This paper discusses the operator's use of pressurized mud cap drilling (PMCD), a form of Managed Pressure Drilling (MPD) technology, to successfully capture reserves in Qatar's North Field that were not accessible with conventional drilling operations. While drilling off one of the platforms, the operator encountered faulting, vugular porosity, and Karst features in one particular carbonate sequence that led to the total loss of returns, which could not be controlled with lost circulation material or cement squeeze operations. After reviewing the drilling alternatives, the operator determined that PMCD was an enabling technology to drill through the problematic lost circulation interval in the 12¼-in. hole section above the reservoir. Risk assessments confirmed that PMCD could be conducted both safely and effectively utilizing a rotating head (RH) rigged up on top of the existing well control package.
The North Field platforms are located in the offshore Arabian Gulf and provide unlimited access to seawater. PMCD requires injection of potentially large volumes of fluid down the drillpipe and by the casing / hole annulus. Since seawater was abundant, it was the consensus fluid of choice. A review of the formations drilled in the 12¼-in. hole section confirmed clays were not present and that borehole instability would not be a concern.
In addition to reducing operational complexity and trouble time, the use of PMCD in this case was also driven by the operator's drilling performance management process which dictates that performance limiters be identified and extended following a specific process. Consequently, this application of PMCD was structured to maximize footage per day rather than only address the lost returns costs. The paper discusses both the lost returns management and the associated efforts to maximize performance gains.
The paper will address PMCD well control and injection equipment requirements as well as the drilling and liner running operations on the first well. The area geology and the field's history with lost returns will also be discussed.
Qatar's North Field is located in the Arabian Gulf and is the world's largest non-associated gas field (see Fig. 1). All drilling operations are conducted with jack-up drilling rigs in water depths that average 225 ft.
The stratigraphy and a typical wellbore configuration are shown in Fig. 2. Most wells are quite similar in terms of lithology, hole size, casing configuration, and casing setting points. Aside from one vertical data well at each platform location, all wells drilled by the operator are approximately 60° S-shaped directional wells with an abbreviated drop section into the reservoir.
Several horizons are prone to intermittent to complete lost returns due to fracturing, vugular porosity, and Karst features. Karsts are void spaces in rock created by dissolution and / or erosion of the carbonates. The size of a Karst can vary from 1 cm up to the size of a cave. It is quite common for the drillstring to drop 1 ft or more when encountering these horizons, as is the case in similar carbonate sequences around the world. Reducing losses to drill ahead safely using lost circulation material and cement squeeze operations have proven to be expensive, time-consuming, and only partially effective. When deemed necessary, the operator has used floating mud cap drilling (i.e., drilling with a static fluid column in the annulus) for short sections through these problem intervals prior to attempting any lost returns remediation work. Drilling an entire section length with total losses had not been attempted due to the higher risk of mechanically sticking the drillstring from cuttings fallout and the complications imposed on well control management.