Best Practices in Developing Deep Tight Gas Reservoirs for Maximum Recovery, Sultanat of Oman

Abstract

The Main Sultanat of Oman Gas Field is comprised of two deep tight gas clastic reservoirs, separated by thick shale layer, and has been on production since 1999. These reservoirs are encountered at depth ranges of 14,000 - 17,000 ft and the top reservoir is a rich gas condensate followed by a lean gas for the lower reservoir. Permeability is in the range of 0.01 to 10 mD, porosities of 1 % to 14% and this field being a swing producer, actual production is in the range of 700 - 1,000 MMsfcd. To ensure achieving a maximum value, from such complex reservoirs, the field has gone through several development stages. The first stage has focused mainly on appraisal and development wells to obtain better understanding of well and reservoir performance and further data acquisition for better reservoir characterisation. The second stage of the Field Development Plan (FDP), in 2005, was an update of the FDP reflecting all the information that have become available since the commission of the field as well as the production performance gained from the previous 5 years of production. Further optimization of total number of wells, better prediction of the field performance and firming up the first stage of depletion compression were the key elements of the FDP update. Hence, first stage depletion compression plant was pursued and commissioned in 2010. In addition, velocity strings from 51/2" to 27/8", for maximum utilisation of low producing wells, were evaluated and more appraisals activities, for further scope maturation, were assigned. The third stage of the FDP update has been completed. The key objectives were to validate the previous FDP, in particular, wells and reservoir performance as a result of stage-1 depletion compression. Besides, phase-2 depletion compression was proposed and the remaining development drilling were re-evaluated, optimised and confirmed. Furthermore, a plan to mature contingency reserves, from poor reservoir intervals, was initiated and associated studies, such as geomechanical models and fracturing optimisation were performed. In this paper, a case study of developing a complex and tight deep gas reservoir will be discussed. The experience of applying phase approach and associated value will be shared and how the multidisciplinary team came up with the best Field Development Plan taking into consideration the different challenges and options taken by every discipline to put the best development strategy to get the optimum ultimate recovery from the field.