MPD can be used to drill carbonates in deepwater on DP drillship

Carbonate formations present drilling challenges due to the high potential for lost circulation and well control situations because they often have flow paths (fractures, wormholes, vugs, caverns, etc.) that are large enough to freely pass whole mud, making pore pressure and the pressure at which returns are lost essentially the same.

If the fractures are small or limited, loss rates may be low, and it may be possible to plug them with drill cuttings or lost circulation material. However, if the fractures are large, even a very slight overbalance can result in total loss of circulation, while a very slight underbalance can result in inflow from the reservoir.

If the formation is thick, this problem is compounded since the formation pressure typically increases at a rate equal to formation fluid gradient while wellbore pressure increases by the drilling fluid gradient (Figure 1). When the top of the formation is balanced, the formation below it will be progressively overbalanced. If the density of the drilling fluid is reduced to balance formation pressure at the bottom, the top will be underbalanced. The proper application of managed-pressure drilling (MPD) can help improve safety and greatly reduce nonproductive time (NPT).

FIGURE 1. The formation pressure typically increases at a rate equal to the formation fluid gradient, while the wellbore pressure increases by the drilling fluid gradient. (Source: Marathon Oil)

Since it is not possible to balance the formation pressure throughout the interval, it may not be possible to control the losses while circulating. When drilling from a floating vessel with synthetic-based mud (SBM), formation influxes masked by losses when drilling fractured reservoirs have resulted in gas entering the riser without the driller’s knowledge. Gas dissolved in the mud at bottomhole pressure expands rapidly when it reaches a depth where wellbore pressure is less than the bubble point.

In deepwater, this may be well above the BOP. In that case, the gas breaks out of solution in the riser, resulting in a rapid increase in volume associated with the phase change. As the gas approaches the surface, it continues to expand with additional breakout adding to the problem. In extreme cases, the riser can be unloaded and even collapse.

This article presents a brief discussion on the planning and execution of two deepwater carbonate exploration wells offshore Indonesia using various forms of MPD and the world’s first-known application of a below tension ring rotating control device (RCD) on a dynamic positioned (DP) drillship.

MPD installation on deepwater rig

The RCD was installed below the telescopic joint to facilitate drillship rotation during station-keeping. A fit-for-purpose buffer manifold incorporating overpressure protection of the riser was incorporated into the fluid circulating system to allow:

1. Conventional drilling with returns from below the RCD to the flowmeter and shakers;
2. MPD using surface backpressure with returns to either the shakers or mud gas separator; and
3. Pressurized mud cap drilling (PMCD) with no returns and a semistatic annulus.

The RCD, 21¼-in. 2,000-psi annular and a flow spool were pre-assembled into a single joint (MPD joint) for ease of handling and installed in the riser just below the tension ring.

MPD equipment. The RCD provided an annular seal around the drillpipe during drilling and tripping under pressure. Wireline logging operations also were conducted under pressure using a pack-off and lubricator joints, which were latched into the RCD.

The MPD choke manifold contained two adjustable chokes and a Coriolis flowmeter. The Coriolis meter measured the flow rate and density of the return fluid. The riser boost pump was used to constantly circulate the riser through the choke to maintain constant wellbore pressure at a selected point by replacing the annular circulating friction pressure with surface backpressure when the downhole pumps stopped.

TABLE 1. Rig critical-path time improvement during MPD operation is shown from Well #1 to Well #2. (Source: Marathon Oil)

Two nonreturn valves and a pressure-while-drilling tool were used in all bottomhole assemblies.

Well control equipment. The rig’s 18¾-in., 15,000-psi subsea BOP stack consisted of two 10,000-psi annular preventers, two 15,000-psi double rams and one 15,000-psi single ram-type preventer. The BOP ram preventers were fitted with one set of blind shear rams, one casing shear ram, two variable bore rams and an extra set of blind rams (BRs) in place of the lower pipe ram. This lower set of BRs was used as the working ram during MPD operations and was excluded from the emergency disconnect system. The lower kill-and-choke lines allowed pressure monitoring and fluid injection below the BOP when the BRs were closed.

Case histories

The first wildcat exploration well was drilled in 1,006 m (3,300 ft) of water, and the second well was drilled in 1,921 m (6,300 ft) of water.

Constant bottomhole pressure (CBHP) techniques were used successfully to restore wellbore stability, control losses and prevent influx when minor fracturing was encountered by maintaining overbalance pressures as low as 35 psi.

Several kicks were detected and immediately contained, with influxes as small as 2 bbl and no more than 9 bbl by comparing metered outflow to inflow rates calculated from pump strokes.

One 10-bbl influx that occurred slowly over an hour was observed by the driller from pressure-volume-temperature data and not detected by the MPD system due to the low inflow rate. When circulated to surface, the influx was distributed throughout such a large mud volume that the only indication of the influx was gas-cut mud.

When large interconnected fractures were encountered, total losses occurred while circulating. The static mud weight was below pore pressure, so when the pumps stopped, so did the losses. PMCD was used to continue drilling by pumping seawater down the drillpipe while maintaining a semistatic SBM fluid column in the annulus.

Multiple wireline logging runs were successfully conducted under pressure, and the abandonment plugs were set while holding surface pressure.

On Well #1, a total of 862 m (2,828 ft) was drilled using MPD and PMCD techniques. A total of 11,572 m (37,956 ft) of pipe was stripped through the RCD on six bearing assembly runs in 761 operating hours, including the four days of wireline logging.

Well #2 penetrated a total of 427 m (1,400 ft) of carbonate. Eight wireline runs were made, and the abandonment plugs were spotted under pressure using MPD.

Table 1 shows the overall rig critical path time associated with MPD operations, the improvement due to the increase in operational efficiency and a decrease in the MPD equipment NPT.

Field experience confirms that fractured carbonate formations can be safely and efficiently drilled and evaluated from a DP drillship with CBHP and PMCD techniques.