pore oressure

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Welcome to P3Introduction___________________________________________

P3 software offers a new standard for 1d pore pressure prediction in deepwater trends.P3 works well for drilling groups doing well planning and for geophysical groups doingcalibration of 2d and 3d pressure prediction tools (e.g. Presgraf). P3 is new. It is not aderivative of Presgraf (also developed by this author).

P3 incorporates two new concepts: a gating-depth concept that defines a depth belowwhich pore pressure predictions become problematic; and, a relative pressure conceptthat describes pressure gradients (PPGo) as a function of depth below sea floor (DBSF).The last concept is key. Intrinsic here is the fundamental principle that water depth (WD)does not affect compaction or the magnitude of overpressure. Also intrinsic is the notionthat pore pressure gradient (PPG) as commonly defined from derrick floor does notdescribe overpressures if WD>0. Basically PPG is a measure of equivalent mud densitywhile PPGo is a measure of overpressures e.g. if PPGo is 1.6 specific gravity (13.34lbs/gal), pore pressure is 1.6 times higher than normal hydrostatic pressure for freshwater (8.335 lbs/gal). For inland wells PPG is approximately equal to PPGo.

A main component of P3 is a spreadsheet called the Stack. Data and pressure

predictions are stored in two partitions in the Stack. One partition (the master) storesdata referenced to DBSF. It includes formation pressure (FTo), velocity (VEL), leak-off pressure (LOTo), resistivity (RSH), mud density (MWo), overburden gradient (OBGo), andpredicted pressure gradient (PPGo). The second partition in the Stack stores a copy of pertinent data referenced to derrick floor (e.g. PPG). Values in the second partitionchange when the user changes water depth (WD) and/or air gap (AG).

The Stack is a powerful feature. It allows the user to compare predicted pore pressureand fracture gradient values to offset FT and LOT data regardless of WD or AG for thatdata, and it allows the user, after the fact, to project predictions to any water depth or righeight (i.e. any WD or AG).

Other features of P3 are:

-An improved dual-overburden Eaton prediction model.

- A bound-water and cation-exchange-capacity calculator.- An experimental resistivity method for estimating Eaton exponents.- A LAS log file reader with multiple shale picking options.- A user-defined equation tool for developing and testing models.- A customize option for the user to add variables to the Stack.- No black boxes. Equations are displayed for scrutiny or modification.

P3 is not comprehensive. It does not include correction functions for hydrocarbon, lateralpressure transfer, or true vertical depth (TVD) e.g. all data must be imported as TVD.

About P3___________________________________________ __

P3 is a Visual Basic application that runs on top of Microsoft Excel. The engines in Excelare used for calculations, plots, and data storage. System requirements are Excel 2000or higher. No Excel experience is required.

P3 is copyrighted. You may not distribute or create any derivative. For more informationcontact Martin Traugott ([email protected] or www.p3help.com).

This software is provided without warranty of any kind, expressed or implied. It isprovided for educational use only. It is not supplied to render an engineering service.The author is not liable for damages resulting from any use of this software.



About this Manual _______________________________ __

This user manual is intentionally concise. The Overview sets out basic P3 workflow.The Tutorial is an auto-instructional guide using examples from the North Sea, Gulf of Mexico, and Canada. The Stack is a glossary of variables in the Stack (study it in detail.)The Appendix is a display of all the menus and panels in P3.

References to menu selections are shown as File>New where New is an item in the File

menu. The nomenclature is simple. PPG, FPG, and OBG are pore pressure gradient,fracture pressure gradient and overburden gradient, respectively. A prefix (n) indicatesnormal compaction and a subscript (o) indicates a sea floor reference e.g. nPPGo is anormal hydrostatic pore pressure gradient referenced to the sea floor.

Background Information ___________________________ __

Pore pressure prediction methods are not new. Ham (1966) defined a depth of sealing(retention depth) below which pore pressure increases at a lithostatic rate. Eaton (1976)defined a power-law relationship between relative effective stress in mudstones andinterval velocity. Both are used in P3 including a resistivity variant of the Eaton method.

An issue with prediction methods is how and why they work, and why they sometimesgrossly under-estimate pore pressure. The picture that is emerging from researchassociated with the development of P3 is a bound water (BW) concept that describes, for mudstones, a retention depth (RD) below which BW has a density slightly less than bulkwater and a gating depth (GD) below which BW has a density sharply higher than bulkwater. Below the retention depth BW tends to behave as a liquid-solid that can support alithostatic load. Pore pressure tends to increase at a lithostatic rate (if not depleted bypermeable intervals) and velocity tends to respond predictably to effective stress. At thegating depth the BW layer condenses and permeability drops sharply. Below the GDexpulsion of dense BW can cause fluid expansion effects and can increase porepressure. Velocity, affected by BW compressibility and density, looses sensitivity toeffective stress below GD. Resistivity is unaffected by compressibility and density.

To understand acoustic prediction methods, consider this derivation of the Eatonequation based on Issler (1992) and Holbrook (1993). From Issler, (1-φ)/(1-φn) equals

(VEL/nVEL)1/2.19

where φn and nVEL are porosity and velocity for normal compaction.

From Holbrook, (OBGo-PPGo)/(nOBGo-nPPGo) equals ((1-φ)/(1-φn))9.348

for shales wherethe term (nOBGo-nPPGo) is effective stress if compaction is normal. Combination andrearrangement gives PPGo=OBGo-(nOBGo-nPPGo)(VEL/nVEL)

4.3. In P3, the exponent is

usually set to 3 above GD and 5 below GD. Note that keeping OBG and nOBG as twoseparate unequal variables is an important modification to the standard Eaton equation.

Overview_____________________________________________

To understand basic P3 workflow, work this example from the North Sea. Given a

prospect in 50 meters of water, predict pore pressure gradient (PPG) if seismic-derivedinterval velocity is 3000 m/sec at a depth of 4050 meters. Assume a rig height of 25meters.

- Select File>New to open a new project. Choose units. Use defaults for other options. - Enter 4050 in column InputD and 3000 in column InputV. - Select Stack>Add Input Data to Stack. Set data type and constants. WD=25, AG=0.- Select Predict>Pore Pressure. Predict PPGo with the acoustic model. Add to Stack.- Select Stack>Associate New WD and AG with Stack. Set AG to 25. - Select File>Export Data File. Select PPG. Read PPG (1.8 SI or 15 lbs/gal).

The Tutorial shows this example in more detail. (As a point of interest, a while-drillingcorrection to the seismic-derived prediction saved a £2 million well-control problem. A

more correct pre-drill PPG-derived seal analysis could have saved a £15 million dry hole.)



Tutorial_______________________________________________ Session 1 – Opening a New Project and Adding Data

Select File>New to open a new project. Choose Stack units and mode e.g. meters andkg/m

3. Set seawater and pore water density in specific gravity units. Ignore maximum

(Stack) depth; it will automatically adjust when needed.

To add data to a new project, enter depth in column InputD and the associated value (e.g.velocity) in column InputV, as shown below, and select Stack>Add Input Data to Stack.

Carefully set Z units, water depth, and air gap for the input data. A more common way toadd data to InputD and InputV is to import a P3 (or Presgraf) data file.



To create a P3 data file select File>New and click Cancel. (Cancel allows you to use thenew worksheet for data entry, not for a new P3 project.) Enter data. Add set-upinformation for WD, AG, and Units in format shown. Select the correct file extensionwhen saving (e.g. VEL for velocity). Select File>Close to return to P3 Stack.

Technically, for interval velocity data, you should enter a value in every layer in the Stack,e.g., you should enter a value at 3610, 3620, etc if granularity is 10 meters. As a shortcut, enter only the top and bottom, select Stack>Fill Gaps and/or Filter, and choose theVEL curve. The result is shown below. This function is also useful with bulk density data.



Session 2 – Predicting Pore Pressure

To predict pore pressure gradient, select Predict>Pore Pressure. As a best practiceselect the acoustic model and the 1d-Model to compare to as illustrated here. Theretention depth for this example is 1050m. Preview the results and select Add to theStack when satisfied with results. Move the cursor to a data point for the series output.

Kan and Swan (2001) suggest that seismic velocity data might have a variance of 10percent. To test the effect on PPG, change (VEL) to (0.9*VEL) as shown. The formula is

active and the plot will automatically refresh with the new results.



Session 3 - Adjusting Normal Compaction Trends

When predicting pressure gradients, evaluate the diagnostic plot that appears whenPredict is selected (as illustrated below). Sediment unloading causes normal compactedvelocities to be faster than expected and bulk density to be denser than expected. Theexample here likely shows the affect of ice unloading. (To display the data shown, importNorthSea.ITT and NorthSea.RHO. Set Stack units to SG.)

To adjust the acoustic or density normal compaction trend (solid red lines) select

Stack>Adjust Normal Trends. All plots change interactively as the sliders are moved.



Import pressure measurements (FT) if available. Use to verify pressure predictions asshown here. The FT data do not have to come from well locations with the same water depth or air gap - P3 corrects to a common sea floor datum. Note that the Compare to drop-down box does not list FT as an item unless FT data exist in the Stack.

Import mud weight data (MW) and use it too to validate predictions. For this North Seacase the acoustic model is predicting sharply higher than MW in the shallow section.Either the prediction is incorrect or the well was drilled underbalanced. Understand thatMW is not necessarily a good measure of actual gradients. In this North Sea example the

tight Tertiary section was likely drilled underbalanced while the deeper Jurassic sectionwas drilled overbalanced.



Also use the 1d Model to validate acoustic-derived predictions. A reasonable practice for the North Sea example is to add acoustic-derived PPG to the Stack for the depth rangebelow 2000 feet and to add the 1d model results for the depth range 0 to 2000 meters.

Session 4 – Predicting Fracture Pressure

To predict fracture pressure select Predict>Fracture Pressure after pore pressure hasbeen predicted. Select a stress ratio model. Import leak off test data, if available, anduse to validate results as show here for the North Sea example.



Session 5 – Working with Data Gaps

The following files (GOM2.ITT and GOM2.RHO) are from the Gulf of Mexico. The Stackunits and mode are feet and lbs/gal, respectively. Note that P3 automatically fills missingdata gaps between the first depth and last depth in the imported file for RHO data files.And note that the velocity gap above 5200 feet is particularly problematic e.g. should thenormal trend line be adjusted to the left to fit the data group around 6000 feet?

Add several files to the Stack for a more complete picture e.g. a second well (GOM1.ITT)

is added here for the depth range 0 to 5200. Now the normal trend looks acceptable.



Session 6 – Opening LAS Files and Picking Shale Points

Select File>Import Data File>Import LAS File to open a LAS file. Choose up to fivecurves from the suite of curves as illustrated below. Maximum and minimum values for each curve are listed for identification purposes. Select a shale discrimination curve (theneutron curve was picked here but generally a gamma ray curve is selected.) Adjust the(red) shale cut-off line with the sliders or use the Smart Cut-Off feature. Select ITT or RSH to save a file of shale value. InputV (red diamonds) is a preview of the shale picks.

Session 7 – Using User-Defined Equations

To use your own equations select Stack>User-Defined Equation. This function is usefulfor redefining variables in the Stack or for creating a new customized variable (e.g. thebottom Panel, where the name KT was added to blank cell Q5 in the Stack). ChooseTest, to display Min, Max and Avg values generated by the specified equation. (An error message appears if the equation fails.) This function is useful for diagnostics e.g. enter LOT and click Test to determine the maximum LOT value in the Stack. Max will be zero if the Stack contains no values. To save for use in other projects, select Add to Library.



Session 8 – Checking Pressure Predictions for Underprediction

Be aware that acoustic and other models can grossly underestimate pressuregradients if the gating depth (GD) has been exceeded. To check for an under-prediction scenario, examine RHOB data. If the top of overpressures occursbelow GD, bulk density will show little sensitivity to overpressures. This exampleis from Venture field, Offshore Nova Scotia (Mudford and Best, 1989) and the topof overpressures is at a depth of about 4500 meters.

The pressure prediction shown here (with a standard exponent of 3.0) illustrates theseverity of under prediction when compared to actual measured data.



Understand that comparison of FT derived PPG to acoustic-derived is not an absolutecheck. Because of lateral pressure transfer or hydrocarbon effects, PPG in reservoirscan be higher than that derived for adjacent mudstones. To test for these effects, predictPPG with a resistivity model as illustrated in this Panel. Clearly the resistivity - which isnot GD affected - confirms that the previous acoustic prediction is in error.

To correct the acoustic predition for GD affects, change the Eaton exponent to a value of 5.0 or, alternately, use the experimental method in P3 to derive the exponent. SelectPredict>Experimental Options as illustrated below. Basically, P3 compares VEL/nVELto cRSH/nRSH i.e. for the experimental method to have meaning the normal compaction

trend (nRSH) has to be adjusted correctly. Note that unlike nVEL, the default value for nRSH is almost never good enough.



Session 9 – Using Bound Water and Cation Exchange Capacity

To predict BW and CEC, import resistivity and slowness data for the same intervals andselect Predict>Bound Water and Cation Exchange Capacity. If CEC is fairly invariantwith depth, the whole interval has similar clay chemistry. If BW is less than about 0.4,there is no indication of a gating depth and pressure predictions should be dependable.(The top Panel is from the Gulf of Mexico example and the bottom from the previousCanadian example.)

If CEC varies and if BW is greater than about 0.6, pore pressure predictions are

problematic. BW values above 0.6 tend to indicate a gating depth. Factors that affect GDare temperature, iron content in the clays, porosity, and water chemistry.



Stack ___________________________________________ _

Select View>View Stack to display the Stack. For online documentation move the cursor to cells marked by a small red triangle in the upper right corner.

The variables in the master partition are described here. Some columns are stackablei.e. new data added to a cell are merged with old data. Other columns are not i.e. newdata replaces old. Stack granularity, units, seawater density, and maximum depth are setwhen a project is initialised (e.g. 10 meters, lbs/gal, 8.7, and 5000 meters, respectively).

DBSF (Depth Below Sea Floor) is sediment thickness. Sea floor is the absolute depthreference. If imported data have depths deeper than the maximum DBSF, the stack sizewill automatically increase.

PPGo is Pore Pressure Gradient referenced to sea floor. PPGo is a direct measure of overpressures (and equal to equivalent mud density for dual-density drilling systems). Topredict PPGo, select Predict>Pore Pressure. Choose a prediction model e.g. Acoustic,1d Model, Resistivity, or User Defined. Use Compare for comparison to MW or FT data.Note that this column is not stackable i.e. when new PPG o data are added to the Stack;all previous values are deleted for that specified depth range.

FPGo is Fracture Pressure Gradient reference to sea floor. To predict FPGo selectPredict>Fracture Pressure. Use Compare for comparison to LOT or OBG data. Thiscolumn is not stackable i.e. a new prediction (for the same interval) and deletes the lastprediction. You can, however, make predictions with different models for differentintervals.

nPPGo is normal (hydrostatic) pore pressure. The default is pore water density (set whenthe Stack is initialised). Edit as needed, e.g., set to seawater density near the seafloor.

OBGo is overburden pressure gradient. OBGo is the average bulk density, RHOB. (Seethe formula in the above Panel.)

nOBGo is overburden pressure gradient for normal compaction. This is a new termintroduced by Traugott and Swarbrick (2002). It is used in the Eaton equation as andoffers a modest improvement in pressure predictions. nOBGo is the average nRHOB.

RHOB is sediment bulk density. Be aware that every cell in the column must contain anon-zero value so that OBGo is correctly computed. The column is populated at

initialisation of the Stack with a default value of 1.955 SG at the sea floor and with anexponential increase to 2.7 SG at infinite depth. The default is for a sand/shale mix andno uplift. Use it with discretion. Import several density files to get a better statisticalrelation. Note that any gaps in the imported file will be linearly interpolated.

nRHOB is sediment bulk density for normal compaction. The default value is the sameas that for RHOB. To adjust interactively to fit data, select Stack>Set VEL, RSH, RHOBTrends. Edit nRHOB values where there is thick salt or carbonate sections.

FTo is formation pore pressure converted to a gradient with a DBSF reference. Thiscolumn is stackable. FT data also appear in column OverP where the integral componentis overpressure above normal hydrostatic, and the fractional component is WD/1000.Ignore column OverP. It is for administration use only.



MWo is mud density converted to a DBSF reference. Do not confuse with equivalent muddensity PPGo. This column is stackable such that mud data from several wells can bemerged into a composite file. P3 does not track which data comes from which well but aheader at the top of the Stack does keeps a chronology of imported files with theassociated WD of each individual file.

LOTo is leak off test data converted to a DBSF reference. To add LOT data, import aLOT data file or import a MW file with embedded LOT data. For MW files, P3 readspositive values as MW and negative values as LOT.

VEL is acoustic interval velocity. All acoustic data added to the Stack are converted tovelocity regardless if imported as slowness, interval velocity, or stacking velocity. Thiscolumn is stackable. When working with seismic data, a good practice is to importseveral records each side of the shot point of interest. (If at least one cell in the VELcolumn contains a non-zero value, pore pressure can be predicted.

nVEL is interval velocity for normal compaction i.e. normal pore pressure. The defaultvelocity at the seafloor (VELo) is given in cell AE4. Compaction rate (VCR) is given in cellAD4. There are three options for describing nVEL versus depth - linear, exponential andlogarithmic. Select Stack>Set VEL, RSH and RHOB Trends to interactively adjust VELo and VCR, or to change options.

RSH is resistivity in mudstones. RSH is affected strongly by cation exchange capacity,temperature and the thickness of bound water layer. RSH is more sensitive to effectivestress than bulk density or velocity. Resistivity is not affected by bulk (or BW) density.

cRSH is temperature corrected resistivity. See Traugott (1998) for more details on thecorrection procedure.

nRSH is corrected mudstone resistivity for normal compaction i.e. normal pressure. Thevalue at the seafloor (RSHo) is given in cell AH4. The compaction rate (RCR) is given incell AG4. Select Stack>Set VEL, RSH and RHOB Trends to adjust RSHo and RCR.

BHT is bottom-hole formation temperature. BHT is required for cRSH, CEC and BWcomputations. The default value tends to be satisfactory but import a BHT file for detailedresistivity-based projects.

GR is gamma ray. GR is not directly used in P3 but has application in user-definedequations such as determining stress ratios as a function of rock type.

X is the (Eaton) exponent in the acoustic method for predicting PPGo. The default is 3.0.Change to 5.0 below the gating depth. There is an experimental option to determine X if resistivity data is available.

k is horizontal to vertical effective stress ratio. The default value is a Gulf of Mexicodeepwater trend (Eaton and Eaton, 1997). There is an experimental option to determine

the stress ratio if LOT and FT data exist in the same layer in the Stack.

PHIsh is mudstone porosity. This column is automatically populated for cells where VELdata exists. The default equation is Issler (1992). PHIsh values are used only in the CECand BW prediction modules.

When a data point is added to a cell in a stackable column, the background colour indexes by one (from white to yellow to orange to red). The index is used to average thenext value e.g. if the index is 4, the next cell value will be (4*OLD+NEW)/5. Whendeleting stackable columns, use P3 clear functions, not Excel clear-contents.

The output columns in the Stack are described here. All except BW and CEC changeinteractively when AG and/or WD is changed. Do not directly edit these columns.



DEPTH is TVD below derrick floor. DEPTH equals DBSF+WD+AG. All plots in P3 are

plotted with DEPTH as the Y-axis. To plot data versus DBSF, switch off Y=Depth whenselecting View>Plot Selection.

PPG is PPGo converted to equivalent mud density using WD and AG to define mud

column height. Do not edit this column. It is linked to PPGo as shown above.

FPG is FPGo converted to equivalent mud density. The same comments as above apply

to FPGo. This above Panel FPG expressed as lbs/gal.

BW is bound water fraction. To predict BW, select Predict>Bound Water and CationExchange Capacity. Values are calculated for stack layers that with VEL and RSH data.

CEC is cation exchange capacity. CEC is computed as part of BW. Use CEC as a

diagnostic tool to detect anomalous clay types that affect pressure predictions.

Po is relative pore pressure. Po equals PPGo/nPPGo. Po is a dimensionless number

(sometimes called an abnormality factor) that defines the magnitude of overpressures e.g.in the above Panel, pore pressure is 1.52 higher than normal hydrostatic (for a columnheight equal to DBSF). PPG in contrast is only 1.26 times higher than normal hydrostatic.Basically PPG is equivalent mud density and Po (or PPGo) is relative pressure gradient.Ambiguity comes because PPG is commonly used as an indicator of overpressures.Clearly that works if WD=0 and AG<<DEPTH when PPG is about equal to PPGo.

nPPG is nPPGo converted to equivalent mud density. Low values near the sea floor arean artefact of AG (nPPG is zero at sea level).

FT is FTo converted to equivalent mud density. Use to test the validity of pressurepredictions. Note that centroid or hydrocarbon effects can cause gradients in mudstonesto be different than that in adjacent reservoirs.

MW is MWo converted to WD and AG conditions. Do not edit this column directly e.g. to

subtract an overbalance make the correction to MWo.

LOT is LOTo converted to the WD and AG conditions. Use to test the validity of fracture

pressure predictions.

User Space is for custom operations. For example, to create a column that containsslowness, type a descriptive name (e.g. ITT) in a blank column. Select Stack>User-Defined Equation. Enter IF(VEL>0,10^6/VEL. Select Apply and choose ITT. Moreblank columns can be added by right clicking a column header.



Appendix – Main Menu Bar, Panels and Dialogue Boxes



P3 References_______________________________________ __

Eaton, B.A., 1976, Graphical method predicts geopressures worldwide, World Oil , July1976, p. 100-104.

Eaton, B.A. and Eaton, T.L., 1997, Fracture gradient prediction for the new generation,World Oil , October Issue, p. 93-100.

Issler, D.R., 1992, A new approach to shale compaction and stratigraphic restoration,Beaufort-Mackenzie basin and Mackenzie corridor, northern Canada, The American Association of Petroleum Geologists Bulletin, v. 76, n. 8, p. 1170-1189.

Ham, H.H.,1966, A method of estimating formation pressures from Gulf Coast well logs,Transactions – Gulf Coast Association of Geological Societies, v. XVI, p. 185-187.

Holbrook, P., 1999, A simple closed form force balanced solution for pore pressureoverburden and the principal effective stresses in the Earth, Marine and PetroleumGeology , v. 16, p. 303-319.

Kan, T.K. and Swan, H.W., 2001, Geopressure prediction from automatically-derivedseismic velocities, Geophysics, v. 66, n. 6, November-December, p. 1937-1946.

Mudford, B. S. and Best, M.E., 1989, Venture Gas Field, Offshore Nova Scotia: CaseStudy of Overpressuring in Region of Low Sedimentation Rate, The American Associationof Petroleum Geologists Bulletin, v. 73, n. 11, November 1989, p. 1383-1395.

Traugott, M., 1997, Pore/fracture pressure determinations in deep water, Deepwater Technology supplement to World Oil , August 1997, p. 68-70.

Traugott, M.O. and Swarbrick. R.E. 2002, Dimensionless Gradients Applied to PorePressure Prediction – a New Standard, CSEG Recorder, September 2002, p. 79-80.

Copyright 2004, Martin Traugott

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