Abstract
Earth stress at any point within a rock mass is made up of the geostatic pressure (or total stress) which is anisotropic and due to the weight of the overburden; the hydrostatic pressure which is isotropic and due to the fluidcontained in the interconnected pore spaces of the rock mass; and the effective stress between the solid particles of the rock mass. Effective stress is the numerical difference between the geostatic and hydrostatic pressures. Effective stress at any point controls the value of the shear strength of the rock at that point. The higher the effective stress, the higher the shear strength and vice versa. Both the geostatic and hydrostatic pressures depend on depth below ground surface such that geostatic/hydrostatic pressure gradients exist at any point below the ground surface. Abnormal pore pressure conditions occur in rock mass when the actual hydrostatic pressure is higher than the estimated value. This condition leads to reduced effective stress and hence reduced shear strength in reservoir rocks particularly, whenever in-situ stress (or applied stress) is greater than the shear strength of the rock mass. Rock mass failure may lead to jointing and faulting. Formation of normal faults is common in deltaic over-pressured environments such as the Gulf Coast of Mexico and the Niger Delta. This has led to the formation of associated geological structures such as growth faults, roll-over anticilines and sealing faults (with shale smears), which are traps for hydrocarbon accumulation.Natural fractures formed as joint sets in shales and carbonate rocks are also good reservoirs for hydrocarbon accumulation. Fracturing at depth can also be effected artificially by creating pressure gradients by means of fluidinjection. Problems associated with drilling oil and gas wells in over-pressured sedimentary basins include wellbore instability. Blowouts and lost circulation. These problems may be prevented by the use of blowout preventers and control of subsurface pressure by use of adequate mud viscosity during drilling operations.KEY WORDS: Geostatic pressure, hydrostatic pressure, pressure gradient, overpressure, faults, reservoirs and blowouts
Highlights
Earth stress or pressure at any point within a rock mass includes the geostatic pressure and hydrostatic pressure
Geostatic pressure at any point within the rock mass is pressure due to weight of the overburden which depends on density of the rock mass. (ρ), acceleration due to gravity (g) and depth (z) below ground surface
Σ = S – Pf - - - - - (3) Or S = σ + Pf - - - - - (4) The geostatic pressure, hydrostatic pressure and effective stress in a rock mass act simultaneously at a point and have the same units: MPa, kPa, kN/m2, psi etc. Since these pressures or stresses depend on the depth below the ground surface (z), and depth below ground surface, they increase with depth below ground surface; geostatic/hydrostatic pressure gradients exist for the stresses (Fig.1)
Summary
Earth stress or pressure at any point within a rock mass includes the geostatic pressure and hydrostatic pressure. Hydrostatic pressure (pore pressure or formation pressure) is pressure due to fluid contained in the interconnected pore spaces of the rock It depends on density of the pore fluid (ρf), acceleration due to gravity (g) and depth below the fluid surface. Σ = S – Pf - - - - - (3) Or S = σ + Pf - - - - - (4) The geostatic pressure, hydrostatic pressure and effective stress in a rock mass act simultaneously at a point and have the same units: MPa, kPa, kN/m2, psi etc Since these pressures or stresses depend on the depth below the ground surface (z), and depth below ground surface (zf), they increase with depth below ground surface; geostatic/hydrostatic pressure gradients exist for the stresses (Fig.).
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