The E&P industry has been facing multitude of challenges including lack of large discoveries coupled with continuously declining production from existing assets. Moreover, the situation is aggravated due to large volumes of unproduced hydrocarbons at abandonment owing to unoptimized development strategies. This paper discusses the strategy adopted to revitalize declining production and target remainder of the sweet spots in a mature gas field on production for the last seven decades. Furthermore, the workflow introduced, integrates the surface and subsurface engineering strategies with an ideal blend of operational considerations that avoids heavy CAPEX involvement.

The field presented in this paper has been producing since 1955, with around 112 wells in four independent reservoirs. Three of these reservoirs are fractured limestone with more than 80% depletion (with very little to moderate aquifer support). At its plateau, the field produced around 1,000 MMscfd gas. However, recently the production decline rose to around 7% annually, as the performance of multiple wells have gone down with declining reservoir pressure and water production. Therefore, to cut water production and arrest the annual field decline, various activities were carried out including cementing, tubing size optimizations, and chemical placements, but to no effect. Resultantly, a two-step workflow is developed that starts with selection of wells faced with rapidly declining production and high water-cut. These are then analyzed using production logging data for zonal contributions and contact movement. Next phase was to develop a comprehensive algorithm to analyze the re-processed seismic and geological parameters, integrated with reservoir simulation to identify the undrained area and quantify potential gain from the optimized sidetracks.

Deploying the proposed approach yielded immediate returns in the form of initial gains of around 10 MMscfd gas production from the first two wells with addition of around 10-15 BCF reserves in the portfolio. Similarly, implementation of the strategy on another set of two wells resulted in production enhancement of 6 MMscfd gas and ~6BCF additional reserves from one of these wells. While the other well did not yield optimized results due to operational challenges which includes uncontrolled losses in complex and intricate fracture network. Consequently, this triggered the addition of fracture attribute study in the earlier developed workflow, further strengthening the sidetrack programs.

While liquid loading and water breakthrough are common challenges pertaining mature assets, stereotyping one solution across the board may not result in optimum results. The proposed workflow delineates multiple factors for production gains including G&G analysis, well construction details, additional fracture-attribute study, and completion types, that can be adjusted to harvest the maximum rewards in such mature fields.

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