By Ian Garrett, Brian Teggart, Alan Dowokpor, Tullow Oil
Tullow oil has interests in two exploration licences offshore Ghana where two successful exploration wells, Mahogany-1 and Hyedua-1, discovered the substantial Tullow-operated Jubilee field in mid-2007. The field straddles the boundary between the two blocks – Deepwater Tano and West Cape Three Points.
Results from three accelerated appraisal wells drilled during 2008 – Mahogany-2, Hyedua-2 and Mahogany-3 – showed that the Jubilee field was a continuous stratigraphic trap with a potential resource upside of 1.8 billion bbl. A fast-track development was embarked upon thereafter.
Significant progress has been made on the Phase 1 development of Jubilee, and first oil remains on track for the end of 2010, just 3 ½ years after the original discovery well. At present, 16 of the 17 Phase 1 wells are ready to be completed, with 9 5/8-in. casing installed and cemented across the reservoir. These 16 wells comprise 13 “new drills” and three utilizations of earlier E&A wells. The wells were drilled by three rigs between October 2008 and February 2010:
• The Blackford Dolphin is a refurbished moored semisubmersible that came out of the shipyard in October 2008 and was released in June 2009. It has:
n Drilled two Jubilee development wells, including a geological sidetrack.
n Performed two DST’s, one of which flowed in excess of 20,000 bbl/day.
n Installed downhole gauges on one well for interference testing on Jubilee.
• The Ocean Rig Eirik Raude, a dynamically positioned fifth-generation semi, was dedicated for three years to the Jubilee Phase 1 program after two early successful E&A wells. It has performed the bulk of the work on the Jubilee field to date, starting in March 2009:
n Drilled 10 Jubilee development wells, including two sidetracks.
n Installed downhole gauges on one well for interference testing on Jubilee.
n At the time of writing, the Eirik Raude remained on contract with Tullow and was installing completions in each of the wells on the Jubilee field.
• The Atwood Hunter is a moored semi that worked briefly within the Jubilee unit in October 2009. It drilled the reservoir section of one Jubilee development well in an E&A drilling programme (the top hole had been previously drilled using the Eirik Raude).
The Jubilee drilling program has been performed under budget for this phase of work with best-in-class performance. Given the limited number of appraisal wells and the challenges associated with starting up operations in a country with little existing oilfield presence, this has been an excellent achievement.
In the course of the drilling programme, a significant learning curve has been realised, resulting in a high level of performance. This has placed Tullow’s deepwater Ghana operations at the top of the Rushmore data sets compared with other operators in a similar West Africa environment (based on a days-per-10,000 ft metric).
This performance has been achieved by the development of a high-calibre team focused on EHS and drilling operations performance. The performance can be attributed to several factors, including:
• A rigorous focus on EHS using offshore EHS coaches, driving participation in various safety observation programmes and providing training on relevant safety initiatives.
• Optimisation of the well design by elimination of the 20-in. casing string where appropriate.
• The batch-drilling of surface holes and subsequent moving of the rig between wells with the BOP suspended.
• Gathering data more efficiently by using the latest LWD technology to replace wireline logs and running longer core barrels on wells that needed to be cored (54 m versus 27 m).
• A focus on bit/BHA design to optimise reservoir drilling performance.
• A focus on flat time improvements.
Overall, consistently challenging existing practices in all areas of the operations has resulted in performance at the highest levels.
Throughout the drilling campaign, the safety and environmental metrics showed an improving trend, with an overall decrease in incident frequency. Tullow implemented an EHS plan that was updated quarterly in response to any deficiencies recorded. Pro-active behavioral change catering to individual personnel training was targeted. Measured on a quarterly basis, trends were monitored and safety initiatives championed to address areas of concern, including lifting training, containment audits and BOP inspections.
As a result of these initiatives, the number of CARE cards submitted increased; CARE is the Ocean Rig safety observation program. This showed that crews on the Eirik Raude were benefitting from the training. This was especially significant as the number of local Ghanaian crew members increased, many with little or no previous oilfield experience. EHS coaches were also placed offshore on all the rigs to further increase awareness levels and encourage participation from all parties in the safety programmes.
Parallel to improved safety performance came an operational “learning curve.” While part of this can be attributed to general improvements associated with the rig and capturing of lessons, the majority of the improvements seen were a result of engineering/design changes and/or doing things smarter or more efficiently.
Elimination of the 20-in. surface casing string
The initial well design on Jubilee included the use of a 20-in. surface casing string (run into a 26-in. hole). Typically this was installed at around 700 m below the mud line (mBML) in order to set a 13 3/8-in. intermediate casing string at around 1,450 mBML, prior to drilling the reservoir section in 12 1/4-in. hole. After drilling the reservoir section on the first few wells and obtaining a good understanding of the pore pressure (and hence mud weight required), the team found that it was able to accept slightly lower leak-off test results. This allowed the team to shallow the 13 3/8 -in. casing shoe setting depth (typically to around 1,100 mBML) and, in so doing, enabled them to drill 17 1/2-in. surface hole in riserless mode.
By eliminating the 20-in. string, the savings was in the region of three to four days per well. At the operating rates experienced on rigs such as the Eirik Raude, this equated to around US$19.6 million over the course of the campaign. The “slim” well design outlined here became the Tullow team’s new “base case” design for Jubilee. From then on, the approach was to examine on a well-by-well basis where this could not be achieved. This included wells that had the potential for shallow gas risks or wells with higher inclinations resulting in the need for higher mud weights through the reservoir sections.
The slim well design was then refined further to drill a “soft S” well profile. The reservoir sands on Jubilee contain a number of hard stringers, which leads to slow rates of penetration – approximately 1-2 m/hr in some places. By utilizing the S-well design and dropping angle back toward vertical through the reservoir, the team was able to minimise the footage drilled and hence reduce overall time and cost.
Batch-drilling/moving with BOP suspended
Wherever possible, a batch-drilling approach was used; this resulted in significant savings. Typical average riser running and pulling speeds on the Eirik Raude have been in the range of 1 joint/hr, which meant approximately four days to round-trip the BOP. Although the focus was initially placed on working with the drilling contractor to improve this, work was concurrently undertaken to enable the team to drill these development wells in batches. This involved jetting the 36-in. conductor, drilling the surface section (either in 26-in. or 17 1/2-in. hole) and installing the surface casing (either 20-in. or 13 3/8-in.) on multiple wells back to back. The BOP was then run on the final well in this sequence, and the remainder of the well was drilled to TD.
After temporarily suspending the well that was ready for the completion to be installed, the BOP was then unlatched and the rig moved with the stack suspended onto the next pre-drilled top hole. This process was repeated for each well in the batch before pulling the BOP back to surface after finishing the last well. A number of challenges had to be overcome to adopt this approach:
• Working closely with the drilling contractor to enable the team to move around the field with the BOP suspended.
• Working in collaboration with the subsurface team to ensure multiple well locations could be agreed up-front.
• Managing spud load outs and associated logistical challenges.
• Optimising the number of wells that could be batched together to account for all of the above and the maximum time the BOP could reliably remain subsea before it needed to be pulled in any way for maintenance and/or repair.
Overcoming these challenges resulted in an overall savings of $24.6 million as the BOP was moved suspended between locations six times during the campaign (the longest distance being 9.8 km). This savings was realised not only from not having to round-trip the BOP on every well but also because of the learning curve associated with performing repetitive activities, particularly when drilling the top-hole sections back to back.
Due to the fast-track nature of the Jubilee development and given the limited number of appraisal wells, the amount of data that needed to be gathered from the development wells was significant. Initially, there was a requirement from the subsurface team for a full wireline logging programme (typically five runs). However, after the first few wells, the team was able to move to an LWD-only data acquisition programme. This included the use of LWD gamma ray/resistivity/annular pressure while drilling/density neutron/sonic, plus the use of formation pressure while drilling.
Initially, there was concern as to the quality of data that could be achieved from an LWD-only approach, but a baseline comparison was performed using both LWD and wireline on one of the earlier wells. From that point forward, wireline logs were almost eliminated from the remainder of the campaign. Working closely with the directional drilling/MWD contractor and using these advanced LWD combinations saved approximately 3.5 days per well.
Another requirement during the campaign was to cut core. Initially this was done using 27-m barrels; the team later moved to 54-m barrels, making an on-bottom connection mid-way through cutting the core. The team averaged approximately 70% recovery on coring in 12 1/4-in. hole and 100% recovery in 8 ½-in. hole using this approach. The time saved as a result of the reduced number of trips to recover the core resulted in savings of around $2 million per well.
Bit/BHA design, selection
As mentioned, a feature of deepwater drilling in Ghana is the hard drilling through the reservoir sections that is tough on bits and LWD tools. Multiple bit runs were required on early wells to reach TD. Hard turbodite stringers were encountered, causing catastrophic damage to PDC bits. Early wells required up to five bit runs to drill the reservoir section, with PDC bits being pulled after as little as 20 m drilled.
Significant work was undertaken on the BHA and bit design to minimise vibration and optimise bit life. Initial work focused on finding a PDC bit design that could drill from the 13 3/8-in. casing shoe to TD in a single run and cope with drilling these hard stringers. As a general rule, formations that are typically “PDC-drillable” have a sonic transit time of greater than 74 µs/ft. The stringers encountered within the Jubilee reservoirs are often much harder than this, at less than 70 µs/ft.
Working with bit suppliers, the team achieved a single bit run for the reservoir section by the fifth well in the campaign. After that, further refinement of both the preferred bit (Smith MDSi816) and the parameters used resulted in significant performance improvements.
Having found a bit that was durable enough to drill the section in a single run, the focus was then on minimising the high levels of vibration that were still being seen. By working closely with both the bit and directional drilling contractors on the BHA design, this was achieved. The result was improved ROPs and, more importantly, the avoidance of damage/early failure of the LWD tools (and hence the requirement for subsequent wireline logging runs).
Flat time improvements
The final area of focus for performance optimisation was “flat-time” improvements. Running bit-retrievable wear bushings eliminated the need for a dedicated trip prior to running casing. Work was put into improving casing-running times that resulted in some initial improvements, but performance appeared to plateau. Consequently improvements in this area were achieved by detailed planning of each sub-operation (such as optimising rig-ups) and performing after-action reviews (AAR’s) after each such event.
Toward the end of the campaign, performance coaches were placed offshore to assist with capturing these lessons, facilitating the AAR’s and assisting with the increased workload that comes with continuous improvement. Every operation was challenged to ensure that every saving, no matter how small, was captured wherever possible. Examples include:
• Offline makeup of BHA’s, either in the bucking unit that was installed offshore or where possible onshore, to reduce the critical path time taken on the rig floor.
• Elimination of leak-off tests at the 20-in. casing shoe on full-bore wells where it is known that subsequent 13 3/8-in. casing would be installed prior to drilling the reservoir section.
• Not thread-locking the shoe-track on the 9 5/8-in. production casing where there are no plans to drill ahead out of the shoe.
Overall, the results of all these improvements were measured on a well-by-well basis, benchmarked and used to set targets for upcoming operations to continue to drive performance.
To continue to improve is a challenge that gets more and more difficult to achieve as you progress through a campaign. The key ingredients required are:
• A well engineering team that is fully committed to driving performance to the ultimate level without compromising any element of EHS.
• A constant challenge on every operation and elimination of unnecessary tasks.
• Capturing learnings and ensuring they are immediately included in the next operation.
• A focus on excellent execution – mitigating risks, avoiding train wrecks and doing the detailed planning work.
• Creating a culture where improvements and changes are easy to take on board and implement.
By having the right team in place and creating the right environment, Tullow has realised best-in-class drilling performance on its Ghana deepwater operations.
This article is based on a presentation at the IADC World Drilling 2010 Conference & Exhibition, 16-17 June, Budapest, Hungary.