Catalyzing Reservoir Revival: A Cutting-Edge Approach for Safe Drilling in Depleted Reservoirs with Unconventional Liner Deployment and Cementing
Authors: Chisti Uzma , Ashraf Qasim , Usmani Mohsin , Dawood Usman 
Published: February 2024

Pakistan has been a major gas producer and self sufficient to meet domestic needs of the country owing to large hydrocarbon reserves in the sedimentary basins of Pakistan. One of the prominent sources of natural gas in Pakistan is the Sui Structure situated in Sulaiman sub-basin of Indus Basin. The Sui structure is a very large, gentle, ellipsoidal anticline exposed at surface, marked by rocky outcrops forming low hills and cliffs.

The Sui gas field, discovered in 1952 by Pakistan Petroleum Limited has evolved into the largest natural gas reservoir in Pakistan, boosting reserves exceeding 9.0 trillion standard cubic feet (TSCF). As the demand of the gas in country escalated, production rates have been increased, leading to the drilling of a total of 115 wells since 1955. However, the gradual decline in pressure within the Sui Main Limestone (SML) reservoir has introduced a multitude of challenges when drilling into a depleted reservoir. These challenges encompass issues such as total circulation loss, reservoir damage, and extended non-productive time thus, requiring innovative solutions to overcome these obstacles.

To effectively address these challenges the operator made the decision to utilize underbalanced drilling (UBD) technique for drilling SML, a depleted reservoir characterized by fractured carbonates. However, the presence of sour gas in the reservoir introduced the potential HSE risks associated with handling H2S gas at surface while drilling underbalanced. Considering these risks, it was sought to develop an innovative approach that would allow drilling with a multiphase fluid, maintaining the equivalent circulating density (ECD) slightly above reservoir pressure to prevent hydrocarbon release into the environment. The application of surface closed- loop system not only provided additional back pressure to manage ECD at the bottom but also enabled handling any reservoir flow during static conditions. Successful drilling of the SML formation in the SUI field was achieved by employing the subsequent technology, maintaining an ECD of 0.33 SG until reaching total depth (TD).

In addition to drilling, formidable challenges were associated with running and cementing an unconventional liner measuring 9 5/8 inches within the fractured reservoir. Given the size of liner and complex nature of reservoir, conventional methods proved inadequate. Hence, it was devised to run and cement the liner under near-balanced conditions, ensuring that pressure differentials were carefully managed.

The utilization of UBD techniques proved effective in drilling the well safely, running the unconventional liner, and successfully cementing it. The operator not only prevented formation losses but also managed to achieve higher rate of penetration compared to conventional methods. The study provides comprehensive details on planning, execution, deliverables, and statistical analysis. The application of this novel technology in an advance manner promises to unlock new horizons in efficient well construction.