By Stephen Whitfield, Associate Editor
While well designs with four casing strings are typical in the Gulf of Thailand, some wells with pressure uncertainties call for a different approach. In 2019, PTTEP called on Weatherford to help devise and execute a new three-string design in two wells that would be drilled using managed pressure drilling (MPD). Not only did MPD provide the pressure management flexibility that the operator needed in these wells, but they also reduced the costs associated with having to use a fourth string of casing.
This marked the first time that Weatherford had used such a design with an operator in the region, and Harpreet Kaur Dalgit Singh, MPD Project Engineer with Weatherford, discussed the planning process, equipment modifications and drilling results at the most recent SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference, held in Kuala Lumpur in September.
“Given the uncertainties of these two wells, we wanted to do things a little bit differently in terms of drilling optimization and the cost optimization,” Ms Kaur said. “Instead of forcing a four-string design, let’s incorporate these uncertainties into our planning and execution. Let’s think of a different way to drill this hole section. That’s where we thought of the using the three-string with the MPD because we know the benefits of MPD in managing bottomhole pressure.”
Weatherford’s work scope involved the drilling of 6 1/8-in. sections in two wells – Well C and Well H – with the lowest and safest mud weights to allow a reduction in equivalent circulating density (ECD) and added surface backpressure (SBP) when drilling and making connections. This would help to minimize the potential for the pore pressure to balloon beyond the range of 1.46 and 1.50, which had been indicated in initial formation integrity testing, and safely stay below the maximum 1.60-SG rating of the casing strings.
To accommodate the small platform of the tender assist rig used in this drilling campaign, Weatherford made several alterations to its MPD system before deployment. This included adding a split skid choke manifold, comprising an MPD choke skid and detection skid. The split was needed as the rig-up crane was unable to lift the MPD choke manifold to the BOP deck, where it is normally located.
The split also allowed the MPD skids to be lifted from the supply vessel onto the edge of the BOP deck using the tender crane, then moved to the deck center with the help of hydraulic jacks and roller skates. Ms Kaur noted that this approach could be a good option for other MPD projects on rigs with small platform decks.
Weatherford also considered alternative rig-up solutions for the rotating control device (RCD) due to the limited space out between the RCD and the rig floor on this rig. A modified 21 ¼-in. bell nipple inner barrel was installed on top of the RCS in lieu of a flowline, and then an RCD rubber leak containment line was added that diverted RCD leak flow directly to the mud trough.
Planning, drilling and execution
Ms Kaur noted that MPD planning typically is guided by the drilling window, with the pore pressure as the lower limits and the fracture pressure as the upper limit. Planning also considers drilling mud weights with wellbore pressures that “walk the line” within a given drilling window.
Prior to drilling the two Gulf of Thailand wells, Weatherford ran two simulations on each well. The worst-case scenario planned for each simulation was a ballooning gradient, which would lead to higher-than-expected pore pressures and a narrow drilling window (approximately 0.1 SG for both wells) to drill the 6 1/8-in. hole section. The best-case scenario was a gradient that allowed the pore pressure to fall within the parameters of the formation integrity test.
The simulation also took into account PTTEP’s plan to drill the section with two BHAs – a motorized BHA and an adjustable gauge stabilizer (AGS) BHA. For Well C, the operator planned to use the motorized BHA to drill out the 7-in. casing shoe to a 3,318.5-m measured depth, then drill the AGS BHA from 3,318.5-m to the section total depth, 4,830 m. For Well H, the motorized BHA would drill out to 3,306.26 m, then the AGS BHA would drill from that point to 4,625.69 m, the section total depth.
The simulations helped Weatherford to determine that drilling with an underbalanced 1.4-1.45 SG mud weight would be sufficient for both wells while keeping the ECD within the drilling window. At that mud weight, the expected pore pressure peak of 1.5 SG for Well C would happen at 3,930-m measured depth. For well H, the expected pore pressure peak of 1.54 SG would be seen at 3,605-m measured depth.
For Well C, the static pore pressure test performed at the end of the section total depth recorded 1.58 SG, higher than the maximum expected pore pressure of 1.5 SG. Thanks to the MPD system, the well was successfully drilled to total depth – Ms Kaur noted that the system allowed for higher flow rates to drill the section at points where it encountered unexpectedly high pressure.
She also said the pressure measured in Well C confirmed the limitations of the conventional four-string design. “Looking at the pore pressure, we knew for certain that if we had drilled this well with the standard four-string design or without an MPD system, it was not going to work. We would not have been able to drill to total depth.”
The 6 1/8-in. section for Well H was drilled from 2,359 m to 3,306 m with mud weights ranging from 1.40 to 1.45 SG. The pore pressure value recorded during the static pore pressure test (1.46 SG) was at the limit of the three-string design planned for the well. However, the well was successfully drilled to total depth, again because of the MPD system.
Compared with the four-string design, the new three-string design with MPD ended up saving the operator approximately $500,000 per well, Ms Kaur said. DC
For more information, please refer to IADC/SPE 209911, “Gulf of Thailand 4-String Well Design Transformation Using MPD System – Cost Saving, Operational Challenges and Learnings.”