Managed pressure drilling episode 2
Projects that have used five of the many different variations of MPD have demonstrated techniques that are proactive in managing the pressure profile, These Techniques are:
- Continuous Circulation System
- ECD Reduction Tool
- Pressurized Mud Cap Drilling
- Controlled Mud Cap System
- Dual-Gradient Drilling Method
In this series, we are going to explain each technique in details.
- Continuous Circulation System:
The continuous circulation system (CCS) is a new technology that enables a driller to make connections without stopping fluid circulation. ACCS enables a driller to maintain a constant ECD (Equivalent Circulating Density) when making connections. In normal drilling operations, a driller must turn the pumps off when making a connection. Numerous problems can occur as pumps start and stop in a drilling operation.
Fig. 5 shows the pressure spikes that occur when making a connection. When the pumps stop, the pressure in the well decreases. This decrease in pressure can cause:
- A kick, formation fluids enter the wellbore.
- The formation could also relax and the formation could collapse on the hole, resulting in stuck pipe.
- The differential pressure between the reservoir and the wellbore can also stick the pipe.
- The drilling fluid starts to form a gel when the pumps are turned off as the fluids stop circulating. When the pumps are restarted,pressure increases to break the gel, causing a pressure spike which could cause lost circulation, where fluids enter the formation, and ballooning of the wellbore.
Also, before a connection is made, the rig has downtime associated with circulating the cuttings out of the bottomhole assembly. This is required so that the cuttings do not settle at the bottomhole assembly.
A CCS could solve these problems when drilling. It would enable a driller to have improved control of the ECD and reduce these problems that can result from shutting down the pumps during a connection.
- How Does the CCS work?
Fig. 6 shows a coupler, the device that enables the continuous circulation of the fluid. The drillstring passes through this device, and during the connection process it provides a seal around the drillstring. The coupler can be divided into an upper and lower section. A sealing devicecan separate the two sections.
- When it is time to make a connection, the fluid flows into the coupler, thus equalizing the pressure around the drillstring. With the pressure equalized, the connection is broken and the tool joint pin is backed out and raised out of the lower section.
- The sealing device then closes and the pressure in the upper chamber is bled off, allowing the tool joint pin to be removed. The fluid that is in the upper chamber drains back into the mud pit. The lower section continues to circulate fluid down the hole during the entire operation.
- The new joint of drillpipe is then lowered into the upper chamber. The chamber is sealed and re-pressured by fluid from the circulating system. Once the pressure is equalized between the two chambers, the dividing seal opens. The drillpipe joint is lowered and a connection is made. Once the connection is made, the pressure is bled off and the seals are opened so that normal drilling operations can continue.
In a field trial in 2003, the CCS was tested for 14 hours drilling a 12 ¼-in. hole. The system made 72 connections. The first 6 were donemanually to calibrate the system. The rest of the connections were controlled automatically by the driller using a touch screen. Circulation rate for the test was 800 gpm and pressure was kept between 2,800 and 3,000 psi.
The test showed that the continuous circulation system can be used to drill sections of the well without turning the pumps off during a connection. After the test was run, the drillpipe used was tested and no damage was found on the drillpipe that was handled by the coupler. The connection times ranged from 13 to 20 minutes. The actual connection time can be reduced to about 8 minutes with improvement in the guidance system when performing the connection
The continuous circulation system is useful in preventing pressure spikes when making connections, thus reducing wellbore problems.Benefits of using the CCS include:
- Reducing non-rotation time (rig downtime) by eliminating the need to circulate the cuttings out of the bottomhole assembly.
- Reducing the possibility of a stuck drillstring by keeping the cuttings from dropping to the bottom.
- Constant ECD can be maintained.