Over balanced, balanced or under balanced – you’re in control
Managed pressure drilling involves controlling the pressure within a borehole so that excessive fluid is not lost into the ground nor produced into the borehole. By controlling pressure within the borehole, problems with piping of unconsolidated material into the hole and liquefaction of material around the hole are avoided. Borehole stability is also improved.
In deeper underground mining operations the control of borehole pressure is frequently essential for the safety of the operation as high pressure gas and water may be present.
In tunnelling operations the control of borehole pressure throughout the drilling process enables the installation of spiles, ground freezing pipes or grouting systems such as tube-a-manchettes, while maintaining pressure within the borehole. This approach enables stable zones to be created ahead of underground excavation. This approach may also be used in the creation of stable ground ahead of cross passage excavation between tunnels.
Managed pressure drilling may also be used to permit the drilling and installation of anchor holes through retaining walls or sheet piles where drilling must take place from below the water table.
Managed Pressure Drilling System in use at the Liang Tang Tunnel Project
Sigra designs and manufactures balanced pressure drilling equipment to suit the application. These can be built to operate at mud pressures from 5 kPa to 7 MPa.
The ancilliary equipment required to achieved this is through the use of four key items of hardware, namely:
- Tunnel wall penetration casing with shut off valve (seals the casing to the wall)
- Static annular seal to seal casing to drill rod (blow out preventer or BOP)
- Rotary seal between the drill rod and the casing or BOP
- Choke to control drilling drilling mud and entrained cuttings at a carefully controlled back pressure within the borehole. This will handle large size cuttings .
These items require bespoke design and manufacture to ensure they are fit for their intended use.
In addition to its surface well control equipment Sigra has manufactured equipment for use in drilling from underground mine roadways to prevent the the uncontrolled inflow of gas and water. Recently it has built a variant used between shallow bored road tunnels in soft, granular soils. This has been successfully used to prevent hole collapse and piping into the tunnel and to prevent the loss of drilling fluid and liquefaction of the ground above. Such events could have has serious consequences for the drillers and for structures above the tunnels.