Continuous-Circulation Technique Drills Narrow-Margin Deepwater Wells
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Ultradeepwater wells are commonly characterized by a narrow margin between pore and fracture gradients. In these wells, even a small variation in pressure profile may lead to severe operational issues. This paper describes the drilling of high-pressure/high-temperature (HP/HT) deepwater wells through an ultranarrow pore-pressure fracture gradient (PPFG) window by means of technology application and strict procedural control.
An exploratory well with a narrow drilling margin can be realized successfully by use of a customized design concept. Such a concept may not follow standard procedures strictly as ordinarily defined but still can fulfill the guidelines and concepts behind conventional approaches.
To apply such a customized concept, a multidisciplinary team was created to design the well through all the possible expected scenarios. An integrated assessment was prepared, evaluating how real-time monitoring could mitigate the operating risk of the exploration activity.
The operator’s guidelines used for standard parameters such as kick tolerance and choke margin were reviewed and reinterpreted for this specific application, because the introduction of new technologies could allow different design factors to control the bottomhole pressure (BHP) without affecting the margin of risk.
Planning. This was the first exploration well drilled in a Mediterranean block with the use of a dynamically positioned rig with continuous-circulation technology installed. The estimated water depth of the area was greater than 2000 m. The distance of the nearest reference well from Well A was more than 90 km. Because of this long distance, the degree of uncertainty of the predicted PPFG was very high. The well was planned to explore sands targets at a total depth (TD) of approximately 6000 m.
From a well-design viewpoint, attention was paid mainly to identify the casing points; otherwise, because the narrow margin between pore and fracture gradients was so tight, the targets could require reaching the TD in 6-in. hole instead of the planned 8½-in. hole. This development could compromise data collection.
Because of these factors, it was decided to use the operator’s proprietary device and continuous-circulation system to add an additional safety margin to the geopressure fluctuation. Continuous circulation was planned to be used from the drilling of the intermediate hole size until the well reached TD and also while running the casing/liner. This project saw the first installation of the system on a drilling ship. Uncertainties were expected to be managed primarily with standard operational precautions; in fact, the ability to place the casing shoes inside the right formation was very important and the correct depth had to be determined while drilling the well.
The salt was supposed to be drilled with a 17½-in. bottomhole assembly. The casing point of this section was planned at approximately 4000 m measured depth, approximately 100 m above the bottom of the theoretical evaporitic sequence but before entering the possible overpressured zone.
Execution. Operation on Well A went as planned through the drilling phase, when the 20-in. casing was run and cemented. The surface casing shoe was placed per the drilling program inside the massive salt halite using brine to drill (pump-and-dump technique). Then the 17½-in. drilling section was drilled through the halite at a planned depth with the use of proprietary valves without encountering any operating issues. The 13⅝-in. casing also was run and cemented without issue.
While drilling with the system per the drilling plan, the team found the pore pressure to be higher than expected, while the fracture profile appeared higher, but not enough to guarantee the drilling margin per the prognosis.
In this challenging scenario, the management-of-change process was applied to create a real-time technical support starting with the well data. All new data were analyzed and updated in order to design a new well model from a multidisciplinary point of view. The only approach in this scenario would be to create a new hydraulic model while designing new mud characteristics. The static mud weight needed to be slightly less than the pore pressure to ensure that the continuous dynamic pressure could guarantee a safe drilling margin. The casing design also was updated to meet the actual high-pressure scenario. This well-design analysis required a nonstandard review practice.
Because of the uncertainties of the geopressure prognosis, especially in an exploration environment, the conventional drilling technique applied for standard wells could prove very risky. The geopressure team provides the pressure prognosis that incorporates safety factors with the aim of minimizing the margin of error. When dealing with an already narrow drilling margin in accordance with deepwater scenarios, a conventional mitigation approach might make such margins even narrower. The direct approach was to modify and control the BHP with the use of the operator’s proprietary system.
The system valves can guarantee continuous circulation, even during connection, in order to have a stable value of annular hole pressure because the dynamic mud pressure is always applied while drilling. Thus, the intrinsic variability of BHP is reduced dramatically. Continuous hole cleaning also enhanced bottomhole stability. The use of these valves was helpful in two ways: They reduced the mud-trip margin while drilling and minimized the continuous circulation that the pressure fluctuation induced by tripping.
Planning. Well B had been abandoned by the previous operators after well-control problems. Because of the very narrow margin between pore and fracture gradients, two contingent casing strings have been used to reach the planned intermediate depth. The budgeted time and cost was exceeded, and, eventually, the well was considered not safely drillable to the final planned well TD.
The operator decided to re-enter the well with confidence that the experience gathered in Well A was useful to redesign the well deepening with a more-efficient approach but with a safe risk margin. The main data gathered from the previous operator were reanalyzed and calibrated, and new operating margins were established.
Narrow-margin engineering adopted to design well re-entry and deepening was focused on equivalent mud density on the bottom while drilling in dynamic conditions. For Well B, the continuous-circulation system (near-balanced drilling) appeared to be the optimal solution from the planning phase (Fig. 1).
MPD technology, in conjunction with the continuous-circulation system, was the natural evolution of the experience performed on Well A. The MPD system allows safe drilling with a static mud weight lower than that of the pore gradient; meanwhile, the continuous-circulation technique will minimize pressure fluctuation created by pump on/pump off during pipe connection. The main difference between this technique and conventional MPD techniques is the capability of ensuring the continuity of the circulation-pressure losses that contribute to build up the BHP. From the management standpoint, this application has another important benefit: The presence of an active surface choke system can guarantee an efficient and prompt virtual mud-weight adjustment.
An engineered mud system was designed with the aim of minimizing the hydraulic effect on the fluid system. Water-based mud was selected because of its lower compressibility properties.
Execution. The 10⅝-in.-hole section was drilled per the plan to 4105 m despite the fact that the gradients encountered were much more challenging than expected. The operating window at section TD was only approximately 0.07 sg. The 9⅝-in. liner was run and cemented. An 8½-in. pilot hole was drilled to 4543 m, where a kick was experienced because of an unpredictable sudden increase in the pore-pressure gradient. The PPFG window at 4490 m was only approximately 0.07 sg.
The following well-control operations ended with a sidetrack to 4498 m: The hole was enlarged to 10¼ in., and 7⅝-in. expandable liner was run and cemented at 4498 m.
The 6¾-in. section was drilled, and the main well target was reached. At 4733 m, a decision was made to stop drilling operations because of the extremely narrow operational window. Nonetheless, the operational objectives were achieved and the well was drilled successfully to the last 6¾-in. section.
The proprietary technique allowed drilling of the 6¾-in. section, where the pore-fracture window was so narrow that use of conventional drilling techniques was impossible. The technique of drilling the well in static underbalanced mud conditions was applied continuously to all three new drilled sections. The static mud condition was almost always below the pore gradient; meanwhile, the dynamic mud condition was always over the pore gradient but below the fracture value.
The actual operating margins encountered were so low that the most-critical operation was not drilling the section but replacing the dynamic mud condition with static mud conditions in order to remain in overbalance while tripping out and tripping in.
A dedicated, accurate hydraulic program was prepared to displace the light mud used to drill the section with heavy mud to be used for tripping out and in. Correct mud-weight and appropriate flow-rate programs were designed for each phase and updated continually to avoid the risk of losses.
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Continuous-Circulation Technique Drills Narrow-Margin Deepwater Wells
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