Analyzing Pressure Transient Test in Semi-Infinite and Finite Reservoirs Using De-Superposition Method

Conventional well test interpretation is dominated by the analysis of pressure BuildUp (BU). The method for analyzing BU is derived by the principal of superposition based on the constant terminal rate DrawDown (DD) solution of an "infinite" reservoir with uniform properties. The presentation of the diagnostic plot (log-log plot) and flow regime identification from the pressure derivative curve - in the Late Time Region (LTR), in particular, critically rely on time functioning. The most popular time functioning used for well test analysis is Horner and Agarwal equivalent time. However, these time functioning methods are not reliable for the LTR analysis. When linear (semi-infinite fluvial channel or compartmentalized systems) and pseudo-steady-state flow (finite or closed system) are present, the flow regime diagnostic at LTR using Horner or Agarwal equivalent time frequently result in a false definition of flow regime and misleads the analyst. The de-superposition method can present the pressure BU in the same way as that for a DD, so the ambiguity in identifying the LTR behavior can be resolved. On the other hand, the DD LTR behavior (pressure derivative) in semi-infinite or finite reservoir systems is very valuable for the identification of the reservoir external boundary conditions (even though it is usually not good to define the radial flow region due to the rate fluctuation and often, incorrect production history input). Experience has proved that BU analysis using the de-superposition method combined with DD LTR analysis can provide much more confident well test analysis. In this paper, two field examples conducted in the fluvial gas reservoirs in the Gulf of Thailand have been interpreted in this way. At first, one of the tests has been analyzed using the conventional method by ignoring the DD. Then, the examination of DD LTR and the analysis of pressure BU using de-superposition method have been conducted to show where analysis was wrong in the first approach. Following the same procedures, the second test has been interpreted to further demonstrate the value of de-superposition method in analyzing test conducted in reservoirs with limited extent.