Mud Cap Drilling and Pressurized Mud Cap Drilling in depleted reservoirs
Both Mud Cap Drilling (MCD) and Pressurized Mud Cap Drilling (PMCD) are Managed Pressure Drilling (MPD) applications. They are applied when a “total loss” scenario is encountered for a given mud weight. When the pore pressure is high enough to support a column of fluid in the annulus, PMCD is used. When the reservoir is depleted or a sub-hydrostatic gradient is present and a column of fluid cannot be supported, MCD is applied.
MCD requires constant injection of fluid down the annulus with sufficient properties to prevent gas migration. With MCD, the annulus is never static: this means the driller is effectively blind to what is happening down hole, with limited ability to monitor the system from surface.
The main benefit of PMCD over MCD, from a safety perspective, is the ability to monitor the migration of gas in the annulus. Both techniques take advantage of the natural ability of the fractured formation to accept losses of drilling fluid and cuttings. PMCD can mitigate severe losses and reduce associated non-productive time when drilling highly fractured formations, including those containing vugs or caverns. As long as the pore pressure in the fractured zone is greater than the hydrostatic pressure of the fluid, drilling can continue using wellsite water (WSW) as a sacrificial drilling fluid – without incurring excessive drilling fluid costs, while well control issues resulting from difficulty in maintaining a fluid column as the primary barrier are avoided.
Rather than attempting to cure losses, PMCD utilises Light Annular Mud (LAM) which is displaced into the loss zone from surface, via the annulus. The pressure shown on the casing at surface, known as surface back pressure (SBP – typically engineered to be +/– 200psi) is the differential pressure between the deliberately underbalanced LAM hydrostatic pressure and reservoir pressure. Figure 1 below illustrates the situation in place with PMCD.
Figure 1: Pressurised Mud Cap Drilling Illustration
Operational Considerations
The positive annular surface pressure ensures that well control is maintained and that the well is always full of fluid. A rising annulus pressure indicates that a gas influx is migrating upwards. The migration rate and influx volume are calculated prior to bull–heading LAM down the annulus to reduce the surface pressure. The pump rate down the annulus is designed to stop gas migration towards the surface, forcing produced gas back into the reservoir or permeable formation. Decreasing annulus pressure – back to the original value when the LAM was first circulated indicates that the gas has been displaced back into the formation.
Figure 2: PMCD gas migration & LAM injection plot
These pumping cycles are repeated during the drilling of the section as and when required. The plot in Figure 2 shows a textbook PMCD cycle of migration and pumping while continuously drilling, with roughly one hour between each pumping cycle. Figure 2 clearly shows the increase in annular pressure (in red) as injection commences (light blue) and LAM is bull-headed back down the well. Each cycle pumps enough fluid to displace the migrated volume, returning annulus pressure to zero. This approach minimises the amount of LAM lost to the well, reducing costs and formation damage.
Pressurized Nitrogen Cap Drilling (PNCD)
When the pressure gradient drops to a point where it can no longer support the lightest column of available fluid to surface, an alternate drilling method must be employed. Typically, MCD is the go-to application under these circumstances. In this case, the annulus is filled with a heavier weight drilling mud. Although lower pressure, these wells still retain the risk of hydrocarbon influx when drilling blind and without the well full and under conventional control.
As a response to these challenges, Pressurized Nitrogen Cap Drilling (PNCD) is a viable alternative. Figure 3 below illustrates the configuration. The technique involves letting the annulus fluid (with a density higher than the pore pressure) drop to an equilibrium level where hydrostatic pressure equals the pore pressure. At this point, the annulus fluid level will not reach the surface. The resulting air gap is pressurized with nitrogen gas. This nitrogen “cap” is contained under the Rotating Control Device (RCD) which maintains pressure communication with the formation. The well is continuously monitored by tracking the wellhead pressure and, if required, casing pressure can be managed by bull heading annulus fluid periodically down the well.
Using this technique, the combination of Heavy Annular Mud and the N2 gas cap ensures that hydrocarbon migration can be monitored and managed during the drilling process, whilst keeping a suitable pressure differential in place below the RCD.
Figure 3: Pressurised Nitrogen Cap Drilling Illustration
