Core drilling making inroads into geothermal exploration

In the past, geothermal exploration has traditionally been done using large oil field equipment. However, advances in equipment and techniques in wireline core drilling, over the past two decades, have given exploration companies an alternative method of drilling. In particular, advances in drill string design, drill bits and core retrieval techniques have led to the use of coring techniques in this application.

Because of the expense involved in using large oil field drilling equipment for geothermal exploration, a cheaper alternative could offer a great advantage.

Tonto Drilling Services has been involved in the exploration for geothermal deposits using coring techniques for some seven years now.

The company became involved when an associated company drilling a geothermal project using rotary techniques ran into problems with a loss of circulation. Circulation could not be regained even after pumping 5,000 sacks of cement into the hole.

Tonto took over the project using coring techniques and successfully completed three holes to a depth of 2,000 ft. each.

Thus began the company’s move into drilling geothermal prospects — including projects in California, Nevada, New Mexico, and the Ascension Island in the South Atlantic.

An interesting kink in the Ascension job was the requirement for special permits allowing the company to drill through a ship, since by some quirk in British law, the island is classified as a British ship, HMS Ascension.

Geothermal energy was sought as a replacement for the island’s only source of energy, which generated using 50,000 gallons of jet fuel per day.

One of the major advances in drilling equipment in recent years is the drill rig itself. Large capacity wireline drills with top head drives and high rotation capacity have enabled companies to drill efficiently to depths of 15,000 ft. using standard wireline equipment.

The steady improvement in diamond drill bits has also played a major role in the application of coring techniques at depth. The range, quality, and performance of impregnated core bits continue to advance while there has been further development of thermally stable polycrystalline diamond drill bits. These bits have the potential to give long life and faster penetration rates in rock under 25,000 psi compressive strength.

A big in advantage using wireline techniques is that the geothermal exploration geologist has a continuous core to examine. This is in contrast to the rotary sample the geologist has traditionally had.

In analyzing core, the geologist can accurately identify rock types and formation characteristics, as well as determine formation dip and strike. Core also provides better information on contact surfaces, discontinuities and voids as well as enabling the geologist to conduct detailed physical and chemical testing of the formation.Certainly of great advantage is that there is far less need for costly downhole logging.

A major problem encountered in geothermal exploration is loss of circulation. With circulation often lost within a few feet of surface, the advantage of core drilling over rotary drilling is evident since the diamond core can advance through lost circulation zones.

For example, while core drilling through a lost circulation zone, Tonto would typically lose 18,000- 20,000 gallons of fluid per day, while an oil field rig’s loss would be in the hundreds of thousands of gallons. In addition, the rotary rig would not likely advance through a lost circulation zone.

Another advantage of a core rig is that environmental legislation makes the smaller and highly maneuverable diamond drill rigs relatively popular with licence- granting agencies which are used to large oil field rigs.

A large oil field rig will typically require between 20 and 50 semis for mobilization to the site — as well as a number of support vehicles, site preparation dozers, and consumable deliveries to the site. In a lost circulation situation, round- the-clock deliveries of cement can be required. In addition, an oil field rig requires up to 20 men to operate, not including support crew. Tonto operates with seven crew members, three per shift, plus a foreman.

The downhole temperature gradient in a geothermal exploration hole can be compared in significance to assay results to an exploration geologist. Temperature results are generally kept under wraps, but temperatures in the 300 degree celsius range are not uncommon.