Geopressurized zones are natural underground formations that are under unusually high pressure for their depth. These areas are formed by layers of clay that are deposited and compacted very quickly on top of more porous, absorbent material such as sand or silt. Water and natural gas that is present in this clay is squeezed out by the rapid compression of the clay, and enters the more porous sand or silt deposits. This natural gas, due to the compression of the clay, is deposited in this sand or silt under very high pressure (hence the term 'geopressure'). In addition to having these properties, geopressurized zones are typically located at great depths, usually 10,000-25,000 feet below the surface of the earth. The combination of all of these factors makes the extraction of natural gas in geopressurized zones quite complicated.
However, of all of the unconventional sources of natural gas, geopressurized zones are estimated to hold the greatest amount of gas. Most of the geopressurized natural gas in the U.S. is located in the Gulf Coast region. The amount of natural gas in these geopressurized zones is uncertain. However, experts estimate that anywhere from 5,000 to 49,000 Tcf of natural gas may exist in these areas! Given the current technically recoverable resources are around 1,100 Tcf, geopressurized zones offer an incredible opportunity for increasing the nation's natural gas supply.
- The technical difficulties of reaching the gas are considerable. Drilling problems related to bore-hole geopressure can present impediments to reaching the proposed depth. Most of the drilling difficulties take place at or near the interface between the seals (shale) and the reservoir quality beds (sand).The frequent disparity between the predicted pore pressure in the shale and the actual pore pressure in the sand causes such drilling problems.
Pressure transgression and regression take place in the subsurface reliant on the sedimentary and structural spatial settings. Sizeable transgressions of the pressure envelope lead to hard kicks, mud cut, bore-hole collapse (instability), hole packing off, stuck pipes, and unset
cement. Conversely, pressure regression causes loss of circulation, hole bridging, sticking pipes and excessive torque.