Method and apparatus for oil and gas exploration.

0001The present invention relates generally to apparatus for and a method of processing a plurality of samples of the earth's subterranean formations, and hence is concerned with geophysical exploration. The invention relates more particularly to a method of and apparatus for obtaining selected physical properties from samples of the earth's subterranean formations surrounding a wellbore and for obtaining inverse logs of the selected physical properties along the length of the wellbore for evaluating the oil and gas bearing potential of the earth's subterranean formations.

0002In the continuing search for oil and gas, explorationists have developed a wide array of geophysical exploration methods for imaging the earth's subterranean formation structures and displaying those images as well as providing estimates of selected formation characteristics such as lithology, porosity, density, and velocity of propagation of seismic energy in the earth's formations. Exemplary of such geophysical exploration methods are seismic surveys, gravity surveys, and magnetic surveys. The principal geophysical method for oil and gas exploration within the earth's subterranean formation is seismic exploration. Simplistically, seismic exploration comprises imparting seismic energy into the earth and recording the earth's response thereto. The earth response can include both refracted and reflected seismic energy. As the seismic energy propagates through the earth, it is controlled by the physical properties of the rocks and the fluids contained therein. Unfortunately, explorationists cannot directly measure these physical properties which constrain the propagation of seismic energy from the recorded seismic survey data. Rather, seismic exploration techniques merely provide a measure of the time it takes the seismic energy to traverse some path. In fact, it is often times impossible to precisely ascertain the path taken by the seismic energy. In view of such uncertainties, how is it that explorationists are able to estimate formation lithology, fluid content or even simple, reconstruct formation geometries through which the seismic energy propagates with only a measure of time? Quite simply, the answer is explorationists cannot without the aid of additional information such as estimates of the velocities of propagation of seismic energy in rock and rock density as well as other rock and fluid physical properties. Methods for estimating the velocity of propagation and a density as a function of depth are generally known to those skilled in the art; however, velocity estimations are the weakest element in the inversion of seismic time data.

0003Often times, wells which have been drilled based on geophysical survey data are logged using one or more of the well logging techniques such as acoustic logging, resistivity logging, neutron logging, etc. Results of such well logging techniques can provide the explorationist with selected physical properties generally over regions of interest along the wellbore length. With such well logging techniques, explorationists are able to obtain better estimates of the formation physical properties for evaluating geophysical survey data. However, it must be recognized that such well logging techniques still only provide an indirect measure of the formation physical properties of interest and are generally limited to only selected regions of interest within a wellbore.

0004Occasionally, core samples are obtained from limited regions of interest within a wellbore for transport to a central laboratory so that physical properties of the formations adjacent the borehole, such as fluid content of the sample, (U.S. Patent 2,458,093 M. Muskat, et al., January 4, 1949), oil content of the sample (U.S. Patent 2,531,083, A. L. Smith, November 21, 1950), and electrical resistivity of the sample (U.S. Patent 2,613,250, H. L. Bilhartz, et al., October 7, 1952), can be obtained. Unfortunately, such core samples generally represent only a small fraction of the formational lithologies traversed by the wellbore along its length. Moreover, the delay associated with transporting the samples to the central laboratory can preclude the measurement of properties of certain friable lithologies or result in seriously degraded measurements of certain properties. During the drilling process, chips which have broken away from the subsurface formations can be captured from the drilling fluid at the surface for analysis. Typically, explorationists have identified the lithologies of the subsurface formations from such chips. However, determining the exact depth in the wellbore from which the chips originated is generally not possible due to the very nature of their transport to the surface. Consequently, core samples and chip samples fail to provide either a regular sampling of borehole formation lithologies along the length of the wellbore or fail to accurately determine the depth of origin of such samples so that logs of formation characteristics, as a function of depth over generally the entire wellbore length, cannot be produced. Moreover, the very nature of the chip samples can often times preclude measuring certain formation characteristics (for example, measurement of rock velocity or compressive strength).

0005Other prior art is as follows. Firstly, a method for <u style="single">in</u><u style="single">situ</u> determination of the cation exchange capacities of subsurface formations is known from United States Patent No. 4,263,509. A method is disclosed for the in situ examination of each subsurface formation penetrated by a borehole to ascertain the cation exchange capacity of such formations within a geological region. Natural gamma ray logging is used to develop signals functionally related to the total gamma radiation and to the potassium -40, uranium and thorium energy-band radiations. A first borehole is traversed by a potential gamma ray spectrometer to provide selected measurements of natural gamma radiation. Core samples are taken from the logged formation and laboratory tests performed to determine the cation exchange capacity thereof. The cation exchange capacities thus are developed then correlated with selected parameters, provided by the gamma ray spectrometer to establish functional relationships. Cation exchange capacities of formations in subsequent boreholes within the region are then determined in situ by use of the natural gamma ray spectrometer and these established relationships.

0006Secondly, apparatus for photographing well core samples is disclosed in United States Patent No. 3,373,440. Well core samples are photographed by arranging them on a conveyor end-to-end in a string in the same order they occupied prior to removal from the well borehole, moving the conveyor continuously while actuating the shutter of a camera directed at the string by automatic means coupled to the conveyor so that the shutter is actuated at uniform intervals during which the string moves a distance equal to the length of the field of view of the camera, and advancing the film between successive exposures a distance substantially equal to the length of the film exposed on each such actuation to form a continuous strip photograph of the string.

0007Thirdly, a method of machine processing well logging data derived from borehole exploring devices which investigate earth formations traversed by boreholes is known from United Kingdom Patent No. 1584060. The method comprises deriving a plurality of measurements related functionally to a respective plurality of different earth formation characteristics along borehole sections traversing earth formations in a field; forming first data sets, each comprising a defined combination of said measurements; combining at least a defined portion of said first data sets to form a statistical model of the traversed earth formations; and combining second data sets derived from a borehole in the same field with said statistical model to determine what modification of the second data sets may be necessary to bring them into a defined statistical conformity with the statistical model.

0008In spite of current geophysical exploration methods, explorationists are not provided with direct measures of the physical properties of the formations surrounding a wellbore along its length. Rather, attempts to evaluate and interpret geophysical survey data must generally depend on estimates of formation physical properties or on indirect measures of the formation physical properties from only limited regions of interest in the earth's subsurface. Consequently, explorationists are unable to accurately calibrate the vertical extent of their geophysical survey data with direct measures of the earth's physical properties so as to better evaluate the oil and gas bearing potential of the earth's subterranean formations.

0009In order to overcome the difficulties noted above, the present invention seeks to provide a novel method of and apparatus for obtaining selected physical properties from samples of the earth's subterranean formations surrounding a wellbore and developing inverse logs of the selected physical properties along the length of the wellbore. Explorationists can employ such inverse logs of the selected physical properties to perform basin analysis whereby the physical properties obtained at one or more wellbores can be used to calibrate geophysical survey data at each wellbore location and then extrapolate such physical properties to other locations in the basin with the geophysical survey data so as to evaluate the oil and gas bearing potential of the earth's subterranean formations.

0010According to the present invention, there is provided apparatus for processing a plurality of samples of the earth's subterranean formations so as to obtain a plurality of physical properties for each sample representative of their in-situ condition, comprising (a) a unique machine readable sample code label for each sample; (b) a plurality of measurement stations for measuring a plurality of selected physical properties, wherein each measurement station is adapted to measure at least one physical property for each sample and wherein there is a unique machine readable station code for each measurement station; (c) reader means for reading the sample codes and station codes for a sample at a selected measurement station; and (d) central processing means (CPU) for initiating the measurement of at least one physical property of a sample at a selected measurement station upon receiving a station code and a sample code from the reader means and for storing said physical property measurement.

0011In a related aspect, the present invention provides a method of processing a plurality of samples of the earth's subterranean formations at a plurality of measurement stations so as to obtain a plurality of physical properties for each sample representative of their in-situ condition, comprising the steps of: (a) labelling each sample with a unique machine readable sample code; (b) assigning each measurement station a unique machine readable station code; (c) machine reading the sample code and station code of a sample at a selected measurement station into a central processing unit (CPU); and (d) arranging the central processing unit to be responsive to receipt of the sample code and the station code, to initiate a measurement of at least one physical property of the sample at the selected measurement station, and to store the measurement of the physical property.

0012In one preferred embodiment of the invention, discrete samples can be obtained at selected intervals along the length of a wellbore so as to be representative of the formation lithologies traversed by the wellbore along its length. The discrete samples can then be processed at the wellsite with a transportable laboratory so as to obtain measures of a plurality of selected physical properties of the subterranean formations representative of their in-situ condition. An inverse log of each physical property as a function of depth can be prepared with which explorationists can evaluate geophysical survey data for oil and gas potential. In more detail, measures of the subterranean formation's compressional wave velocity, shear wave velocity, grain density, saturated density, dry density, magnetic susceptibility, compressive strength, and static elastic moduli can be determined from which one can also calculate the formation's acoustic impedance and porosity.

0013Inverse logs of one or more of the physical properties can be used to perform basin analyses. When used for basin analysis, the physical properties of each inverse log can be used to calibrate geophysical survey data of the basin at one or more wellbore sites within the basin from which the inverse logs were obtained. Having thus calibrated the geophysical survey data, explorationists can then extrapolate the physical properties to other locations in the basin remote to the wellbore site with the geophysical survey data to evaluate the oil and gas potential of the earth's subterranean formations at such remote locations.

0014In another preferred embodiment of the invention, a transportable laboratory is provided which is especially adapted for processing, at a wellsite, large numbers of discrete samples obtained along the length of a wellbore in a minimum amount of time so as to obtain measures of the formations physical properties which more properly reflect their in-situ condition. Timely measurement of the physical properties of certain friable lithologies (such as shales) is essential if physical properties are to be obtained at all. Additionally, the in-situ measures of certain physical properties can be seriously degraded by a delay in measurement e.g., seismic energy velocities of propagation in the rock, and porosity. Surprisingly, such properties have been found to vary over a wide range of values simply due to a delay in the time of their measurement.

0015The transportable laboratory comprises a plurality of highly automated measurement stations each connected to a central processing unit (CPU). By labeling each sample with a unique machine readable sample code and each measurement station with a unique machine readable station code, reader means can be provided for reading the sample code of each sample and the station code of a selected measurement station into the CPU. Upon receipt of valid sample and station codes, the CPU initiates the measurement of the sample at the selected measurement station and stores the measurements in a data file associated with the unique sample code. The CPU can also calculate additional physical properties from selected measured physical properties. Thus, the transportable laboratory becomes in effect a "paperless" laboratory since manual recordation of a sample's identity and its measured and calculated physical properties has been eliminated. The paperless laboratory greatly reduces errors generally associated with manually recording such information. In addition to initiating such measurements and recordation, the CPU can initiate a prescribed sequence of measurements at a given measurement station as well as collect, collate and evaluate the physical properties measured to provide inverse well logs of the measured properties as a function of depth. Additionally, the CPU can calculate additional physical properties from the physical properties measured as well as reinterpret geophysical survey data and models at a remote site.

0016The present invention will be better understood with reference to the following drawings and detailed description, which are purely by way of example. In the drawings:- <ul id="ul0001" list-style="none"><li>Figure 1 is a schematic representation of a transportable laboratory according to the present invention;</li><li>Figure 2 is a generalized flow diagram for the operation of each measurement station in the transportable laboratory;</li><li>Figures 3 (a, b, c) depict inverse logs of velocity measured from samples obtained at selected intervals along three separate wellbores;</li><li>Figures 4 (a, b, c) depict inverse logs of porosity measured from samples obtained at selected intervals along three separate wellbores;</li><li>Figures 5 (a, b, c, d, e, f) depict inverse logs of formation properties density, compressional velocity (V_p), shear velocity (V_s) V_p/V_s ratio, impedance and magnetic susceptibility from samples obtained at generally 3 metre (ten foot) intervals along the length of the wellbore; and</li><li>Figures 6 (a, b, c, d, e) depict smoothed inverse logs of density, compressional velocity (v_p), shear velocity (v_s), the ratio v_p/v_s, and formation impedance, respectively, while Figure 6 (f) represents synthetic seismic traces obtained by convolving a source wavelet with the inverse log of formation impedance shown in Figure 5 (e);</li><li>Figures 7 (a, b) show a section of a seismic survey in Fig. 7a, and Fig. 7b depicts an inverse log of formation impedance obtained from samples collected along the length of the wellbore indicated by the derrick structure in Fig. 7a.</li></ul>

0017The following discussion is provided in order to more completely understand the present invention. In the continuing search for oil and gas, explorationists employ a variety of geophysical and geological survey techniques to identify regions having potential oil and gas formations. By their very nature, such geophysical and geological survey techniques generally provide only indirect estimates of selected subsurface formation properties which the explorationist uses to interpret potential oil and gas formations. Moreover, such survey techniques generally depend on at least estimates of selected physical properties of the earth's subterranean formations before any meaningful interpretation can be made of either the formations' structure or their oil and gas bearing potential

0018By way of example, explorationists often times employ the time it takes seismic energy to travel from the earth's surface to a reflecting horizon and back again to infer structure, lithology, porosity fluid content as well as the path traversed by the seismic energy. Similarly, magnetic surveys can provide estimates of formation structure. However, geophysical and geological survey techniques do not generally directly measure the properties of interest, but rather simply provide indirect measures of other characteristics of the formation from which it is possible for the explorationist to estimate the physical properties of interest.

0019After a wellbore is drilled, well logging techniques (such as sonic logging, electric resistivity logging, gamma ray logging, gravity logging, neutron logging, etc.) can provide additional estimates of the subsurface formation properties of interest. However, additional sources of error (such as borehole rugosity, mudcaking, varying borehole dimensions, variation of and borehole fluids) can be introduced by such logging techniques which may adversely affect the quality of the estimates of the formation physical properties. On occasion, wellbores have been cored to provide a few samples of selected formations over limited intervals adjacent the wellbore from which direct measures can be obtained. Generally, such samples are obtained at a wellsite and then transported to a central facility for measurement some time thereafter. However, it has now been found that certain formation properties (such as velocity and porosity) can be substantially altered if they are not measured shortly after removal from the wellbore, while the physical properties for certain friable lithologies are simply not obtainable at all if not done within a few hours after their removal from the wellbore. Moreover, even for those formation physical properties not affected by time, the physical properties measured are generally measured on only a handfull or so of samples over a very short interval of the wellbore. Consequently, the explorationist is simply not provided with direct measures of formation physical properties for substantially the entire length of the wellbore. Hence, the value of such information is limited to regions adjacent the borehole from which the samples came and the explorationist is left to evaluate and interpret the balance of his geophysical survey data with only estimates of the subterranean formation physical properties.

0020In one preferred embodiment of the invention, core samples can be obtained over substantially the entire length of a wellbore; however, an important aspect of the present invention is that discrete samples of the earth's formations surrounding a wellbore need only be taken at selected intervals along the length of the wellbore. One criteria for establishing the sampling intervals is that discrete samples generally be obtained for every formation lithology traversed by the wellbore along its length. Since the physical properties of the formations surrounding the wellbore can also be used to evaluate and calibrate geophysical survey data (e.g., seismic survey data), the minimum resolution interval of the geophysical survey data being evaluated establishes another criteria for the sampling interval. Typically seismic data has the finest resolution interval of any geophysical survey technique (approximately 30 m (100 ft)). In practice, we have found that a sampling interval of one sample every 3 m (10 ft) adequately addresses the criteria of lithology changes as well as the geophysical survey minimum resolution interval criteria so as to address the problems of heterogeneous rather than homogeneous earth formations so that the physical properties of the samples can be correlated with geophysical survey data without introducing sampling bias.

0021In another preferred embodiment of the invention, a transportable laboratory is provided so as to process samples obtained from the earth's formations surrounding a wellbore along its length. Since timely measurement of certain physical properties and of certain lithologies can be important, the transportable laboratory is preferably configured for field operation in the vicinity of the wellbore. Generally, the transportable laboratory comprises a plurality of measurement stations, each adapted to measure at least one physical property of each sample. Additionally, the transportable laboratory includes a central processing unit for identifying samples, initiating measurements on the samples at selected measurement stations, and storing the measured physical properties in a sample data file.

0022Prior to discussing the operation of each measurement station of the transportable laboratory A, the basic sequence of steps for all operations will be discussed with reference to both Figures 1 and 2. Every sample which enters the transportable laboratory A is given a unique machine readable sample code (which includes a well identifier, date and depth). Preferably, such sample codes are included on a label adapted to be mounted on each sample. Every measurement station is assigned a unique machine readable station code. Preferably, the sample codes and station codes are bar codes which can be read by a laser bar code reader into a central processing unit (CPU). As such, the transportable laboratory A is capable of processing a large number of samples through each of the various measurement stations employing multiple, asynchronous flow paths, until each sample has been through each measurement station. To initiate a measurement sequence at any measurement station, the machine readable station code (preferably a bar code) assigned to a selected measurement station is read into the CPU using a laser bar code reader 70 at step 100. The sample to be measured at the selected measurement station then has its unique machine readable sample code (preferably a bar code) read into the CPU with the laser bar code reader 70 at step 110. If the station code and the sample code are both valid codes, the CPU directs the selected measurement station at Step 120 to initiate a sequence of measurements unique to the selected measurement station and the set of measurements obtained are then stored in the CPU at step 130 in a sample data file associated with the sample's unique sample code.

I. SYSTEM

0023Looking now to Figure 1, a schematic representation of the transportable laboratory A is shown. Although the transportable laboratory A is particularly suited to field needs, those skilled in the art will appreciate that the transportable laboratory satisfies many of the needs of a central stationary laboratory. Generally, the transportable laboratory A comprises a plurality of uniquely identified measurement stations connected to a central processing unit (CPU) such as a Hewlett-Packard 9000 series 500 computer and a laser bar code reader 70 as manufactured by Intermec. The CPU also includes a mass storage system (not shown) for collecting and storing the physical properties obtained from the various samples which are processed by the transportable laboratory A.

0024In particular, the transportable laboratory A includes an electronic dimension jig 10 for digitally measuring sample lengths and diameters. The dimensioning jig 10 includes pairs of opposed linear voltage displacement transducers (LVDT) adapted to measure axial and lateral dimensions of each sample. The transportable laboratory A also includes an electronic balance 20 adapted to measure the weight of each sample. Further, an electronically driven pycnometer 30 is provided to measure grain density of each sample; a triaxial loading apparatus 40 is provided to obtain measures of each sample's compressive strength, as well as provide an input for calculating Young's modulus and Poisson's ratio for each sample; an electronic magnetic susceptibility meter 50 is provided to obtain a measure of the magnetic susceptibility for a given weight of sample; and a pair of electronically driven pressure velocity cells 60 are provided to measure the velocities of propagation of compressional (P), horizontal shear (SH), and vertical shear (SV) waves in each sample as a function of pressure. A laser bar code reader 70 is provided to read both the unique sample codes assigned to each sample and the unique station codes assigned to each measurement station into the CPU such that the CPU can initiate a measuring sequence on a sample at a selected measurement station and store the results of such measurement in a sample data file within the CPU memory. The CPU accumulates all the measurements from the various measurement stations for a particular sample in the sample data file. The individual sample data files can be displayed at a printer or plotter 80 or a CRT screen 90, or be transmitted via satellite to a central location with a satellite communication station 95. Additionally, each physical property from the samples can be sequenced according to depth to produce inverse logs of selected physical properties representing the formations surrounding the wellbore along its length or used to form cross plots of selected physical properties for trend analysis.

0025The transportable laboratory A of the present invention provides means for obtaining measurements of the following physical properties from discrete samples taken at selected intervals along the length of a wellbore: grain density, dry bulk density, saturated bulk density, magnetic susceptibility, compressive strength, elastic moduli, and seismic velocities (P, SH, SV) versus pressure. With these direct measurements one can calculate many other physical properties of interest. For example, the difference between the grain density and the dry bulk density yields a porosity estimate; likewise, the difference between the saturated and dry bulk densities yields another estimate of porosity. Further, combinations of saturated density and seismic velocities yield dynamic elastic moduli, acoustic impedance and reflection coefficients. Piecewise integration of the seismic velocity over the depth interval sampled can yield an estimate of the two-way traveltime, which can be used to calibrate the time observed on a seismic section to a true depth. Additionally, synthetic seismic traces can be produced by convolving an inverse log of formation impedance with a wavelet having known characteristics for the calibration of seismic survey data. A more complete discussion below will detail uses of the measured physical properties and the inverse logs derived therefrom.

II. OPERATION

0026Each discrete sample obtained at selected intervals along the length of a wellbore is assigned a unique machine readable sample code, preferably a bar code, which can be mounted with the sample or attached with its sample holder. All measurement sequences begin by entering the measurement station code and sample code via the bar code reader 70 into the CPU. Hereafter, the step of reading the station code and sample code will be understood in the operation of each measurement station. In order to carry out the desired measurements at the various measurement stations shown in Figure 1, three separate samples are cut from each discrete sample. The three samples obtained from each discrete sample are each generally right circular cylinders having lengths of approximately 13mm, 38m and 70mm (0.5", 1.5" and 2.75"). With reference to Figures 1 and 2, generally the detailed steps to obtain at least one physical property of each sample at each measurement station will be discussed. In order to simplify the following discussion, the operation of the transportable laboratory A will be made with respect to the operations which occur on each separately sized sample

0027The 13mm sample will be referred to hereafter as the disc sample. The initial measurements made on each disc sample are its length and diameter at 10 and its saturated weight at 20. The disc sample is dried and then placed in a vacuum oven for an additional period. The dried disc samples are then weighed again at 20 to obtain their dry weight. The dry weight can be used by the CPU in calculating the dry bulk density of each sample. About 20 gms of the disc sample are subsequently crushed to a particulate size of 6mm (0.25") or less. A portion of the fragmented disc sample is then placed in a pycnometer cup and reheated in a vacuum oven. The pycnometer 30 is used to obtain a measure of the disc sample's grain density. The pycnometer 30 measures a volume of space occupied by the fragmented sample and pycnometer cup using helium gas. Because the measurements by the pycnometer 30 takes the longest time of any measurement station, the system includes multiple pycnometers 30 which are each interfaced to an IBM AT computer (IBM) (not shown) so as to permit automated operation of each pycnometer which in turn is connected with the CPU.

0028After receiving a valid sample code and pycnometer station code, the CPU signals the selected pycnometer to begin its measurements. The fragmented disc sample is inserted into the pycnometer's sample chamber. Upon completion of the measurement cycle, the IBM reads a measured volume and divides by the recorded sample weight to arrive at a grain density for the fragmented disc sample. This value along with the sample code and pycnometer station code are sent back to the CPU for storage and further processing.

0029The fragmented disc sample can also be used for magnetic susceptibility measurement at 50. Here the fragmented disc sample is poured into a polyethylene vial of known weight and reweighed at 20 and its specific magnetic susceptibility is measured using a Bartington magnetic susceptibility meter at 50. When the magnetic susceptibility measurement has been completed, an average of five readings can be sent to the CPU memory.

0030The 38mm sample can be used to measure the seismic velocities of the discrete samples at pressures appropriate for their in situ depth. The magnitude of seismic velocity in rock is sensitive to the concentrations of cracks and pores. Since cracks exhibit a strong pressure dependence, seismic rock velocities can exhibit a strong pressure dependence. Therefore, to make meaningful velocity measurements on rock, one must subject it to the pressure and saturation it experiences in situ. This is done most effectively by making velocity measurements on the 38mm samples at a plurality of pressures which bracket the in-situ pressure appropriate for each discrete sample.

0031The flat faces of the 38mm samples are first polished to remove cutting marks and to make the faces more parallel for dimension measurements at 10 and velocity measurements at 60. A bar code label can be affixed to the perimeter of the 38mm sample with mylar tape. One side of the 38mm sample is marked to indicate a preferred bedding direction for alignment with respect to the shear wave polarizations. The 38mm samples are kept in a water bath while awaiting velocity measurement so as to maintain their fluid saturation. The 38mm samples are typically processed in batches since each pressure velocity cell 60 has positions for multiple samples.

0032The 38mm sample processing begins by measuring its dimensions at measuring station 10. The 38mm sample weight can then be obtained at measurement station 20. Having the 38mm sample dimensions and weight, one is ready to measure the velocities of the compressional (P) and shear (S) waves in the 38mm samples as a function of pressure using a pulse transmission technique.

0033Each 38mm sample is inserted between a set of end caps. The relative orientation of each 38mm sample with the end cap provides a mechanism for determining the relative anisotropic orientation of the sample. Piezoelectric transducers assemblies are mounted within both end caps. The 38mm sample and end cap assembly are then installed in the pressure velocity cell 60 and electrical connectors are attached. These connectors permit the transmitted and detected signals to be applied and received from the appropriate transducers. The operator selects a pressure velocity cell by reading the appropriate station code and sample code via the bar code readers. Once the CPU decides there is a valid sample code, a control program begins filling and pressurizing the pressure velocity cell 60. An acquisition program is activated by the CPU when equilibrium at programmed pressure set points is achieved. Five time series are collected for each sample at each pressure point. When the measurements at the pressure velocity cell 60 are complete, the samples are removed and reweighed at 20. The new weight is the saturated weight. Saturated weight plus sample volume obtained from sample dimensions yields a saturated density.

0034With the measurement of the compressional (P) and shear (S) wave velocities of each 38mm sample, the CPU can calculate a measure of the 38mm samples anisotropy, the magnitude of the difference between the fast and slow shear velocities, and saturated density. When these measurements are coupled with disc sample measurements, one can estimate sample porosity i.e., difference between saturated and dry density provide a saturated porosity estimate.

0035The remaining 70mm sample (hereafter plug) is plugged parallel to its long axis with a 25mm (1") diameter hollow core bit. The ends are trimmed parallel and flat with a slow speed precision cutoff saw. The plug sample is used in the triaxial loading apparatus 40 to determine its compressive strength, Young's modulus, and Poisson's ratio. The plug sample length and diameter are measured at 10. A radial LVDT is positioned at the midpoint of the plug sample and an axial LVDT is mounted on one end of the plug sample and extension rod on the other. The plug sample can then be inserted into the triaxial loading apparatus 40. The CPU first sets a confining pressure within the triaxial loading apparatus 40 to a value appropriate for the sample's depth within the wellbore and then raises the pore pressure to a fixed value to assure sample saturation. The CPU then initiates axial loading of the plug sample at a programmed rate, nominally 10-6 sec-1, until failure of the plug sample. Axial and radial displacements from the LVDT's are constantly recorded by the CPU during loading. These values can be normalized by the original dimensions to provide axial and radial strains. The CPU can also determine the slope of the axial strain versus axial stress to yield a value for Young's modulus. The incremental ratio of radial to axial strain can provide values of Poisson's ratio. The maximum stress (force/sample area) reached during the test is the compressive strength of the plug sample.

0036Separate operating programs allow the CPU to process a plurality of samples through multiple, asynchronous flow paths until each sample has gone through each of the various measurement stations. These programs run in their own virtual machines within the CPU.

0037The most complicated program is MAIN. This program directly interfaces to the bar code readers 70, magnetic susceptibility meter 50, balance 20, pycnometer, digital dimension jig 10 and memory storage devices (not shown) and indirectly with the pressure velocity cell 60 and the triaxial loading apparatus 40. All bookkeeping and sample data file entries are done through the program MAIN. When a sample code is recognized by the CPU, it is assigned an external sequence number which consists of year, Julian day and daily run number. An active discrete sample code list is maintained by MAIN. The sample code list includes filenames and mass storage locations, The filenames correspond to the measurement results from selected measurement stations. When a sample code is entered by the laser code reader 70, MAIN first checks the active sample code list; if the sample code exists, it will proceed to the proper measurement subroutines. If the sample code is not found on the active sample code list, a set of data files are created and the sample code is added to the active sample code list and measurements can then be carried out. Whenever the active sample code list is updated or a new measurement is performed, the sample codes and status can be displayed on the CRT 90.

0038There are a few measurements which are more involved than weighing or measuring sample dimensions such as velocity and compressive measurements. For such measurements, additional programs which vary extrinsic variables and programs to measure intrinsic properties of the samples as a function of the variation in extrinsic variables are provided.

III. FORMATION EVALUATION

0039With the direct measures of a plurality of selected physical properties of the formation surrounding one or more wellbores along their length, explorationists are now in a position for the first time to more intelligently use geophysical survey data to evaluate the oil and gas bearing potential of the earth's subterranean formation both during the well drilling process at the wellsite as well as after completion of the well at a remote facility. Looking first to Figures 3, 4, 5, and 6, inverse logs of measured and calculated physical properties obtained from samples of the earth's formations at selected intervals along the length of a wellbore are shown. In particular, Figures 3 (a, b and c) each represent inverse logs of the compressional wave velocity (V_p) in samples obtained at selected intervals along the length of three separate wells. The velocities depicted in Figure 3 represent the velocity in a particular sample unlike previous velocity estimates which are really no more than an averaged or integrated velocity. The importance of such distinction is that acoustic impedance changes in the earth which provide an indication of the nature of the reflection and refraction of seismic energy depend not on averaged or integrated velocities; but rather, on the interval velocity of seismic propagation in each formation lithology.

0040Figures 4 (a, b, and c) depict inverse logs of formation porosity measured from samples obtained at selected intervals along the length of three separate wellbores. Additionally, Figures 5 (a, b, c, d, e and f) depict inverse logs of formation density, compressional wave velocity (V_p), shear wave velocity (V_s), the ratio V_p/V_s, formation impedance and magnetic susceptibility, respectively. The sample interval for the inverse logs of Figure 5 is much finer than the minimum resolution thickness of conventional geophysical survey data. Consequently, such oversampling tends to produce inverse logs which have a very "spiky" appearance as depicted in Figure 5. However, when the inverse logs of Figure 5 are smoothed with a smoothing operator having a wave length generally equal to the minimum resolution thickness of the geophysical survey data with which it is to be compared, the inverse logs take on an appearance similar to those in Figures 6 (a-e). In fact, the inverse logs of Figure 6 (a-e) were smoothed with a smoothing operator having a minimum resolution thickness appropriate for seismic survey data. The synthetic traces of Figure 6 (f) were produced by convolving a wavelet signal having characteristics of a seismic source with the smoothed inverse impedance log of Figure 5 (e) and correspond well with actual seismic survey data.

0041The importance of having such inverse logs of formation properties along the length of a well both during the drilling process and upon completion of the drilling process can be very important. By way of example with the inverse logs of formation velocity to the point of current drilling within the wellbore, the original geophysical survey data can be reprocessed to forecast depth to target. Additionally, inverse logs of formation compressive strength obtained during the drilling process can provide the drillers with information important for controlling drilling parameters.

0042With inverse logs of formation physical properties obtained at selected intervals along the wellbore, the geophysical survey data can be calibrated to the physical properties represented in the inverse log. In particular, Figures 7a and 7b represent a seismic time section and an inverse log of formation impedance measured from samples obtained at selected intervals along the length of a wellbore. In particular, the inverse impedance log of Figure 7b was obtained at a well site indicated by the derrick structure in Figure 7a. With such comparison of the inverse log of impedance and the seismic section one can calibrate the physical properties of the formation at the location of the wellbore site. Having calibrated the geophysical survey data (in this case the seismic survey data) at the wellbore site, geophysicists can then use the calibrated geophysical survey data to extrapolate the physical properties to remote locations in a basin.

0043Thus, the present invention seeks to provide a novel method of and apparatus for obtaining selected physical properties from samples of the earth's subterranean formations surrounding a wellbore and developing inverse logs of the selected physical properties along the length of the wellbore. Explorationists can employ such inverse logs of the selected physical properties to perform basin analysis whereby the physical properties obtained at one or more wellbores can be used to calibrate geophysical survey data at each wellbore location and then extrapolate such physical properties to other locations in the basin with the geophysical survey data so as to evaluate the oil- and gas-bearing potential of the earth's subterranean formations.

0044It should thus be understood that the invention is not limited to any specific embodiment set forth herein as variations are readily apparent and thus the invention is to be given the broadest possible interpretation within the terms of the following claims.