Highly pressured formations have caused severe drilling and completion problems in almost every area in the world. Failure to control these high pressures can cause an uncontrolled flow of formation fluids (blowout), which can result in extreme financial losses for the operator, possible pollution of the environment, loss of petroleum reserves and potentially unsafe conditions for workers.
Subsurface pressures are result of gravitational forces acting on oveerlying formations and fluids. This is similar to what occurs in the atmosphere, whcih the accumulated weight of the atmosphere causes the air pressure at sea level to be about 14.7 psi.
Overburden is the volume and weight of all formations and fluids above a given formation. The total stress imposed by the overburden on a subsurface formation is called the geostatic, lithostatic or total overburden pressure.
In addition to saying that, Overburden pressure is equal to the total pressure from the weight of sediments plus the pressure from the weight of the fluids that exist aboe a particular formation and which must be mechanically supposed by the formation.
Since sediments bulk densities vary from area to area and with depth is usually taken as 144 lb/ft3 or specific gravity 2.3; therefore, the geostatic or overburden gradient is 1 psi/ft. The overburden pressure gradient varies, depending on the formation density, percent pore space and pore fluid density. These variables are dependent on historical geological conditions, such as chemical composition and distance of transport of the sediments.
Total overburden pressure is supported by the rock in two ways. The first is through intergranular pressure, a matrix stress caused by the force transmitted through grain-to-grain mechanical contact. As formations are compacted by the overburden with increasing burial depth, pore water escapes so that pore pressure is equal to the hydrostatic pressure of the pore water density.
Pore pressure is the pressure of the formation fluids (water, oil and gas) which must be balanced with mud weight. Therefore the total overburden pressure is equal to the sum of the intergranular pressure and the pore pressure.
Hydrostatic pressure (PHYD) is the pressure that is caused by the vertical height of a column of fluid. Hydrostatic pressure is independent of aerial size and shape of the fluid body; pressure at any depth is equal in all directions. The pore pressure gradient is affected by the concentration of salt in the fluid density of the column. So, depending on the salinity of the water in the depositional environment of the particular geological region, normal pore pressure will have different values.
A “normally” pressured formation has a pore pressure equal to the hydrostatic pressure of the pore water. Since many more wells are drilled in sediments characterized by 8.95-lb/gal saltwater, a “normal” pressure gradient, for the purposes of this discussion, is considered to be 0.465 psi/ft. Any deviation from the normal hydrostatic pressure environment is referred to as abnormal. High pressures are called geopressures, overpressures or sur-pressures. Low pressures are called underpressures or subpressures.
Abnormal pressure is caused by the geological processes that have occurred in a given geological area and involves both physical and chemical actions within the earth. Pressures that are lower than what
is considered normal can be detrimental and problematic to the drilling process. Conversely, abnormally high pressures are common and can cause severe drilling problems.
Abnormal overpressures are always caused by a particular zone becoming “sealed” or isolated. Seals are impermeable layers and boundary zones that will not permit the release of pressure generated by the percolation of fluids and gases to higher zones and subsequently to the surface. These seals may consist of many rock types: dense shales, calcareous shales, cemented limestone, calcareously cemented sandstone solidified volcanic ash (tuff), anhydrite and/or others.
Subnormal (low) pressures are encountered in zones with pore pressures lower than the normal hydrostatic pressure. Severe lost-circulation problems may occur in these zones when muds are used in drilling. Subnormal pressure conditions often occur when the surface elevation of a well is much higher than the subsurface water table or sea level. The most common example occurs when drilling in hilly or mountainous locations.
One cause of abnormally low pressures is depleted sands. These sands whose original pressure has been depleted or drained away. Depleted sands are found most frequently in reservoirs from which oil and gas have been produced, a common phenomenon in many so-called “mature” oil and gas areas.