BACKGROUND

Certain substances (e.g., salt, etc.) have low permeability, i.e., do not readily transmit fluids. In some cases, a low permeability substance may be located above a substance that is targeted by a drilling operation.

FIG. 1 shows an exemplary diagram of a drilling operation. Those skilled in the art will appreciate that the drilling operation shown in FIG. 1 is provided for exemplary purposes only and accordingly should not be construed as limiting the scopeof the invention. For example, the drilling operation shown in FIG. 1 is a seafloor drilling operation, but the drilling operation may alternatively be a land drilling operation.

As shown in FIG. 1, a drilling rig (105) is configured to drill into a formation (e.g., a formation below a seafloor (110)) using a drill bit (not shown) coupled to the distal end of a drill string (125). Specifically, the drill bit is used todrill a borehole (130) extending to a target substance (120). The target substance (120) may be hydrocarbon or a mineral resource targeted by a drilling operation. Further, as shown in FIG. 1, a low permeability substance (115) may be located above thetarget substance (120). Thus, to reach the target substance (120), the borehole (130) may be required to pass through the low permeability substance (115).

When sediment of low permeability substance is buried or compacted, fluid may be trapped in pores within the resulting structure (i.e., within the low permeability substance itself and/or within substances beneath the low permeability substance(e.g., sand, etc.). Fluid trapped in this manner exerts pressure on the surrounding formation referred to as pore pressure. Formations in which pore pressure exceeds hydrostatic pressure at a given depth are referred to as overpressured.

When drilling in an overpressured formation, the mud weight (i.e., the weight of drilling fluids transmitted to the borehole) must be high enough to prevent the pore pressure from moving formation fluids into the borehole. In the worst case,formation fluids entering a borehole may result in loss of the well and/or injury to personnel operating the drilling rig. Accordingly, for safe and economic drilling, it is essential that the pore pressure be predicted with sufficient accuracy. Inparticular, it is beneficial to predict pore pressure pre-drill, i.e., either before any drilling has commenced and/or at a location that the drill bit has not yet reached.

Conventionally, pre-drill pore pressure prediction is based on the use of pre-drill seismic velocities and a velocity-to-pore pressure transform calibrated using offset well data (i.e., data from other wells near the drilling site). However, insome cases (e.g., when drilling under salt), conventional pre-drill pore pressure predictions may not be sufficiently accurate. Further discussion of conventional pre-drill pore pressure prediction techniques can be found in Sayers C M, Johnson G M, andDenyer G., 2002, "Pre-drill Pore Pressure Prediction Using Seismic Data," Geophysics, 67, 1286-1292.

Mud is used in drilling operations to cool the drill bit, to transport cuttings generated by the drilling operation to the surface, to prevent the influx of formation fluids into the borehole, and to stabilize the borehole. With respect topreventing the influx of formation fluids, the drilling operator must maintain the mud weight at or above the pore pressure. With respect to stabilizing the borehole, drilling operators adjust the mud weight (i.e., the density of the mud being used) tocounter the tendency of the borehole to cave in. However, the drilling operator must be careful not to fracture the formation by using an excessively high mud weight.

Moreover, too high a mud weight may result in an unacceptably low drilling rate. Accordingly, the mud weight must be low enough to maintain an acceptable drilling rate and avoid fracturing the formation. In such cases, the allowable mud weightwindow (i.e., the range of allowable mud weights) may be small when drilling in overpressured formations. Specifically, the force exerted by the mud must fall within the range between the pore pressure (or the pressure to prevent a cave in, if higherthan the pore pressure) and the pressure required to fracture the formation.

Further, when drilling in overpressured formations, the number of required casing strings (i.e., structural supports inserted into the borehole) may be increased. Specifically, if a sufficiently accurate pre-drill pore pressure prediction is notavailable, additional casing strings may be inserted prematurely, to avoid the possibility of well control problems (e.g., influx of formation fluids) and/or borehole failure. Prematurely inserting casing strings may delay the drilling operation and/orreduce the size of the borehole and result in financial loss.

SUMMARY

In general, in one aspect, the invention relates to a method for pore pressure prediction. The method includes obtaining a stress sensitivity coefficient, obtaining a compressional wave (P-wave) velocity and a shear wave (S-wave) velocity for apre-drill location, obtaining a first predicted pore pressure, iteratively performing: calculating a total stress value associated with the pre-drill location using the first predicted pore pressure associated with the pre-drill location; and calculatinga second predicted pore pressure associated with the pre-drill location using a stress-pressure relationship equation, a stress-velocity relationship equation, the stress sensitivity coefficient, a reference location, and at least one selected from agroup consisting of the P-wave velocity and the S-wave velocity for the pre-drill location, wherein iterating is complete if a difference between the first predicted pore pressure and the second predicted pore pressure is less than or equal to athreshold difference value, and wherein the first predicted pore pressure is updated using the second predicted pore pressure if the difference is greater than the threshold difference value, and adjusting a drilling operation associated with thepre-drill location, based on the second predicted pore pressure.

In general, in one aspect, the invention relates to a computer readable medium comprising instructions to perform a method for pore pressure prediction. The method includes obtaining a stress sensitivity coefficient, obtaining a compressionalwave (P-wave) velocity and a shear wave (S-wave) velocity for a pre-drill location, obtaining a first predicted pore pressure, iteratively performing: calculating a total stress value associated with the pre-drill location using the first predicted porepressure associated with the pre-drill location; and calculating a second predicted pore pressure associated with the pre-drill location using a stress-pressure relationship equation, a stress-velocity relationship equation, the stress sensitivitycoefficient, a reference location, and at least one selected from a group consisting of the P-wave velocity and the S-wave velocity for the pre-drill location, wherein iterating is complete if a difference between the first predicted pore pressure andthe second predicted pore pressure is less than or equal to a threshold difference value, and wherein the first predicted pore pressure is updated using the second predicted pore pressure if the difference is greater than the threshold difference value,and adjusting a drilling operation associated with the pre-drill location, based on the second predicted pore pressure.

In general, in one aspect, the invention relates to a downhole tool configured to perform a method for pore pressure prediction. The method includes obtaining a stress sensitivity coefficient, obtaining a compressional wave (P-wave) velocity anda shear wave (S-wave) velocity for a pre-drill location, obtaining a first predicted pore pressure, iteratively performing: calculating a total stress value associated with the pre-drill location using the first predicted pore pressure associated withthe pre-drill location; and calculating a second predicted pore pressure associated with the pre-drill location using a stress-pressure relationship equation, a stress-velocity relationship equation, the stress sensitivity coefficient, a referencelocation, and at least one selected from a group consisting of the P-wave velocity and the S-wave velocity for the pre-drill location, wherein iterating is complete if a difference between the first predicted pore pressure and the second predicted porepressure is less than or equal to a threshold difference value, and wherein the first predicted pore pressure is updated using the second predicted pore pressure if the difference is greater than the threshold difference value, and adjusting a drillingoperation associated with the pre-drill location, based on the second predicted pore pressure.

Other aspects of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary diagram of a drilling operation.

FIG. 2 shows a diagram of a system in accordance with one or more embodiments of the invention.

FIG. 3 shows a flowchart in accordance with one or more embodiments of the invention.

FIG. 4 shows a diagram of a computer system in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency.

In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the artthat the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.

In general, embodiments of the invention provide a method and system for pre-drill pore pressure prediction. Specifically, pre-drill pore pressure is predicted using reference data, acoustic or seismic wave velocities associated with a pre-drilllocation, at least one stress-velocity relationship equation, and a stress-pressure relationship equation. In one or more embodiments of the invention, the pore pressure is iteratively predicted until a threshold condition is satisfied. A drillingoperation associated with the pre-drill location is adjusted based on the predicted pore pressure.

FIG. 2 shows a diagram of a system in accordance with one or more embodiments of the invention. Specifically, FIG. 2 shows a diagram of a computing environment (205) in accordance with one or more embodiments of the invention.

In one or more embodiments of the invention, the computing environment (205) may include one or more computer systems (e.g., computer system A (210), computer system N (215)) configured to perform drilling-related tasks. In one or moreembodiments of the invention, the computer system(s) (e.g., 210, 215) may be web servers, embedded systems (e.g., a computer located in a downhole tool), desktop computers, laptop computers, personal digital assistants, any other similar type of computersystem, or any combination thereof.

Specifically, in one or more embodiments of the invention, one or more of the computer systems (e.g., 210, 215) may include a stress sensitivity coefficient calculator (220), a total stress calculator (230), and/or a pore pressure calculator(235). In one or more embodiments of the invention, the aforementioned components (i.e., 220, 230, 235) may be located in a single computer system (e.g., 210, 215), distributed across multiple computer systems (e.g., 210, 215), or any combinationthereof. In one or more embodiments of the invention, each of the aforementioned components (i.e., 220, 230, 235) may include one or more software modules, one or more hardware modules, or any combination thereof. Further, in one or more embodiments ofthe invention, the aforementioned components (i.e., 220, 230, 235) may be configured to communicate with each other via function calls, application program interfaces (APIs), a network protocol (i.e., a wired or wireless network protocol), electroniccircuitry, any other similar type of communication and/or communication protocol, or any combination thereof.

In one or more embodiments of the invention, the stress sensitivity coefficient calculator (220) may be configured to calculate at least one stress sensitivity coefficient using reference data. Calculating stress sensitivity coefficients usingreference data is discussed below with respect to FIG. 3. In one or more embodiments of the invention, the total stress calculator (230) may be configured to calculate a total stress value associated with a pre-drill location using a predicted porepressure. Calculating a total stress value is discussed below with respect to FIG. 3.

In one or more embodiments of the invention, the pore pressure calculator (235) may be configured to calculate a predicted pore pressure using the total stress value calculated by the total stress calculator. Further, in one or more embodimentsof the invention, calculating the predicted pore pressure may involve using the stress sensitivity coefficient(s) calculated by the stress sensitivity coefficient calculator (220). Calculating a predicted pore pressure is discussed below with respect toFIG. 3.

Further, in one or more embodiments of the invention, once the pore pressure calculator (235) has calculated a predicted pore pressure, the total stress calculator (230) may use the predicted pore pressure to re-calculate the total stress value. The re-calculated total stress value may then be used by the pore pressure calculator (235) to re-calculate the predicted pore pressure. In one or more embodiments of the invention, the pore pressure calculator (235) and total stress calculator (230)may be configured to re-calculate the total stress value and the predicted pore pressure iteratively, until the predicted pore pressure meets a threshold condition. Iterative re-calculation of a total stress value and predicted pore pressure isdiscussed below with respect to FIG. 3.

FIG. 3 shows a flowchart in accordance with one or more embodiments of the invention. Specifically, FIG. 3 shows a flowchart of a method for pre-drill pore pressure prediction in accordance with one or more embodiments of the invention.

In one embodiment of the invention, a pre-drill location corresponds to a location that has not yet been drilled. Said another way, the drill bit has not reached the pre-drill location, however, the pre-drill location is in the intended path ofthe drill bit and, unless the trajectory of the borehole changes, the drill bit will eventually reach the pre-drill location. In one embodiment of the invention, the method described in FIG. 3 may be performed while drilling, prior to drilling (i.e.,prior to commencing the drilling of the borehole), or any combination thereof.

Turning to FIG. 3, one or more stress sensitivity coefficients may be calculated using reference data (Step 305). Specifically, in one or more embodiments of the invention, the reference data may include values associated with one or morereference locations. In one or more embodiments of the invention, the reference locations may include locations in one or more offset wells, simulated geologic formations, any other similar type of location, or any combination thereof.

In one or more embodiments of the invention, the reference data may include, but are not limited to total stress, effective stress, P-wave velocity, S-wave velocity, and pore pressure. The reference data may be obtained using sonic logmeasurements, ultrasonic measurements in a laboratory (see e.g., Winkler, K. W. & Xingzhou, L., 1996, "Measurements of third-order Elastic Constants in Rocks" J. Acoust. Soc. Am., Vol. 100, pp. 1392-1398), borehole seismic measurements, a verticalseismic profile (VSP), a computer simulation, any other method, or any combination thereof.

For example, the pore pressure at a location in an offset well may be obtained using the Modular Formation Dynamics Tester (MDT) tool or the Repeat Formation Tester (RFT) tool. Alternatively, the pore pressure may be obtained using a numericalmodel such as the one described in Malinverno, A. et al., 2004, "Integrating Diverse Measurements to Predict Pore Pressure with Uncertainties While Drilling" SPE Paper 90001.

Further, the total stress at the reference location may be obtained using a stress model. The following references describe various stress models that may be used with the invention: Lee, D., et al., 2003, "Using a Dynamic Mechanical Earth Modeland Integrated Drilling Team to Reduce Well Costs and Drilling Risks in San Martin Field" SPE Paper No. 84557 and Moos, D & Castillo, D. A., 2000, "Reservoir Geomechanics Applied to Drilling and Completions Programs in challenging formations: North Westshelf Timor Sea, North Sea and Columbia," APPEA Journal, February 16, pp. 509-521.

In one or more embodiments of the invention, the stress sensitivity coefficient(s) calculated in Step 305 may be coefficients that relate compressional wave (P-wave) and shear wave (S-wave) velocities associated with a location in a formation toeffective stresses at the location. Accordingly, in one or more embodiments of the invention, the stress sensitivity coefficient(s) may be calculated using one or more stress-velocity relationship equations. The stress-velocity relationship equation(s)may be first-order, second-order, n-order (i.e., where n is any positive integer), or any combination thereof.

In one embodiment of the invention, the stress-velocity relationship equation(s) may include one or more of the following equations: .nu..sub.P(.sigma..sub.V,.sigma..sub.h)=.nu..sub.P.sup.(0)+.alpha..sub.P.-sup.V.DELTA..sigma..sub.V+.alpha..sub.P.sup.h.DELTA..sigma..sub.h (1) .nu..sub.S(.sigma..sub.V,.sigma..sub.h)=.nu..sub.S.sup.(0)+.alpha..sub.S.- sup.V.DELTA..sigma..sub.V+.alpha..sub.S.sup.h.DELTA..sigma..sub.h, (2) where .alpha..sub.P.sup.V is avertical compressional stress sensitivity coefficient, .alpha..sub.P.sup.h is a horizontal compressional stress sensitivity coefficient, .alpha..sub.S.sup.V is a vertical shear stress sensitivity coefficient, and .alpha..sub.S.sup.h is a horizontal shearstress sensitivity coefficient.

.alpha..sub.P.sup.V is a coefficient that relates the change in the compressional wave (P-wave) velocity to the change in .sigma..sub.V (i.e., the effective vertical stress). .alpha..sub.P.sup.h is a coefficient that relates the change in thecompressional wave (P-wave) velocity to the change in .sigma..sub.h (i.e., the effective horizontal stress). .alpha..sub.S.sup.V is a coefficient that relates the change in the shear wave (S-wave) velocity to the change in .sigma..sub.V (i.e., theeffective vertical stress). .alpha..sub.S.sup.h is a coefficient that relates the change in the shear wave (S-wave) velocity to the change in .sigma..sub.h (i.e., the effective horizontal stress).

In one or more embodiments of the invention, .nu..sub.P.sup.(0), .nu..sub.S.sup.(0), .sigma..sub.V, and .sigma..sub.h may be related according to the following equations: .nu..sub.P.sup.(0)=.nu..sub.P(.sigma..sub.V.sup.(0),.sigma..sub.h.sup.(0)-) (3) .nu..sub.S.sup.(0)=.nu..sub.S(.sigma..sub.V.sup.(0),.sigma..sub.h.s- up.(0)) (4) .sigma..sub.V=.sigma..sub.V.sup.(0)+.DELTA..sigma..sub.V (5) .sigma..sub.h=.sigma..sub.h.sup.(0)+.DELTA..sigma..sub.h (6)

The aforementioned variables are defined as follows: .sigma..sub.V.sup.(0) is an effective vertical stress value associated with a common reference location, .sigma..sub.h.sup.(0) is an effective horizontal stress value associated with the commonreference location, .sigma..sub.V is an effective vertical stress value associated with a location, .sigma..sub.h is an effective horizontal stress value associated with the location, .DELTA..sigma..sub.V is the difference between the effective verticalstress value at the common reference location and the effective vertical stress value at the location, .DELTA..sigma..sub.h is the difference between the effective horizontal stress value at the common reference location and the effective horizontalstress value at the location, .nu..sub.P.sup.(0) is a reference P-wave velocity associated with the common reference location, .nu..sub.S.sup.(0) is a reference S-wave velocity associated with the common reference location,.nu..sub.P(.sigma..sub.V,.sigma..sub.h) is a P-wave velocity associated with the location, and .nu..sub.S(.sigma..sub.V,.sigma..sub.h) is a S-wave velocity associated with the location.

Using equations (1)-(6), the following method may be used to obtain the stress sensitivity coefficients (i.e., .alpha..sub.P.sup.V,.alpha..sub.P.sup.h,.alpha..sub.S.sup.V, and .alpha..sub.S.sup.h). Initially, a common reference location (L0), afirst location (L1), and a second location (L2) are selected. Those skilled in the art will appreciate that the terms "first" and "second" are used herein solely to distinguish values associated with one location from values associated with anotherlocation. Accordingly, no order should be implied from the use of these terms. Continuing with the example, L0, L1, and L2 may be located at different depths within the offset well. In one embodiment of the invention, L0 is located at the top of aformation in the offset well. Alternatively, L0, L1, and L2 may be located at a common depth but at different distances from the axis of the borehole of the offset well. For example, L0, L1, and L2 may be located at 10,000 feet below the seafloor,where L0 is located on the borehole wall, L1 is located 3 inches into the formation measured along a line perpendicular to the borehole axis, and L2 is located 6 inches into the formation measured along a line perpendicular to the borehole axis.

In one embodiment of the invention, the formation at L0, L1, and L2 includes substantially similar physical properties (e.g., lithology, porosity, etc.). The similarity of physical properties may be verified using core data or logs (e.g.,resistivity logs, neutron porosity logs, density logs, nuclearmagnetic resonance logs, etc.).

Once L0, L1, and L2 have been selected, .nu..sub.P(.sigma..sub.V,.sigma..sub.h) and .nu..sub.S(.sigma..sub.V,.sigma..sub.h) are obtained for each of the locations (i.e., L0, L1, and L2). In one or more embodiments of the invention, the P-waveand S-wave velocities may be obtained using a look-ahead walkaway vertical seismic profile (VSP). In one embodiment of the invention, the VSP is a multi-azimuth walkaway VSP. Further discussion of multi-azimuth VSPs may be found in Leaney, W. S.,Sayers, C. M., and Miller, D. E., 1999, "Analysis of Multiazimuthal VSP Data for Anisotropy and AVO," Geophysics, 64, 1172-1180.

Alternatively, P-wave and S-wave velocities may be obtained using a borehole sonic tool as described in Harrison, A. R., 1990, "Acquisition and Analysis of Sonic Waveforms from a Borehole Monopole and Dipole Source and the Determination ofCompressional and Shear Speeds and their Relation to Rock Mechanical Properties and Surface Seismic Data." SPE Paper No. 20557, 267-282. Additional methods for obtaining P-wave and S-wave velocities as functions of distance from the borehole axis aredescribed in U.S. Pat. Nos. 5,838,633 and 6,904,365. All of the aforementioned patents are commonly owned by the assignee of the present application.

Alternatively, in one or more embodiments of the invention, the P-wave and S-wave velocities may be obtained using a multi-component seismic survey (e.g., a 3C survey, a 4C survey, etc.). For example, the multi-component seismic survey may beperformed on land using P-wave and S-wave vibrators, P-wave and S-wave receivers, or any combination thereof. As another example, the multi-component seismic survey may be performed in a marine environment using P-wave and S-wave sensors on theseafloor.

As another alternative, in one or more embodiments of the invention, the P-wave and S-wave velocities may be obtained using PP and SS reflections, PP and PS reflections (e.g., in cases when an S-wave receiver is available, but no S-wave source isavailable), any other similar type of method, or any combination thereof. Further, in one or more embodiments of the invention, the P-wave and S-wave velocities may be obtained by inverting amplitude-versus-offset (AVO) data associated with PPreflections. Similarly, in one or more embodiments of the invention, AVO data associated with PS reflections and SS reflections may be used.

Returning to the discussion of using equations (1)-(6) to determine the stress sensitivity coefficient, the pore pressure is obtained at each of the locations (i.e., L0, L1, L2). As discussed above, pore pressure may be obtained using variousdownhole tools or numerical methods. In addition, the total stress, which is also expressed as a tensor, is also obtained for each location (i.e., L0, L1, L2).

Using the pore pressure and the total stress at each of the locations, the effective stress at each of the locations may be determined. In one embodiment of the invention, the effective stress (.sigma.) is defined as the difference between thetotal stress and the pore pressure expressed as a tensor. The aforementioned effective stresses (.sigma.) along with equations (5) and (6) are then used to determine .DELTA..sigma..sub.V and .DELTA..sigma..sub.h at L1 and L2. The values of.nu..sub.p(.sigma..sub.V,.sigma..sub.h), .nu..sub.S(.sigma..sub.V,.sigma..sub.h), .DELTA..sigma..sub.V, and .DELTA..sigma..sub.h for L1 and L2 are the used to create four equations (two equations corresponding to equation (1) and two equationscorresponding to equation (2)). At this stage, there four equations and four unknowns (i.e. .alpha..sub.P.sup.V, .alpha..sub.P.sup.h, .alpha..sub.S.sup.V, and .alpha..sub.S.sup.h) and, accordingly, the equations may be rearranged and solved to obtainthe values of .alpha..sub.P.sup.V,.alpha..sub.P.sup.h,.alpha..sub.S.sup.V, and .alpha..sub.S.sup.h.

In one embodiment of the invention, equations (1) and (2) may be used in situations where the minimum horizontal stress and the maximum horizontal stress are equal or substantially the same. However, in cases where the minimum horizontal stressand the maximum horizontal stress are not equal or substantially the same, the following expanded version of equations (1) and (2) may be used: .nu..sub.P(.sigma..sub.V,.sigma..sub.h,.sigma..sub.H)=.nu..sub.P.sup.(0)+-.alpha..sub.P.sup.V.DELTA..sigma..sub.V+.alpha..sub.P.sup.h.DELTA..sigma..- sub.h+.alpha..sub.P.sup.H.DELTA..sigma..sub.H (7) .nu..sub.S1(.sigma..sub.V,.sigma..sub.h,.sigma..sub.H)=.nu..sub.S1.sup.(0-)+.alpha..sub.S1.sup.V.DELTA..sigma..sub.V+.alpha..sub.S1.sup.h.DELTA..sig- ma..sub.h+.alpha..sub.S1.sup.H.DELTA..sigma..sub.H, (8) .nu..sub.S2(.sigma..sub.v,.sigma..sub.h,.sigma..sub.H)=.nu..sub.S2.sup.(0-)+.alpha..sub.S2.sup.V.DELTA..sigma..sub.V+.alpha..sub.S2.sup.h.DELTA..sig- ma..sub.h+.alpha..sub.S2.sup.H.DELTA..sigma..sub.H, (9)

Those skilled in the art will appreciate that when S.sub.H and S.sub.h are not equal, there are two shear waves with different polarizations having different velocities.

In one or more embodiments of the invention, the stress sensitivity coefficient(s) may be obtained by applying a micromechanical model to the reference data. For example, a micromechanical model may be used that accounts for thestress-dependence and orientation distribution of the contacts between grains (e.g., in a sand formation). Further discussion of micromechanical models may be found in Sayers, C. M., 2002, "Stress-dependent elastic anisotropy of sandstones," GeophysicalProspecting, 50, 85-95, and Sayers, C. M. (1999) Stress-dependent seismic anisotropy of shales, Geophysics, 64, 93-98.

In some cases, the vertical stress may not be a principal stress. In such cases, more general forms of the equations (1) and (2) relating effective stress to the P-wave and S-wave velocities may be used. For example, equations (1) and (2) mayinclude shear stress components or principal stress components that are not aligned parallel or perpendicular to the vertical axis (with respect to a gravity vector).

Additional methods for obtaining stress sensitivity coefficient(s) are described in U.S. Pat. Nos. 7,042,802; 6,351,991; 5,838,633; 5,619,475; 5,475,650; and 5,398,215. All of the aforementioned patents are commonly owned by the assignee ofthe present application.

Those skilled in the art will appreciate that the exemplary stress-pressure relationship in equations (1) and (2) relies on the fact that while vertically propagating P-waves are mostly sensitive to changes in the component of the effectivestress acting parallel to the direction of propagation, S-waves are sensitive to changes in both the component of the effective stress acting parallel to the direction of propagation and the component of the effective stress acting parallel to thedirection of polarization. Further discussion of these underlying facts can be found in Sayers, C. M., 2004, "Monitoring production-induced stress changes using seismic waves," 74.sup.th SEG Annual Meeting, Extended Abstracts. Specifically, since thedirections of propagation and polarization are approximately parallel for the P-wave and orthogonal for the S-wave, the combination of the P-wave and S-wave velocities allows perpendicular components of the effective stress to be determined separately,thus allowing a determination of the pore pressure, and principal stresses.

Continuing with discussion of FIG. 3, in one or more embodiments of the invention, in Step 310, P-wave and/or S-wave velocities associated with the pre-drill location may be obtained using one or more of the methods described above. In oneembodiment of the invention, P-wave and/or S-wave velocities may be obtaining using multi-offset Vertical Seismic Profile (VSP), often referred to as a walkaway VSP. In such cases, the VSP may be acquired using shots arranged on a line designed tomaximize the extraction of information about the principal stresses. A multi-offset, multi-azimuth VSP may also be used.

Continuing with discussion of FIG. 3, in one or more embodiments of the invention, in Step 315, P-wave and/or S-wave velocities associated with a reference location may be obtained using one or more of the methods described above.

In Step 320, total stress values associated with the pre-drill location may be calculated, using a predicted pore pressure. Specifically, in one or more embodiments of the invention, the first time that Step 320 is performed, the predicted porepressure may be obtained from an offset well, a computer simulation, or any other source. For example, the predicted pore pressure may be obtained using a velocity-to-pore-pressure transform on offset well data. Further discussion ofvelocity-to-pore-pressure transforms can be found in Eaton, "The Equation for Geopressure Prediction from Well Logs," SPE 5544 (Society of Petroleum Engineers of AIME, 1975) and Bowers, "Pore Pressure Estimation from Velocity Data: Accounting forPore-Pressure Mechanisms besides Undercompaction," SPE Drilling and Completion (June 1995) 89-95.

In one or more embodiments of the invention, calculating total stress values may involve performing a finite element analysis that includes the pre-drill location and surrounding formations. For example, a finite element model used in the finiteelement analysis may include the pre-drill location and one or more locations in one or more offset wells. Performing the finite element analysis may also involve calculating a total stress variation across one or more depths (e.g., above, below, and/oraround the low permeability substance) using the predicted pore pressure. For example, boundary stress conditions may be obtained from offset wells and used to estimate total stress values at the pre-drill location, Further discussion of finite elementanalysis can be found in Fredrich, J. T., et al., 2003, "Stress Perturbations Adjacent to Salt Bodies in Deepwater Gulf of Mexico." SPE Paper No. 84554.

Alternatively, a finite difference or any other similar numerical method may be used. The following article discusses a numerical method that may be used instead of the aforementioned finite element method: Poliakov, A. N. B. et al., 1993,"AnExplicit Inertial Method for the Simulation of Viscoelastic Flow: An Evaluation of Elastic Effects on Diapiric Flow in Two-and Three-layer Models," Stone, D. B. & Runcorn, S. K. (eds.) Flow and Creep in the Solar System: Observations, Modeling andTheory, pp. 175-195.

Continuing with discussion of FIG. 3, in one or more embodiments of the invention, in Step 325 a second predicted pore pressure is calculated using the total stress values obtained in Step 320. The following discussion details an embodiment forobtaining the second predicted pore pressure.

Initially, a reference location is selected. The reference location used in Step 325 may be the common reference location used in Step 305 or it may be another location in one of the offset wells or in the target borehole (i.e., the boreholewhose current trajectory intersects with the pre-drill location). In one embodiment of the invention, the reference location and the pre-drill location include substantially similar physical properties (e.g., lithology, porosity, etc.). The similarityof physical properties may be verified using logs (e.g., resistivity logs, porosity logs, density logs, magnetic resonance logs, etc.).

The relationship between the effective stress, the total stress, and the pore pressure is given by the following equation: .sigma..sub.ij=S.sub.ij-.alpha.p.delta..sub.ij, (10) where i and j refer to components of a tensor, .sigma..sub.ij is theeffective stress component, S.sub.ij is the total stress component, .alpha. is a poroelastic coefficient, .delta..sub.if is 1 if i=j, and .delta..sub.ij is 0 if i.noteq.j. .alpha. may be obtained using a variety of methods such as those described inWang. H F., 2000, "Theory of Linear Poroelasticity--with Applications to Geomechanics and Hydrogeology" Princeton University Press. 287 pp. Those skilled in the art will appreciate that methods described in the aforementioned references are notintended to limit the scope of the invention. Continuing with the discussion of Step 325, replacing the effective stress in equations (1) or (2) using equation (8) yields .nu..sub.P(S.sub.V,S.sub.h,p)=.nu..sub.P.sup.(0)+.alpha..sub.P.sup.V(.DEL-TA.S.sub.V-.alpha..DELTA.p)+.alpha..sub.P.sup.h(.DELTA.S.sub.h-.alpha..DEL- TA.p) (11) .nu..sub.S(S.sub.V,S.sub.h,p)=.nu..sub.S.sup.(0)+.alpha..sub.S.sup.V(.DEL- TA.S.sub.V-.alpha..DELTA.p)+.alpha..sub.S.sup.h(.DELTA.S.sub.h-.alpha..DEL- TA.p), (12)

With respect to the reference, the variables in equations (11)-(12) are defined as follows: .nu..sub.P(S.sub.V,S.sub.h,p) is the P-wave velocity associated with the pre-drill location, and .nu..sub.S(S.sub.V,S.sub.h,p) is the S-wave velocityassociated with the pre-drill location; .nu..sub.P.sup.(0) is a reference P-wave velocity associated with the reference location; .nu..sub.S.sup.(0) is a reference S-wave velocity associated with the reference location; .alpha..sub.P.sup.V is thevertical compressional stress sensitivity coefficient obtained in Step 305; .alpha..sub.P.sup.h is the horizontal compressional stress sensitivity coefficient obtained in Step 305; .alpha..sub.S.sup.V is the vertical shear stress sensitivity coefficientobtained in Step 305; .alpha..sub.S.sup.h is the horizontal shear stress sensitivity coefficient obtained in Step 305; S.sub.v is the total vertical stress; S.sub.h is the total horizontal stress; p is the pore pressure; the terms preceded by .DELTA. represent the difference between values at the pre-drill location and the reference location; and .alpha. is the poroelastic coefficient.

Using equation (11) and/or equation (12) with values obtained in the previous steps, .DELTA.p may be determined. .DELTA.p may then be added to the pore pressure at the reference location to obtain the second predicted pore pressure.

Continuing with discussion of FIG. 3, in one or more embodiments of the invention, in Step 330, the predicted pore pressure used in Step 320 (hereinafter the "first" predicted pore pressure) may be compared with the predicted pore pressurecalculated in Step 325 (hereinafter the "second" predicted pore pressure). Specifically, one or more metrics of the differences between the first and second predicted pore pressures may be compared with one or more threshold difference values. In oneor more embodiments of the invention, a threshold difference value may be a maximum value of the metric of differences indicating that the second predicted pore pressure is sufficiently accurate. In one embodiment of the invention, a second predictedpore pressure is determined to be sufficiently accurate when |(PP1-PP2)/PP1|.ltoreq.0.001, where PP1 denotes the first predicted pore pressure and PP2 denotes the second predicted pore pressure. Those skilled in the art would appreciate that othervalues for the threshold difference value may be used and that other mathematical constructs for the metric of the differences may be used. The threshold difference value may correspond to any other value less than 1.

In one or more embodiments of the invention, if the differences between the first and second predicted pore pressures are greater than the threshold difference value in Step 330, then in Step 335 the first predicted pore pressure is updated usingthe second predicted pore pressure. In this case, Step 320 is subsequently repeated using the first updated predicted pore pressure. In one or more embodiments of the invention, if Step 320-325 are repeated, current data gathered while drilling may beused (e.g., seismic wave velocities, stress data, etc.). For example, the current data may be gathered using measurements while drilling (MWD), logging while drilling (LWD), any other similar type of drilling data, or any combination thereof. Thoseskilled in the art will appreciate that using more current data and/or data from a location closer to the pre-drill location may improve the accuracy of the calculated total stress value. Further, those skilled in the art will appreciate that "updating"the first predicted pore pressure may simply imply that Step 320 is repeated using the predicted pore pressure calculated in Step 325.

In one or more embodiments of the invention, Steps 320 and 325 may be iteratively performed (i.e., via Steps 330 and 335) until the difference between the first and second predicted pore pressures are less than or equal to the thresholddifference values in Step 330. Said another way, Steps 320 and 325 may be repeated until the first and second predicted pore pressures have sufficiently converged (e.g., the difference between the first and second predicted pore pressure is.ltoreq.0.1%).

In one or more embodiments of the invention, if the differences between the first and second predicted pore pressures are less than or equal to the threshold difference value(s) in Step 330, then in Step 340 the drilling operation associated withthe pre-drill location may be adjusted based on the second predicted pore pressure. Specifically, in one or more embodiments of the invention, adjusting the drilling operation may involve adjusting a drilling fluid density (i.e., increasing ordecreasing the drilling fluid density as appropriate), adjusting a drilling trajectory (e.g., to avoid an overpressured area, to pass through a low-pressure area, etc.), optimizing the number of casing strings in the borehole (i.e., adding a casingstring, delaying addition of a casing string, etc.), or any other similar type of adjustment.

In one embodiment of the invention, if a proposed well trajectory is situated in the vicinity of a complex structure that perturbs the surrounding stress-field (e.g., salt) and the proposed well trajectory passes through a highly permeableformation (e.g., sand), then the pore pressure in the highly permeable formation may be determined using any of the available techniques for determining pore pressure in such a formation.

For example, if shale (a low permeability substance) interfaces with a sand (a high permeability), then an offset well may be drilled in the shale and the pore pressure in the shale may be determined using, for example, acoustic or seismicvelocities. The pore pressure in the shale may then be used to obtain the pore pressure in the sand using, for example, the centroid method. A particular version of the centroid method is described in the Patent Cooperation Treaty (PCT) Applicationwith the serial number WO 2004/022905 and assigned to the assignee of the present application. Alternatives to the centroid method include numerical basin modeling. Once the pore pressure in the sand has been determined, the pore pressure may be usedto validate the various stress sensitivity coefficients and/or the model used to generate the total stress at the pre-drill location.

In one embodiment of the invention, wireline and/or logging while drilling (LWD) tools are used to obtain pore pressure measurements for a location in an offset well, where the location is in a sand and the sand extends to a pre-drill location. In such cases, the pore pressure at the pre-drill location may be estimated using the pore pressure obtained at the location in the offset well and information about the hydrostatic gradient (defined by the fluid density) between the location in theoffset well and the pre-drill location. Once the pore pressure in the sand has been determined at the pre-drill location, the pore pressure may be used to validate the various stress sensitivity coefficients and/or the model used to generate the totalstress at the pre-drill location.

In one embodiment of the invention, downhole measurements obtained while drilling in the target borehole (i.e., the borehole whose trajectory intersects with the pre-drill location) may be used to update the stress sensitivity coefficients and/orthe model used to generate the total stress at the pre-drill location. These coefficients may then be used to predict the pore pressure ahead of the bit using seismic velocities or velocities obtained using a multi-offset VSP.

One or more embodiments of the invention provide a means for accurately predicting a pre-drill pore pressure. Accordingly, one or more embodiments of the invention may prevent formation fluids from entering a borehole, thereby preventing damageto the well and/or personnel operating a drilling rig. Further, one or more embodiments of the invention may prevent the financial overhead of prematurely inserting casing strings.

The invention may be implemented on virtually any type of computer regardless of the platform being used. For example, as shown in FIG. 4, a computer system (400) includes a processor (402), associated memory (404), a storage device (406), andnumerous other elements and functionalities typical of today's computers (not shown). The computer (400) may also include input means, such as a keyboard (408) and a mouse (410), and output means, such as a monitor (412). The computer system (400) maybe connected to a network (414) (e.g., a local area network (LAN), a wide area network (WAN) such as the Internet, or any other similar type of network) via a network interface connection (not shown). Those skilled in the art will appreciate that theseinput and output means may take other forms.

Further, those skilled in the art will appreciate that one or more elements of the aforementioned computer system (400) may be located at a remote location and connected to the other elements over a network. Further, the invention may beimplemented on a distributed system having a plurality of nodes, where each portion of the invention (e.g., stress sensitivity coefficient calculator, total stress calculator, pore pressure calculator, etc.) may be located on a different node within thedistributed system. In one embodiment of the invention, the node corresponds to a computer system. Alternatively, the node may correspond to a processor with associated physical memory. The node may alternatively correspond to a processor with sharedmemory and/or resources. Further, software instructions to perform embodiments of the invention may be stored on a computer readable medium such as a compact disc (CD), a diskette, a tape, a file, or any other computer readable storage device. Inaddition, in one embodiment of the invention, the predicted pore pressure (including all the pore pressures calculated using the method described in FIG. 3) may be displayed to a user via a graphical user interface (e.g., a display device).

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of theinvention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

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