Underbalanced Well Intervention to Re-Enter a Dead Well Changed the Future Dynamics of the Largest Gas Field in Pakistan – A Successful Pilot Project

Abstract

The world was feared to hit peak oil early in the 21st century but oil demand continued to grow and most oil and gas companies dedicated a large budget into developing advanced techniques to maximize production from existing maturing fields while minimizing costs in future exploration. Some of these techniques focused on managing the reservoir heterogeneities, enhancing the production lift mechanisms and reducing reservoir damage while drilling. The end goal however was always to increase the ultimate recovery of hydrocarbons from proven reserves.

Pakistan an energy deficient country and a net importer of oil and gas is in dire need of self sustainability in oil and gas. Located in southern Pakistan, Qadirpur field once produced upto 600 MMSCFD of gas, currently produces only 250 MMSCFD of gas. The primary reservoir in the field is a water drive reservoir which after extensive pressure depletion has caused water conning and fingering in a large number of wells. Recent attempts to drill new wells have suffered massive lost circulation resulting in severe reservoir invasion damage, and ultimately zero production was observed in a most recently drilled well.

As a final resort the operator opted to perform a well intervention in an old and dead well with underbalanced drilling techniques to drill a horizontal leg in the narrow pay zone of the reservoir. The underbalanced drilling campaign was expected to eliminate lost circulation and the ensuing invasion damage to the reservoir formation, characterize the reservoir while drilling and identify sweet spots, and finally increase efficiency of the drilling process and reduce the associated costs.

For the pilot well, a multiphase underbalanced drilling fluid was designed to achive extremely low bottom hole pressures, then appropriate directional drilling equipment with electromagnetic measurement while drilling tools were selected to operate with multiphase fluids, and finally a downhole isolation valve was incorporated in the last casing string to allow tripping without the need of ever killing the well.

The execution of underbalanced drilling in the pilot well resulted in an unprecedented gas production rate of 14 MMSCFD, identified a sweet spot of hardly 0.5 meters in true vertical depth (TVD) of the pay zone, and increased drilling performance upto 3 times of a conventionally drilled well. The success of the pilot well changed the dynamics of the depleting Qadirpur field as a well intervention on an old and dead well could yield the same production as on a new well while costing half as much. The paper elaborates on the techniques applied and lessons learnt on the pilot well.