Patent Publication Number: US-10768324-B2

Title: Method to predict pore pressure and seal integrity using full wavefield inversion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/338,799 filed May 19, 2016 entitled METHOD TO PREDICT PORE PRESSURE AND SEAL INTEGRITY USING FULL WAVEFIELD INVERSION, the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     Exemplary embodiments described herein pertain generally to the field of geophysical prospecting, and more particularly to geophysical data processing. More specifically, an exemplary embodiment can predict subsurface pore pressure and characterize seal integrity. 
     BACKGROUND 
     This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art. 
     Seismic inversion is a process of extracting information about the subsurface from data measured at the surface of the Earth during a seismic acquisition survey. In a typical seismic survey, seismic waves are generated by a source  101  positioned at a desired location. As the source generated waves propagate through the subsurface, some of the energy reflects from subsurface interfaces  105 ,  107 , and  109  and travels back to the surface  111 , where it is recorded by the receivers  103 . The seismic waves  113  and  115  that have been reflected in the subsurface only once before reaching the recording devices are called primary reflections. In contrast, multiple reflections  117  and  119  are the seismic waves that have reflected multiple times along their travel path back to the surface. Surface-related multiple reflections are the waves that have reflected multiple times and incorporate the surface of the Earth or the water surface in their travel path before being recorded. 
     Full Wavefield Inversion (FWI) is a seismic method capable of utilizing the full seismic record, including the seismic events that are treated as “noise” by standard inversion algorithms. The goal of FWI is to build a realistic subsurface model by minimizing the misfit between the recorded seismic data and synthetic (or modeled) data obtained via numerical simulation. 
     FWI is a computer-implemented geophysical method that is used to invert for subsurface properties such as velocity or acoustic impedance. The crux of any FWI algorithm can be described as follows: using a starting subsurface physical property model, synthetic seismic data are generated, i.e. modeled or simulated, by solving the wave equation using a numerical scheme (e.g., finite-difference, finite-element etc.). The term velocity model or physical property model as used herein refers to an array of numbers, typically a 3-D array, where each number, which may be called a model parameter, is a value of velocity or another physical property in a cell, where a subsurface region has been conceptually divided into discrete cells for computational purposes. The synthetic seismic data are compared with the field seismic data and using the difference between the two, an error or objective function is calculated. Using the objective function, a modified subsurface model is generated which is used to simulate a new set of synthetic seismic data. This new set of synthetic seismic data is compared with the field data to generate a new objective function. This process is repeated until the objective function is satisfactorily minimized and the final subsurface model is generated. A global or local optimization method is used to minimize the objective function and to update the subsurface model. 
     The exploration and production industry is moving into frontier oil and gas provinces with higher well costs and drilling risks. In order to execute a drilling program in a safe, efficient and cost effective manner, accurate pre-drill prediction of pore pressure and fracture pressure is useful. In addition, seal integrity analysis is useful in understanding how much hydrocarbons can be trapped by prospect seals before mechanical leak. Conventionally, these analyses have been done using seismic velocities obtained from tomography. Tomographic velocities are used to predict pore pressure using Eaton&#39;s normal compaction trend and Bower&#39;s effective stress methods. In general, such velocity profiles are suitable for pore pressure prediction in background shales, but are too smooth to detect embedded sand reservoirs. In comparison, FWI provides higher resolution and more accurate velocity models, which provides an opportunity to do pressure prediction for the sands as well. Pressure prediction for sands allows for the appraisal of mechanical seal failure risks of overlying shales. 
     SUMMARY 
     A method, including: generating a velocity model for a subsurface region of the Earth by using a full wavefield inversion process; generating an impedance model for the subsurface region of the Earth by using a full wavefield inversion process; and estimating pore pressure at a prediction site in the subsurface region by integrating the velocity model and the impedance model with a velocity-based pore pressure estimation process. 
     In the method, the prediction site can include a reservoir rock, and the velocity-based pore pressure estimation process can include: identifying, within the impedance model, the reservoir rock; using the velocity model to determine or estimate shale pore pressure at a base of the reservoir rock in the impedance model; determining a buoyancy pressure of a fluid assumed to fill a column height of the reservoir rock; and the estimating the pore pressure for the reservoir rock can include combining the shale pore pressure at the base of the reservoir rock and the buoyancy pressure. 
     The method can further include: predicting a lithology volume for the subsurface geologic formation from the impedance model; determining Poisson&#39;s ratio within the reservoir rock based on the lithology volume and P-wave velocity from the velocity model generated using the full wavefield inversion process; and determining a full wavefield inversion lithology-based minimum horizontal stress from the Poisson&#39;s ratio. 
     The method can further include: analyzing seal integrity of the reservoir rock based on a difference between the estimated pore pressure and the minimum horizontal stress. 
     In the method, the reservoir rock can be sand. 
     In the method, the predicting the lithology volume can include using impedance values from an offset well. 
     In the method, the fluid can be one of oil, gas, or water. 
     The method can further include extracting hydrocarbons from the reservoir rock. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. It should also be understood that the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating principles of exemplary embodiments of the present invention. Moreover, certain dimensions may be exaggerated to help visually convey such principles. 
         FIG. 1  illustrates an example of a conventional seismic acquisition. 
         FIG. 2  illustrates an exemplary method for predicting pore pressure and seal integrity. 
         FIG. 3  illustrates an exemplary plot of velocity vs. effective stress. 
         FIG. 4  illustrates an exemplary plot of porosity vs. depth. 
         FIG. 5  illustrates an exemplary plot of impedance vs. depth. 
         FIG. 6  illustrates an exemplary Poisson&#39;s ratio cross-plot. 
         FIG. 7  illustrates an exemplary plot of velocity vs effective stress from FWI shale velocity. 
         FIG. 8  illustrates an exemplary plot of impedance vs. depth from FWI full bandwidth impedance and FWI band limited impedance. 
         FIG. 9  illustrates an exemplary plot of Vclay vs. depth. 
         FIG. 10  illustrates an exemplary FWI derived lithology sand volume. 
         FIG. 11  illustrates an exemplary buoyancy pressure determination. 
         FIG. 12  illustrates an exemplary plot of pore pressure vs. depth for lithology based pore pressure. 
         FIG. 13  illustrates an exemplary derivation of FWI derived Poisson&#39;s ratio at a predicted location. 
         FIG. 14  illustrates an exemplary plot of minimum horizontal stress vs. depth for FWI lithology-based minimum horizontal stress. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments are described herein. However, to the extent that the following description is specific to a particular embodiment, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the invention is not limited to the specific embodiments described below, but rather, it includes all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims. 
     In recent years, FWI has been widely used in industry and academia to improve seismic imaging quality (Mothi, et al., 2012; Liu et al., 2014), particularly for regions with significant structural complexity. The present technological advancement can improve prediction of subsurface pore pressure estimates and can characterize seal integrity. In a non-limiting example, subsurface pore-pressure can be predicted using higher resolution and more accurate FWI velocity models (as compared to tomographic velocity) in conjunction with conventional effective stress methods. Lithology inversions can be integrated from high-resolution model domain (FWI impedance) products with velocity-based pore pressure estimation. The coupled analysis allows for the reduction in uncertainty in pressure prediction due to lithology (primarily, sand vs. shale, but other reservoir rock could be used in place of sand) changes. Further, pressure prediction in sand allows for preparation for seal integrity analysis. The results from the integrated lithology inversion can be used to appraise the risk of mechanical seal failure, given fluid-fill (or hydrocarbon type) and fluid-column height scenarios. Fracture pressure or minimum stress in the seals can be predicted by using either a constant fracture gradient measured from regional or global leak off test (LOT) databases or through geomechanical method-based estimations (viz., stress-ratio equations). 
     Inputs for the present technological advancement include: (1) FWI model domain products (velocity and impedance); and (2) rock physics transformations for lithology classification, velocity pressure relations and lithology-based Poisson&#39;s ratio estimation. 
     Compared to conventional techniques, the present technological advancement can use higher resolution (˜0-15 Hz) velocities from FWI instead of lower resolution tomographic velocities, uses lithology (sand vs. shale)—dependent pressure-velocity relations for estimating pore-pressure from FWI P-velocity inversion instead of using a single pressure-velocity relation, and uses data-driven seal strength from seismic methods to analyze seal integrity instead of using spatially continuous or homogenous seal properties. 
       FIG. 2  illustrates an exemplary method for predicting pore pressure and seal integrity. Step  201  includes building a shale-velocity vs. effective stress plot (see  FIG. 3 ) for an offset well. Offset well density and pressure data can be used to calculate overburden pressure from density logs. As shown below in equation 1, effective stress is equal to overburden pressure minus pore pressure.
 
Effective Stress=Overburden Pressure−pore pressure  (Eqn. 1)
 
     Pore pressure can be either obtained by direct measurement in the field or estimated from drilling mud weight. Sand pore pressure is usually obtained from the direct measurement in the reservoir. Shale pore pressure can be measured in the formation or estimated from drilling parameters and well events. 
     Step  202  includes building a lithology based porosity plot for an offset well (see  FIG. 4 ). Offset well log data and core analyses can be used to build a shale porosity vs. depth compaction trend  401  and a sand porosity vs. depth compaction trend  402  (see  FIG. 4 ). 
     Step  203  includes building a lithology-based impedance plot (see  FIG. 5 ). Offset well velocity and density are used to obtain acoustic impedance by multiplying velocity and density. As shown in  FIG. 5 , a shale acoustic impedance vs. depth trend  501  is generated and a sand acoustic impedance vs. depth trend  502  is generated. 
     Step  204  includes building a lithology-based Poisson&#39;s ratio cross plot at an offset well (see,  FIG. 6 ). The offset wells primary velocity (Vp) and secondary velocity (Vs) are used to calculate Poisson&#39;s ratio  6 , as shown in equation 2.
 
σ=½( V   P   2 −2 V   S   2 )/( V   P   2   −V   S   2 )  (Eqn. 2)
 
The cross-plot in  FIG. 6  is generated based on the Poisson&#39;s ratio and P-wave velocity. (VES). Shale pore pressure at the prediction location can be obtained via equation 1.
 
     Step  205  includes calculating the overburden pressure at the prediction location. The velocity vs. effective stress plot from step  201  is shown as  701  in  FIG. 7 , along with the FWI shale velocity  702 , and FWI shale velocity is used to obtain the effective stress. 
     Step  206  includes generating high frequency FWI full bandwidth (BW) and band-limited impedance data using seismic shot gathers. The following is an example of an algorithm for performing local cost function optimization in FWI. This procedure is iterated by using the new updated model as the starting model for another gradient search. The process continues until an updated model is found that satisfactorily explains the observed data. Commonly used local cost function inversion methods include gradient search, conjugate gradients and Newton&#39;s method. 
     Local cost function optimization of seismic data in the acoustic approximation is a common geophysical inversion task, and is generally illustrative of other types of geophysical inversion. When inverting seismic data in the acoustic approximation the cost function can be written as: 
                     S   ⁡     (   M   )       =       ∑     g   =   1       N   g       ⁢       ∑     r   =   1       N   r       ⁢       ∑     t   =   1       N   t       ⁢     W   ⁡     (         ψ   calc     ⁡     (     M   ,   r   ,   t   ,     w   g       )       -       ψ   obs     ⁡     (     r   ,   t   ,     w   g       )         )                     (     Eqn   .           ⁢   3     )               
where:
 
S=cost function,
 
M=vector of N parameters, (m 1 , m 2 , . . . m N ) describing the subsurface model,
 
g=gather index,
 
w g =source function for gather g which is a function of spatial coordinates and time, for a point source this is a delta function of the spatial coordinates,
 
N g =number of gathers,
 
r=receiver index within gather,
 
N r =number of receivers in a gather,
 
t=time sample index within a trace,
 
N t =number of time samples,
 
W=minimization criteria function (a preferred choice is W(x)=x 2 , which is the least squares (L2) criteria),
 
ψ calc =calculated seismic data from the model M,
 
ψ obs =measured seismic data.
 
     The gathers can be any type of gather that can be simulated in one run of a seismic forward modeling program. Usually the gathers correspond to a seismic shot, although the shots can be more general than point sources. For point sources the gather index g corresponds to the location of individual point sources. For plane wave sources g would correspond to different plane wave propagation directions. This generalized source data, ψ obs , can either be acquired in the field or can be synthesized from data acquired using point sources. The calculated data ψ calc  on the other hand can usually be computed directly by using a generalized source function when forward modeling. For many types of forward modeling, including finite difference modeling, the computation time needed for a generalized source is roughly equal to the computation time needed for a point source. 
     Equation (3) can be simplified to: 
                     S   ⁡     (   M   )       =       ∑     g   =   1       N   g       ⁢       W   ⁡     (     δ   ⁡     (     M   ,     w   g       )       )       .               (     Eqn   .           ⁢   4     )               
where the sum over receivers and time samples is now implied and,
 
δ( M,w   g )=ψ calc ( M,w   g )−ψ obs ( w   g ).  (Eqn. 5)
 
     Inversion attempts to update the model M such that S(M) is a minimum. This can be accomplished by local cost function optimization which updates the given model M (k)  as follows:
 
 M   (k+1)   =M   (k) −α (k) ∇ M   S ( M )  (Eqn. 6)
 
where k is the iteration number, α is the scalar size of the model update, and ∇ M S(M) is the gradient of the misfit function, taken with respect to the model parameters. The model perturbations, or the values by which the model is updated, are calculated by multiplication of the gradient of the objective function with a step length a, which must be repeatedly calculated.
 
     From equation (4), the following equation can be derived for the gradient of the cost function: 
     
       
         
           
             
               
                 
                   
                     
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     So to compute the gradient of the cost function one must separately compute the gradient of each gather&#39;s contribution to the cost function, then sum those contributions. Therefore, the computational effort required for computing ∇ M S(M) is N g  times the compute effort required to determine the contribution of a single gather to the gradient. For geophysical problems, N g  usually corresponds to the number of geophysical sources and is on the order of 10,000 to 100,000, greatly magnifying the cost of computing ∇ M S(M). 
     Note that computation of ∇ M W(δ) requires computation of the derivative of W(δ) with respect to each of the N model parameters m 1 . Since for geophysical problems N is usually very large (usually more than one million), this computation can be extremely time consuming if it had to be performed for each individual model parameter. Fortunately, the adjoint method can be used to efficiently perform this computation for all model parameters at once. The adjoint method for the least squares objective function and a gridded model parameterization is summarized by the following algorithm: 
     1. Compute forward simulation of the data using the current model and the gather signature w g  as the source to get ψ calc (M (k) ,w g ), 
     2. Subtract the observed data from the simulated data giving δ(M (k) ,w g ), 
     3. Compute the reverse simulation (i.e. backwards in time) using δ(M (k) ,w g ) as the source producing ψ adjoint (M (k) ,w g ), 
     4. Compute the integral over time of the product of ψ calc (M (k) ,w g ) and ψ adjoint (M (k) ,w g ) to get ∇ M W(δ(M,w g )). 
     Step  207  includes generating the FWI band limited impedance. The FWI band limited impedance is determined by: FWI Band Limited Impedance=FWI Full Bandwidth Impedance−Shale Impedance Trend.  FIG. 8  shows a comparison of the FWI band-limited impedance and the FWI full bandwidth impedance. In  FIG. 8, 801  identifies the reservoir prediction of an anomaly (i.e., a sand body). FWI is able to show the anomaly  801 , whereas tomographic velocity would not. 
     Step  208  includes producing lithology volumes (shale volume and sand volume). FWI full bandwidth impedance minus shale impedance  501  is used to obtain shale lithology identification. FWI full bandwidth impedance minus sand impedance  502  is used to obtain sand lithology identification. FWI band-limited impedance is used to check the sand lithology classification and shale lithology classification when the curves  501  and  502  are off compaction trend.  FIG. 9  shows lithology for shale and sand, including the sand anomaly  801 . 
     Step  209  includes determining the reservoir column height. The FWI derived lithology from step  208  is used to determine reservoir and column heights. As indicated in  FIG. 10 , the FWI derived lithology from step  208  is consistent with offset well lithology log at an offset well location. At the new prediction location, FWI derived lithology is used to digitize the top  1002  of the sand body and base  1001  of the sand body  801 . The column height of sand body  801  is the elevation between top of reservoir  1001  and the deepest portion of the entire reservoir body  1002 . 
     In step  210 , hydrocarbon buoyancy pressure is determined. Fluid is assumed to fill the column height of the sand body  801  (sand thickness). This is further illustrated in  FIG. 11 . The buoyancy pressure due to the existence of the fluid (gas/oil/water) in the sand body  801  is calculated.
 
 F   b   =gρV=ρghA   (Eqn. 8)
 
F b  is the buoyant force, ρ is the density of the fluid, g is gravity, V is the volume of the immersed part of the body in the fluid, h is the height of the immersed part, and A is the surface area of the immersed part. The fluid, whether gas, oil, or water, can be determined from amplitude vs offset (AVO) analysis or from data provided by the offset well.
 
     In step  211 , lithology-based pore pressure is predicted. This is determined from the shale pore pressure from step  205  and buoyancy pressure from step  210 . At the prediction site, shale pore pressure is used as a background pore pressure. At the base of the sand body, the shale pore pressure will be used. The pressure within the sand is calculated from the sum of the fluid buoyancy pressure from step  210  and the base of shale pressure. This is further illustrated in  FIG. 12 . 
     Step  212  includes producing an FWI derived Poisson ratio (PR) at the prediction location (i.e., the sand anomaly  801 ). A high-resolution FWI velocity model, offset well lithology-based Poisson&#39;s ratio vs. velocity cross-plot from step  204  and lithology prediction from step  208  are used to predict the Poisson&#39;s ratio with the specific FWI P-wave velocity. This is further illustrated in  FIG. 13 . 
     Step  213  includes determining FWI lithology-based minimum horizontal stress. The minimum horizontal stress is determined from:
 
Minimum horizontal stress=pore pressure+ Ko *(overburden pressure−pore pressure)+tectonic term,  (Eqn. 9)
 
where the tectonic term is the lateral stress introduced by lateral movement of formation due to tectonic forces, and
 
               Ko   =     a   *     σ     1   -   σ           ,         
wherein σ is Poisson&#39;s ratio, and a=constant that can be estimated from the offset well. The constant a can be obtained by matching offset well fracture pressure with calculated minimum horizontal stress using equation 9.  FIG. 14  provides a comparison of the pore pressure and the minimum horizontal stress.
 
     Step  214  includes obtaining the FWI derived mechanical seal integrity, which is calculated by the difference between the minimum horizontal stress and the pore pressure. As minimum horizontal stress gets closer to pore pressure, the mechanical seal will begin to fail. The values of mechanical seal integrity are close to zero. 
     Pore pressure predictions and mechanical seal integrity can be used to manage hydrocarbons. As used herein, hydrocarbon management includes hydrocarbon extraction, hydrocarbon production, hydrocarbon exploration, identifying potential hydrocarbon resources, identifying well locations, determining well injection and/or extraction rates, identifying reservoir connectivity, acquiring, disposing of and/or abandoning hydrocarbon resources, reviewing prior hydrocarbon management decisions, and any other hydrocarbon-related acts or activities. 
     In all practical applications, the present technological advancement must be used in conjunction with a computer, programmed in accordance with the disclosures herein. Preferably, in order to efficiently perform FWI, the computer is a high performance computer (HPC), known to those skilled in the art. Such high performance computers typically involve clusters of nodes, each node having multiple CPU&#39;s and computer memory that allow parallel computation. The models may be visualized and edited using any interactive visualization programs and associated hardware, such as monitors and projectors. The architecture of system may vary and may be composed of any number of suitable hardware structures capable of executing logical operations and displaying the output according to the present technological advancement. Those of ordinary skill in the art are aware of suitable supercomputers available from Cray or IBM. 
     The present techniques may be susceptible to various modifications and alternative forms, and the examples discussed above have been shown only by way of example. However, the present techniques are not intended to be limited to the particular examples disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the spirit and scope of the appended claims.