Air/Foam Drilling Coupled with Drilling with Casing Technique Enables Operator to Drill and Isolate Troublesome Section to Target Depth on Multiple Wells in Pakistan

Abstract

The Kirthar fold belt located in southern Pakistan, contains some of the largest gas reserves of the country. Operators when attempting to drill in the locality face a major hurdle in the surface hole sections. The surface section on most wells contain a group of three formations, Kirthar Limestone, Ghazij Shale, and Laki Limestone. Both the Limestone formations are highly fractured and exhibit total circulation losses, while the Shale formation is highly reactive and exhibits a swelling and sloughing behavior when drilled with a conventional water-based mud system. The operator on many occasions suffered stuck pipe incidents up to 3 times per well due to the massive circulation losses and shale instability problems. An air/foam system was initially used to eliminate the major problem of total lost circulation. The foam base fluid was formulated to contain a special blend of shale inhibitors to address the reactive nature of the shale formation. A special combination of polymers was also added to the base fluid to stabilize the foam in the presence of these shale inhibitors. The designed air/foam system was able to eliminate lost circulation completely and minimize the swelling and sloughing of the shale formation in a few wells drilled. While running the surface casing the operator observed high compression against the shale formation and in most instances the casing was set prematurely leaving a part of the vulnerable limestone formation exposed in the next section. As the next section required a much higher mud weight to drill, numerous cement plugs had to be peformed to bridge off the exposed limestone formation. The operator then further desired a solution that would enable them to drill without losses and allow them to land the surface casing to target depth. A drilling with casing system was first considered for this objective, but it was found to be incompatible with the air/foam system. The operator then finally decided to drill the section with an air/foam system and run the casing with a drilling with casing system, reaming through the troublesome shale formation. A series of wells were then drilled with an air/foam system, and the casing was run with a special drillshoe laced with pdc cutters. As expected high compression was observed against the shale formation. The casing was then connected to the top drive utilizing a specifically designed tool and reamed joint by joint to bottom. A conventional mud system was used while reaming the casing to bottom. The operator by the application of these two unique methods was able to drill and isolate the section in all future wells to target depth and achieve a cost and time saving of as much as 50 percent.