Anomalous layer (reservoir) pressure

ANOMALOUS LAYER (RESERVOIR) PRESSURE (EN: abnormal seam pressure; DE: anomaler Flozdruck; FR: pression anomale des соuches; ES: presion anomal en capas; RU: аномальное пластовое давление) is the pressure, which acts onto the fluids (water, oil, gas), which are contained within the porous space of the rock, the magnitude of which differs from the normal (hydrostatic) one.

The layer pressures, which are greater than the hydrostatic one, that is the pressure of the column of the fresh water (with the density of 103 kilograms per cubic metre), which is equal by the height to the depth of the layer at the point of measuring, they name them as anomalously high layer pressures (RU: AVPD, АВПД, аномально высокие пластовые давления); the layer pressures, which are lesser than the hydrostatic one, they name them as anomalously low layer pressures (RU: ANPD, АНПД, аномально низкие пластовые давления).

Anomalous layer pressure exists within the isolated systems. There is no unified opinion on the genesis of anomalous layer pressure. They consider as the major causes of the formation of anomalous layer pressure the densification of the clayish rocks, the processes of osmosis, of catagenetic transformation of rocks and organic matter, which is contained within them, the processes of tectogenesis, and the geothermal conditions of the deep of the earth. Each of these factors may prevail with dependence on the geological structure and the history of development of region. However, according to the opinion of certain researchers, the temperature factor, apparently, is the most important one, because the coefficient of thermal expansion of various fluids, which are enclosed within the isolated volume of the rocks, is significantly greater, than such coefficient of the mineral components of the rocks.

Anomalous layer pressures have been identified with the help of drilling of numerous wells on land and within water areas during prospecting, exploration, and development of the oil and gas reservoirs within the sediments from Pleistocene to pre-Cambrian within the wide interval of depths. AVPD may be found more often, especially they are widely developed at the great depths (more than 4 kilometres). AVPD usually exceed the hydrostatic pressure by 1.3-1.8 times, much less often by 2.0-2.2 times; in this case they usually do not reach the values of the geostatic pressure, which is applied by the weight of the overlying rocks. However, in the unique cases, there have been recorded at the great depths the AVPD, which are equal or exceed the values of the geostatic pressure, which is apparently caused by the action of the additional factors (for example, as the result of manifestation of earthquakes, mud volcanism, growth of the salt-dome structures). AVPD may be found within the USSR within the Volga-Urals, South Caspian, Dnieper-Donets, Western Siberia, Afghan-Tajik, North Carpathian, and other oil-gas-bearing basins; abroad within the basins of the Persian Gulf and the Gulf of Mexico, Sahara - Eastern Mediterranean, Central European, and others. ANPD may be induced artificially during the extraction of oil, gas, and water, if there does not occur the replenishment of the fluids, which are taken from the layer. The surface indicator of such decreasing of pressure is the subsidence of the earth surface. ANPD have been recorded within the USSR within the North Carpathian, Dnieper-Donets, North Black Sea, Irkutsk, and other oil-gas-bearing basins; they are known abroad within the basins of San Juan, pre-Appalachian, of Denver, and others.

The existence of AVPD beneficially influences the collector properties of the host rocks, increases the time of the natural operation of oil and gas fields without the usage of expensive secondary methods, increases the specific reserves of gas and yields of drilled wells, is favourable in relation to the preservation of accumulations of hydrocarbons, indicates the existence of isolated places and zones within the oil-gas-bearing basins. AVPD zones, which are developed at the great depths, especially where they enjoy regional distribution, contain significant resources of methane, which exists in dissolved state within the superheated (up to 150-200 degrees Celsius) water. It is possible to extract methane, and it as also possible to use the hydraulic and heat energy of water. On the other hand, AVPD are the the source of the accidents during the process of drilling. Unexpected unsealing of the AVPD zones is the cause of many complications, the mitigation of which leads to the high material costs. During the drilling within the AVPD zones, they thicken the drilling fluid in order to prevent the throw-outs from the wells. But the layers with hydrostatic pressure and ANPD can absorb such fluid. Because of this, before the unsealing of the rocks with AVPD, they isolate the overlying absorbing layers with the help of the drilling column. If the distribution of pressure within the rocks according to the depth is known, then it is possible to choose the optimal design of the drilled well, the technology of drilling and cementing, and to prevent the possible complications and accidents. The existence of the AVPD zones significantly increases the cost of the drilled wells. There are used for the forecasting of AVPD mainly the seismic exploration, data of the drilling, and various types of logging (electrical, acoustical, gamma ray, neutronic, and others).