Pore Pressure
Monday, September 21, 2009
by ShoXee
INTRODUCTION This Topic will present the origins of pore pressure and principles its determination. It should be emphasized here that this subject alone requires more than one book to cover in detail. Hence the emphasis will be placed on the practical utilisation of pore pressure in the well planning process. It is hoped that the ideas presented here will help the engineer to better understand lithological columns and deduce potential hole problems before producing a final well plan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . All formations penetrated during the drilling of a well contain pressure which may vary in magnitude depending on depth, location and proximity to other structures. In order to understand the nature, extent and origin of formation pressures, it is necessary to define and explain basic wellbore pressure concepts. HYDROSTATIC PRESSURE Hydrostatic pressure is defined as the pressure exerted by a column of fluid. The pressure is a function of the average fluid density and the vertical height or depth of the fluid column.Mathematically, hydrostatic pressure is expressed as: POROSITY & PERMEABILITY Porosity is the total pore (void) space in a rock OVERBURDEN PRESSURE The overburden pressure is defined as the pressure exerted by the total weight of overlying formations above the point of interest. The total weight is the combined weight of both the formation solids (rock matrix) and formation fluids in the pore space. The density of the combined weight is referred to as the bulk density (ρb).The overburden pressure can therefore be expressed as the hydrostatic pressure exerted by all materials overlying the depth of interest: A list of typical matrix and fluid densities is included in Table 1.1 below: Substance Density (gm/cc) GENERATION OF OVERBURDEN VS. DEPTH GRAPH The calculation and compilation of the overburden gradient for a given field or area is the building block for a well plan. In addition, the overburden gradient is used in the analysis ofpore and fracture pressures.There are many techniques for the quantification of pore pressureand fracture pressure from drilling and petrophysical data which all require input of overburden gradient data. Figure 1.1 a shows a plot of bulk density vs. depth, which is generated from wireline logs. This figure can then be used to generate an overburden gradient vs. depth plot by merely applying Equation (1.4) at selected depths EFFECTS OF WATER DEPTH ON OVERBURDEN GRADIENT: In offshore operations, the depth of the sea (length of the water column) determines how much the overburden gradient is reduced. The reduction in overburden gradient is due to water being less dense than rock and for a given height; the hydrostatic head caused by water is less than that caused by any rock. The resultant effect is that as the water depth increases, the numerical value of the overburden gradient and in turn the fracture gradient reduce. Hence, offshore wells will have lower overburden gradient near the surface due to the influence of seawater and air gap and the uncompacted sediments. In onshore wells, the near surface overburden gradient is influenced mainly by the uncompacted surface sediments. MATRIX STRESS Matrix stress is defined as the stress under which the rock material is confined in a particular position in the earth’s crust. The matrix stress acts in all directions and is usually represented as a triaxial stress, using the Greek symbol , pronounced Sigma Pore pressure is defined as the pressure acting on the fluids in the pore spaces of the rock. This is the scientific meaning of what is generally referred to as formation (pore) pressure.Depending on the magnitude of pore pressure, it can be described as being either normal, abnormal or subnormal. A definition of each follows. NORMAL PORE PRESSURE Normal pore pressure is equal to the hydrostatic pressure of a column of formation fluid extending from the surface to the subsurface formation being considered In other words, if the formation was opened up and allowed to fill a column whose length is equal to the depth of the formation then the pressure at the bottom of the column will be equal to the formation pressure and the pressure at surface is equal to zero.Normal pore pressure is not a constant. The magnitude of normal pore pressure varies with the concentration of dissolved salts, type of fluid, gases present and temperature gradient. For example, as the concentration of dissolved salts increases the magnitude of normal pore pressure increases. ABNORMAL PORE PRESSURE Abnormal pore pressure is defined as any pore pressure that is greater than the hydrostatic pressure of the formation water occupying the pore space. Abnormal pressure is sometimes called overpressure or geopressure. Abnormal pressure can be thought of as being made up of a normal hydrostatic component plus an extra amount of pressure. This excess pressure is the reason why surface control equipment (e.g. BOPs) are required when drilling oil and gas wells. SUBNORMAL PORE PRESSURE Subnormal pore pressure is defined as any formation pressure that is less than the corresponding fluid hydrostatic pressure at a given depth.Subnormal pore pressures are encountered less frequently than abnormal pore pressures andare often developed long after the formation is deposited. Subnormal pressures may have natural causes related to the stratigraphic, tectonic and geochemical history of an area, or may have been caused artificially by the production of reservoir fluids. The Rough field in the Southern North Sea is an example of a depleted reservoir with a subnormal pressure. CAUSES OF ABNORMAL PORE PRESSURE Abnormal pore pressure is developed as a result of a combination of geological, geochemical, geophysical and mechanical process as will be discussed in the following paragraphs. These causes may be summarised under: • Depositional Effects
Knowledge of formation pressures is vital to the safe planning of a well. Accurate values of formation pressures are used to design safe mud weights to overcome fracturing the formation and prevent well kicks. The process of designing and selection of casing weights/grades is predominately dependent on the utilisation of accurate values of formation pressure. Cementing design, kick control, selection of wellhead and Xmas trees and even the rig rating are dependent on the formation pressures encountered in the well.
DEFINITIONS
HP = g x ρf x D where:
HP = hydrostatic pressure
g = gravitational acceleration
ρf = average fluid density
D = true vertical depth or height of the column
In field operations, the fluid density is usually expressed in pounds per gallon (ppg), psi per foot, pounds per cubic foot (ppf) or as specific gravity (SG).
In the Imperial system of units, when fluid density is expressed in ppg (pounds/gallon) and depth in feet, the hydrostatic pressure is expressed in psi (lb/in2):HP (psi) = 0.052 x ρf (ppg) x D (ft)
For the purposes of interpretation, all wellbore pressures, such as formation pressure, fracture pressure, fluid density and overburden pressure, are measured in terms of hydrostatic pressure.
When planning or drilling a well it is often more convenient to refer to hydrostatic pressures in terms of a pressure gradient. A pressure gradient is the rate of increase in pressure per unit vertical depth i.e., psi per foot (psi/ft). It should be noted that fluid densities, measured in ppg or SG, are also gradients. Hydrostatic pressures can easily be converted to equivalent mud weights and pressure gradients.Hydrostatic pressure gradient is given by:
HG = HP / D … (psi/ft) (1.3)
It is usual to convert wellbore pressures to gradients relative to a fixed datum, such as seabed, mean sea level or ground level. The resulting figure (pressure gradient) allows direct comparison of pore pressures, fracture pressures, overburden pressures, mud weights and Equivalent Circulating Density (ECD) on the same basis. In addition the use of pressure gradients accentuates variations in pressure regimes in a given area when values are plotted or tabulated.
When pressure gradients are used to express magnitudes of wellbore pressure, it is usual to record these as Equivalent Mud Weight (EMW) in ppg.
Permeability is the ease with which fluids can flow through the rock.
σov = 0.052 x ρb x D (1.4)
where
σov = overburden pressure (psi)
ρb = formation bulk density (ppg)
D = true vertical depth (ft)
And similarly as a gradient (EMW) in ppg:
(1.5
σovg = overburden gradient, ppg
ρb = formation bulk density (gm/cc)
(the factor 0.433 converts bulk density from gm/cc to psi/ft)
In a given area, the overburden gradient is not constant with depth due to variations in formation density. This results from variations in lithology and pore fluid densities. In addition the degree of compaction and thus formation density, increases with depth due to increasing overburden.
Note the densities in Equation (1.6) are expressed in gm /cc, instead of the usual units of ppg. With the exception of the oil industry, all other industries use the Metric system of units where density is usually expressed in gm/cc. The oil industry borrows many of its measurements from other industries
Sandstone 2.65
Limestone 2.71
Dolomite 2.87
Anhydrite 2.98
Halite 2.03
Gypsum 2.35
Clay 2.7 - 2.8
Freshwater 1.0
Seawater 1.03 - 1.06
Oil 0.6 - 0.7
Gas 0.15
To convert densities from gm/cc to gradients in psi/ft simply use:
Gradient (psi/ft) = 0.433 x (gm /cc) 
The vertical component of the matrix stress is that portion which acts in the same plane as the overburden load. The overburden load is supported at any depth by the vertical component of the rock matrix stress ( mat) and the pore pressure. This relationship is expressed as:
σov = Pf +σmat (1.9)
The above simple expression is used in many mathematical models to quantify the magnitudes of pore pressure using data from various drilling or petrophysical sources.
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PORE PRESSURE
Abnormal pore pressure can occur at any depth ranging from only a few hundred feet to depths exceeding 25,000 ft. The cause of abnormal pore pressure is attributed to a combination of various geological, geochemical, geothermal and mechanical changes. However for any abnormal pressure to develop there has to be an interruption to or disturbance of the normal compaction and de-watering process as will be outlined later in this topic
• Diagenetic Processes
• Tectonic Effects
• Structural Causes; and
• Thermodynamic Effects