Pinnacle carbonate drilling challenges Petronas engineers

The majority of the world’s hydrocarbons are trapped in carbonate reservoirs. However, carbonates typically present drilling challenges due to their heterogeneous nature. Within the same well it is common to encounter tight formations, highly fractured rocks, and vugular or karstified textures. It is difficult to predict when or if these karsts will be encountered, and the results can be potentially disastrous. High-conductivity fractures can quickly drain drilling fluid, leading to total loss of circulation, which can precede loss of well control.

For almost every challenge there is a solution, and with persistence Petronas Carigali has found one. Moreover, the company has recently implemented its solution on 11 wells offshore Sarawak, Malaysia, six of which experienced total losses. The solution is called pressurized mud-cap drilling (PMCD).

Once total losses are encountered, a series of procedures are executed that result in converting the well from a conventional drilling operation to a PMCD operation. Evaluating loss rate, determining formation pressure, and testing formation injectivity are all vital steps that must be carried out prior to initiating a PMCD operation.

FIGURE 1. Drilling a stand down in PMCD mode with two bull-heading cycles is shown. (Images courtesy of Schlumberger)

Not as easy as it sounds

Petronas engineers, working in collaboration with Schlumberger, took advantage of the high percentage of high-loss carbonates found in the pinnacle carbonates offshore Sarawak to develop and prove a safe, efficient technique to deal with the problem. The chance of encountering such formations is estimated at 33% in pinnacle reefs.

Such high probabilities encouraged a careful study and planned development of a safe and efficient procedure to detect and react to unexpected loss of circulation. Many if not most wells plagued with high-loss potential are exploration wells where detailed geological and geomechanical information is sparse.

In the Sarawak wells the objective was to determine and confirm the gas/water contact and maximum column height of the reservoir and volume of reserves in place. To achieve these objectives all wells were required to be drilled to their planned total depth (TD). Experience gained from previous encounters with pinnacle reefs led Petronas to specify PMCD as an enabling technique to reach TD.

Preparation and planning

To prepare for the 11-well drilling campaign, the company could not simply replicate techniques used on previous wells. Each pinnacle structure is unique, so a meticulous planning session must precede any activity. Six planning areas were involved including geology and geophysics, well design and engineering, rig equipment, operations, personnel, and logistics.

From the planning stage, a recommended well architecture evolved. Petronas engineers dedicated themselves to detailed bottomhole assembly (BHA) and drillstring design. The addition of a mud motor was introduced to reduce rotary. Three pressure-tested float valves were installed in the drillstring above the bit. This is a major safety requirement to maintain well integrity. Depending on formation pressure, the floats may be responsible for holding up to several thousand psi of pressure during connections.

FIGURE 2. Decreasing pressure indicates gas being bullheaded into the formation.

Fluid logistics is perhaps the most challenging aspect of planning the PMCD operation. Light annular mud (LAM) is designed to be underbalanced with some pressure at surface – the LAM density and pressure at surface is equal to the formation pressure. Drilling proceeds while annulus pressure is allowed to rise to a predetermined level due to hydrocarbon – usually gas – migration as dictated by the riser pressure operating limit (Figure 1). LAM is injected to flush migrating gas back into the formation, which reduces surface pressure to its original value.

Wellbore pressure never changes; the surface pressure is changing (Figure 2). Therefore, the LAM must be properly mixed and available in sufficient quantities. In an offshore environment this can be challenging, and a high degree of coordination with onshore mixing facilities and the drilling fluids provider is required.

The presence of experienced personnel on location for PMCD operations is paramount considering that decisions must be made in a timely manner during critical operations, and good judgment is critical. During a recent PMCD operation involving coring, 16 trips were required.

Altogether, three different PMCD stripping techniques were employed. The different techniques were applied at different times depending on the condition of the wellbore to minimize the amount of LAM consumed while at the same time allowing the driller to pull out of hole quickly and safely. The optimization of the 16 trips was based on the good judgment and experience of the PMCD team on location. There is no one-size-fits-all approach for PMCD operations.

PMCD in action

Experience has shown that trying to mitigate severe fluid loss in carbonates by pumping lost circulation material or gunk caused more problems than it solved. The PMCD technique can solve the problem without damaging the formation or plugging up the bit nozzles or drillpipe. Of 11 wells drilled in the area, six exhibited total losses. Valuable lessons were learned and applied to subsequent wells. Some key findings included:

• Experienced PMCD advisors, a capable trained rig crew, and an engaged drilling team are essential to a successful operation;
• Riser pressure limitations need to be evaluated based on expected drilling parameters;
• Gas migration rates averaged 1 m/min to 3 m/min (3 ft/min to 10 ft/min) as opposed to the 27 m/min (90 ft/min) theorized;
• Doubling LAM viscosity reduced gas migration somewhat but created unacceptable friction pressure while injecting LAM down the annulus. Evaluating formation pressure at the reservoir top and injectivity is essential;
• Brine reduces mixing time for LAM significantly over barite;
• Use of a mud motor is advised to increase operational flexibility; and
• Rig suitability is crucial. Pumps, pit space, standpipe configuration, and riser limits need to suit PMCD operations.

Petronas is continuing to drill carbonate pinnacles using the PMCD technique with good results. Improvements have been suggested based upon observations and lessons learned during the 11-well campaign. PMCD operations will continue to be refined as operations continue.

One significant improvement that has been successfully adopted is the use of a composite bridge plug set at the bottom of the 9 5/ 8 -in. casing to seal the openhole section from the wellbore. Although it is possible to run liner in PMCD conditions, this technique was used to mitigate risk. By sealing the loss zone from the wellbore, the 7-in. liner can be run conventionally before installing the rotating control device’s stripping element, drilling out the composite bridge plug and resuming PMCD operations. The liner is then run and set at TD.

Penetrating the carbonate structure down-flank of the pinnacle and beginning PMCD operations in a lower pressure zone can and will reduce logistical and operational complexity. Near-bit gamma-ray and seismic-while-drilling logs will improve carbonate top detection.

As experience increases, more challenging PMCD operations will be implemented. A successful PMCD coring operation has recently been completed with plans to incorporate PMCD wireline logging into future wells.

This article was prepared from Society of Petroleum Engineers (SPE)/International Association of Drilling Contractors (IADC) paper 163479 presented during the 2013 SPE/ IADC Drilling Conference and Exhibition held in Amsterdam March 5 to 7, 2013.