One of several efforts to develop dual-gradient drilling technology has drilled its first test well in the Gulf of Mexico.

The world's first dual-gradient well, Texaco's Green Canyon 136 No. 8, has been successfully drilled. The well is the culmination of the Subsea MudLift Drilling (SMD) joint industry project (JIP). The 51/2-year, US $50 million cooperative effort was undertaken to develop dual-gradient drilling technology and deliver that technology to industry. The project was a tremendous success. For the first time, dual-gradient technology has been used to drill a well. This drilling success confirms that the dual-gradient concept is sound, the SMD equipment performs as intended and the operational procedures are effective.
Field testing
The intent of the field test was to transform previous work into a prototype drilling system and test the system by drilling a well. The SMD field test was the culmination of the JIP. The work required a convergence of the hardware and trained personnel on the rig for the test. Success was to be measured by evaluating system performance against a list of 130 objectives that the SMD system must perform successfully and repeatedly during the course of well construction operations. Eight objectives were of primary interest:
• verification of the dual-gradient drilling concept;
• constant pressure and rate control for the MudLift pump (MLP);
• MLP capable of pumping gas from the well;
• MLP capable of pumping solids from the well;
• proven drilling procedures;
• proven well control procedures;
• proven MLP controls and logic; and
• evaluation of major subcomponents' performance.
A great deal of work had to be done before the 130 objectives could be tested. A test rig and drilling site had to be identified. A rig-specific MLP and ancillary equipment had to be designed and fabricated. All of the equipment had to be tested individually and as a system. A training program had to be developed for all SMD drilling procedures, well control procedures and equipment and pump control systems. The training had to be imparted to the personnel tasked with drilling the test well. A test well dual-gradient drilling program had to integrate the test well operator's objectives with those of the JIP. The SMD system had to be installed, integrated and commissioned on the test rig. The test well had to be drilled and any lessons learned factored into the follow-on commercial system (Figure 1).
Rig testing, personnel training
Hyperbaric chamber testing was performed on all pressure-sensitive components, and it was determined that they were robust to at least 9,000-ft (2,750-m) water depth, the limit of the test facilities. What remained was to prove that the SMD system and the drilling and well control procedures could be used in the field environment.
Identification of a test rig was a critical first step in the development of a rig-specific SMD test system. The rig selected dictated the dimensions of the MLP package configuration since it had to pass through the moonpool and fit in the rig's moonpool area. The Diamond Offshore Ocean New Era was selected as the test rig. Built in 1975, it is a second-generation semisubmersible suitable for the water depths being sought for the test well. The rig was warm-stacked in Galveston, Texas, which was very attractive because of the ease of access for rig integration purposes. The New Era had a large, uncluttered moonpool area, but a small moonpool. This challenged the design team to pack the MLP into as small a package as possible.
JIP participants and nonparticipants were solicited in an effort to identify a test well location. BP and Texaco offered locations. A Texaco location in 910-ft (275-m) waters in Green Canyon Block 136 was selected primarily because there were synergy opportunities; the well would be a field delineation well, and therefore Texaco agreed to share some drilling costs.
Texaco prepared a conventional drilling program for the well and obtained the necessary permits and regulatory approvals. The SMD team modified the Texaco well plan to accommodate dual-gradient drilling operations.
Once the rig and operator were decided, the SMD training program was applied. About 130 people attended the various 12-day schools and prespud meeting designed to ensure that all staff members were prepared to safely and efficiently perform the test operations.
Designing the system
With the test location and rig identified, the team turned its attention to designing a rig-specific SMD system. The system had to include not only the MLP but also the ancillary equipment required to operate it. A seawater power fluid system was provided to drive the MLP. The system had to supply filtered seawater to the MLP at sufficient rates and in sufficient quantity to enable the pump to perform at its designed rates and pressures. Supplemental power was supplied to augment the rig power supply. The New Era's riser was modified to support the seawater power fluid line, mud return line and control umbilical. A control and monitoring system was developed. Control touch screens allowed the driller to monitor system performance and select the appropriate commands.
Factory acceptance testing and endurance testing of the complete system was conducted in an attempt to identify outstanding problems and issues onshore, in the shop, where they could be rectified more easily and economically than on the rig.
Rig integration
Rig integration was critical to the success of the JIP. The equipment had to be installed correctly, but just as importantly the rig-up had to be performed as quickly and efficiently as possible. Prefabrication of system components was critical. All skids, piping, riser modifications and control consoles were prefabricated as much as possible.
The MLP package was transported from the Hydril manufacturing facility to the New Era in one piece. The MLP traveled by Heavy Haul transport to Channelview, Texas, where the MLP package was driven onto a barge for the trip down the Houston Ship Channel to Galveston. In Galveston, the barge was positioned beneath the rig, between the pontoons, and the MLP was hoisted through the moonpool. This proved to be a great time-saver.
A rig-up plan was prepared to optimize bringing the skids and equipment on board the rig. A load master was appointed and given the responsibility of coordinating all lifts and crane usage during the rig-up. Likewise, a welding master was chosen to coordinate the welding activity aboard the rig and at dockside. At times, 40 welders were working on the New Era.
With rig integration complete, a pump test was performed on the complete offshore test system as rigged up on the New Era, providing one last opportunity to uncover potential problems. None were found.
The Galveston rig integration took less than 3 weeks to perform. This was a notable achievement considering that many in the industry had expressed the view that SMD rig integration might require several months of shipyard time. Most of the rig-up was performed outside of critical path; this should be true of future installations.
On Aug. 20, 2001, the New Era was towed out of Galveston en route to Texaco's location.
Drilling the well
The well was spudded Aug. 24. Several problems were discovered and corrected before the MLP was ready to drill. Most were electrical and easily repaired in the field. The debugging took about 3 weeks, after which the system functioned properly for the remainder of the well.
Several tests were conducted inside the 20-in. casing prior to drill-out to confirm system readiness and performance. The 20-in. shoe then was drilled, followed by 2,700 ft (825 m) of 171/2-in. hole to the 133/8-in. casing point at 4,870 ft (1,485 m). The SMD system performed quite well. Several "gumbo attacks" were encountered and handled effectively by the Solids Processing Unit. Connection times fell into the normal range as the crews became familiar with the technology. Although nearly all of the individual components exhibited some type of problem, the system functioned well as a whole. The well was successfully drilled, and 133/8-in. casing was run and cemented.
After setting the 133/8-in. casing, the well was drilled conventionally to total depth.
Lessons learned
The root cause of every failure was quality control (QC) during the installation of the components and not a result of the design. There were no repeat failures; once a problem was identified and repaired, that particular problem did not recur. Improvements in the QC program will eliminate these types of problems.
The well was successfully drilled. About 90% of the test objectives were attained. The MLP is functional as designed. The test proved definitively that it and the associated procedures could effectively manage the bottomhole pressure (BHP) at all times. This holds true with the pump operating in the constant pressure mode (routine drilling operations) and constant rate mode (well control operations). Further, BHP can be manipulated by adjusting the inlet pressure at the MLP inlet.
The system fit on a relatively small semisubmersible, suggesting it is compatible with most rigs that would be candidates for deepwater drilling operations.
Drill cuttings were not degraded, gas indicators are much sharper and kick detection is quicker because bottom-up time is reduced drastically.
The procedures and training were effective, and the control system logic is robust and performs as designed.
The prototype MLP is complex and similar to the first remotely operated vehicle, subsea blowout preventer, subsea tree or the first MUX system. Although the initial problems were frustrating, the team is pleased the system was debugged in only 3 weeks.
Path forward
Hydril has formed SMDC LLC to commercialize the SMD dual-gradient technology. The company is actively marketing the technology to the industry while the redesign progresses. The design team is committed to commercial success.
Following the completion of the test well, a 3-day meeting took place to capture lessons learned during the test. In attendance were the people who were present on location during the rig-up and test well operations. Several hundred comments were recorded. These pertained to all aspects of the SMD system test - equipment, operations or procedures. All of these observations are being considered and incorporated into the commercial system's redesign.
Most importantly, the prototype system has been tested; the next SMD system will be a commercial system with these observations incorporated.
Everyone associated with the project recognizes the potential significance of the accomplishment. It required a tremendous amount of commitment and diligence to prove that dual-gradient drilling could be done, but it may be only the beginning of a revolutionary change in the way drilling is done in ultradeep water.

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