Underbalanced Nitrified Foam Drilling Enabled Operator to Drill Without Damage and Achieve an Unprecedented Production Rate from a Depleted and Fractured Limestone Formation in Pakistan

Abstract

The southern part of Pakistan is known for its rich hydrocarbon potential. Most fields in the locality have matured, and pressure depletion has created problems in further field development. Located in northern Sindh, Qadirpur is one such field, which once accounted for 20 percent of Pakistan's total gas production. The highly fractured Sui Main Limestone (SML) formation is the primary reservoir in Qadirpur field, which has depleted over the years. Recent attempts to drill and produce from the SML formation were plagued with numerous problems including total circulation losses, extreme formation invasion damage, stuck pipe, and well-control events.

Severe circulation losses not only led to excessive drilling time, but the heavy lost circulation material pumped into the wellbore invaded the formation and completely choked it off. The operator made multiple attempts to clean off the wellbore and kick off the well, but eventually no production was observed, ultimately leading to plugging and abandonment.

The operator thus sought a solution that would enable drilling through the SML formation with maximum efficiency while minimizing formation damage during the drilling process. The operator desired to keep the SML formation in a "virgin" state from the beginning of drilling to the final production stage. The SML formation currently exhibits a low depleted pressure of about 3.9 ppg equivalent mud weight (EMW) and drilling through it requires a very low bottom hole pressure to mitigate circulation losses and subsequent invasion damage. Underbalanced drilling was thus considered for drilling through the SML formation in a proposed well, but a multitude of issues had to be addressed for successful execution of an underbalanced program. Some of the challenges in designing an underbalanced program on the subject well included maintaining underbalanced bottom hole pressures in the presence of high annular pressure losses, selection of appropriate directional drilling equipment for use with a lightweight multiphase fluid, maintaining efficient hole cleaning in the horizontal leg, designing an appropriate technique for tripping in and out of a live well, and finally achieving greater drilling performance to offset the cost incurred in underbalanced drilling.

A nitrified foam system was selected to achieve an equivalent circulating density below 3.9 ppg. A special formulation of polymers was used in the base fluid for foaming to achieve enhanced cuttings carrying and suspension capacity. The directional drilling equipment was also customized to be used with a multiphase fluid medium. Lastly, a downhole isolation valve was deployed in the casing string to allow tripping of the drill string in a live well.

By the application of underbalanced nitrified foam drilling, the operator was able to cut down drilling time by half and keeping the SML formation in a "virgin" state at all stages delivered an unprecedented production rate from the subject well.