GEOTHERMAL DRILLING

Geothermal drilling technology is adapted from the oil industry. Most of the geothermal wells are drilled with a rotary-type drilling rig. Because of the hard, abrasive, and high-temperature volcanic rocks of geothermal systems, the drilling bits require hard abrasive resistant teeth such as diamond or tungsten carbide. Hence, geothermal drilling is an expensive activity in geothermal development. Most geother­mal wells are drilled 1 to 2 km deep to intersect faults and fractures so as to increase the success rate of production from permeable zones. This is enhanced by directional or deviated wells as opposed to vertical wells of earlier development. Directional drilling also allows drilling of more than one well on a single well pad, especially in difficult terrains. A geothermal well typically costs U. S. $1 million and increases exponentially with depth. Geothermal drilling may be divided into three types: exploration, production, and reinjection.

Exploration drilling is carried out to assist earth scientists investigating a geothermal resource. Geol­ogists will study the drill cuttings and drill cores of exploration wells that are normally drilled vertically and in small diameters (100-150 mm or 4-6 inches). It is not uncommon to make continuous drill cores from exploration wells so that the subsurface geology of the geothermal area can be confirmed. Exploration wells may be productive, and geothermal fluids discharged from the wells will confirm the chemistry of the fluids from earlier geochemistry studies.

Production wells are drilled after data from exploration wells have proven that a geothermal resource is economically viable in terms of its expected generating capacity and life. Geothermal fields of earlier development, such as Wairakei and Cerro Prieto, have vertical wells drilled to approxi­mately 1km deep. The fields have flat terrains, and wells are spaced as close as 20 m from each other. Nowadays, it is common to have several production wells drilled from a single well pad using directional drilling. Modern production wells are typically 2 km deep and may be deviated 1 km away from the well pad. Typical production wells have 250-mm (95/8-inch) diameter production casing. Some large geothermal fields in The Philippines have large (500­mm or 20-inch) diameter production casing. (Note: As drilling is adapted from the oil industry dominated by the United States, the unit used is imperial.)

For environmental reasons, modern practices reinject most, if not all, geothermal fluids produced from the production wells back into the ground after the energy has been used. This is achieved by piping the spent geothermal fluids to reinjection wells. Reinjection wells are drilled similarly to the produc­tion wells. Good reinjection wells require as much permeability as do production wells to swallow the reinjected fluids. Hence, a good production well makes a good reinjection well. The reinjection may require pumping to achieve the required injection rate. However, reinjection of cooler geothermal fluids back into the ground may cool the hot fluids from production wells. Hence, reinjection wells are normally located either downstream of the flow direction of the geothermal fluids in the reservoir or at the edges of the production field so that reinjected fluids flow away from production zones.

A well is drilled by a rotating drilling bit (commonly a tricone bit with a tungsten carbide insert), with an additional load applied to it from the weight of a thick wall tube called the drill collar. The rotating motion comes from a rotating table with a square hole that transmits torque to a square tube known as the kelly. The kelly is connected to the drill collar by drill pipes to form the drill string. Drill pipes have a thinner wall than does the drill collar. The drill string is smaller in diameter than the hole being drilled, allowing a drilling fluid to be pumped down inside the drill string, clearing the cuttings around the drill bit, and carrying the cuttings up via the annulus between the hole and the drill string.

The drilling of a typical production well (Fig. 9) commences by drilling a large diameter hole (24-26 inches) up to approximately 50 m. A 20- to 22-inch outer diameter pipe (casing) is then cemented in the hole. The cementation is achieved by pumping a cement slurry down inside the casing and flowing up the annulus. The cement inside the casing is then displaced by water to just above the bottom of the casing (shoe). When the cement has set, a drilling wellhead with blowout preventer (BOP) is erected, controlling accidental discharge of geothermal fluid during drilling. Drilling continues with a smaller diameter hole (17.5 inches) for a 133/8-inch casing from the top of the cement near the casing shoe to the next casing depth. This sequence is repeated until three or four casings are cemented to a depth assessed to be the top of the desired production zone in the reservoir. The final casing is called the production casing, usually 95/8 inches in a 121/4-inch hole. In general, the shallow outer casings are required for drilling operations, whereas the inner casings are for production purposes. These casings not only control the geothermal fluids flowing inside the production casing but also prevent cold fluids outside from entering the well.

When the final section of the hole (8.5 inches) is completed in the production zone, a 7-inch casing with holes (slotted liner) is usually used to line the well. Unlike other casings, the liner is not cemented into place. It can be sitting at the bottom of the well or hanging from the production casing shoe. The liner prevents the production formation at the hole from collapsing and allows the geothermal fluids to

enter the well and flow up inside the production casing. A wellhead consisting of flanges, valves, and pipe spools is usually attached to the top of one of the intermediate cemented casings. The production casing is allowed to expand into a wellhead pipe spool during production of the hot fluids.

Because the safety of the drilling rig and the completed well can be at risk for an unstable well site, a complete geotechnical investigation might be necessary before site construction. Of particular concern is the possibility of a shallow blowout during drilling in areas with shallow permeable rock formation. To prevent this, consolidated grouting of the drilling site is carried out. This involves drilling and cementing slim holes to depths of 10 to 30 m

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