OA16230A - Apparatus and method for predicting vertical stress fields. - Google Patents

Abstract

A method of estimating at least one of stress and pore fluid pressure in an earth formation is disclosed. The method includes: discretizing a domain including at least a portion of the earth formation into a plurality of cells, each cell including a respective density value; dividing the domain into a first region and a second region, the first region including a surface of the earth formation; vertically integrating the respective density values in the first region; and estimating the total vertical stress for each cell in the first region and the second region by estimating a point load based on the respective density value.

Description

APPARATUS AND METHOD FOR PREDICTING VERTICAL STRESS FIELDS CROSS REFERENCE

This application claims priority to U.S. Non Provisional Patent Application Serial No. 12/568,094 entitled, APPARATUS AND METHOD FOR PREDICTING VERTICAL STRESS FIELDS, fïled September 28, 2009

BACKGROUND [0001] Détermination of pore fluid pressure is an important aspect of subterranean drillîng, exploration and completion operations. Détermination of pore fluid pressure is important in maintaining proper fluid pressures to maximize the efïectîveness of drillîng, production or other operations. For example, the drillîng fluid pressure applied by drillîng fluid pumped downhole through a drillstring must be sufficient to control hydrostatic pressure in a wellbore to prevent blowouts and maintain optimum drillîng rates.

[0002] Typically, the pore fluid pressure at a point in a formation has been calculated by considering a différence between total vertical and effective vertical stress at the point of interest. Conventionally, total vertical stress is estimated by vertical intégration of density data. On the other hand, there are different approaches for estimation of effective vertical stresses.

[0003] Total vertical stress distribution in the Earth may be affected by many factors including surface topology and density heterogeneities. The effect of these factors on total vertical stresses decays with depth below the surface or below the heterogeneity. For cxample, total vertical stresses arc signîficantly affected by topology dose to the surface, but with increasing depth, they approach the stress distribution for a horizontal surface at average élévation.

[0004] Conventionally used vertical intégration of density implicitly assumes that the gravitational load of an infinitésimal rock élément is completely transferred to the element below it. As a resuit of this assumption, the influence of the gravitational load of an element on the vertical stress distribution does not decay with depth but is transferred to ali the cléments below ît. Depending on the surface topography and density distribution, this assumption can resuit in overestimation or underestimation of the total vertical stresses and in

tum, overestir mation pore pressures derived from the total vertical stress

BRJEF DESCRIPTION [0005] Disclosed herein is a method of estimating at least one of stress and pore fluid pressure in an earth formation, including: discretizing a domain including at least a portion of the earth formation into a plurality of cells, each cell including a respective density value; dividing the domain into a first région and a second région, the first région including a surface of the earth formation; vertically intégrâting the respective density values in the first région; and estimating the total vertical stress for each cell în the first région and the second région by estimating a point load based on the respective density value.

[0006] Also disclosed herein is a System for estimating at least one of stress and pore fluid pressure in an earth formation, including: a downhole tool configured to bc disposed in a borehole in the earth formation; at least one sensor associated with the downhole tool configured to generate data relating to a density of the earth formation; a processor in opérable communication with the sensor, for receiving the data, the processor performîng: discretizing a domain including at least a portion of an earth formation into a plurality of cells, each cell including a respective density value; dividing the domain into a first région and a second région, the first région including a surface of the earth formation; vertically integrating the respective density values in the first région; and estimating the total vertical stress for each cell in the first région and the second région by estimating a point load based on the respective density value.

[0007] Further disclosed herein is a computer program product stored on machine readablc media for estimating at least one of stress and pore fluid pressure in an earth formation by executing machine implemented instructions, the instructions for: discretizing a domain including at least a portion of the earth formation into a plurality of cells, each cell including a respective density value; dividing the domain into a first région and a second région, the first région including a surface of the earth formation; vertically integrating the respective density values in the first région; and estimating the total vertical stress for each cell in the first région and the second région by estimating a point load based on the respective density value.

BRIEF DESCRIPTION OF THE DRAWINGS [0008] The following descriptions should not be considered limîting in any way. With référence to the accompanying drawîngs, like éléments are numbered alike:

[0009] FIG. I depicts an exemplary embodiment of a well drilling, production and/or logging System;

[00ΙΟ] FIG. 2 depicts a flow chart providing an exemplary method of predicting a force such as vertical stress and/or formation pore fluid pressure in an earth formation;

[0011 ] FIG. 3 depicts a cross-sectional view of a domain includîng an earth formation and an associated density data matrix;

[0012] FIG. 4 depicts a cross-sectional view of a domain and a cross-sectional représentation of an associated distributed surface load;

[0013] FIG. 5 depicts density data matrices of the domain of FIG. 4;

[0014] FIG. 6 depicts a cell of a two-dimensional density data matrix and an associated idealized point load; and [0015] FIG. 7 depicts a cell of a three-dimensional density data matrix and an associated idealized point load;

DETAILED DESCRIPTION [0016] A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures.

[0017] Referring to FIG. 1, an exemplary embodiment of a portion of a well drilling, production and/or logging System 10 includes a conduit or string 12, such as a drillstring or production string. The string 12 is configured to be disposed in a borehole 14 for performîng operations such as drilling the borehole 14, making measurements of properties of the borehole 14 and/or the surrounding formation downhole, and facilitating hydrocarbon production. As a matter of convention, a depth of the borehole 14 is described along a z-axis, while a cross-section is provided on a plane described by an x-axis and a y-axis.

[0018] In one example, the drill string 12 includes lengths of drill pipe or drill segments 16 which drive a drill bit 18. Drilling fluid 20 is pumped or otherwîse flows through the drill string 12 toward the drill bit 18, and exits into the borehole 14. The drilling fluid 20 (also referred to as “drilling mud”) generally includes a mixture of liquide such as water, drilling fluid, mud, oil, gases, and formation fluids as may be indigène us to the surroundings.

[0019] The string 12 may include equipment thercin such as a logging instrument or logging tool 22 for performing various measurements of the borehole, downhole components and/or the formation. In one embodiment, the logging tool 22 is configured as a “measurement while drilling” (MWD) or “logging while drilling” (LWD) tool. In another embodiment, the logging tool 22 is configured to be lowered into the borehole 14 after drilling, such as by a cable or wireline. Exemplary tools 22 include sensors for generating data such as resistivity, density, gamma ray, pressure, straîn and stress data. In one embodiment, the tool 22 is configured to collect and/or process data for predictîng or estimating a vertical stress field and/or pore fluid pressures of the formation.

[0020] The logging tool 22 includes at least one sensor 24 for sensing various characteristics of the borehole 14, the formation and/or downhole components. In one embodiment, the at least one sensor 24 is în communication with downhole electronîcs 26 that may receive input from the sensor 24 and provide for at least one of operational control and data analysis. The downhole electronîcs 26 may include, without limitation, a power supply, a transformer, a battery, a proccssor, memory, storage, at least onc communications interface and the like.

[0021] In one embodiment, the logging tool 22, sensor 24 and/or electronîcs 26 are operably coupled in communication with surface equipment 28. The surface equipment 28 may provide power to the tool 22 and/or other downhole components, as well as provide computing and processing capabilities for at least one of control of operations and analysis of data. A communications channel is included for communication with the surface equipment 28, and may operate via pulsed mud, wired pipe, and other technologies as are known in the art.

[0022] In one embodiment, the System 10 is operably connected to a downhole or surface processing unît, such as surface equipment 28, which may act to control various components of the System 10, such as drilling, logging and production components or subs. Other components include machinery to raise or lower segments and to operably couple segments, and transmission devices. The downhole or surface processing unit may also collect and process data generated by the System 10 during drilling, production or other operations.

[0023] FIG. 2 illustrâtes a method 40 of predictîng or estimating a force in an earth formation. Such a force includes stress and pressure, such as the vertical stress field and/or formation pore fluid pressures in an earth formation. Prédiction of the vertical stress field and/or formation pore fluid pressures includes estimating the total vertical stress of the formation. Tn this method, estimation of total vertical stress includes a calculation approach utilizing the Boussincsq's solution for a point load on a half space.

[0024] The method 40 includes onc or more stages 41-49. The method 40 is descrîbcd herein in conjunction with the system 10, although the method 40 may be performed in conjunction with any number and configuration of sensors, tools, processors or other machmery. The method 40 may be utîlized as a workflow or as part of one or more workflows, such as vertical stress, pore fluid pressure and horizontal stress estimation workflows. In one embodiment, the method 40 includes the execution of all of stages 41 -49 in the order described. However, certain stages may be omitted, stages may be added, or the order of the stages changed.

[0025] In the first stage 41, density data is calculated or measured at various depths (z-axis locations). In one embodiment, data is collected at each selected depth at a plurality of x-axîs and/or y-axis locations. The plurality of x-axîs and/or y-axis locations may correspond to sensor locations in one or more boreholes 14 and/or in a sensor array disposed with the tool 22. In onc embodiment, density data is estimated from data gencratcd by one or more gamma ray detectors. ' [0026] In the second stage 42, referring to FIG. 3, a région or domain 50 is selected that includes locations at which density data has been generated. In one embodiment, the domain 50 is a two-dimensional plane. In another embodiment, the domain 50 is a threedimensional région. For convention purposes, the domain 50 has a z-axis corresponding to a depth of the location, and an x-axis and/or y-axis that is orthogonal to the z-axis. In one embodiment, the domain 50 includes a surface région 52 including a surface topology 54 and a subterranean région 56.

[0027] In the third stage 43, the domain 50 is discretized into a plurality of cells forming a matrix. In one embodiment, the domain is two-dimensional and the cells are rcctangular cells having dimensions referred to as “Δχ” and “Δζ”. The matrix includes a number of rows “M” in the z-dîrection and a number of columns “N” in the x-direction.

[0028] In another embodiment, the domain 50 is three-dimensional and the cells are rectangular prism cells having dimensions “Δχ”, “Δγ”, and “Δζ”. In this embodiment, the matrix includes a number of rows “M” in the z-dîrectîon, a number of columns “N” in the xdirection and a number of columns “R” in the y-direction.

[0029] In the fourth stage 44, the cells are populated with density data. The domain is partitioned into two régions 58, 60 separated by a “half space” 62. For two-dimensional domains, the half space 62 is a line extending along the x-axis. For three-dimensional domains, the half space 62 is a plane extending along the x- and y-axes. Tn one embodiment, the half space 62 is positioncd at a sclcctcd depth relative to the surface topology 54. For examp le, the half space 62 is positioned at a depth corresponding to a lowest depth of the surface topology 54.

[0030] In one embodiment, for a two-dimensional domain, each cell thus includes a density value ρψ where “i” is a row number 1 through M and “j” is a column number 1 through N. The domain is sectioned into two régions 58 and 60, which are bounded by the half space line 62. A first région 58 includes rows above the half space 62, shown as rows 1 through p, and a second région 60 includes rows below the half space 62, shown as rows r through M.

[0031] In another embodiment, for a three-dimensional domain, each cell includes a density value pij.k, where “i” is a row number 1 through M, “j” is an x-axîs column number 1 through N and “k” is a y-axis column number 1 through R. The two régions 58, 60 are bounded by a half space plane 62, the first région 58 including rows 1 through p and the second région 60 including rows r through M.

[0032] In the fîfth stage 45, referring to FIGS. 4 and 5, the total vertical stress (i.e., overburden stress) in each column in the first région 58 (i.e., above the half space 62) is estimated or calculated, for example, by using vertical intégration of the density data. The total vertical stress data is stored and the total vertical stress corresponding to the depth of the half space 62 is applied on the second région 60 as a distributed surface load 64.

[0033] Implémentation of this stage is shown in FIG. 5. A new discrète domain 66 is created that includes rows p and r through M, and density values in the original domain 50 are transferred to the new discrète domain 66. Density values in the row above depth to half space (row p) are replaced by équivalent density values, p*.

[0034] In one embodiment, where the domain 50, 66 is two-dimensional, /j* is represented as “pi*”, where:

is the total vertical stress value calculated by vertical intégration for the first région in each cell (p,î) locatcd in the row p, “Az” is the vertical dimension of the cell, and N is the number of éléments “i” in the x-direction.

[0035] In another embodiment, where the domain 50, 66 is three-dimensional, p* is represented as “pij*”, where:

is the total vertical stress value calculated by vertical intégration for the first région 58 in each cell (p,i,j), and ‘W” and “7î” are the number éléments “i” and “j” in the xand y-directions, respectively.

[0036] In the sixth stage 46, the gravitational load in each cell is idealîzed as a point load acting at the bottom center of the cell. The point load location in each cell is exemplary, as other locations such as a center or top center location can be used.

[0037] In one embodîment, shown in Figure 6, the domain 66 is a two-dimensional domain having a plurality of two-dimensional (such as rectangular) cells 68. In this embodîment, the cell numbers “i” and “k” are the coordinates of each cell 68 in the xdirection and z-direction, respectively, and the density associatcd with the cell 68 is denoted as The induced total vertical stress, Δσ_ν, at each cell 68 is calculated for a selected point load location 70, designated (xi,Zk). In one embodîment, the total vertical stress Λσ_ν is calculated for a plurality of points (x_m,zi) relative to the selected point (xj,Zk) as:

>

where is the induced vertical stress at a point (x_m,zi), x_m and zi are the x- and zcoordinates of each of the plurality of points relative to the cell 68, and xi and Zk are the xand z-coordinates of the location of the idealîzed point load 70. δζ is equal to the différence between zi and zt, and δχ is equal to the différence between x_m and xj. Pk,i is a vertical point load value at the idealîzed point and may bc calculated as:

Ph ⁼ />V^x ·Δζ.

Δχ is a dimension of the cell along the x-axis, and Δζ is a dimension of the cell along the zaxis, [0038] In another embodîment, shown in FIG. 7, the domain 66 is a threedimensional domain. In this embodîment, the cell numbers “i”, “j” and “k” are the numbers in the x-dîrection, y-direction and z-direction, respectively, and the density associated with the cell 68 is denoted as “/?k,ij“ The induced total vertical stress, Δσ_ν, at each cell 68 is calculated for a selected point load location 70, designated (xj,yj,Zk). In one embodîment, the total vertical stress Δσ_ν is calculated for a plurality of points (x_m, y_n,zi) relative to the selected point (xj,yj,z_k) as:

whcrc Δσ(_ν>,„,/ is the induccd vertical stress at a point (x_m,y_n,Z|), x_m, y_n and zj arc the respective x-, y- and z-coordinates of each of the plurality of points relative to the cell 68, and Xi, yj and z_k are the x-, y- and z-coordinates of the location of the idealized point load 70. δζ is equal to the différence between Z] and z_k, δχ is equal to the différence between x_m and Xi, and Sy is equal to the différence between y_n and yj. P_k,ij is a vertical point load value at the idealized point 70 and may be calculated as:

Pk,i,j^pk,i.fEx-Ay -Δζ,

Δχ is a dimension of the cell along the x-axis, ûy is a dimension of the cell along the y-axis, and Δζ is a dimension of the cell along the z-axis.

[0039] In the seventh stage 47, the total vertical stress a_v at each cell 68 is calculated by summing ail of the induced vertical stresses The total vertical stress o_v for each cell 68 in the first région 58 is merged with the total vertical stress σ_ν for each cell in the second région 60 to form a complété total vertical stress field of the domain 50.

[0040] In the eighth stage 48, effective vertical stress is estîmated or calculated based on any suitable method or technique. For example, the effective vertical stress is calculated for a selected cell 68 from interval velocities based on empirical relationships that are calibrated against well-based data.

[0041] In the nînth stage 49, the pore fluid pressure is estîmated by subtracting the effective vertical stress from the total vertical stress. In one embodiment, the effective vertical stress for a selected cell 68 is subtracted from the total vertical stress a_v for the selected cell 68.

[0042] In addition, various other properties of the formation can be estimated using the vertical stress calculations described herein. For example, the estimated total vertical stress and pore fluid pressure are used to estimate horizontal stresses. Total maximum and total minimum horizontal stresses can be calculated as:

ESR(min) = (shmin - Pp) / (Sv - Pp),

ESR(max) = (sHmax - Pp) / (Sv - Pp), where ESR(min) is the effective stress ratio for minimum horizontal stress, ESR(max) is the effective stress ratio for maximum horizontal stress, Pp is the pore fluid pressure, Shmin is the total minimum horizontal stress, sHmax is the total maximum horizontal stress, and Sv is the total vertical stress.

[0043] As described herein, “drillstring” or “string” refers to any structure or carrier suitable for lowering a tool through a borehole or connecting a drill bit to the surface, and is not limited to the structure and configuration described herein. For example, the string 12 is configured as a hydrocarbon production string or formation évaluation string. The term carrier as used herein means any device, device component, combination of devices, media and/or member that may be used to convey, house, support or otherwise facilitate the use of another device, device component, combination of devices, media and/or member. Exemplary non-limitîng carriers include drill strings of the coiled tube type, of the jointed pipe type and any combination or portion thereof. Other carrier examples include casing pipes, wirclines, wireline sondes, slickline sondes, drop shots, downholc subs, BHA's and drill strings.

[0044] In support of the teachings herein, various analyses and/or analytical components may be used, including digital and/or analog Systems. The System may hâve components such as a processor, storage media, memory, input, output, communications link (wired, wireless, pulsed mud, optical or other), user interfaces, software programs, signal processors (digital or analog) and other such components (such as resîstors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well-appreciated in the art. It is considered that these teachings may be, but need not be, împlemented in conjunctîon with a set of computer exécutable instructions stored on a computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the présent invention. These instructions may provide for equîpment operation, control, data collection and analysis and other fùnctions deemed relevant by a System designer, owner, user or other such personnel, in addition to the fùnctions described in this disclosure.

[0045] The apparatuses and methods described herein provide various advantages over existing methods and devices, in that the apparatuses and methods produce described herein resuit in superior pore fluid pressure prédictions and improved calculation methods as compared to prior art techniques.

[0046] The pore pressure prédiction methods described herein provide an improvement in workflows pertaining to calculation of formation stresses, pressures and other properties. For cxample, the improvement in the total vertical stress, î.e. overburden stress, calculation provides an improvement in any related workflow. In addition, most of the currently used methods start with calculation of total vertical or overburden stress. Thus, the methods described herein improve not only pore pressure prédiction workflows but also other workflows such as estimation of total horizontal stresses.

[0047] Prior art methods calculate pore fluid pressure and generally estimate total vertical stress by vertical intégration of densîty data. Such intégration cannot capture the decay of the effect of topology and heterogeneities as a fonction of depth, and thus these prior art methods can lead to unrealistic pore fluid pressure prédictions and unrealistic input for borehole stabîlity prédictions during drilling and production. In contrast, the apparatuses and methods described herein produce résulte that reflect the decay with depth of the total vertical stress, and thus producc more accurate and realistic results.

[0048] One skilled in the art will recognize that the various components or technologies may provide certain necessary or bénéficiai functionality or features. Accordîngly, these fonctions and features as may be needed in support of the appended daims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed.

[0049] Whilc the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and équivalents may be substituted for éléments thereof without departing from the scope of the invention. In addition, many modifications will be appreciated by those skilled in the art to adapt a particular instrument, situation or materîal to the teachings of the invention without departing from the cssential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include ail embodiments falling within the scope of the appended claims.

Claims (20)

1. What is claimed is:

   1. A method of estimating at least one of stress and pore fluid pressure in an earth formation, comprising:

   discretizing a domain including at least a portion of the earth formation into a plurality of cells, each cell including a respective density value;

   dividing the domain into a fîrst région and a second région, the first région including a surface of the earth formation;

   vertically integrating the respective density values in the fîrst région; and estimating the total vertical stress for each cell in the fîrst région and the second région by estimating a point load based on the respective density value.
2. 2. The method of claim 1, further comprising estimating the respective density values, each of the respective density values being représentative of a selected location in the domain.
3. 3. The method of claim 1, wherein the stress încludes at least one of total vertical stress, total minimum horizontal stress and total maximum horizontal stress.
4. 4. The method of claim 1, wherein the first région and the second région are divîded by a half space that is positioncd at a selected depth relative to a topology of the surface.
5. 5. The method of claim 1, wherein the point load is located at a bottom center location of each cell.
6. 6. The method of claim 1, wherein the domain is two-dimensîonal and includes a z-axis corresponding to a depth of the earth formation and an x-axîs orthogonal to the z-axis, and the total vertical stress in each cell is calculatcd based on the following équation:

   wherein Δσ^χ/ is an induced vertical stress at a point (x_m, Z]) in the cell, x_ra and Zi are coordinatcs of at least one location on the x-axis and the z-axis respectively, x, and are coordînates of the point load on the x-axîs and the z-axis respectively, pk.^:. is the respective density value, δζ is equal to the différence between zi and z_k, δχ is equal to the différence between x_ra and x,, Pk,i is a vertical point load value represented by:

   Pk,i = Pk,r&x ·Δζ,

   Δχ is a dimension of the cell along the x-axis, and Δζ is a dimension of the cell along the zaxis.
7. 7. The method of claîm 1, wherein the domain is three-dimensional and includes a z-axîs corresponding to a depth of the earth formation, an x-axis orthogonal to the z-axis, and a y-axis orthogonal to the x-axis and the z-axîs, and the total vertical stress in each cell is calculated based on the following équation:

   < 0, z_K > z_t

   0, = z_t & Φ x_m

   -- y_rt =t ^& = Jj —----------------r- > Z_k k (δχ^£ -F &y² -f- δζ²) * >

   wherein Δσ(_ν>._η./ is an înduced vertical stress at a point (xm,y_n,zi) in the cell, x_m, y_n and z_t are coordinates of at least one location on the x-axis, the y-axis and the z-axis respectively, x,, yj and Zk are coordinates of the point load on the x-axis, the y-axis and the z-axis respectively, pk,ij is the respective density value, Ôz is equal to the différence between zi and Zk, δχ is equal to the différence between x_m and x„ 5y is equal to the différence between y_n and yj, Pkjj is a vertical point load value represented by:

   Pà.î,/ = Pu/Ax -Δγ-Δζ,

   Δχ is a dimension of the cell along the x-axis, Ay is a dimension of the cell along the y-axis, and Δζ is a dimension of the cell along the z-axis.
8. 8. The method of claîm 1, further comprising estimatîng an effective vertical stress for each cell.
9. 9. The method of claîm 8, further comprising estimatîng a pore fluid pressure for each cell based on the effective vertical stress and the total vertical stress.
10. 10. A System for estimating at least one of stress and pore fluid pressure in an earth formation, the System comprising:

    a downhole tool configured to be disposed in a borehole in the earth formation;

    at least one sensor associated with the downhole tool configured to generate data relating to a density of the earth formation;

    a processor in opérable communication with the sensor, for receiving the data, the processor performing;

    discretizing a domain including at least a portion of an earth formation into a plurality of cells, each cell including a respective density value;

    dividing the domain into a first région and a second région, the first région including a surface of the earth formation;

    vertically integratîng the respective density values in the first région; and estimating the total vertical stress for each cell in the first région and the second région by estimating a point load based on the respective density value.
11. 11. The System of claim 10, wherein the first région and the second région are dîvided by a half space that is positioned at a selected depth relative to a topology of the surface.
12. 12. The System of claim 10, wherein the domain is two-dimensîonal and includes a z-axis corresponding to a depth of the earth formation and an x-axis orthogonal to the zaxis, and the total vertical stress in each cell is calculated based on the following équation:

    / z% > Zi

    0, = z_l & Xj Φ x_m i Ρκ,ιΑ^ϊ ²R ~ ²l & ~ î^2PM ^δζ3 k π ' (δχ² + δζ²)² wherein Δσ^η,ι îs an induced vertical stress at a point (x_m, zi) in the cell, x_m and zi are coordinates of at least one location on the x-axis and the z-axis respectively, x, and z_k are coordinates of the point load on the x-axis and the z-axis respectively, />k,i îs the respective density value, δζ is equal to the différence between zi and z_k, δχ is equal to the différence between x_m and x,, Pk,i is a vertical point load value represented by;

    Pit.i = p*.ï-Ax ·Δζ,

    Δχ is a dimension of the cell along the x-axis, and Δζ is a dimension of the cell along the zaxis.
13. 13, The System of claim 10, wherein the domain is three-dîmensional and includes a z-axis corresponding to a depth of the earth formation, an x-axis orthogonal to the z-axis, and a y-axis orthogonal to the x-axis and the z-axis, and the total vertical stress in each cell is calculated based on the following équation:

    0, z_k = zj & Χί Φ x_m

    0, 2_k = z_t & Φ yj

    Ζχ = z_i & Xj = x_m & y_n = y- ’ δζ³

    Il t — - I ' < + Sy² + δζ²')^ wherein Δσ(_ν>,_π,/ is an induced vertical stress at a point (x_m,y_n,zi) in the cell, x_m, y_n and zi are coordinates of at least one location on the x-axis, the y-axis and the z-axis respectively, xj, yj and zr are coordinates of the point load on the x-axis, the y-axis and the z-axis respectively, Pk,ij is the respective density value, δζ is equal to the différence between zj and zt, δχ is equal to the différence between x_m and xj, ôy is equal to the différence between y_n and yj, Pk.ij is a vertical point load value represented by;

    Pk.ij=pk.i./àx Ay ·Δζ,

    Δχ is a dimension of the cell along the x-axis, Ay is a dimension of the cell along the y-axîs, and Δζ is a dimension of the cell along the z-axis.
14. 14. The System of claim 10, forther comprising estimating an effective vertical stress for each cell.
15. 15. The System of claim 14, fiirther comprising estimating a pore fluîd pressure for each cell based on the effective vertical stress and the total vertical stress.
16. 16. A computer program product stored on machine rcadable média for estimating at least one of stress and pore fluid pressure in an earth formation by executing machine împlemented instructions, the instructions for:

    discretizing a domain including at least a portion of the earth formation into a pluralîty of cells, each cell încludîng a respective density value;

    dividing the domain into a first région and a second région, the first région including a surface of the earth formation;

    vertically integrating the respective density values in the first région; and estimating the total vertical stress for each cell in the first région and the second région by estimating a point load based on the respective density value.
17. 17. The computer program product of claim 16, wherein the first région and the second région are divided by a half space that is positioned at a selected depth relative to a topology of the surface.
18. 18. The computer program product of claim 16, wherein the point load is located at a bottom center location of cach cell.

    i

    I i j

    J
19. 19. The computer program product of claim 16, wherein the domain is twodimensional and includes a z-axîs corresponding to a depth of the earth formation and an xaxis orthogonal to the z-axis, and the total vertical stress in each cell is calculated based on the following équation:

    wherein is an induced vertical stress at a point (x_m, z_t) in the cell, x_m and zj are coordinates of at least one location on the x-axis and the z-axis respectively, x; and Zk are coordinates of the point load on the x-axis and the z-axis respectively, pt.i is the respective density value, δζ is equal to the différence between zj and Zk, Ôx is equal to the différence between x_m and x_;, Pk,i is a vertical point load value represented by:

    Pk.i = pk.i'&x -Az,

    Δχ is a dimension of the cell along the x-axis, and Δζ is a dimension of the cell along the zaxis.
20. 20. The computer program product of claim 16, wherein the domain is threedimensional and includes a z-axis corresponding to a depth of the earth formation, an x-axis orthogonal to the z-axis, and a y-axis orthogonal to the x-axis and the z-axis, and the total vertical stress in each cell is calculated based on the following équation:

    wherein &O(_V)_m,_n,i is an induced vertical stress at a point (x_m,y_n,Z[) in the cell, x_m, y_n and zi are coordînates of at least one location on the x-axis, the y-axis and the z-axis respectivcly, Xi, yj and Zk are coordînates of the point load on the x-axis, the y-axis and the z-axis respectîvely, Pk,ij is the respective density value, Ôz is equal to the différence between zj and zt, δχ is equal to the différence between x_m and Xj, δ y is equal to the différence between y_n and yj, Pk.îj is a vertical point load value represented by:

    Pk,i,j = pk,i,jAx Ay ·Δζ,

    Δχ is a dimension of the cell along the x-axis, Ay is a dimension of the cell along the y-axis, and Δζ is a dimension of the cell along the z-axis.