Severe to total loss during a drilling operation leads to nonproductive time, instances of stuck pipe and an increase in drilling costs and prevents operators from reaching the desired total depth. Lost circulation material (LCM), underbalanced drilling or managed pressure drilling (MPD) can be implemented in wells encountering severe to total losses. Mudcap drilling (MCD) is a major variant of MPD.

This drilling technology helps drill the formations with total lost circulation, where LCM and cement squeeze operations are very expensive, time-consuming and ineffective. There are no returns to surface during an MCD operation. Benefits of MCD applications increase in the presence of hazardous reservoir fluids such as hydrogen sulfide and CO2 in high concentration and in absence of facilities to store the returns from the well.

MCD has multiple variations, such as floating MCD (FMCD), pressurized MCD (PMCD) and dynamic MCD. Four distinct zones develop during an MCD operation, namely fluid zone, mixture zone, interface zone and interaction zone. Fluid zone is present in the wellbore annulus and prevents the formation fluids from migrating to the surface by using the hydrostatic head of a viscous mudcap fluid (with or without additional pressure).

The mixture zone contains a mixture of drilling fluid, formation fluid and cuttings. Interface denotes the dynamic boundary between the mudcap fluid and the mixture zone. The interaction zone is a part of the subsurface formation that interacts with well fluids through intake or expulsion. Interaction zones mostly take in the sacrificial fluid, cuttings and formation fluids from other zones based on the pressure and energy equilibrium conditions.

Limitations of MCD

There are several limitations of MCD. It requires a subsurface zone that can take the cuttings and the drilling fluid used to drill that section for a prolonged period of time. Large reserves of sacrificial fluids are needed along with sufficient quantities of mudcap fluid.

Regulatory and logistics requirements must be available before the operations are performed. MCD does require specialized and trained people.

When it comes to selection of mudcap fluid, a fluid suitable for MCD should be nondamaging to the matrix; not form damaging or blocking emulsions with either reservoir crude, reservoir water or drilling fluid; be inexpensive; be easy to weight up at the rig site based on the required density changes during drilling, tripping and other rig operations; be mixable in high volumes; have low rheology for normal drilling; and have high downhole rheology under the mudcap to minimize mixing with the reservoir fluid.

Introduction to PMCD

The International Association of Drilling Contractors defines PMCD as a variation of MPD that involves “drilling with no returns to surface, where an annulus fluid column, assisted by surface pressure, is maintained above a formation that is capable of accepting fluid and cuttings.” PMCD grew out of practical experiences with FMCD technology in the deeper trends of the Austin Chalk.

Also, development of rotating control devices (RCDs) rated at 2,500-psi rotating pressure and 5,000-psi static pressure facilitated PMCD technology. During a PMCD operation, the top of the mudcap fluid is held under some pressure at the surface to provide a better control on MCD operations. Mudcap fluid is kept at or underbalanced against the lowest reservoir pressure. The surface pressure is referred to as the choke pressure or backpressure.

The mudcap controls the reservoir pressure regardless of what is happening in high-permeability zones. Mudcap fluid density is deliberately configured to provide a positive underbalance at the top of the interaction zone or the top of the first fracture.

Annulus fluid is an integral part of PMCD operation. The mudcap is relatively underbalanced compared to reservoir or formation pressure, allowing it to float above the loss zone since it is being supported by fluids coming from the reservoir. Since PMCD is performed on a closed system, the difference between the reservoir pressure and hydrostatic pressure of the mudcap is equal to the surface backpressure being exerted by the RCD.

The backpressure is monitored to record any changes in the reservoir pressure or to detect the annular migration of gas, allowing the control of the same by bullheading the fluid in the annulus whenever a predetermined backpressure value is reached. An increase in choke pressure indicates migration of formation fluids through the mudcap. Mudcap fluid is filled up in the well at a rate that exceeds the gas percolation rate or applies a backpressure from the annulus to prevent the entry gas from escaping to the surface.

Drill cuttings are sandwiched by the annulus and drillpipe pressures, thereby getting injected into the fractured section. The injected cuttings act as natural LCM. The type of BOP required for this method is similar to conventional drilling, but a pressure control device is installed on the top of the BOP stack as an additional safeguard.

The RCD is a key equipment component of PMCD. It is used for sealing the annulus while still allowing rotation and movement of the drillpipe. This feature makes underbalanced, near-balanced or overbalanced drilling possible in a safe manner when facing an elevated risk of gas kicks. An RCD should never be used as a shut-in device since it is the last pressure barrier.

If any backpressure is observed from the annulus, the pipe rams are closed and an optimum fluid is injected into the annulus. An optimum density mud is required to maintain the well in overbalanced condition. The annulus must be full of mud to maintain effective mudcap sealing.

In offshore drilling when available mud volume becomes critical due to heavy losses, seawater is used as the drilling fluid. A polycrystalline diamond compact bit is the recommended type as it creates fine drill cuttings by its grinding effect. Such fine grains easily penetrate the naturally fractured loss zones.

A choke pressure gauge is required to monitor hydrostatic pressure in the annulus. Generally, when annulus pressure reaches 1,000 psi at the surface, the annular BOP is closed and a kill mud is bullheaded until the annulus pressure is 0 psi. When using an LCM pill or barite, a circulating valve is used to remove coarse particles. This reduces plugging problems in the drill string due to settling of mud materials. Before tripping out, the string capacity should be displaced with mud having an optimum density considering a trip margin.

References:

Terwogt, J. H., Mäkiaho, L. B., van Beelen, N., Gedge, B. J., & Jenkins, J. (2005, January 1). Pressured Mud Cap Drilling from A Semi-Submersible Drilling Rig. Society of Petroleum Engineers. doi:10.2118/92294-MS

Niznik, M. R., Elks, W. C., & Zeilinger, S. C. (2009, January 1). Pressurized Mud Cap Drilling in Qatar North Field. Society of Petroleum Engineers. doi:10.2118/122204-MS

Goodwin, B., Nauduri, S., & Medley, G. (2014, April 8). MudCap Drilling: New Variations, Drivers, Limitations, and Lessons Learned - Case Histories. Society of Petroleum Engineers. doi:10.2118/168956-MS

Comments