Pressured Mud Cap Drilling from A Semi-Submersible Drilling Rig

Abstract

Pressured Mud Cap Drilling (PMCD) is an advanced technique to control wells which encounter heavy mud losses and has been applied on onshore and offshore wells with surface wellheads. The key to successfully apply the technique is the use of a Rotating Control Device (RCD), which is a technology taken from Under Balanced Drilling (UBD). PMCD has not been applied to date on subsea wells as rigging up the RCD on the drilling riser brings additional challenges compared to surface wells. Three subsea wells were drilled Offshore Sarawak, utilizing the PMCD technique. A detailed review was made to rig up an RCD on the drilling riser of a Semi-Submersible drilling rig. The riser between the subsea BOP and the RCD at surface formed a low-pressure system to control the well. The first 2 wells did not see any losses but on the 3rd well, total losses occurred and the system was activated. The final result gave reason to look back on a successful operation but also good learnings were made for future operations. Introduction Since the 1970's, Shell Malaysia has drilled more than 150 wells into the carbonate structures offshore Sarawak. These carbonates normally exist of reef build-ups, many of them are more than 1000 feet high, are sometimes charged with gas and can be over-pressured up to 1300 psi. The biggest hazard when drilling through these carbonates are total losses, which historically has occurred in one out of six wells drilled. These losses are mainly caused by karsts, a common geological feature in the Sarawak carbonates. Severe well control situations have been the result of these losses, where total losses sometimes lead to the entire well being evacuated to gas. Measures to control these losses were previously managed by placing Diesel Oil Bentonite (DOB) plugs and in the later years more sophisticated fiber cement plugs were used. These methods resulted in the additional risk of getting the drill string plugged and stuck creating even worse well control situations. In year 2000, together with the development of more reliable Rotating Control Device (RCD), Pressured Mud Cap Drilling (PMCD) techniques were explored on the platform wells with Tender Assist and Jack-Up rigs. PMCD has since then evolved to be the safest method to control the carbonate losses. "There are two types of people who have applied PMCD; the ones who planned for it but never needed it and the ones who did need it but never planned for it". With the increasing amount of horizontal wells with lower overbalance compared to vertical or deviated wells, no significant losses have been recorded in some 20 carbonate wells drilled since year 2000 offshore Sarawak. With the increasing activity of subsea developments and still having some carbonates unexplored, the idea of taking the PMCD set-up from Tender Assist - and Jack-Up rigs to Semi Submersible Drilling rigs was born. The barrier of doing this was lowered due to the pioneering of other technologies, e.g. Under Balanced Drilling (UBD) and Surface BOP (SBOP) for deepwater drilling operations in Shell Malaysia. Installing a RCD on top of the drilling riser could be accomplished relatively easy based on the learnings of these technologies. Additionally, equipment was provided by a contractor who had experience with a similar project offshore Brazil, where an RCD was installed on top of a drilling riser. Pressured Mud Cap Drilling Technique Pressured Mud Cap Drilling (PMCD) is a method to control wells with heavy losses. When drilling conventionally, drilling fluid with a slightly higher hydrostatic head (300 psi) than reservoir pressure is used to keep hydrocarbons from flowing. When losses occur and the fluid level cannot be maintained at surface, the hydrostatic head will eventually drop to balance reservoir pressure at the loss zone. When this happens the wellbore pressure at the top of the Carbonate can be several hundred psi below the reservoir pressure. With such large under-balance and permeable formation, gas starts flowing into the well immediately after that the level has dropped down. Primary well control can now only be maintained by filling up the well with mud at a rate, which exceeds the gas percolation rate (Figure 1).

Real-Time Detection of Karstification Hazards While Drilling in Carbonates

“…For example, Shell reported that total losses were observed in each of the six wells when drilling in carbonates in Sarawak (Peninsular Malaysia) [30]. Similar problems exist on the Norwegian Continental Shelf when drilling into open fractures or karsts resulted in total losses during a week [20,31,32].…”

Lessons Learned and Safety Considerations for Installation and Operation of a Managed Pressure Drilling System on Classed Floating Drilling Rigs

“…5,6,7,8,9 Industry's First MPD Installation on ABS-classed Drillship The plans for equipment installation, testing, and operations of the various MPD elements were well under way, with equipment on order, before it became clear that the Class of the rig (ABS-CDS) meant that additional verifications, a validation of all engineering designs, and an ABS survey/witnessing would be required. The service company, who has historically been in a rental-equipment business providing a service for specific jobs at the request of the operator, was now tasked with a permanent rig installation on a drillship that was underclass.…”

Evaluation of MPD Methods for Compensation of Surge-and-Swab Pressures in Floating Drilling Operations

“…Figure 5 illustrates a schematic of a set-up arrangement with the RCH located above the slip joint which is locked in closed position in order to increase the pressure rating. This arrangement has been used in several cases 9 .…”