Deep Workover Challenges of an HP/HT Sour Gas Well - Case History of Mutriba 12

Abstract

Abstract Mutriba 12 is a deep 20,000 psi exploration well originally drilled to search for non-associated gas in the Triassic/Permian formations. During final testing operations the tubing became plugged with salt deposits and the well was suspended. The decision was taken to permanently abandon the lower zones and examine the possibilities of using the well to explore the hydrocarbon potential of the upper Jurassic horizons. The main re-entry objectives were to clean out the salt from the tubing string, permanently abandon the lower gas zone, cut and recover the tubing, perforate through multiple casing strings and safely test the upper formations. To meet these objectives the operations were planned in two parts with a Heavy Duty HP/HT Snubbing Unit used for tubing clean out / gas zone abandonment followed by a Deep Workover Rig to remove the tubing string, prepare the well and conduct the testing. Extensive planning and preparation were necessary involving detailed evaluation of all possible scenarios for key operations. During operations, significant challenges were encountered which dictated the employment of innovative methodologies and practices. These included confirming the top of abandon plug of 20K well test zone inside the tubing, working with high mud weights and restricted geometries, coiled tubing use under high pumping pressures, cutting / recovery of the tubing filled with solid laden heavy mud, adaptation and deployment of special tubing cutting mechanisms, perforation through multiple casings, and extended well testing in highly corrosive environment with fluids containing up to 24% H2S and 40% CO2. This paper will present a case history of the well, mainly concentrating on the operations with the workover rig, and including details on planning and contingencies, job design options, special equipment items, program development and operations along with conclusions and lessons learned. Introduction Mutriba-12 (MU-12) is a deep vertical exploration well originally drilled to investigate the hydrocarbon potential of the Triassic/Permian formations. The well was drilled and cased to a depth of 22,094 ft. and production testing was carried out to evaluate the Khuff and Sudair formations. In the course of testing these zones, complications occurred where saline water was produced along with gas, and salt deposits were formed which eventually plugged the tubing string. The operations were suspended at this time and following a review of the well situation it was subsequently decided to re-enter the well to permanently abandon these lower zones. The objectives of the first workover were to clean out the salt from the tubing string and permanently abandon the lower gas zone. At the time of the intervention the well was exhibiting high surface pressures, and a Heavy Duty Snubbing Unit was used to clean out the salt bridges and kill the well with an 18.9 ppg Oil Based Mud. A cement plug was then placed above the perforations, from production packer bore to up inside the tubing string in order to isolate the lower tested interval. As there was 19.8 ppg mud in the casing / tubing annulus above the packer, the Snubbing Unit work string was run in to tag the top of cement and to increase the mud weight inside the tubing string to 19.8 ppg. No firm cement top could be located with the Snubbing Unit string and high pressures were recorded while attempting to displace the tubing contents, therefore the well was left with 18.9 ppg mud in the tubing.