Better recoveries, better economics make new projects more drillable.

Many drilling-related challenges facing the offshore drilling industry today contribute to more than half of all offshore prospects being economically undrillable. In water depths greater than 7,500 ft (2,287 m), the percentage of economic "undrillability" is greater. And, as zonal and reservoir pore pressures deplete over time, and as the requirement to drill in ever-deeper water intensifies, the economic undrillability percentage will surely increase.

Clearly, conventional offshore drilling tools and techniques require a step-change technology now more than ever. Solutions to overcoming, remediating or otherwise dealing with drilling-related challenges beg for more precise wellbore pressure management; thus the interest in applying managed-pressure drilling (MPD) technology in marine environments.

Conventional drilling challenges

Conventional drilling methods often contribute to mud and cuttings invasion and formation damage that diminish a well's production potential. Conventional drilling-related obstacles include slow rate of penetration (ROP); drilling flat time, often associated with circulating out influx or kicks; lost circulation when drilling into zones of depleted pore pressure and when the margins between formation pore pressure and fracture gradient are narrow; and difficulty drilling ahead during kick-loss situations.

The obstacles to conventional drilling are particularly onerous and costly when they arise in marine environments, where they worsen progressively with water depth. Drilling-related obstacles to cost-effective drilling, from fixed/platform rigs with surface blowout prevent (BOP) stacks in shallow water and from floating rigs with subsea and surface BOP stacks in deep water, include the following:

• Excessive casing programs requiring larger, more expensive floating rigs to buoy the weight;
• Shallow geohazards, including shallow gas and abnormally pressured aquifers (shallow-water flows);
• Slow ROP;
• Drilling flat time associated with circulating out kicks and well control;
• Lost circulation and mud costs;
• Kick-loss cycles associated with narrow pore-to-fracture-pressure margins; and
• Failure to reach total depth objective and/or with large enough open hole for the well to be economically viable.

These obstacles to conventional drilling often contribute greatly to authorizations for expenditure (AFEs) being exceeded.

Underbalanced drilling option

Underbalanced drilling (UBD) tools and technology have evolved over the last several decades to permit drilling ahead with a mud weight that imparts less hydrostatic head pressure than the pore pressure of the formation being drilled and does so with excellent safeguards against an uncontrolled blowout at the surface. Hydrocarbons are produced to the surface while drilling ahead and must be dealt with using specialized surface equipment. In addition to avoiding unnecessary damage to a well's potential to produce hydrocarbons, lighter drilling fluid increases the ROP and minimizes the occurrence of exceeding formation fracture gradient and associated lost circulation. Tools developed for the safe practice of UBD enable more precise wellbore pressure management, containment and diversion of mud returns.

Inherent challenges to practicing UBD in marine environments include handling of hydrocarbons produced during the drilling process; rig space limitations for UBD specialized equipment and personnel; and, in some cases, regulatory regimes.

MPD potential

MPD is enabled by some of the tools and technology developed for UBD; however, MPD focuses on more precise wellbore pressure management rather than creating a pressure drawdown across formations, as UBD. More precise wellbore pressure management has the potential to see fewer hydrocarbons produced to the surface than most conventional overbalanced drilling operations experience in reality. What is important here is the fact that more precise wellbore pressure management has the potential to address - at least to some degree and, in some cases, eliminate entirely - perhaps up to 80% of the drilling-related obstacles facing conventional drilling programs today.
A good litmus test for this point is a review of an offset well's drilling-related excesses, perhaps a well for which the drilling program significantly exceeded its AFE. Envision where more precise wellbore pressure management could reduce lost-circulation incidents and reduce mud cost. Look at the root causes of drilling flat time, especially time spent circulating out kicks. Ask yourself: Would a closed and pressurizable mud returns system enhance well control and perhaps enhance employee health and safety and environmental integrity (HSE)?

Where practicing UBD in marine environments may today appear to many offshore drilling decision-makers be somewhat of a quantum leap, MPD technology offers the offshore industry a more readily acceptable step change toward the sometime-in-the-future inevitable. No more hydrocarbons are produced to the surface with MPD than with today's conventional drilling programs - perhaps less, in fact. Most drilling-related challenges that contribute to excessive drilling costs can be addressed and in some cases eliminated. Rig modifications for several variations of MPD are modest, if at all. Specialized equipment for practicing MPD from all types of rigs has been developed and in most cases well proven.
Several offshore wells-in-progress around the world are practicing reactive (or contingency) MPD. In reactive MPD, a closed and pressurizable mud returns system enables response to surprises allowed by conventional wisdom fluids programs - usually to reduce drilling flat time associated with circulating out kicks. Reactive MPD encompasses using such a system for HSE or enhanced well-control purposes.
Others are practicing proactive MPD, in which the benefits of drilling with a closed and pressurizable mud returns system have prompted planning of the well construction design, casing set points and fluids program from the beginning with this capability in mind.

Variations of MPD

Early adopters of the technology are currently applying several adaptations or variations of MPD to drilling in marine environments:

Constant-bottomhole-pressure MPD. In this variation, the objective is to "walk the pore pressure line" with a nearer-balanced-than-conventional-wisdom fluids program as a means of overcoming kick-loss issues associated with narrow margins between formation pore pressure and fracture gradient. When drilling ahead, surface annulus pressure is near zero. During shut-in for jointed pipe connections, a few hundred psi backpressure is required.

Dual-gradient MPD. Several methods of dual-gradient deepwater drilling are being practiced. A predetermined quantity of nitrogen from a nitrogen production unit (membrane) is injected at some predetermined depth into the casing or marine riser. A gas, mud and cuttings pressure-depth gradient results from injection point to the surface. Below the injection point, a different mud and cuttings pressure-depth gradient results; thus the term dual-gradient. This technique is helpful as a means of adjusting the effective bottomhole pressure without having to change base fluid density and with fewer interruptions to drilling ahead, usually to avoid lost circulation in a theft zone or to minimize differential sticking of the drillstring. Nitrogen may be injected by concentric casing, concentric riser, parasite line or, on fourth- or fifth-generation deepwater rigs, by way of the rig's existing booster pump and line.

Pressured-mud-cap MPD (PMCD). In zones with a proven ability to readily accept mud and cuttings and where offset wells have indicated depleted pressure, a "cap" of heavy mud is pumped down the backside, into the annulus, to reduce mud loss. Drilling "blind" with no returns, a lighter-than-conventional drilling fluid and cuttings are single-passed into the otherwise troublesome zone. Drilling with a lighter-weight fluid usually means lower overall mud cost and higher ROP, as compared to offset wells. Equipment required for practicing PMCD includes: a rotating control device (RCD), dedicated mud cap pump, choke manifold and UBD hydraulic flow modeling to estimate density and column height of the mud cap and ensure surface pressure is within the capability of surface equipment being used.

Riserless dual-gradient MPD. As a means of effecting zero-discharge riserless drilling, or as a means of recovering expensive mud in riserless drilling, this variation of MPD incorporates the use of a subsea RCD, subsea pump and a returns line back to the rig. The rig's mud pumps plus drilling fluid density and cuttings create one pressure-depth gradient from the mud line down. The adjustable speed of the subsea pump contributes to another pressure-depth gradient with mud and cuttings from the mud line to the rig. A major operator has recently used riserless drilling with success in the Caspian Sea. The technique offers promise as an alternative to the industry standard method of dealing with shallow geohazards by pumping and dumping, single-passing a heavy viscous mud to the mud line when establishing subsea locations by drilling without a marine riser.

Conclusion

Drilling programs using MPD technology are in progress from jackup/platform rigs with surface BOP stacks, floating rigs with subsea BOPs, floating rigs with surface BOPs and riserless drilling of top holes in deep water. Enabling more precise wellbore pressure management, MPD is now being widely recognized as a step-change technology more readily acceptable to offshore drilling decision-makers than true-state UBD.

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