Formation testing in a challenging environment

Talisman Energy Norge AS's formation testing program called for formation pressure measurements and sampling in a deviated HPHT well in North Sea. In addition to the challenging well environment, logging with the CMR-Plus combinable magnetic resonance tool indicated that the formation had very low permeability, which would further complicate both testing and obtaining formation fluid samples. Another concern was the likelihood of differential sticking because of the high overbalance in the well. If the formation testing operation was not successful, an expensive well test would be necessary.

Expanding the isolated interval

Using the Dual-Packer Module fitted with high-performance packers expands the operating envelope for formation testing and sampling with the MDT tester to a temperature rating of 350 degF [177 degC]. The dual packers effectively isolate a large area of the formation, with a 3.2-ft [0.98-m] interval between the packers that can be extended up to 11.2 ft [3.4 m]. The 3.2-ft interval of the borehole is about 3,000 times larger than the area isolated by a single-probe module. In low-permeability formations, using the MDT tester configured with the Dual-Packer Module enables fluid withdrawal with only a minimum pressure drop through the larger flowing area. Phase separation is avoided because the larger flowing area minimizes pressure drop.

Reliably evaluating reservoir properties

Using the TLC tough logging conditions system for conveyance because of the high overbalance, the MDT tester toolstring was deployed and retrieved without any abnormal tension readings. Operational reliability was further enhanced by the autoretract mechanism (ARM) of the Dual-Packer Module, which applies a longitudinal tensile force to assist in retracting the packers after deflation.

Three mini-DST stations were attempted. The first collected high-quality data and representative formation fluid samples. Two 450-cm³ bottles and a 1-galUS single-phase sample were recovered. Although the other two station tests were successfully performed despite the extremely high differential pressure across the packers, they did not fully stabilize because of the very low permeability. The absence of any fracture permeability was also confirmed.