Geothermal gradients

The most important factor for maturation and hydrocarbon type is therefore heat. The increase of temperature with depth is dependent on the geothermal gradient which varies from basin to basin An average value is about 3°C per 100 meters of depth. 

Geothermal gradient of resource, °C/km Breakeven electricity price, /kWh Cost of thermal energy, /GJ... 

Geothermal sources are categorized into various types hydrothermal reservoirs, geopressurized zones, hot di y rock, normal geothermal gradient, and magma. 

In principle the normal geothermal gradient produces a useful temperature difference anywhere on the globe. If a hole is drilled to a depth of 6 kilometers (which is feasible), a temperature difference of about I80°C (324°F) is available, but no technology has been developed to take advantage of this resource. At this depth, water is unlikely, and the problems of extracting the energy are similar to the difficulties encountered with hot, dry rocks near the surface. 

In general there is considerable variation in the geothermal gradient throughout the United States and the world. Also, in many regions of the world where there is evidence of rather thin crust, the relationship between temperature at depth and depth may not be approximated by the linear function given in Equation 2-163. The increase in temperature with depth has important consequences for drilling and production equipment that is used in the petroleum industry. The viscosity... 

The average geothermal gradient used in most areas of the United States for initial predictions of subsurface temperatures is a value of0.016°F/ft [32]. 

Nichols, E. A., Geothermal Gradient in Mid-Continent and Gulf Coast Oil Fields Transactions ASME, Vol. 170, 1947. 

T = surface atmospheric temperature in °R T = average temperature of flow in annulus in °R = geothermal gradient in °F per ft K = drilling rate in ft/hr... 

Sometimes it may become necessary to shut-in a gas well when the demand for gas is low. In such instances, the well is shut-in for an indefinite period, after which it is reopened and production is resumed. It often has been found that the production rate of gas from the reopened well is substantially less than it was before the well was shut-in. During production, the inner wall of the production tubing will be coated with a film of condensed freshwater because of the geothermal gradient. This water flows down when production is interrupted and can cause formation damage. This may occur because clays are normally saturated with brine water and not with freshwater. This swelling can be prevented with the injection of some additive, for example, sodium chloride, potassium chloride, calcium chloride, or an alcohol or a similar organic material [1853]. 

A continental warming means that the underground is also warming up, which eventually will show by a reduced geothermal heat flow as the geothermal gradient decreases (Chapman, 1998). 

For the local process especially the borehole resistance is important. The borehole resistance depends mainly on the loop type and material, loop dimensions, circulation fluid properties, temperature of the process, borehole engineering (Hellstrom, 1991). Furthermore the far field temperature in the ground and geothermal gradient needs to be measured. 

The relevance of the remarks on sulfur content is that, for reasons explained above, it is usually a valid index of the salinity of the environments of deposition. It was remarked earlier that the Eastern and Interior provinces have experienced different temperature/pressure/time histories. It should be added that coals of the Rocky Mountain, Pacific and Alaskan provinces most probably experienced yet further sets of conditions of metamorphism a locally increased geothermal gradient that produced relatively high temperatures at relatively low depths of burial and hence at relatively low pressures of overburden. 

In VMS systems in tectonic environments which have sufficiently low geothermal gradients and sufficiently constant permeability with depth, we would expect to see metal leaching at the underside and through the intrusion as well as along the top surface. 

For heat pumps based on flooded mine systems, where the heat abstracted is replenished seasonally by inputs from solar radiation and geothermal gradient, the longevity of the reservoir is, in principle, limitless. The sustainability of the operation will only be limited by the durability of the infrastructure (e.g., biofouling, clogging or physical instability of boreholes, longevity of collector coils). 

It would appear that their frequence decreases in older rocks, especially the Paleozoic (Weaver, 1959). The assembled studies of Perry and Hower (1970), Dunoyer de Segonzac (1969), Muffler and White (1969), Browne and Ellis (1970), Weaver (1959), Weaver and Beck (1971), Burst (1959), van Moort (1971) and Iijima (1970) demonstrate that the conversion of montmorillonite to other minerals in sequences of deeply buried sedimentary rocks is independent of time or geologic age and appears to be a function of the geothermal gradient which the rocks have experienced. These studies indicate that fully expandable dioctahedral montmorillonite is not stable above 100°C at depths of two kilometers or more. The occurrence of these minerals in sedimentary rocks can be considered to be controlled by their orogenic history. 

The studies cited above deal with deeply buried sediments which occur in areas of low geothermal gradients. Further the rocks are basically sodi-potassic, at least in the silicate aggregates, and most often contain a potassic phase such as illite, feldspar or zeolites. In instances where these chemical conditions prevail and where the geothermal gradient is high (Muffler and White, 1969 Browne and Ellis, 1970) the same temperature-mineralogy relations seem to hold, 100-120°C appears to be the upper limit of fully expandable montmorillonite stability. However, R. 0. 

---