Integrated Workflow to Evaluate and Understand Well Performance in Multi Layer Mature Gas Reservoirs, Bahrain Case Study

Abstract

Natural gas is the primary source of energy in Bahrain, and the Khuff reservoir is the main supplier for this gas 4 . The Khuff reservoir, which was discovered in 1948, is composed of four limestone layers (K0, K1, K2, and K3) separated by tight limestone and anhydrite beds that can act as permeability buffers in some areas of the reservoir.In such a mature carbonate gas reservoir as the Khuff, managing and optimizing the performance of wells represents a challenge for the reservoir and production engineers because of the reservoir heterogeneity created by dissolution process which induced good porosity and permeability zones within a lower quality matrix rock; differential depletion, which occurs because reservoir heterogeneity creates a pressure difference between layers; and the commingle production as Khuff wells are completed comingled using either openhole or perforated completion.An integrated workflow applied to Khuff wells helped to understand the well performance and to optimize the workover activities. The process integrates the geological information, openhole data, multirate production logging test data, time-lapse corrosion logs, multilayer transient testing, and nodal analysis and is used to determine the remedial job that is required to achieve the most favorable well performance.The process was applied to one of the Khuff wells that was originally completed as K2 producer and then K1 and K0 were added in double casing using a small gun size because of completion limitations. The well performance after adding K1 and K0 was below expectation, and conclusion was made that the new perforations are not performing as been expected because of insufficient formation penetration. Thus, proposed remedial actions were to reperforate the K0 and K1 intervals using a bigger gun size and then stimulate them. However, before starting this costly operation, the integrated workflow process was used to assess the need for this workover operation. The integrated evaluation showed a different conclusion, which led to different workover plan, thus saving the cost of a more expensive workover operation.