Geothermal Drilling in California
Authors: Cromling John 
Published: December 2007

Although drilling for geothermal energy is quite similar to drilling for oil and gas, there are some aspects of it that are unique. The high temperatures associated with geothermal wells affect the circulating system and the cementing procedures as well as the design of the casing and drilling string. The unique problems must be coped with along with those normally encountered.

Introduction
Geothermal energy is simply the energy derived from the earth's magmatic heat. This would include all the products of geothermal processes, naturally occurring products of geothermal processes, naturally occurring and artificially induced. Steam, hot water, and minerals are some of the most important direct products of geothermal resources.Geothermal power was recognized first in Italy in 1904 as a future source of energy. It was not until the mid-1950's that the U. S. became seriously interested in geothermal power, the chief reason being that until this time other energy sources were plentiful.In 1955, a drilling program began in the Geysers area of California. The first geothermal power generator in the U. S. began operation in the Geysers in 1960. Since that time the drilling activity hasincreased substantially and several generating plants have been constructed, The total capacity of the Geysers is presently in excess of 300 megawatts (Mw), which ranks the U. S. as the second leading nation in geothermal power production. This activity stimulated the exploration for geothermal resources in other areas of the country, especially in other parts of California.The Imperial Valley of California is another center of geothermal interest. Unlike the Geysers area, which produces dry superheated steam, the Imperial Valley produces hot brine. There are presently several wells being drilled in the latter area and there have been many proposals to utilize the available geothermal power.

The Imperial Valley Geology
The Imperial Valley is essentially a flat, featureless, alluvium-filled basin trending northwestward from the Gulf of California to and including the Coachella Valley. The sediments filling the Imperial depression are primarily sandstones, shales, claystone, and conglomerates deposited by the ancestral Colorado River as it formed its delta. The sediments range in thickness from 8,000 ft in the northeastern Coachella Valley to 17,000 ft in the southwestern Imperial Valley.Three major fault systems, all with northwesterly trends, are present in the Imperial Valley. The San Andreas zone travels along the northeast side of Coachella Valley. The San Jacinto and Elsinor fault zones are on the northwest side of the Imperial Valley. Most of the numerous faults in the three systems have right lateral and vertical movement, with the northeast block structurally higher than the south-west block.

Surface Locations
Most locations for drilling operations pose no real problem since the terrain is nearly flat. The primary problem since the terrain is nearly flat. The primary surface problem in the Imperial Valley is the extreme heat to which the personnel and the machinery are exposed. Water for the drilling operations is usually obtained from one of the many irrigation canals that cross the Valley or from water wells.
JPT
P. 1033