The successful application of managed-pressure drilling (MPD) to enable the use of a solids-free drilling fluid system proves the validity of a new technique for drilling high-pressure carbonate reservoirs prone to severe circulation losses.

The high probability of losing large volumes of costly solids-free drilling fluid to the formation led to the novel use of MPD to safely reduce fluid density requirements. In two initial wells offshore Sarawak, Malaysia, MPD successfully compensated for the lower fluid density while precisely managing downhole pressure.

Carbonate challenges

The carbonate field, located in about 78 m (255 ft) of water, is the first subsea gas field development program in Malaysia. Discovered in 2005, the development objective was a challenging high-pressure carbonate reservoir. Plans called for drilling and completion of two wells from a semisubmersible drilling rig.

Experience with offset wells prompted multiple concerns. The high-pressure reservoir is in an area prone to severe and total lost circulation. Avoiding losses while preventing kicks was critical to reaching target depth, enhancing safety, and minimizing nonproductive time.

In addition, offset wells raised concerns about potential production impairment caused by exposing the reservoir to drilling mud and introducing drilling materials to the gas processing system during production. These factors recommended a solids-free drilling fluid.

The high reservoir pressure, which required an equivalent mud weight of more than 15 ppg (1.8 specific gravity), demanded a high-density system fluid such as cesium formate. However, the cost of these systems and the strong probability of losing a large volume of fluid when drilling the fractured carbonate reservoir made this option economically impractical.

MPD multiples

MPD provided a cost-effective option that allowed the use of a less dense solids-free drilling fluid by compensating for the difference in density with annular back-pressure. The solution was achieved using the same core MPD capabilities for monitoring and managing down-hole pressure in real time that were already in the well plan to mitigate kicks and losses.

MPD constant bottomhole pressure (CBHP) and pressurized mud-cap drilling (PMCD) methods were specified to deal with the pressure and loss extremes. In the event of a total loss of circulation, the PMCD method allows drilling to safely continue with no returns to the rig.

Lost circulation risk also is mitigated using the CBHP method to precisely and safely reduce the hydrostatic overbalance to a minimum. In addition, MPD enables early kick and loss detection (EKLD) to quickly identify flow and pressure fluctuations and respond appropriately.

EKLD, in conjunction with a hydrostatically underbalanced drilling fluid, also allows the collection of actual geopressure data to inform drilling and completion operations. The two wells were the first drilled by the operator using MPD hydrostatically underbalanced techniques. Both were tested with a maximum potential flow rate that exceeded the technical potential in initial projections.

System design

MPD was selected primarily to reach the target depth with minimal drilling complications and to avoid uncontrolled events by managing bottomhole pressure in the 12/ 4 -in. and 8 1/ 2 -in. hole sections. Engineering considerations included hydraulic and flow modeling, circulation system design, and equipment selection.

Calculations were performed to guide EKLD actions in the 12 1/ 4 -in. hole section. In addition, contingency preparations were made for PMCD operations in the event that severe circulation losses were encountered while drilling the top of the carbonate reservoir and while running and cementing the 9 5/ 8 -in. casing.

Similar preparations were made for CBHP operations in the 8 1/ 2 -in. hole section. Engineering calculations were performed to determine the MPD backpressure required during drilling and while making connections. A contingency plan was developed for PMCD operations in the 8/ 2 -in. hole section in the event of severe circulation losses. Kick simulations also were conducted, and procedures were defined in an MPD well control matrix.

The MPD system, which includes a rotating control device (RCD) and an automated MPD choke manifold, was configured and deployed aboard the semisubmersible rig, and the circulating system components were examined and modified to ensure pressure parameters were met.

Carbonate drilling

The 12 1/ 4 -in. hole sections of the two wells were drilled conventionally but with MPD equipment ready as a contingency in the event that either PMCD or CBHP techniques were required to reach the section target depth. The automated MPD manifold also was placed online to provide EKLD. However, no problems were encountered that required a response with the MPD system.

The 8 1/ 2 -in. hole section of the first well was drilled with a 13-ppg brine composed of sodium and potassium formate that was both statically and dynamically underbalanced in relation to the pore pressure. A 150-psi overbalance was maintained by the MPD system throughout the operation.

No major incidents were encountered in reaching total depth; the well was stable, and the risk of severe circulation loss was minimal. As a result, a high-density solids-free cesium formate brine of 15 ppg was selected as the tripping fluid to balance the pore pressure.

The brine was circulated using the automated MPD system and the pressure-while-drilling (PWD) data to maintain an approximately 15-ppg bottomhole pressure equivalent. When wellbore temperature reduced the density of the tripping fluid lower than 14.8 ppg, the MPD system detected the variance and offset it immediately by applying backpressure. A second circulation of cesium formate was required to bring the overall fluid density up to the desired tripping margin.

For the second well, the 8½-in. section was drilled to target depth with a drill-in fluid (DIF) of approximately 13-ppg sodium/potassium formate brine. Approximately 500 psi of backpressure was held at surface to dynamically maintain overbalanced conditions. Backpressure was increased to 750 psi during drillpipe connections to compensate for the loss of annular friction. The DIF was circulated, and MPD fingerprinting was performed prior to drilling out the casing shoe.

Dynamic flow checks safely performed by the automated MPD system provided information on the highest actual pore pressure value of the formation being drilled. Enabled by the hydrostatically underbalanced environment, the test was performed by reducing surface back-pressure in 25-psi increments while monitoring flow returns. Because the influx size was minimal due to MPD early detection capabilities, the anomaly was safely circulated out of the system at full drilling rate.

A dynamic formation integrity test also was performed on both wells in the 8½-in. hole section. Backpressure was increased until the automated MPD manifold detected a loss, effectively determining the fracture pressure and the upper boundary of the drilling window for the reservoir section. The dynamic formation integrity test was used to acquire wellbore integrity data without formation damage.

Deployment of the MPD system to manage kicks and losses also provided the basis for a new technique for drilling high-pressure Malaysian carbonate reservoirs.