A Novel Approach for Underbalanced Well Completions in Depleted and Highly Fractured Carbonate Reservoirs

Abstract

A prime objective of oil and gas operators is to maximize reservoir productivity and increase the ultimate recovery from all depleting fields. Underbalanced drilling is one such enabling technology that has been adopted world wide to achieve a number of objectives in maximizing the reservoir potential. Chief among these objectives are to reduce formation invasion damage, identify sweet spots in the reservoir, and reduce well costs.

Underblanaced operations however introduces more complexity into the successful drilling and completion of a candidate well. An improperly executed underbalanced operation can result in having less productivity in contrast to a conventionally drilled and completed well.

Pakistan a country currently highly dependent on foreign hydrocarbon fuels, once had total independence in at least natural gas. The southern part of Pakistan is known for its rich hydrocarbon potential, but most fields were discovered decades ago and have depleted at a rapid pace. Numerous fields in the vicinity have depleted to an extent that the reservoir pressure has reduced to a mere 3.9 PPG in EMW.

In the most recently drilled well the pressure depletion caused massive circulation losses while drilling the reservoir formation and the operator had resorted to pumping of heavy LCM pills and blind drilling to complete the section. After completing the well conventionally the operator made multiple attempts to kick off the well but observed no production. Subsequently multiple acid stimulation jobs were performed to reduce the formation damage, but all efforts were in vain. It was evaluated that the heavy LCM and drilled cuttings had bridged off and choked the reservoir skin completely from which there was no return. Ultimately the well had to be plugged and abandoned.

In relatively higher pressured and non-fractured formations the option exists to drill a well in underbalanced mode and trip the running string by balancing the well with a light weight fluid. For the subject case however, this option was impossible due to the highly fractured nature of the formation.

A plan was devised to include a downhole casing isolation valve in the last casing string and drill the well with an extremely light weight multiphase fluid. A rotating control device would be used to strip the running string in and out of the well. The completion packer was also to be stripped into a live well and set in place without the need of ever killing or balancing the well. By executing the mentioned methodology, the operator was able to drill and complete a well all the while keeping the reservoir formation in a virgin state. The paper discusses the planning, design, execution, and lessons learnt in underbalanced drilling and completion operations in the subject field.