Underbalanced Nitrified Foam Drilling Enabled Extensive Reservoir Characterization and Production from a Highly Depleted Formation in Dhodak Field in Pakistan: A Case History

Abstract

The Dhodak field is located in the eastern Sulaiman Range in Pakistan. The first well drilled there in 1975 produced gas and condensate from the Pab sandstone intervals created in the Cretaceous Age. Drilling any well in the field results in severe circulation losses because of the younger carbonate horizons of the Eocene Age, which includes the Habib Rahi and Rubbly Limestone. Operators cure these losses by spotting lost-circulation material (LCM) pills and deploying numerous cement plugs. In one particular well in the Dhodak field, an operator encountered an unexpected well-control scenario when drilling through the Rubbly Limestone, which further confirmed the potential for hydrocarbons in the horizon.

To explore the potential of the Rubbly Limestone, the operator drilled a dedicated well named Rubbly-1 using a conventional mud system and, as expected, encountered total loss of circulation. Drilling continued to the target depth, with heavy LCM pills and four cement plugs to cure losses. A prospective production interval was identified, and subsequent drill stem testing (DST) was performed. The well did not flow in the DST even after numerous attempts to kick it off. An evaluation determined that bridging materials had extensively invaded the formation when attempting to cure losses. The invading materials had clogged the pores of the reservoir formation, which resulted in no production and subsequent abandonment of the well.

Underbalanced drilling provided a feasible solution that allowed the well to flow while drilling and eliminated overbalanced pressures and invasion damage. The reservoir pressure measured just 5 lb/gal (599 kg/m3) in equivalent mud weight. The low pressure of the formation meant it could only be drilled with a foam system.

A suitable foam system provided a low equivalent circulating density that enabled drilling the Rubbly Limestone formation. By applying an underbalanced nitrified foam system, the operator drilled the horizontal to the target depth (TD) without any issues. The well started flowing while drilling, and the operator evaluated the reservoir potential by performing surface testing and openhole wireline logging, which resulted in logs with excellent resolution. This paper details the design criteria, equipment selection, and wellsite execution program used for drilling and evaluating the Rubbly Limestone formation.