13) and the partial completion is not significant (sp=2.15). The permeability estimated from well testing is good agreement with past studies. Permeability, k from Horner plot is 201.6 mD and type curve is 236 mD. k was also estimated decline analysis method. Both Fetkovich and Blassingame methods provide ‘k’ value close to each other but order of magnitude lower than well testing results.