An operator drilled an inland waters well in the 1960’s in southern Louisiana. By the time the well was scheduled for plug and abandonment (P&A), the canal had been transformed into wetlands and significant silt had built up around the wellhead, thus increasing the complexity of the abandonment operations. The environmental impact to the wetlands would be minimized by laying mats around the well in preparation for coiled tubing (CT) operations. However, these mats laid out on the soft wetlands would not safely support the excessive weight of the CT unit and reel, so they were rigged up on a barge 125-ft away from the wellhead. A CT drilling tower was used to provide stability to the wellhead stack. The wellbore conditions (11,300 psi MASP and 313 deg F BHT) increased the complexity of this operation further, requiring a very thorough job planning process. A mock rig-up of equipment was performed before mobilizing equipment to location to ensure the stability of the wellhead stack. The CT operations were completed successfully which allowed the operator to complete the decommissioning of the well. This paper presents a case study that demonstrates the adaptability of CT technology to meet the special requirements of well abandonment in the inland waters environment of the Gulf of Mexico.

As a result of production having drained once-prolific reservoirs, Gulf of Mexico (GOM) operators are dealing with an increase in well abandonment activity spurred by environmental concerns and government regulatory agencies. Many well plugging and abandonment (P&A) activities are performed using coiled tubing (CT), which eliminates the need for a rig. Cement placement is an essential part of every P&A operation. Deep wells (exceeding 20,000-ft measured depth) with high bottomhole static temperature (BHST) (greater than 400°F) and an aggressive H2S/CO2 environment add additional challenges to the cement slurry design. Furthermore, operations need to be executed safely and within the limits of CT equipment. An operator needed to P&A a well in the Mobile Bay block of the GOM. No previous high-pressure, high-temperature cementing operations through CT at BHSTs greater than 400 °F in the Gulf had been performed. One of the key challenges for this CT intervention was designing a cement system for 425 °F BHST to be pumped through a 23,620-ft, 1-1/2-in. OD CT string. Concentrations of 75 ppm H2S and 4% CO2 increased the complexity of the operation even further. After several unsuccessful attempts with wireline to dump-bail cement, the operator successfully used CT to plug the bottom zone of this deep, high-pressure well with cement.

Coiled tubing (CT) is a well-established well intervention technique in deep water (1,001–4,999 ft [305–1,523 m] water depth) offshore environments in the Gulf of Mexico (GOM). Wells as deep as 25,000 ft [7,620 m] measured depth (MD) are routinely accessed, serviced, and abandoned with CT at the present time. However, as demand for oil and gas continues to increase, the industry is now expanding its activities into ultradeep waters (water depths greater than 5,000 ft [1,524 m]), ultradeep reservoirs (well depths greater than 30,000 ft [9,144 m]) and high pressures (maximum shut-in pressures close to 15,000 psi [100 MPa] at the seafloor and greater than 10,000 psi [69 MPa] at surface). Ultradeep water projects are becoming central to the overall deep water investments being made by the major oil & gas operators, with several projects slated for 2015. Coiled tubing is considered one of the critical contingency operations that may be required during the completion of these ultradeep high pressure wells. CT interventions are also expected to be part of future well workover/maintenance operations. The extreme downhole and surface conditions that exist in these ultradeep high pressure wells pose many significant challenges to the CT industry, as it strives to provide safe and reliable access to these wells. In order to address some of these challenges, the CT industry has to re-evaluate various aspects of its coiled tubing design and operations, including the minimum yield stress of the pipe material, surface equipment, downhole tools, fluids, and wellbore access modeling software in order to identify any technology gaps that may affect its ability to effectively service these ultradeep, high-pressure wells. Once these gaps have been identified, they must be converted into specific action items to be addressed by all parties, ranging from service companies and equipment manufacturers to operators. This technical feasibility study on coiled tubing deployment for an ultradeep high pressure project in the GOM describes the evaluation criteria used, identifies technology gaps and outlines the specific solutions proposed.

Demand for subsea well intervention work has continued to be strong for the last 10 years. Even though subsea well interventions from an intervention vessel represent only a fraction of the total well workover activity in the Gulf of Mexico (GoM), its operating environment poses challenges in meeting economic goals for the Operators. The Q4000, a purpose-built, multi-service dynamically positioned (DP) vessel, was first introduced in the GoM in April 2002 and has been extensively used to meet demand of subsea well intervention and abandonment. Coiled tubing (CT) intervention, along with Slickline and Electric Wireline, has been instrumental in supporting the Q4000 ability to maintain safe, reliable, and cost efficient subsea through tubing well interventions in such a demanding operating environment. This paper will summarize the operational experience and will focus on the CT-specific best practices and lessons learned that have resulted from over than 10 years of continuous operations on the Q4000. The document will highlight critical stages in the learning curve, standardized procedures, synergies between the CT and the Q4000 crews, and their impact on safety, efficiency and cost reduction. The paper will also discuss the needs for further improvements that will be required to meet the new challenges that subsea well intervention will be facing in the future.

The future of exploration and production (E&P) operations in the Gulf of Mexico (GoM) is focused on deep water, that is, >5,000 ft [1,524 m] water depth. As the industry pushes the envelope to reach deeper reservoirs, subsea wells have become an important part of the offshore production. Although subsea completions may offer significant economic and environmental advantages over seafloor-founded structures, they require floating drilling or well intervention vessels to enable wellbore access should a workover operation be required to maintain production at an economic level or even to plug and abandon (P&A) the wells once they become uneconomical to produce. Interventions and P&A operations are very costly and may involve multiple services. The P&A of subsea wells represents no financial upside to the operators, other than the possible recycling of the subsea trees. Consequently, reducing costs and avoiding future environmental liability are the prerequisites for successful subsea decommissioning. Riserless subsea P&A operations are often completed by setting the plugs with drillpipe after the production tubing is removed with the rig. As part of the operational improvement process, a coiled tubing (CT) service company evaluated the option of performing this task using the riserless CT technique from a multiservice vessel (MSV) after the zonal and casing isolation packer had been successfully set and tested. This technique would allow the operator to save time to complete these operations, thus reducing the cost. Between 2010 and mid-2015, the CT service company has successfully completed 18 riserless subsea interventions at well depths ranging from 4,900 ft to 21,000 ft [1,494 m to 6,400 m] and water depths from 4,600 ft to 7,000 ft [1,402 m to 2,133 m]. Riserless wellbore access with CT has proved to be an efficient P&A method that has helped reduce operating time by up to 40% as compared to riserless workover rig-based operations. Detailed engineering and planning as well as continuous improvements have allowed implementing the CT riserless technique in accordance with the Bureau of Safety and Environmental Enforcement regulations.