AFR™ service helps major operator isolate problematic shale formation

Overview

In the Antelope field, Total needed to accurately pick the 9-5/8-inch casing point in the uppermost portion of a fractured carbonate reservoir above a total loss transition zone, while being able to isolate the reactive and overpressurized shale above the reservoir. The Sperry Drilling team utilized at-bit resistivity (ABR) measurements from the AFR™ azimuthal focused resistivity sensor to successfully set the casing shoe without any down time.

Challenge: Selecting correct casing point is crucial in total loss zones to minimize NPT

Due to the differing pressure regimes between the shale and the reservoir, accurately picking the 9-5/8-inch casing shoe depth was crucial to the success of this well. If the 12-1/4-inch hole section was drilled too deep into the reservoir, it would have encountered total mud losses, which could potentially lead to an underground blowout. If the casing shoe had been set too shallow, a section of the shale would have been exposed below the shoe. This shale is both chemically and mechanically unstable, and deteriorates with time. The need to prevent this shale from sloughing and swelling could potentially have required an unplanned 7-inch liner to be run. To add more complexity to the challenge, the geological depth uncertainty at the top of the reservoir was as much as 984 feet (300 meters) because of poor seismic resolution.

Solution: At-bit-resistivity from AFR™ sensor is recommended solution for real-time geostopping

The Sperry Drilling team recommended that the customer use the real-time ABR measurement from the AFR sensor to “geostop” in the uppermost section of the reservoir. The AFR sensor is primarily a high-resolution borehole-imaging and compensated laterolog resistivity tool, but, in this case, the tool was used primarily for its ABR measurement. To make the ABR measurement, the AFR tool sends current down the bottomhole assembly (BHA), through the bit and into the formation. By measuring the current, the AFR tool can calculate the resistivity of the formation around the bit and the lower BHA. With the ABR data, the operator has an early indication of when the bit enters into a different formation.

Result: Abr solution proves optimal for determining correct casing point, saving operator costly potential rig time

During this operation, the ABR data was pulsed up in real time together with other formation evaluation data. When the real-time logs indicated that the well was approaching the top of the reservoir, the rate of penetration was controlled, ensuring that high-quality, real-time data was available, and allowing for accurate correlation with offset well data and correct identification of the optimal casing point. Even with 984 feet (300 meters) of uncertainty in the pre-well seismic model, the ABR data successfully detected the top of the fractured carbonate reservoir, and the casing point was picked above the total loss transition zone. The casing was run to depth, isolating the problematic shale above the reservoir, and saving Total potential downtime associated with setting the casing at the wrong depth