EP0491841A4 - Pore pressure prediction method - Google Patents
Pore pressure prediction methodInfo
- Publication number
- EP0491841A4 EP0491841A4 EP19900914581 EP90914581A EP0491841A4 EP 0491841 A4 EP0491841 A4 EP 0491841A4 EP 19900914581 EP19900914581 EP 19900914581 EP 90914581 A EP90914581 A EP 90914581A EP 0491841 A4 EP0491841 A4 EP 0491841A4
- Authority
- EP
- European Patent Office
- Prior art keywords
- seismic
- curve
- well
- itt
- pore
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
- E21B49/006—Measuring wall stresses in the borehole
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B47/00—Survey of boreholes or wells
- E21B47/06—Measuring temperature or pressure
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
- E21B49/003—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells by analysing drilling variables or conditions
Definitions
- the present invention relates to a method of predicting pore pressures over known depth intervals of a well bore prior to drilling at a given location.
- it relates to a method of estimating pore pressures of earth formations along the length of a well bore to reach a subterranean reservoir from a proposed drilling site using seismic data recorded at the earth's surface over the drilling site as calibrated in accordance with similar recorded seismic data and well logs recorded in and around a well bore at an offset location proximate to the proposed well location.
- An estimate of the pore pressures in a well bore to be drilled at a selected well location is obtained by utilizing seismic data collected and recorded down to depths extending through both an area below a proposed well site and below or adjacent to a drilled well near the proposed well. In both areas, time-amplitude seismic "traces" having a common mid-point (CMP) between a plurality of pairs of sources and detectors are recorded for a multiplicity of different source-detector distances or offsets, each pair having the same common mid-point.
- CMP common mid-point
- Such separate traces are selectively corrected and stacked or combined to correct for "move-out” (to compensate for different distances between each pair of sources and detectors after reflection from various depths along the mid-point) .
- Such traces are also corrected for dip of the reflecting horizons and other errors due to difference in geometry and geology between the sources and detectors.
- Such combined traces yield so-called “stacking velocities" over the various subterranean geological intervals underlying both the adjacently drilled and proposed well location. The drilled well is selected as close as possible to the given or proposed drill site.
- the only information contained in the combined seismic traces are time and amplitude; that is, the time required for a seismic wave to travel from a source to a seismic discontinuity, which then reflects the wave back to a detector, and the amplitude of the reflected wave.
- the actual depth of each reflector in such traces must be converted to depth-amplitude by relying entirely upon a knowledge of the velocity of each interval of the rock sequences through which the seismic wave has traveled before it is imaged by the seismic trace.
- traces indicate only the average of all strata through which the seismic energy has passed, from the source to a reflector and back to a detector.
- the depth of the strata acting as reflectors are shallow, say a few thousand feet, and the bedding planes are not complex, few of the amplitudes represent multiple reflections, and such velocities correlate well with the true depths of such strata.
- such recorded traces velocities may change with depth, and geometry of the reflecting beds, as well as many factors depending upon physical or chemical structure, or both.
- the depth of such seismic reflectors may vary in actual depth over several tens or even hundreds of feet. Accordingly, a particular difficulty in previously known methods of predicting pore pressures at given depths under an exploratory well from such seismic data alone lies in the lack of detailed information as to the actual velocities of portions, or intervals, of the geological column, through which the seismic waves travel.
- the true values of pore pressures at a given depth in a proposed well are developed by properly calibrating a suite of pore pressures calculated from the selected normal trend line and the common mid-point (CMP) seismic curve representing the drilled well to the well measured pore pressures over at least the critical depths of the well.
- CMP common mid-point
- the normal trend of the overburden pressure by which all values of pore pressure are calculated from interval transit time values, are generated by iterative comparison of such values with a similar series of discrete pore pressures over selected intervals of earth formations traversed by the adjacent drilled well.
- the suite of pore pressures may then be used through the critical drilling depths in the proposed well with pore pressures calculated from the calibrated normal pressure trend line and the synthetic sonic log derived from the stacked interval velocity seismic trace at the selected drill site.
- the iterative procedure may include selecting a multiplicity of normal trend lines having different slopes relative to a semi-log plot of interval transit time versus depth for direct comparison of a selected suit of successive pore pressures over a given depth interval which extends above and below the critical depths of the drilled well, or over the full length of the well by means of:
- FIG. 1 is a log-log scale plot of seismic velocities, in interval transit times, (microseconds per foot) versus depth for strata of common geological ages and based on a normally pressured environment extrapolated to cover depth normally recorded by seismic surveys.
- FIG. 2 is a plot, similar to Fig. 1, but on semi- logarithmic scale representative of interval transit times versus depth, strata of Pliocene age only.
- FIG. 3 is a plot on semi-logarithmic scale, similar to FIG. 2, generated utilizing both stacking velocities and seismic amplitudes, of interval velocities versus depth for a given common depth, or mid-point, of stacked traces on a seismic line. This curve is termed an Interval Transit Time or "ITT" curve.
- FIG. 4 is a refined ITT plot of FIG. 3 with the addition of smooth "trend lines" representing the compactional trend along the recorded curve, representing subterranean geological structure along the downward projection of the surface stacking point.
- FIG. 5 is a plot of the trend lines of FIG. 4 comparing graphically the calculated normal interval velocities of rocks of a single age, such as those shown in FIG. 2, with the addition of representative information regarding various lithological sections, and notes comparing the curves with one another.
- FIG. 6 is a plot of Interval Transit Times (ITT) versus depth for a previously drilled well bore at an offset location, where the smooth curve is representative of a gather of seismic common mid-point traces taken from seismic lines extending downward from a point very close to the offset, (previously drilled) wellbore.
- the second, more erratic curve is a sonic log from the same offset wellbore.
- FIG. 7 is the ITT plot of FIG. 6 with the addition of actual pore pressures in equivalent drilling fluid (or mud) weights (lbs/gal) at depths indicated by dash lines to the left of the number values, as taken from logs run in the offset wellbore and wellbore records.
- FIG. 8 is a graphic representation of pore pressures calculated from a first attempt to graphically fit calculated pore pressure values (lbs/gal) from an assumed overburden normal velocity trend line and the seismic ITT curve to the actual pore pressures, as shown in FIG. 7.
- K g and "Pore Pressure ITT" are respectively aligned with the Depth scale values.
- FIG. 9 is the resulting plot of calculated pore pressures, similar to Fig. 8, from a second attempt to graphically fit a revised normal trend line to the seismic ITT curve.
- FIG. 10 is similar to FIGS. 8 and 9 showing the resulting plot of pore pressures calculated from a third attempt to graphically fit a further revision of the normal trend line and seismic ITT curves to the well data logs, the normal trend line of FIG. 9 has been rotated for a better match with the log-derived data.
- FIG. 11 is a plot of the corrected normal trend line and the known suite of pore pressures, derived from the logs after a sixth trial. The calculated values are also shown after depth adjustment, as noted, to demonstrate a good match.
- FIG. 12 is a plot resulting from the depth adjustment of the seismic generated ITT to match the corrected normal trend with respect to the sonic log recorded in the drilled well. This plot represents the graphical solution of the curve matching process as calculated from FIG. 11.
- FIG. 13 compares a pair of curves respectively representing (1) a suite of pore pressures derived from ITT data, as correctly adjusted for depth and (2) a suite of pore pressures derived from a well bore resistivity log, showing excellent agreement between the two curves.
- FIG. 14 is a plot of an ITT curve generated for a proposed drilling location in the vicinity of the offset well bore and seismic pore pressures derived from data as shown in Figs. 6 to 13. It will be noted that, numerical solutions of the Eaton equations, based on the calibrated normal trend, are depth corrected as indicated by the column "Corr. Depth”.
- FIG. 15 is a plot similar to Fig. 13 of predicted pore pressures versus depth from the data of Fig. 14, for a proposed drilling location as derived from the seismic ITT data corrected by using the method of the present invention to calibrate the normal trend curve.
- FIG. 16 is a plot similar to Fig. 7 but in which the normal interval transit times ⁇ n are calculated from the observed travel time ⁇ f ⁇ , pore pressure gradients, G p and the overburden gradient, G 0 which are listed at depths corresponding to the known pore pressures. The calculated values are plotted to establish the normal trend gradient of the overburden with respect to the ITT at the offset well.
- FIG. 17 is an example of the method of the present invention in which the predicted pore pressures from a properly calibrated normal trend line and a seismic ITT curve are compared to show close correlation to measured pore pressures and mud weights used in actual drilling of the well.
- Figures IB to 45 are plots of graphs, similar to
- Figure 1 to 15 useful in connection with Example II of this specification, as particularly directed to predicting pore pressures in wells in a North Sea drilling environment.
- Figures 46 to 60 are similar plots useful in connection with Example III of this specification, as directed to predicting pore pressures in a carbonate drilling environment.
- pore pressures at given depths below a proposed well site are calculatable from seismic data and a knowledge of the "normal" trend of geostatic pressures through the geological column of earth strata through which the well is to be drilled.
- Seismic information is collected by conventional means known to those skilled in the art as common mid-point (CMP) stacking at both a proposed location and at an offset location adjacent a drilled well.
- CMP common mid-point
- Such seismic information inherently includes velocity information that may be recorded, as "stacking" velocities.
- interval velocities over different portions of the geologic column may be used to generate a curve of interval transit time velocities (in microseconds per foot) versus depth beneath any point on a seismic line. It will be appreciated that this curve is essentially a synthetic sonic log. It is also referred to by those skilled in the art as a "slowness curve".
- Fig. 1 illustrates a series of such "slowness curves" representing interval transit times through five different types of rocks of increasing geological eras or ages. Interval velocities are extrapolated from the earth's surface to a depth of burial of 30,000 feet.
- interval velocities, or ransit times uniformly increase with depth when plotted on a log-log scale.
- the data thus appears as linear curves.
- This interval velocity versus depth data may then be replotted on semi- logarithmic scale, with the vertical linear scale representing depth, in feet, and the horizontal log scale representing interval transit times in microseconds per foot.
- a representative curve 11, such as that shown in Fig. 2 may be drawn to represent normally pressured strata, all of the same geological age, with interval velocity increasing uniformly with depth.
- Curve 11 represents interval transit time (ITT) versus depth for Pliocene strata at all depths on such a semi-logarithmic scale.
- an ITT curve 12 indicates a seismic trace of stacked seismic amplitudes versus interval transit times.
- Recorded ITT curve 12 then may be refined, as in Fig. 4, by generating smooth or "trend" lines 13 and 14 adjacent curve 12 to represent the seismic co pactional trend.
- Trend lines 13 and 14 also serve to highlight abrupt changes in the "normal" velocity trend for lithology changes encountered, as indicated by abrupt changes or breaks at a given depth. It is at these velocity breaks or change points in the velocity curve, where grain to grain contact may be observed in rocks making up the earth formations, rather than the fluid pressures within the rock pore spaces.
- curve 13 progresses downwardly with no additional abrupt shift in the curve, that would indicate additional significant lithological changes.
- another ITT seismic curve 15 is taken from seismic data for CMP's at a location where (1) the top of the abnormal pressure zone was known to be relatively deep and (2) strata above that zone is normally pressured.
- Such synthetic sonic log is plotted along a plot of a normal trend of velocities as indicated by curve 16. If one compares the normal velocity trend of an ITT curve of normally pressured formations, as in Fig. 2, but that curve is shifted to the left, as is done to curve 17, the two curves do not overlap, and may be more easily interpreted. Additional information as to the geological environment may be added to the plot. As shown in Fig.
- Trend line 16 may then be connected across the lithological "tops" (changes in seismic velocities) and the resulting curve used as the ITT curve.
- the method of the present invention for calibrating the normal trend line is then developed from ITT seismic data for CMP's at an adjacent site where a well has been drilled in conjunction with well logs, as confirmed by the geology and other well based measurements.
- interval transit times similar to the seismic ITT data may be directly determined from a sonic log or computed from other logs, such as resistivity, conductivity, density or d c exponents.
- Pore pressures may be determined from such logs by equations developed by Eaton ("The Effect of Overburden Stress on Geopressure Prediction from Well Logs" Journal of Petroleum Tech. Aug. 1972). Specifically the Eaton equation for pore pressures based on an Interval Transit Time, derived from a well bore sonic log, is as follows:
- G pore pressure gradient, psi/ft
- This equation relates pore pressures for a selected depth interval to a relationship between observed values of a parameter (transit time, t) and what the normal values would be for a normally pressured formation occurring at the same depth.
- R 0 , C 0 , d co observed readings; respectively, of resistivity R, conductivity C, and density, d c .
- Example I With pore pressures known from the Eaton equations, the following describes a method for predicting pore pressures at a proposed location utilizing seismic interval transit time curves. The method is described utilizing example data and representative plots, shown in Figs. 6 to 15 by which the various steps achieved the desired calibration of the normal trend. It will be particularly noted that a precise knowledge of the normal trend value is critical to calculate each of the above formulae.
- An offset location should be selected where a well has been drilled through comparable depths of interest and the quality of well log information is satisfactory.
- an ITT curve is generated using seismic information recorded so that a mid-point of a substantial number of pairs of sources and detectors each generates a trace whose reflector point, or mid-point, is common, but each pair has a different path through the earth.
- the common raid, or reflection, point method provides a multiplicity of wave travel paths which allow direct determination of velocities associated with such paths.
- Hyperbolic searches for semblance among appropriately gathered arrays of traces form the basis upon which velocities are estimated. Measured semblances are presented as a velocity spectral display. Velocity spectral displays help to determine the velocity function needed for optimum stacking. A further description of stacking velocities and modern linear seismic reflection methods is given in Section 5.2 of Reflections Seismology. 2nd Edition, by Kenneth H. Waters (1981, John Wiley & Sons),and is incorporated by reference herein.
- Smooth curve 18 of Fig. 6 is such a seismic ITT curve representative of data obtained from multiple traces, suitably time adjusted for differences in distance from each shot point to a detector on seismic lines, which have a common mid-point as close to the wellbore as reasonably possible.
- the second, more erratic, curve 19 represents a sonic log from the same well.
- a straight line, termed a normal trend line 21 is constructed upon seismic ITT curve 18, and pore pressures from these curves are calculated using the Eaton equations. For convenience these may be displayed as shown in Fig. 8.
- Normal trend line 21 is then realigned to whatever position is necessary so that the pore pressures derived from the ITT more closely match those derived from the logs.
- This matching procedure improves the accuracy of the ultimate prediction of pore pressures through the method of the present invention.
- the matching process may be accomplished using a trial and error technique and in some cases several attempts may be required to achieve a desired match.
- the match may also be through an appropriately programmed digital computer to accomplish the trial and error matching process. Many least squares programs for curve matching either graphically or statistically are available to perform and optimize this matching process.
- Fig. 8 it will be noted that the ITT pore pressures (right side) calculated from the Eaton equations are too high when compared to the resistivity "Known Pore Pressures". From this first attempt at drawing a trend line upon the log derived data of Fig. 8, it is observed that the normal trend has a transit time of SI ⁇ sec./ft. at a depth of 13,000 ft. and 170 1 Sec./ft. at 2,000 feet. The Eaton equations then predict a pore pressure too high when compared to the known pore pressure derived from a resistivity log at the same depth, and accordingly it is necessary to adjust trend line 21. Fig.
- FIG. 9 shows a second trend line 23 that is shifted slightly to the right, but parallel to trend line 21 from Fig. 8. Again the Eaton equations are used to calculate pore pressures as predicted from ITT trend line 23, and compared to known pore pressures derived from the resistivity the log. Fig. 9, therefore, shows a somewhat better fit, but a still close match of the ITT and log derived pore pressures may be achieved by better adjustment of the trend line. While the top of the abnormal pressure coincides for the two curves and the seismic ITT pore pressures at the bottom are correct, the ITT pore pressures through the critical mid-section of the well are still too high.
- trend line 25 has been rotated about a point at the top to give a better fit through the mid- section that more closely match pore pressures predicted by the ITT trend line and pore pressure solutions by the Eaton equations of the resistivity log.
- a rotation of trend line 25 as shown in Fig. 10 does not achieve an optimum match.
- the calculated pore pressures at the bottom of ITT curve 25 are too low.
- the ITT curve 18 may, as in this case, also require adjustment for depth.
- sonic log curve 19 drifts to the left while seismic ITT curve 18 drifts right.
- the two curves drift in opposite directions.
- a light table may be useful in analyzing a proper depth adjustment, allowing independent movements for a proper alignment of the curves 18 and 19.
- Fig. 12 shows that a proper match is obtained by shifting the ITT curve 18A 1,700 feet upward relative to sonic log 19.
- a corrected normal trend line 24 may be drawn as shown by Fig. 11.
- an ITT curve is developed in the same manner for the proposed location and plotted as curve 28 in Fig. 14.
- the normal velocity trend line 29 developed and adjusted or calibrated for the drilled well is then used over the same stratigraphic intervals.
- the Eaton equations for Interval Transit Time are similarly used to calculate pore pressures with comparable values of observed to normal Interval Transit Times and to calculate pore pressures, after making the required depth adjustment at the offset location.
- Fig. 15 shows the resulting plot 30 of predicted pore pressures at the proposed drilling location versus depth using the calibrated normal trend line and the seismic ITT computed values of pore pressure (in equivalent mud weight in pounds per gallon) .
- the normal velocity trend of the drilled offset well may be independently computed from a suite of measured pore pressures at known depths and the seismic ITT.
- the pore pressures are desirably actual pore pressures measured from geological or core data.
- the Eaton equations are rearranged to complete a corresponding ⁇ .T n rather than pore pressure gradient, G 'P,' as follows: f * %
- Fig. 16 particularly illustrates the application of the above-noted calculations to define a calibrated normal velocity (or pressure) trend line relative to the seismic ITT curve.
- the suite of ⁇ TJ values over the significant portions of the two curves are plotted in Interval Travel-Time (microseconds per foot) rather than as pore pressures (pounds per gallon) .
- the known pore pressures are plotted numerically along the left side of Fig. 16 and the corresponding AT o or ITT velocities, are plotted alongside the seismic ITT curve 18.
- the calculated suites of values of ⁇ n the normal trend of interval transit times, (for normally pressured and normally compacted rock of the same type at the same depths) is indicated in the right hand column.
- a plot of a properly calibrated normal trend may then be drawn through plotted points 30 as straight line 31.
- the foregoing method of constructing a calibrated normal pressure trend curve may be used either alone or as confirmation of the accuracy of curve fitting the pore pressures calculated from graphic fitting of a normal trend curve and the seismic ITT curve to the suite of measured borehole pore pressures.
- the position of normal curve 31 may be fitted to points 30 by least square calculations generated by a computer program, in a manner well known in the art.
- Fig. 17 shows application of the present method to a well drilled after computing predicted pore pressures.
- substantial increases in pore pressure were expected below 6,500 ft.
- the well was drilled with drilling fluid having a mud weight of 9.5 pounds per gallon down to 5,000 feet and then gradually increased to 10 pounds per gallon at about 6,200 feet and 10.5 pounds at 6,500 feet.
- the pore pressures both predicted and actual (as measured by resistivity) correspond closely through the vertical course of the well.
- Example II The present method was applied to analysis of seismic correlative and seismic volocity (ITT) to predict pore pressures in a North Sea exploratory well prior to drilling. Actual mud weights used in drilling the prospect well are compared to the predicted pressures encountered in the well.
- ITT seismic correlative and seismic volocity
- the steps used to predict such pore pressures included sonic log, pore pressure analysis correlated to the prospect, development of a regional pore pressure overlay for interval transit times, and analysis of ITT information developed from seismic shot mid-points both at the prospect and at a previously drilled offset well.
- the predicted pore pressures show that the actual pressures, determined after drilling the well, indicate the effectiveness of the present invention to predict the measured pore pressure.
- This example identifies the steps necessary to predict pore pressures in most wells, including exploratory and delineation wells. It also illustrates that even with little knowledge of the actual formations to be drilled, whether or not a proposed prospect well is hydraulically related to a drilled offset well so that the pore pressure profile can be reliably predicted for the prospect well.
- Drilling summaries and mud logs Drilling summaries and mud logs.
- Geologic descriptions including lithology types and approximate depths, types of faults (depositional, post depositional) , and geologic age.
- ITT curves generated from seismic shot points at offset and proposed well locations (pseudo sonic logs) .
- the next step in this process was to identify the closest offset wells to the prospect location and ascertain information on these wells that would be sufficient to conduct a comprehensive pore pressure analysis in such a well or wells. Ideally more than one offset well is suggested, but one well can provide enough information to predict pressures to be encountered. The more control wells available the more reliable the prediction will be.
- the offset well for this example was drilled to approximately 13,800 ft. The well reached total depth with a 13.0 ppg. mud weight.
- the prospect location was approximately 6 miles from this offset well.
- the objective in the prospect well was to test sands at approximatley 11,500 feet.
- Figure 18 shows a redisplayed sonic log for the offset well.
- the log is displayed at one inch equals one thousand feet (along the well bore) to enhance determination of changes in lithology penetrated by the well bore.
- This log identifies abrupt shifts in the gamma ray and SP log tracks which correspond to shifts in the interval transit time logs. These shifts show that interval transit times are primarily influenced by lithology and not by pore pressure changes. Such shifts are marked on the log and used as recalibration points in these analysis.
- the indicated lithology changes are preferably compared to lithological information provided by dull chips, cores or other geological data.
- the lithology changes affecting interval transit time can thus be confirmed or amended.
- the identified lithology shifts were then relocated to an expanded sonic log display and marked accordingly. This is depicted in Figure 19.
- a sonic log display of one inch equals one hundred feet is most suitable for determination of interval transit time trends and values.
- Trend lines are drawn on the ⁇ ttrace identifying those areas considered to represent the most uniform shales for each lithological section as shown in Figure 19. As shown, it is not necessary to connect trend lines across recalibration points on the log since changes in absolute £&" values are deemed to be due wholly to lithology changes, which result in different log measured values.
- the trend lines are shifted horizontally, starting with the uppermost trend for reference to form one continuous curve.
- This "recalibration" across lithologies is based on the assumption that the lithology picks are the influential factor in C-T variations, not pore pressures. This then allows for recalibration since the pore pressures immediately above and below a lithology change are normally be the same.
- a continuous trend curve depicts interval transit time values for different lithologies and pore pressure environments penetrated by the drilled offset well. This curve eliminates the influence of lithology changes on £fc, and thus the remaining factor that influences the curve is pore pressure.
- the offset well had a known pore pressure equivalent to 11.8 ppg at 12,175 ft. as measured in a drill stem test.
- Figure 22 shows the interpreted normal trend line for the offset well. As seen in this figure, the trend is based on that portion of the curve above 3000 ft.
- a normal pressure trend line must favorably recalibrate that curve. This step is important for this analysis and those following to form usable ITT curves, as generated from seismic data. Desirably, all available information should be evaluated to best determine which part of the recalibrated trend line represents normal pressure trends.
- the exponent x was experimentally determined to be 3 for the Gulf of Mexico, based on an evaluation of regionally averaged data, the present example required evaluation of pore pressures for the North Sea case to determine the correct exponent for this area .
- Equation 1 This value of x can now be used in equation 1 to generate a sonic pore pressure overlay by solving for ⁇ 7; for various assumed pore pressures. Equation 1 is rearranged to solve for * Z ⁇ as follows:
- 9 ppg .468 psi/ft
- 10 ppg - .520 psi/ft etc.
- the next step in the method of the present invention is to correlate known pore pressure points in the offset well to the prospect well location.
- This step requires interpreted seismic lines which pass through or very near both the offset well, and the prospect well, locations. Additional interpreted lines may be required to tie the two well lines together.
- a seismic base map may be necessary to determine which lines to evaluate. Desirably, as many pore pressure points as feasible are plotted on the offset well trajectory for the best correlative prediction.
- this step requires use of the time/depth conversion from the offset. It is possible that the correlation may identify significant thickening or thinning of some horizons at the offset well which will affect the accuracy of the time/depth conversion at that well for correlation with the offset well. If thickening or thinning occurs, time/depth conversions are determined by converting velocity values at the offset well into interval velocities for each plotted horizon. These interval velocities from the offset well are then applied to the corresponding formations at the prospect well to determine the correct depths correlated to the pore pressure horizons. The three columns represent (1) depth in the well (2) pore pressure (PP) in mud weight equivalents of pounds per gallon and (3) two-way seismic wave travel time.
- PP pore pressure
- the pore pressures were then corrected for structural relief between wells. This analysis assumed that the two locations were hydraulically related and that the pore fluid was water, which is usually the case. Pressures were then corrected in Figure 28 by simply adding or subtracting the hydrostatic pressure of the normal hydrostatic pressure (HP) fluid gradient, exerting pressure equal to .442 psi/ft. For example, the pressure in the offset at 10,800 ft determined from analysis of the sonic log was:
- a pore pressure profile for the prospect is displayed in Figure 29.
- This profile may or may not be accurate. because it assumes a hydraulic relationship between the two locations. However, as noted above, this may not be the case, because a prospect well may be in an entirely different pore pressure environment than the one that exists at the evaluated offset well.
- seismic velocity information is desirably redisplayed into interval transit time (ITT) curves.
- Figure 30 shows the offset well ITT curve comparison to the sonic log in the offset well. This display is on semi-logarithmic paper with interval transit times in micro seconds per foot versus depth on a linear scale of one inch equal 1000 ft.
- the sonic log for the offset well is compared to the ITT to determine where major lithological shifts may occur as shown in Figure 31. Even without a gamma ray log the ITT character closely resembles the sonic log presentation so that it is satisfactory for selecting changes in lithologys. This procedure is performed to determine recalibration or shifts for ITT trend lines, as was done with the sonic log.
- trend lines were established, as seen in Figure 32. These trend lines were moved to a continuous curve and displayed on a semi-logarithmic scale, as seen in Figure 33.
- the ITT curve for the North Sea offset well is shown in Figure 34, with pore pressure overlay developed as was done for the sonic log. In this case, the ITT curve had to be corrected 600 ft to match depths from the sonic log. Note that the exponent x determined from ITT analysis is .338 versus .5219 for the sonic log. This is expected since the interval transit time information is developed from entirely different sources.
- Figure 35 shows a plot of the sonic log and ITT pore pressure interpretations and mud weights used on the well, and to check the accuracy of the overlays. As shown, the two pore pressure profiles track closely, giving confidence in the interpretation.
- the overlay is additionally useful for applications to other seismic generated ITT curves to predict pore pressures in other prospect wells in the same general area of the
- the overlay generated for the ITT on the first offset can be applied to such other ITT curves. In this manner, the accuracy of the overlay can be determined, and further refined, as it is based on additional information from such other wells.
- the overlay generated for the ITT curve at the prospect location can be further evaluated. Lithologies are determined by matching the ITT's between the wells. Similarly, trend lines and recalibration of the curves are performed as before, and from these a continuous ITT curve is developed on a semi-log scale.
- Pore pressures are then determined from this curve by using the overlay developed for the offset ITT.
- the normal compaction, normal pressure trend line is identified using the same procedure as for the offset well.
- the normal pore pressure line on the overlay is then matched to the ITT normal trend line.
- Pore pressures are then determined by reading values from the overlay corresponding to various points and inflections on the ITT curve.
- RFT information was obtained from sand at 11,401 ft. Recorded pressure was 6,884 psi. which equates to a 11.6 ppg equivalent pore pressure. At this depth the present method prediction had estimated an 11.8 ppg pore pressure.
- Figures 44 through 60 illustrate the procedure for predicting and verifying the application to a well drilled in the Dentin Dome area of the Gulf of Mexico.
- the gamma ray and sonic are then displayed in a one inch equals one hundred foot scales. Again smoothing may be required.
- the lithology tops previously determined are translated to this display.
- Next to this data are plotted an unsmoothed version of the gamma ray, as well as an SP (self potential) resistivity and conductivity curves, as in Figures 45 through 49.
- the gamma ray peaks show a trend to the right in the shale direction, as circled in Figures 45 through 49.
- the sonic log velocity trend lines are drawn so that the sonic velocities correspond to the gamma ray intervals previously circled.
- These corresponding sonic velocities have also been circled in Figures 45 through 49 and the corresponding trend lines drawn. It is to be noted that in some instances in these figures that the velocity trend lines appear on the left of the sonic log, and in others on the right.
- the sonic velocity trend lines are then drawn on semilogrithmic paper, honoring lithology tops as in Figure 50.
- the pressure of this formation was approximately 15.2 ppg pore pressure so that formation fluid continued to flow at a rate of roughly 3/4 a barrel per hour with a mud weight as high as 14.9 ppg in the hole.
- the bottom portion of the well experienced a pressure regression and mud weights could be reduced.
- Figures 55 through 60 illustrate excellent results obtained in carbonate regions using this method.
- An appropriately programmed computer may be used to perform any number of the steps in the above- described method and exemplary description.
- Many statistical and graphical software programs are also available and may be adapted by those skilled in the art to perform the method of the present invention.
- the present invention is directed to the use of seismic data recorded over a proposed well site to predict the depths at which over pressured (or underpressured) formations will be encountered at depth.
- Such methods depended upon a detailed knowledge of two measurements of values that are seldom known. These are (1) the "normal” trend of hydrostatic or geostatic fluid pressures down through the same depth intervals and (2) the correlatable seismic "events” (distinguishable seismic reflections) represented as the changes in lithology at depth from the seismic amplitude-time trace.
- the "depths" assigned to the composite seismic trace depend upon a detailed knowledge of the age of the strata traversed by the transmitted and received seismic waves.
- the "interval” velocities of each layer of rock, or strata, or seismic wave from the earth's surface to depth and reflected back to a group of detectors.
- the present invention makes possible accurate prediction of such values from a seismic trace at a proposed drill site by correctly calibrating the normal geopressure trend of strata extending over critical depths of the proposed well so that pore pressures calculated from seismic ITT curves and well logs in a drilled well correspond to pore pressures measured in strata penetrated by a proposed well bore at the correct depth.
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Abstract
Description
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US40865089A | 1989-09-20 | 1989-09-20 | |
US408650 | 1989-09-20 |
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EP19900914581 Withdrawn EP0491841A4 (en) | 1989-09-20 | 1990-09-20 | Pore pressure prediction method |
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EP (1) | EP0491841A4 (en) |
CN (1) | CN1052530A (en) |
AU (1) | AU644106B2 (en) |
BR (1) | BR9007668A (en) |
CA (1) | CA2066760A1 (en) |
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US7490028B2 (en) | 2002-04-10 | 2009-02-10 | Colin M Sayers | Method, apparatus and system for pore pressure prediction in presence of dipping formations |
CN103375162B (en) * | 2012-04-18 | 2016-01-20 | 中国石油化工股份有限公司 | The method of monitoring slit formation formation pressure gradient |
CN103375161B (en) * | 2012-04-18 | 2015-12-16 | 中国石油化工股份有限公司 | The method of monitoring pore type formation pressure gradient |
CN104453879B (en) * | 2014-11-14 | 2017-04-05 | 中国海洋石油总公司 | The Forecasting Methodology of pressure before boring |
US10370955B2 (en) * | 2015-03-12 | 2019-08-06 | Statoil Gulf Services LLC | Method of calculating pore pressure while drilling |
CN106401574B (en) * | 2015-07-28 | 2020-06-19 | 中国石油化工股份有限公司 | Method for predicting formation pressure of high-temperature geothermal well before drilling |
CA3010908C (en) | 2016-02-12 | 2021-01-12 | Landmark Graphics Corporation | Transferring logging data from an offset well location to a target well location |
CN108561127B (en) * | 2018-03-26 | 2022-04-01 | 上海电力学院 | Stratum pressure prediction method based on random simulation |
CN114729564A (en) * | 2019-09-16 | 2022-07-08 | 精准代码人工智能 | Machine learning control for automatic overflow detection and blowout prevention |
CN110929383A (en) * | 2019-10-28 | 2020-03-27 | 中国石油大港油田勘探开发研究院 | Method for calculating height of oil-gas column under source |
US20220229201A1 (en) * | 2021-01-19 | 2022-07-21 | Saudi Arabian Oil Company | Pore pressure in unconventional formations |
CN115680638A (en) * | 2021-07-26 | 2023-02-03 | 中国石油化工股份有限公司 | Method for identifying overpressure top seal layer by utilizing pressure attenuation gradient |
CN113803063B (en) * | 2021-10-29 | 2023-08-22 | 中国石油天然气股份有限公司西南油气田分公司勘探开发研究院 | Method for defining flow state limit of reservoir cracks of natural gas reservoir |
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US3382933A (en) * | 1966-01-21 | 1968-05-14 | Shell Oil Co | Process for drilling geopressured formations without encountering a kick |
US4399525A (en) * | 1979-10-05 | 1983-08-16 | Chevron Research Company | Method for interpreting well log records to yield indications of gas/oil in an earth formation such as a sandstone, limestone, or dolostone |
US4817062A (en) * | 1987-10-02 | 1989-03-28 | Western Atlas International, Inc. | Method for estimating subsurface porosity |
-
1990
- 1990-09-20 BR BR909007668A patent/BR9007668A/en not_active Application Discontinuation
- 1990-09-20 EP EP19900914581 patent/EP0491841A4/en not_active Withdrawn
- 1990-09-20 AU AU64434/90A patent/AU644106B2/en not_active Withdrawn - After Issue
- 1990-09-20 CN CN90108841.2A patent/CN1052530A/en active Pending
- 1990-09-20 WO PCT/US1990/005263 patent/WO1991004500A1/en not_active Application Discontinuation
- 1990-09-20 CA CA002066760A patent/CA2066760A1/en not_active Abandoned
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CA2066760A1 (en) | 1991-03-21 |
CN1052530A (en) | 1991-06-26 |
AU644106B2 (en) | 1993-12-02 |
EP0491841A1 (en) | 1992-07-01 |
BR9007668A (en) | 1992-06-02 |
AU6443490A (en) | 1991-04-18 |
WO1991004500A1 (en) | 1991-04-04 |
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