Determination of reservoir properties of a gas field by advanced decline analysis of pressure build-up test

Abstract

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 importance of performing accurate analysis and interpretation of reservoir behavior using only rate and pressure data as a function of time is fundamental to assessing reservoir properties such as permeability, skin and reservoir drainage area. The equations used for well test analysis are derived from the constant terminal rate solution of the radial diffusivity equation (RDE). 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 gas wells using Type Curves. This technique is also used to estimate Skin Factor for near wellbore drainage area, Formation Permeability, Reservoir Drainage Area and gas in place. As opposed to well test analysis, the equations used for modern decline analysis attempt to plot rate versus time with different transformations. Therefore theoretically these two independent methods should yield same results. It is of interest to investigate whether in real case two opposite approaches can be used to obtain sufficiently close results of the same properties such as skin, permeability etc. In this study two real cases were analyzed using both well testing and decline analysis. Commercial software Ecrin v4.20 (Saphir and Topaze) was used to carry out this work. It is found that the data quality is the greatest challenge with well testing data is obtained from a relatively short period of time in a controlled environment. If properly done, the data quality is good and results obtained can be reliable. However well testing is done only occasionally in Bangladesh, then developing a good understanding of the reservoir from well testing alone is often difficult. On the other hand decline analysis uses well pressure and production data which is usually available for the entire operational life of a well. Despite the volume of the data it is usually full of noise and difficult to discern the true reservoir signal from the dataset. However, sufficiently close results were obtained from the two approaches. For Well # 4, k was 19.4 from DCA and 25.1 from PBU respectively; S was 0.996 for DCA and 0.64 for PBU. For well A#3, k was 52.53 from DCA and 83 from PBU respectively, S was 1.504 for DCA and 2.97 for PBU. For DCA, two separate techniques (Fetkovich and Blasingame) were applied. They also showed reasonably close estimate of k (15.8 and 18.9) respectably and STGIIP (82.7 and 76.4 bscf).