RIASSUNTO
Abstract
In late 2007, while drilling Chevron's Big Foot #3 Appraisal well in the deepwater Gulf of Mexico, an extremely mobile tar zone was encountered subsalt at ~20,800 ft. Numerous unsuccessful attempts were made to conventionally drill thru the tar to deepen the well. The tar would have to be cased off in order to continue drilling.
Conventional casing methods were considered and it was determined that - due to the extreme tar mobility - the liner would have to be drilled thru the tar. Since extreme torque was encountered while drilling with a conventional drilling BHA, a novel approach to liner drilling was pursued.
A Service Provider had recently developed a tar liner drilling tool for another Operator which allowed the concurrent running of a 11-7/8"" liner while drilling oversized hole below the liner shoe utilizing a bit, motor and concentric reamer with returns taken up the inside of the casing. This tar casing drilling tool had never previously been utilized in a wellbore.
This paper will present how Chevron worked in conjunction with the other Operator, and the Service Provider, to implement the tool on three successive attempts before finally successfully casing off the tar zone 55 days after first encountering it. Details of the application of the tool, lessons learned and future tar casing drilling tool enhancements will also be presented in the paper.
Encountering Tar on Big Foot #3
The Big Foot #3 appraisal well was spud in Walker Ridge Block 29 July 1, 2007, utilizing the Ensco 7500 dynamically positioned semi-submersible. The well is located 210 miles SSW of New Orleans, Louisiana in 5,230 ft of water as illustrated in Fig. 1. The well was drilled uneventfully thru a massive 9,400 ft thick salt interval, exiting salt at approximately 17,500 ft. A 13-5/8?? x 14?? liner was successfully run and cemented at 20,194 ft in 14-¾?? x 16-½?? hole.
The 13-5/8?? liner shoe was drilled out and a shoe LOT conducted to a 13.36 ppg surface equivalent mud weight. A 12- ¼?? x 13-½?? hole was drilled using a rotary steerable BHA with a concentric reamer positioned 141 ft behind the bit. SOBM density was maintained at 11.8 ppg. On August 11, 2007, 40 days after arriving on location, while drilling in the Upper Miocene at 20,798 ft, the rotary stalled. The jars were fired to free the stuck pipe and drilling progressed to 20,803 ft with additional rotary stalling and BHA packing off. With the first indication of tar (or more correctly bitumen) (Han et al, 2008) over the shakers as shown in Fig. 2, the cuttings were routed to cuttings boxes and the rig went to zero discharge operations. The tar zone was encountered 600 ft below the 13-5/8?? liner shoe and 3,300 ft below the base of salt. The circulating downhole temperature was recorded at 155o F / 68o Celsius.
The well was deepened to 20,830 ft with additional packing off and high torque stalling the rotary. The mud density was raised to 11.9 ppg in an attempt to control the tar. The magnitude and size of the tar returns to surface plugged off the cuttings auger. The BHA became stuck on numerous occasions requiring increasing jarring loads to free it and regain circulation. The BHA was pulled above the tar interval to allow unplugging of the augers and to increase the mud density to 12.1 ppg. The bit was reamed back to bottom and an additional 5 ft drilled to 20,835 ft where the decision was made to pull out of the hole to reposition the concentric reamer in the BHA closer to the bit. This was done to minimize the rathole required to be drilled below the tar interval before running an 11-7/8?? liner to case off the mobile tar.