Underbalanced Drilling

Lessons learned in major underbalanced sidetrack drilling project

Key management practices that led to successful multilateral sidetracks in 20 existing wells in the Sajaa onshore gas field.

Robert E. Snyder, Executive Engineering Editor, World Oil

This article is an overview of planning and operations of an underbalanced coiled tubing drilling (UBCTD) program in the Sajaa onshore gas field in the Emirate of Sarjah, United Arab Emirates. The gas producing reservoir is a Thamama Group limestone ranging from 11,000 ft to 13,000 ft true vertical depth. Its bottomhole temperature ranges from 260°F to 310°F, and BHPs range from 1,100 to 1,900 psi. 

Fig. 1. Coiled tubing drilling tower used in sidetrack drilling of multilaterals in Sajaa gas field.

In early 2002, a major project was initiated by operator BP Sharjah to utilize existing wellbores comprising vertical, perforated 7-in. production casings to drill, with coiled tubing, Fig. 1, multiple sidetracked small diameter, openhole laterals into the gas reservoir to contact additional reservoir matrix and interconnecting microfractures. The results have been significant, and as of early 2005, the team was drilling the 20th well of the multilateral UBCTD program. 

This program has been documented by three major SPE-sponsored technical papers: 1) IADCSPE 87146, presented at the IADC/ SPE Drilling Conference, Dallas, Texas, March 2 – 4, 2004,¹ and summarized in the Journal of Petroleum Technology, June 2004; 2) SPE 89644, presented at the SPE/ ICoTA Coiled Tubing Conference and Exhibition, Houston, March 23 – 24, 2004;² and 3) IADC 92513, prepared for presentation at the SPE/ IADC Drilling Conference, February 23 – 25, 2005.³ This article was summarized from information in these papers. 

BACKGROUND

In April of 2002, the concept was conceived to utilize UBCTD to unlock additional gas production rate and reserves by exposing additional net pay with multilateral wellbores from the existing wells in Sajaa. After a full year of evaluating/ planning, the first well began in April 2003. This full year of Front End Loading (FEL) was key to success of the project. BP identifies the following as principal tasks within the FEL: 

• Concept identification: A “peer assist” was held with internal and external industry experts to discuss technical options that might achieve the multilateral objectives. One principal outcome was identification of Key operational challenges.³
• Basis of design: A concept basis of design paper¹ provided equipment sizing and parameters. The design was necessary to prepare tenders, and this effort looked at: pumping parameters, formation inflow performance, and hole cleaning performance to size surface equipment; and coiled tubing drill string size and need for electric line to provide surface read-out, and drilling assembly selection to efficiently operate in a 2-phase/ high-temperature environment. 
• Tender: Responses were evaluated on their technical merit as well as their safety and environmental management systems. Tenders were awarded to multiple service companies, as no one company could adequately provide the full range of services required. 
• Assurance: Testing was conducted on the drilling assemblies by pumping a 2-phase fluid (nitrogen and water) and inducing vibration to simulate downhole conditions at 300°F. 
• Fabrication: Both local and international suppliers completed fabrication work. A majority of this “purpose-built kit” was fabricated locally, including the substructure, deployment lubricator, tool skid and underbalanced equipment.
• Mobilization: Most required equipment was sourced locally; three exceptions were, coiled tubing with internal electric cable; bottomhole drilling assemblies (BHAs); and the surface separation package.
• People, procedures and training: The core team of four drilling engineers and four well-site leaders assembled three months prior to startup. This allowed time for training, logistics coordination and preparation of detailed procedures for both normal operations and potential emergencies. 
• Startup: A “Putting it all together” paper² provided detailed information about the UBCTD kit. The initial rig up was slow; early in the project, there were significant equipment failures of both surface and downhole equipment. These led to several important learnings so the team could systematically eliminate other problems as operations progressed. 
• Drilling program: The Sharjah UBCTD campaign has evolved as performance improved and technical capabilities were established. The original concept was for only two laterals of 1,000 ft in length each per well. This soon became obsolete and the campaign eventually expanded to at least four laterals with a total footage in excess of 10,000 ft per well. Record footage drilled to date is 13,206 ft in four laterals using only two BHAs.³ Gas production performance results speak for themselves.

PROJECT LEARNINGS

      Vision. A safe and long-term drilling campaign for unlocking rate and reserves was the organizers’ vision. It was recognized at the onset that a multiple well program was required for the best chance of proving the concept. Planning for success included tendering for a 10-well campaign with triggers for early termination, if needed, after the first three wells. 

FEL was vital to the program success with respect to planning, procedure preparation and training. Some major concerns during the FEL period were two-phase (nitrogen gas and water) effects on motor life at 300°F. To date, this has not been an issue. The longest BHA run was 178 hours (more than 7 days) of continuous downhole drilling with a total of 223 hours of continuous circulating through the motor. Another lesson was, do not stop drilling if a downhole assembly is working!

Real-time data is extremely important and is used to make online decisions about, ex., operation of the: underbalanced, nitrogen pumping and water pumping equipment, and directional drilling, gas export and rate of penetration. This project has all data on one single screen vs. multiple screens. These single-screen data displays are placed at strategic locations around the location, along with the over 20 radios (and radio protocol) to keep the operation online and “drilling ahead.” 

      Getting health, safety and environment (HSE) right is a team effort. Initially, one of the main quality HSE goals was to build an interface document that would serve as a reference for any HSE matter. With more than 10 companies and 20 different nationalities involved, it was obvious at the start of the project that such a document would evolve based on lessons learned during the actual project. Each operational step has been fully described and reviewed by crew leaders and management. 

An onsite rig safety meeting is held twice per day, where a brief summary of actual operations are discussed, and whereby all participants can express their thoughts about safety issues. After nearly 20 months and 700,000 hours of safe operations, the team is now working the problem of complacency at all levels. To help fight against this false sense of HSE security, the team is re-working the entire contents of risk assessments and Tool Box Talk information.³

      Plan-Do-Learn-Evaluate-Share. The hallmark of this project has been the extraordinary degree of collaboration among the four major service companies. Success of the project required a shared success among all the service companies. On a larger scale, UBCTD worldwide would be directly affected by success or failure of this project

BP shared this global perspective and a desire that this project be an incubator for the development of UBCTD technology. Thus it has opened the Sharjah drilling site to visitors from other oil companies, and the project has become a showcase for the industry. It is also a showcase for future customers of the technology. In this respect, BP has provided an example of openness and cooperation. Service providers are expected to follow this example. 

Minimizing natural gas flaring to the environment was also a goal. Extraordinary measures were put in place to capture produced gas and condensate while drilling forward. All hydrocarbon products are sold to the market on the same day, an amazing accomplishment. Success of “flowing-to-sales-while-drilling” added an extra measure of environmental success and efficiency, as well as an additional economic benefit for the project. 

All safety and environmental information is freely shared with service providers at safety meetings, operational briefings and “toolbox” talks. The information is also verbally translated into other languages (such as Arabic and Hindi) to ensure all personnel onsite are aware of key issues and understand planned activities for the day. Operational results are shared and service providers are respectful of propriety technologies of other companies. Each service provider understands sharing success from this project helps the future of underbalanced drilling on a larger scale. 

      Leadership. The style of leadership on any project is important; especially when a multinational and multicultural group has to work together. Effective leadership is achieved by several means. There is clear commitment from the top down, as shown by high visibility. Senior leaders frequently visit the location, attend crew safety meetings and set a good example by conducting advanced safety audits with the crews. This behavior has created a close knit working team that has a pride and passion for the job.

Personnel holding key wellsite leadership positions are continually evaluated. Those who do not demonstrate effective leadership skills are replaced using the “promotion from within” process. Any personnel change is performed using a specific management of change process to ensure any change to key personnel is implemented with minimum disruption to site operations.

External visitors from other oil and gas operators, as well as other service providers, have been to the wellsite. Feedback about the people, equipment and operation is consistently good. The project is relatively high profile for the coiled tubing drilling sector. This is recognized by the work crews and they understand that what they are doing is important and widely recognized by many – a “hidden” motivator.

In building the Sharjah project team, the foundation was to bring the right knowledge, skills, experience, as evidenced by safety, efficiency, teamwork and production performance results.

      Equipment. Other papers have addressed equipment, work during front end loading, and putting it all together. During the early phases, the importance of having backup for major equipment components was not recognized. During the course of the campaign, the team has added a significant amount of backup equipment including: an injector head, power pack, BOP stack and nitrogen pumping unit. The original program began with 10 wells in mind and the team is now about to complete the 20th well. Preventative maintenance has evolved and is now very efficient. 

Vibration effect on downhole drilling tool electronics was underestimated early in the campaign. Since then, significant trouble-shooting and problem solving has been completed. Interventions include a change in the way some components are secured inside state-of-the-art drilling assemblies, changing the type of drilling bit, and installation of downhole stabilizing equipment such as heavy subs, to name a few. Results have been impressive, resulting in substantially improved BHA reliability, Fig. 2. 

Fig. 2. Example drilling rate improvement with experience, evidenced by performance record of the first seven-well sidetrack operations.2

Another key learning is to thoroughly surface test downhole equipment before it leaves the shop and goes into a well. This may be an obvious step, but when the rig is calling for tools, equipment has the potential to leave the shop before it is completely checked out or modified with new components. Stopping this trend has gone extremely well and is clearly reducing overall non-productive time for the rig. 

Early in the project, a pessimistic view was taken about the effects of nitrogen on rubber for downhole tools. After nearly 20 wells, the negative effects were overstated and equipment is handling this potentially corrosive environment much better than originally predicted.

Procedures. Those used for the Sharjah project have evolved considerably since the first well. In fact, nearly all the procedures in use now are different than the ones from the original pre-spud version used on the first well. Some of the procedures are substantially different. An important lesson is that the more time spent on procedures before project startup will ultimately pay huge dividends even when the procedures change later in the project life.

One benefit of spending pre-spud time for writing and intensively reviewing procedures is that this process uncovers many of the associated risks and hazards. A best practice is to have review procedures by as many people as practical. This provides a certain level of external assurance. 

Another lesson learned about procedures is that, once the operation starts, there must be an established process in place for procedural change and modification. The Management of Change (MOC) process used in the Sharjah project fulfills several important criteria: 1) it is streamlined; all procedural changes are revised and approved at the well site; 2) it is rigorous from an HSE standpoint; as the risk assessments are done as part of the MOC process; and 3) it is signed off and documented with all changes going into a master procedure listing. 

The last item is particularly important because the operation is run 24 hr/day and 7 days/week. Two 12-hr shifts work each day and crews normally work 28-day hitches. The MOC process is a great way to ensure all changes have been communicated across all shifts and hitches. 

A good example of how the MOC process worked is on the “live well” pressure deployment and undeployment procedure for downhole tools. There have been many other procedural improvements that allowed this reduction in deployment time. A few key procedures included using wellbore gas to leak test the deployment lubricator instead of hydrotesting; using predrilled mouseholes to reduce drilling assembly makeup/ handling time; and using flowback lines to bleed off stored pressure instead of a 1/2-in., high-pressure hose. Each of these small changes used the MOC process, resulting in a safer, more efficient pressure deployment process. 

And regarding Extended teams, one year prior to startup, it was clear that the Sarjah team needed help. The project required more expertise than was available. The first extended team was the BP coiled tubing drilling operation in Alaska. That team provided a ready source of information about how to get started, and helped Sharjah focus on critical issues related to the reservoir, equipment and personnel. The Alaska team was also a key source of expertise for real-time trouble shooting of complex problems, rig-site to rig-site – a best practice, especially for external assurance. 

And a BP subsurface team of engineers was assembled in the UK to define the “prize.” The initial impression about this split-site team setup was lack of teamwork. Making use of modern communication technology helps reduce inherent problems caused by having split-site teams. 

Fig. 3. Initial gas production rate increases with multilateral wellbores.2

Subsurface lessons. The single key subsurface lesson derived from this project is that the technology works. UBCTD has delivered a substantial rate and reserve volume from this tight (low permeability) and pressure-depleted gas reservoir. To date, the sustained incremental production exceeds 140 million scf/d from 19 wells, a staggering volume of additional incremental gas volume from what was previously considered a mature reservoir. Fig. 3 illustrates initial gas production increases in the first seven wells. 

Reservoir access was achieved by drilling the multiple sidetracks into relatively higher pressures away from the original motherbore. After nearly 20 months of operations, there is very little interference between individual laterals or wells. This learning has allowed the campaign to continue into 2005. 

As expected, understanding the reservoir is essential. The Sharjah downhole and surface surveillance data gathering effort was completely overhauled prior to the campaign. Reservoir pressure data was the largest gap. A key learning was a redesigned way of collecting pressure buildup data.

Another learning was determining how to land the drilling assembly into the best quality formation and then keep it there while drilling forward. Using existing stratigraphic biozone data with real-time rig site paleontology, coupled with gamma ray and ROP data, allows the team to keep the drilling assembly in the zones of interest. 

Seismic information is not able to predict minor faulting or fracturing. Since the project uses underbalanced drilling, occasional minor faulting and fracturing are evident. The best indicator is, while drilling in a high porosity zone, a sudden gas rate and pressure increase is recorded. Paleo samples and gamma-ray data are eventually collected to confirm the fault or fracture area. This data is later used for seismic imaging by the UK-based subsurface team. 

Underbalanced drilling in Sarjah has several direct and indirect benefits. A direct benefit is that it has prevented need for any remedial stimulation treatments usually required after drilling. Wells are immediately placed on full flow production to the Sajaa gas plant without any time delay for completion work. This results in a significant cost savings and keeps deferred production to a minimum. 

An indirect benefit of the “flowing-to-gas-sales-market-while-drilling” is economic return. To date, 2-billion scf has been sold instead of flaring. And, over 15,000 bbl of condensate has been recovered instead of flaring. While the environmental impact with greenhouse gas emissions has been mitigated, the recovered hydrocarbon during forward drilling operations has a positive impact on economic performance.

Rate and reserve performance from this UBCTD campaign has had a positive impact on the local economy of Sharjah. The Government recently recognized BP with its highest award for industrial excellence. 

ACKNOWLEDGMENT

The authors of Paper IADC 92513 offer a special thanks to the Government of Sharjah, and particularly the Sharjah Petroleum Council, for having the patience to pursue this project. BP also appreciates its permission to present the paper to the wider global audience. The following teams are recognized:

• Sharjah Petroleum Council
• Weatherford-GSI
• BP in Sharjah, United Arab Emirates
• Blade Energy Partners
• Schlumberger
• Al Farris Cranes
• Baker Hughes Inteq
• Emirates Industrial Gases
• Baker Oil Tools
• Maritaine Industrial Services
• Halliburton Energy Services

LITERATURE CITED

  ¹  Suryanarayana, P.V, et al., “Basis of design for coiled tubing underbalanced through-tubing drilling in the Sajaa field,” Paper SPE/ IADC 87416, Presented at the 2004 IADC/ SPE Drilling Conference, Dallas, Texas, March 2 – 4, 2004.

  ²  Pruitt, R. et al., “Sajaa underbalance coiled tubing drilling “Putting it all together” Paper SPE 89644, presented at the SPE/ ICoTA Coiled Tubing Conference and Exhibition, Houston, March 23 – 24, 2004.

  ³  Pruitt, R. et al., “Underbalanced coiled tubing drilling update on a successful campaign,” Paper IADC 92513, Prepared for presentation at the SPE/ IADC Drilling Conference, Amsterdam, February 23 – 25, 2005.