Deepest Horizontal Well Drilled to TD Using an Underbalanced Nitrified Foam System in a Depleted Limestone Formation in Pakistan: An Analysis of the Design, Execution, and Results

Abstract

Located in Sindh on the right bank of the Indus River, the Kandhkot Gas Field was discovered in 1959 and was estimated to contain up to 680 Bcf of gas reserves. The field contains three producing formations: Habib Rahi Limstone, Sui Upper Limestone, and Sui Main Limestone. The Sui Main Limestone formation has been depleted because of extensive production, and it currently exhibits a very low pressure of approximately 3.5 lb/gal (419 kg/m3) equivalent mud weight (EMW).

An operator drilling with a conventional mud system encountered total losses and resorted to heavy lost-circulation material (LCM) pills and cement plugs to cure them. All bridging materials pumped in the hole to cure losses invaded the reservoir formation. As a result, the completed well required excessive cleanup operations to bring it into production. Therefore, the operator faced costs of loss curing materials, damage to the reservoir formation, and a substantial amount of time for drilling and cleanup operations.

The operator sought a solution to not only reduce the costs associated with drilling this highly depleted reservoir formation, but also mitigate invasion damage that inevitably affects overall production volumes. Although underbalanced drilling had been used in similar situations, the operator considered the challenges of this well before execution. The horizontal well design contained a vertical section of about 1,640 ft (500 m). Challenges to account for while drilling the horizontal leg included designing appropriate directional drilling equipment for use with a multiphase fluid, cleaning the hole in the horizontal leg, maintaining underbalanced wellbore pressures in the presence of high annular pressure loss, and achieving drilling performance to offset the cost incurred by underbalanced drilling.

For the given reservoir pressure of 3.5 lb/gal (419 kg/m3) EMW, a nitrified foam system was designed to achieve an equivalent circulating density (ECD) below 3.5 lb/gal (419 kg/m3) EMW. The foam system used a specially formulated polymer system to yield a high foam quality for suspension and cutting carrying capacity. The directional drilling equipment consisted of a special mud motor and extended range electromagnetic measurement-while-drilling (MWD) system to enable drilling with a multiphase fluid system. In addition, a downhole isolation valve deployed inside the casing string enabled isolating the live well while tripping. Using the custom underbalanced system, the operator drilled the well to total depth (TD) in just 3 days and achieved an ROP average of 49 ft/hr (15 m/hr). The paper outlines the planning and design of the underbalanced drilling package, the wellsite execution, and the achieved results.