An injection well is drilled for the safe disposal of geothermal fluids, whose total dissolved solid concentrations in the state can reach 250,000 parts per million, about seven times higher than sea water. Some production wells are converted to injection wells.
Besides safe fluid disposal, injection wells provide the following benefits:
Subsidence can be a significant problem in Southern California’s Imperial Valley. Most of the area is farmland and even a few feet of subsidence can have serious consequences. Ground-level surveys are performed annually and fluids are injected to compensate for any elevation changes.
After 1986, production at The Geysers Geothermal field began to decline and in 1998 treated sewage effluent from Lake County was piped to the field to inject and produce as steam. Since the injection began, the production decline has slowed significantly.
After a well is drilled, often to depths over 5,000 feet, casing (steel pipe) is inserted in the well and cemented in place by pumping cement into the well annulus--the space between the casing and the rock formation. The casing and cement prevent fluids in the formation from mixing with each other or with injected fluids. If the casing is cemented across the injection zone, small holes are punched through the casing with a special gun. Usually the casing is cemented from just above the injection zone back to the surface and fluid is injected below the casing into fractures in the injection zone. Tests may be required to ensure the well is operating properly and that injected fluids are confined to the intended injection zone.
An injection zone in a geothermal field may be a permeable rock, like sandstone, but usually it is a fractured rock. Rock beds chosen as injection zones often are overlain by impermeable, finer-grained beds—like shale. These are called cap rocks and they keep injected fluids inside the injection zones.
The Department of Conservation, Division of Oil, Gas, and Geothermal Resources (DOGGR), under provisions of the Public Resources Code and the federal Safe Drinking Water Act, regulates all geothermal wells drilled on private and state lands. California geothermal injection wells are considered federal Class V injection wells, and the state injection program includes permitting, inspection, enforcement, mechanical integrity testing, sealing and closing (plugging and abandonment) oversight, data management, and public outreach.
All drilling, rework, and plugging and abandonment operations on geothermal wells require a permit. DOGGR engineers evaluate the geologic and engineering information proposed in the permit. The regional water quality control board receives a copy of the proposal for comment. DOGGR also solicits public comments and may hold a public hearing. An approved permit will include conditions, such as the injection zones, maximum injection pressures, and future testing and monitoring requirements.
DOGGR engineers monitor all injection wells to make sure they are operating properly. They review operational data and witness tests to ensure well casing soundness. In addition, surface pipes at most well sites are inspected every year.
There are about 160 geothermal injection wells located in about a dozen geothermal fields in California. In 2009, over 300 billion kilograms of water was injected into the wells.