Looking into Geothermal Drilling
A fantastic breakdown of geothermal drilling components from Energy.gov
MAIN COMPONENTS OF THE DRILL RIG
Derrick – The part of the drilling equipment that you see above the ground. It holds the drill pipe in place, and contains the motor that turns the drill pipe.
Drill String – A long column of pipe composed of sections that are connected to each other. The column transmits the force to turn the drill bit as it digs through the layers of earth and rock. It also transmits the drilling fluid that helps clear the way for the drill string.
Rotary Table – A device that turns the drill string and the drill bit. This rotational motion provides the force required to drill the well.
Drill Bit – The rotating device at the tip of the drill string that breaks, cuts, and crushes through rock to drill the well.
The drilling starts with a large diameter tricone drill bit. After the initial well is “spudded” or drilled, a steel casing is cemented into place. The steel casing protects fresh-water aquifers and allows for deeper drilling. As the drilling continues, the diameter of the hole, casings, and bits become smaller, allowing each new section to fit inside the previous one.
DRILLING WITH MUD
A viscous drilling fluid, called mud, is circulated through the drill string into the well. Mud lubricates and cools the drill bit, and carries rock cuttings back to the surface.
Trips in and out of the well are completed to change the drill bits as they wear out. Tripping is also done to adjust the bottom hole assembly, which is the equipment near the drill bit. It takes approximately 60 days to drill 10,000 feet. It is common to have up to 45 trips in and out of the well.
DRILLING WITH AIR
Once the well reaches the geothermal reservoir (hot permeable rock), the drilling process switches to using air instead of mud as the circulating fluid. Air is used instead of mud to prevent clogging of the permeable reservoir. This high pressure air also cools the drill bit and lifts the cuttings out of the well.
The bottom hole assembly guides the drill bit toward the optimal location to tap the geothermal energy. Most wells are directionally drilled because it is much less costly than moving the derrick to a new site. Also some areas are inaccessible above ground due to the terrain. When complete, wells at The Geysers range from 8,000 to 12,000 feet deep. They can be angled up to 45 degrees from the vertical well.
When the water underground is heated by the hot rock, it produces steam. Once the production well is drilled, steam is allowed to flow up the well and through a meter to determine the flow rate.
POWER FROM STEAM
The steam flows through pipelines from the production well to the power plant. It then turns a turbine, which produces electricity.