Pore Pressure 6n4n2q

Pore Pressure M.V.V.S Anuprakash

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Table of contents

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Hydrostatic pressure Pore pressure Causes of abnormal pore pressure Abnormal pore pressure evaluation Mud logging methods Measurement while drilling & logging while drilling data • Direct measurements of pore pressure • Summary of pore pressure determination. 2

Overview What do you learn? • This unit will present the origins of pore pressure and principles its determination. • Hence the emphasis will be placed on the practical utilization of pore pressure in the well planning process. Why should we learn? • 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. Why do we require a planning? • 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 deg and selection of casing weights/grades is predominately dependent on the utilization 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. 3

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: Where HP = hydrostatic pressure g = gravitational acceleration ρf = average fluid density D = true vertical depth or height of the column •

Note: 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). 4

Hydrostatic pressure •• 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): • For the purposes of interpretation, all wellbore pressures, such as formation pressure, fracture pressure, fluid density and overburden pressure, are measured in of hydrostatic pressure. • When planning or drilling a well it is often more convenient to refer to hydrostatic pressures in 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. 5

Hydrostatic pressure • 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. 6

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: 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:

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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. 7

OVERBURDEN PRESSURE • A useful equation for calculating the overburden gradient under field conditions of varying lithological and pore fluid density is given by: σovg= 0.433[(1 – φ)ρma + (φxρf)] where σovg= overburden gradient, psi/ft φ= porosity expressed as a fraction ρf= formation fluid density, gm/cc ρma= matrix density, gm/cc • To convert densities from gm/cc to gradients in psi/ft simply use: Gradient (psi/ft) = 0.433 x (gm /cc) • • •

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.

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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 of pore and fracture pressures. • There are many techniques for the quantification of pore pressure and 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, as shown in Figure 1.1 b.

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Pore pressure

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Causes of abnormal pore pressure

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Abnormal pore pressure evaluation

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Mud logging methods

• Mud logging is carried out in a specially instrumented unit designed to: 1. Measure Drilling parameters (ROP,WOB, RPM, flow rate) 2. Measure properties of drill cuttings from samples collected at the shale shaker 3. Measure gas levels from well 4. Produce a lithological column as well is drilled 5. Determine where to drill to and when to stop 6. Give warnings of increasing pressures 7. Help determine accurately the depth of casing seats 14

RATE OF PENETRATION (ROP) • Drillbits break the rock by a combination of several processes including: compression • (weight-on-bit), shearing (rpm) and sometimes jetting action of the drilling fluid. • The speed of drilling is described as the rate of penetration (ROP) and is measured in ft/hr. • The rate of penetration is affected by numerous parameters namely: Weight On Bit (WOB); Revolutions Per Minute (RPM); bit type; bit wear; hydraulic efficiency; degree of overbalance; drilling fluid properties, hydrostatic pressure and hole size. • The difference between the mud hydrostatic pressure and pore pressure is called the overbalance or "Chip Hold Down Pressure (CHDP)". • This overbalance prevents formation fluids from entering the wellbore while drilling. However, this overbalance (CHDP) also acts to keep the rock cuttings held to the bottom of the wellbore. The effects of bit rotation and hydraulics offset this force and ensure that cuttings are lifted from the bottom of the hole.

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RATE OF PENETRATION (ROP) • The CHDP (differential force) has one of the largest effect on ROP especially in soft to medium strength formations. • If all parameters affecting ROP are held constant whilst drilling a uniform shale sequence then ROP should decrease with depth. This is due to the natural increased compaction with depth reflecting a decrease in porosity and increased shale density and increased shale (compressive) strength. • When entering an abnormally pressured shale, the drillbit sees a shale section which is undercompacted. The increased porosity and decreased density of the undercompacted section results in the formation becoming more ‘drillable’ as there is less rock matrix to remove. • Consequently ROP increases, assuming all drilling parameters were kept constant. • In addition, the reduced differential pressure (less CHDP) between the mud hydrostatic and pore pressure further increases ROP. 16

RATE OF PENETRATION (ROP) • The increase in ROP on entering an abnormally pressured zone is shown in Figure 1.7. As can be seen from this figure a sharp drillbit would pick up the onset of the transition zone much faster than a dull bit. • Careful monitoring of ROP during drilling is useful in detecting the onset of an abnormal pressure, however, the value of the ROP parameter on its own is limited as outlined below. Limitations of ROP • ROP is affected by changes in WOB, RPM, bit type, hole size, bit wear, hydraulics, mud parameters and lithology. • Hence, a sudden increase in ROP may not be due to penetrating an abnormal pressure zone, but may be caused by changes in any one of the above parameters. • Experimental work has shown that the effect of increasing pore pressure on ROP is limited to a differential pressure of 500 psi. • when the overbalance is greater than 500 psi, ROP shows little change over large changes in differential pressure.

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CORRECTED D EXPONENT •• From the previous section it is clear that a   method of correcting, or normalising ROP for changes in drilling parameters is desirable to make interpretation of drilling rate easier and improve its effectiveness as an indicator of pore pressure. • The D exponent is an example of one such ‘normalised’ drilling rate. The D exponent is the culmination of the work of Bingham (1965) and Jordan and Shirley (1967) 2. The authors provided the following mathematical expression for calculating the D exponent: 18

CORRECTED D EXPONENT

•   where d= D Exponent, in d-units ROP= penetration rate (ft/hr) RPM= rotary speed (rpm) WOB= weight on bit (lbs.) B= diameter of the bit, inch

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CORRECTED D EXPONENT • As can be seen from Equation (1.10), the D Exponent basically attempts to correct the ROP for changes in RPM, weight on bit and hole size. • The D exponent is proportional to rock strength and for normally pressured formations, the D exponent increases linearly with depth, reflecting increased rock strength with depth. • For abnormally pressured shales, the D exponent deviates from the normal trend and actually decreases with depth. • A further modification of the D exponent was proposed by Rehm et al (1971) 3 who attempted to correct the D Exponent for the effect of changes in mud weight. The resultant equation for the corrected D Exponent (dc) is as follows:

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where dc= corrected D Exponent (dimensionless) NPP= normal pore pressure gradient (ppg) ECD= equivalent circulating density (ppg) 20

CORRECTED D EXPONENT • The correction has no sound theoretical basis, however it makes the dc exponent more sensitive to changes in mud weight (therefore overbalance) and increasing pore pressure and is therefore universally used. • An idealised response of the dc exponent or abnormal pore pressure with depth is shown in Figure 1.8. • As the overpressured zone is encountered the formation becomes less dense and will have a higher porosity than a normally pressured rock at the same depth. • The decrease in rock strength results in a greater rate of penetration and hence a reduction in dc exponent.

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Measurement while drilling & logging while drilling data

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Direct measurements of pore pressure

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Summary of pore pressure determination.

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