Rock Pressure

a collective concept in rock geomechanics referring to the fields of force (states of stress) that form in the interior of the earth as a result of natural influences and production activity.

Gravity is the primary activator of rock pressure; additional activators, which differ in extent, duration, and strength of action, are geotectonic processes and production activity related to extracting minerals and building underground and aboveground structures. Continuously or discon-tinuously redistributed in rock masses because of various factors, rock pressure is capable of being destructive or useful (for example, making it easier to extract minerals). The phenomena caused by rock pressure (deformations, displacements and breaking up of elements of rock masses and of the earth’s surface, and interaction between elements of the rock masses and engineering structures, including the load on supports of underground workings) are known in engineering under the name “manifestations of rock pressure.” Expedient control of these phenomena is one of the most important practical tasks of mining production and mining science. Correct location, excavations, and support of underground workings are means to effective local control of rock pressure and are used extensively in mines (for example, to combat mining shocks resulting from digging out “protective” layers too quickly). The concept of rock pressure appeared in the 19th century. The theory of controlling rock pressure was discussed in the works of M. M. Protod’iakonov, Sr., P. M. Tsimbarevich, V. D. Slesarev, and numerous other Soviet investigators. In foreign countries significant research has been done by K. Bach (Germany), G. Spackeler (German Democratic Republic), R. Fenner (Chile), F. Mohr (Federal Republic of Germany) and H. Labasse (Belgium).

the pressure on the liquid (petroleum, water) and gas that saturate the pore space or cracks of reservoirs in oil and gas fields. Rock pressure is an important parameter characterizing the energy of oil-, gas-, and water-bearing beds. Before the exploitation of a pool begins, the rock pressure is in most cases roughly equivalent to the hydrostatic pressure, or the pressure of a column of water whose height is equal to the depth of the deposit. Rock pressure usually increases about 0.1 meganewton/m² for every 10 m of depth; however, there are many pools in which the initial rock pressure does not correspond to the hydrostatic pressure. The origin, variation, and state of rock pressure in oil and gas pools depend primarily on hydrostatic pressure, geostatic pressure (determined by the mass of the overlying rock), and geotectonic pressure (which forms in beds as a result of tectonic processes); the existence of dynamic pathways between beds with different pressure; the chemical interaction of water and rock; and the secondary phenomena of cementation of porous, permeable beds.

When wells are in operation, areas of reduced pressure form in the zone of the borehole bottoms. The pressure at the bottoms of flowing wells is called dynamic; if the well is not in operation, the pressure is called static. Rock pressure decreases during production unless measures are taken to maintain pressure. To compare the rock pressures at different points of a bed, the pressure is related to a certain conventional plane—usually the plane of the initial position of the water-oil contact in the bed. As the deposits are worked, the variations in rock are continuously recorded. It is consequently possible to assess the processes occurring in the bed and to regulate the extraction operations. Rock pressure is determined through measurements in the wells with depth manometers.