Abstract:
Data from a string of multiple formation evaluation data sensor are evaluated by an expert system. Based on the analysis, the logging speed is increased if all the sensors justify it, and is reduced if any of the sensors require a reduced logging speed. Alternatively, the sensitive volume of a NMR sensor is altered based on a determination of a fraction of the sensitive volume that includes a borehole fluid. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This is a divisional application of U.S. patent application Ser. No. 11/179,990, filed on Jul. 12, 2005. U.S. patent application Ser. No. 11/179,990 is a continuation-in-part of U.S. patent application Ser. No. 10/819,665, filed on Apr. 7, 2004 (now U.S. Pat. No. 7,117,733) and is also a continuation-in-part of U.S. patent application Ser. No. 10/828,812 filed on Apr. 21, 2004 (now U.S. Pat. No. 7,193,414) which is a continuation of U.S. patent application Ser. No. 09/928,768 filed on Aug. 13, 2001 which has issued as U.S. Pat. No. 6,727,696. 

   FIELD OF THE INVENTION 
   This invention relates generally to methods of improved logging using oilfield borehole tools and more particularly to dynamic adjustment of the logging speed and acquisition parameters based on the quality of the acquired data and the formations being evaluated. 
   BACKGROUND OF THE ART 
   Oil or gas wells are often surveyed to determine one or more geological, petrophysical, geophysical, and well production properties (“parameters of interest”) using electronic measuring instruments conveyed into the borehole by an umbilical such as a cable, a wireline, slickline, drill pipe or coiled tubing. Tools adapted to perform such surveys are commonly referred to as formation evaluation tools. These tools use electrical, acoustical, nuclear and/or magnetic energy to stimulate the formations and fluids within the borehole and measure the response of the formations and fluids. The measurements made by downhole instruments are transmitted back to the surface. In many instances, multiple trips or logging runs are needed to collect the necessary data. Additionally, the logging speed is usually a predetermined fixed quantity. 
   In order to reduce the amount of rig time needed for wireline logging, it is common practice to run multiple sensors in a single run. FOCUS™, from Baker Atlas Inc., is an open hole logging systems suitable for use with the present invention. All of the downhole instruments have been redesigned, incorporating advanced downhole sensor technology, into shorter, lighter, more reliable logging instruments, capable of providing formation evaluation measurements with the same precision and accuracy as the industry&#39;s highest quality sensors, at much higher logging speeds. Logging speeds are up to twice the speed of conventional triple-combo and quad combo logging tool strings. Speeds of up to 3600 ft/hr (1080 m/min) are possible. The logging system may include four standard major open hole measurements (resistivity, density, neutron, acoustic) plus auxiliary services. 
   The resolution and accuracy of logging measurements is determined by the type of measurement and the type of formation being logged. The measurement may be tailored to the type of formation. For example, U.S. Pat. No. 5,309,098 to Coates et al. teaches a method and apparatus in which a variable time-window echo-recording system is used to obtain significant improvements in signal quality and logging speed for NMR measurements. An initial test is performed to provide an assessment of the relaxation qualities of the sample. If the test reveals that the sample is a slow-relaxation rock, then the full time is allocated to measuring echoes. However, if the test reveals that the sample is a fast decay rock, then the echo acquisition time window is reduced. This provides increased efficiency since the system is able to maximize the number of measurements made by optimizing the individual sampling intervals to the particular geologic structure being tested. 
   Generally, prior art methods have conducted logging at a uniform logging speed. A fixed logging speed is used for the entire logging interval. This flies in the face of logic since reservoir intervals form only a small portion of the entire geologic section and it is only in reservoir intervals is it necessary to get precise and accurate measurements with high resolution: in the non-reservoir intervals, high precision and accuracy are not usually necessary. 
   It would be desirable to have a method and apparatus of logging a borehole in which the inefficiencies of the prior art are overcome. Such an invention should preferably be able to accommodate a variety of logging tools. The present invention satisfies this need. 
   SUMMARY OF THE INVENTION 
   One embodiment of the invention is a method of conducting logging operations of a borehole in an earth formation. A plurality of formation evaluation (FE) sensors are conveyed into the borehole using a conveyance device. The conveyance device is used to move the plurality of FE sensors at a logging speed while making measurements with the FE sensors. An expert system is used for analyzing said measurements made by the FE sensors. A signal for alteration of the logging speed is provided based on the analysis. The sensors may include a resistivity sensor, a natural gamma ray sensor, a porosity sensor, a density sensor, a nuclear magnetic resonance sensor and/or an acoustic sensor. The conveyance device may be a wireline or a slickline. The logging speed may be altered based on the provided signal. In addition, a wait time for NMR signal acquisition, and/or the number of echoes acquired in NMR signal acquisition may be altered. The signal to alter may be based on a determination of a spin relaxation time and/or an identification of a bed boundary. 
   Another embodiment of the invention is an apparatus for conducting logging operations of a borehole in an earth formation. The apparatus includes a plurality of formation evaluation (FE) sensors, a conveyance which moves the plurality of FE sensors at a logging speed while making measurements with said FE sensors, a processor including an expert system which analyzes the measurements made by the FE sensors, and a processor which provides a signal for alteration of the logging speed based on the analysis. At least one of the processors is at a downhole location. The FE sensors may include a resistivity sensor, a natural gamma ray sensor, a porosity sensor, a density sensor, a nuclear magnetic resonance sensor, and/or an acoustic sensor. The conveyance device may be a wireline or a slickline. The apparatus may further include a device which positions one of the FE sensors at a different distance from the borehole wall than the distance of another of the FE sensors from the borehole wall. 
   Another embodiment of the invention is a method of conducting logging operations of a borehole in an earth formation in which a nuclear magnetic resonance (NMR) sensor is conveyed into the borehole on a conveyance device. A fraction of at least one sensitive volume of the NMR sensor that includes borehole fluid is determined. The sensitive volume is altered based on the determination. There may be a plurality of sensitive volumes from which NMR signals are obtained, the determination being based on the NMR signals from the plurality of sensitive volumes. The determination may also be made based on a standoff measurement. Altering the sensitive volume may be done by altering a frequency of operation of the NMR tool, altering a standoff distance of the tool, using a field shifting magnet and/or altering a distance of the sensitive volume from the borehole wall. An additional sensor responsive to a property of the formation may be used and the logging speed altered based on analysis of outputs of the additional sensor and the NMR sensor. 
   Another embodiment of the invention is an apparatus for conducting logging operations of a borehole in an earth formation. The apparatus includes a conveyance device which conveys a nuclear magnetic resonance (NMR) sensor into the borehole, and a processor. The processor determines a fraction of at least one sensitive volume of the NMR sensor including a borehole fluid, and alters the at least one sensitive volume based on the determination. The NMR sensor may have a plurality of sensitive volumes from which NMR signals are obtained. The processor may make the determination based on the NMR signals from the plurality of sensitive volumes. The processor may alter the sensitive volume by changing a frequency of operation of the NMR sensor and/or by activating a field shifting magnet. The apparatus may include a caliper which determines a standoff of the NMR sensor and the processor may make the determination base don the standoff. The caliper may be an acoustic caliper or a mechanical caliper. The apparatus may include a device that alters the sensitive volume by changing the position of the NMR sensor inside the borehole. The apparatus may include at least one additional formation evaluation sensor and the processor may alter the logging speed based on the outputs of the NMR sensor and the at least one additional sensor. 
   Another embodiment of the invention is a machine readable medium for use with an apparatus for conducting logging operations of a borehole in an earth formation. The apparatus includes a conveyance device which conveys a nuclear magnetic resonance (NMR) sensor into the borehole. The medium includes instructions which enable determination of a fraction of at least one sensitive volume of the NMR sensor including a borehole fluid and altering the at least one sensitive volume based on the determination. The medium may be a ROM, an EPROM, an EEPROM, a Flash Memory and/or an Optical disk. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, reference should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawing and in which: 
       FIG. 1  (prior art) is a schematic illustration of a wireline logging system including a plurality of sensors; 
       FIG. 2  (prior art) is an embodiment of a system using a radially adjustable module adapted for use in logging operations; 
       FIG. 3  (prior art) illustrates a sectional view of one embodiment of a positioning device made in accordance with the present invention; 
       FIG. 4  (prior art) is a schematic elevation view of radially adjustable module positioned in an open hole portion of a borehole; 
       FIG. 5  is a schematic illustration of steps involved in the method of the present invention; 
       FIGS. 6   a  and  6   b  show the alteration of the sensitive volume by movement of the tool within the borehole; and 
       FIGS. 7   a  and  7   b  show alteration of the sensitive volume by use of a field shifting magnet and/or altering the frequency of operation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is discussed with reference to specific logging instruments that may form part of a string of several logging instruments for conducting wireline logging operations. It is to be understood that the choice of the specific instruments discussed herein is not to be construed as a limitation and that the method of the present invention may also be used with other logging instruments as well. 
   A typical configuration of the logging system is shown in  FIG. 1 . This is a modification of an arrangement from U.S. Pat. No. 4,953,399 to Fertl et al. having the same assignee as the present invention and the contents of which are incorporated herein by reference. Shown in  FIG. 1  is a suite of logging instruments  10 , disposed within a borehole  11  penetrating an earth formation  13 , illustrated in vertical section, and coupled to equipment at the earth&#39;s surface in accordance with the method and apparatus for determining characteristics of clay-bearing formations of the present invention. Logging instrument suite  10  may include a resistivity device  12 , a natural gamma ray device  14 , and two porosity-determining devices, such as a neutron device  16  and a density device  18 . Collectively, these devices and others used in the borehole for logging operations are referred to as formation evaluation sensors. Resistivity device  12  may be one of a number of different types of instruments known to the art for measuring the electrical resistivity of formations surrounding a borehole so long as such device has a relatively deep depth of investigation. For example, a HDIL (High Definition Induction Logging) device such as that described in U.S. Pat. No. 5,452,761 to Beard et al. having the same assignee as the present invention and the contents of which are fully incorporated herein by reference may be used. Natural gamma ray device  14  may be of a type including a scintillation detector including a scintillation crystal cooperatively coupled to a photomultiplier tube such that when the crystal is impinged by gamma rays a succession of electrical pulses is generated, such pulses having a magnitude proportional to the energy of the impinging gamma rays. Neutron device  16  may be one of several types known to the art for using the response characteristics of the formation to neutron radiation to determine formation porosity. Such a device is essentially responsive to the neutron moderating properties of the formation. Density device  18  may be a conventional gamma-gamma density instrument such as that described in U.S. Pat. No. 3,321,625 to Wahl, used to determine the bulk density of the formation. A downhole processor may be provided at a suitable location as part of the instrument suite. 
   Instrument suite  10  is conveyed within borehole  11  by a cable  20  containing electrical conductors (not illustrated) for communicating electrical signals between instrument suite  10  and the surface electronics, indicated generally at  22 , located at the earth&#39;s surface. Logging devices  12 ,  14 ,  16  and  18  within instrument suite  10  are cooperatively coupled such that electrical signals may be communicated between each device  12 ,  14 ,  16  and  18  and surface electronics  22 . Cable  20  is attached to a drum  24  at the earth&#39;s surface in a manner familiar to the art. Instrument suite  10  is caused to traverse borehole  11  by spooling cable  20  on to or off of drum  24 , also in a manner familiar to the art. 
   Surface electronics  22  may include such electronic circuitry as is necessary to operate devices  12 ,  14 ,  16  and  18  within instrument suite  10  and to process the data therefrom. Some of the processing may be done downhole. In particular, the processing needed for making decisions on speeding up (discussed below) for slowing down the logging speed is preferably down downhole. If such processing is done downhole, then telemetry of instructions to speed up or slow down the logging could be carried out substantially in real time. This avoids potential delays that could occur if large quantities of data were to be telemetered uphole for the processing needed to make the decisions to alter the logging speed. It should be noted that with sufficiently fast communication rates, it makes no difference where the decision making is carried out. However, with present data rates available on wirelines, the decision making is preferably done downhole. 
   Control circuitry  26  contains such power supplies as are required for operation of the chosen embodiments of logging devices within instrument suite  10  and further contains such electronic circuitry as is necessary to process and normalize the signals from such devices  12 ,  14 ,  16  and  18  in a conventional manner to yield generally continuous records, or logs, of data pertaining to the formations surrounding borehole  11 . These logs may then be electronically stored in data storage  32  prior to further processing. The processor  28  includes the ability, such as that described in U.S. Pat. No. 4,271,356 to Groeschel et al, for separating radiation measurements from natural gamma ray device  14  into individual energy bands centered about energy peaks of selected elemental sources of radiation, preferably the energy peaks of potassium, uranium and thorium. This processing of the natural gamma ray device could also be done by the downhole processor. 
   Surface electronics  22  may also include such equipment as will facilitate machine implementation of the method of the present invention. Processor  28  may be of various forms but preferably is an appropriate digital computer programmed to process data from logging devices  12 ,  14 ,  16  and  18 . Memory unit  30  and data storage unit  32  are each of a type to cooperatively interface with processor  28  and/or control circuitry  26 . Depth controller  34  determines the longitudinal movement of instrument suite  20  with borehole  11  and communicates a signal representative of such movement to processor  28 . The logging speed is altered in accordance with speedup or slowdown signals that may be communicated from the downhole processor, or provided by the surface processor, as discussed below. This is done by altering the rotation speed of the drum  24 . Offsite communication may be provided, for example by a satellite link, by the telemetry unit  36 . 
   While running different logging instruments in a single wireline run, the present invention may use a configuration disclosed in U.S. Pat. No. 7,045,737 to Frost et al. The teachings of Frost recognize the fact that different logging instruments operate best at different standoffs from the borehole wall. 
   Referring next to  FIG. 2 , there is shown a rig  10  on the surface that is positioned over a subterranean formation of interest. The rig  10  can be a part of a land or offshore a well production/construction facility. A borehole formed below the rig  10  includes a cased portion  42  and an open hole portion  11 . In certain instances (e.g., during drilling, completion, work-over, etc.), a logging operation is conducted to collect information relating to the formation and the borehole. Typically, a tool system  100  is conveyed downhole via a wireline  20  to measure one or more parameters of interest relating to the borehole and/or the formation  13 . The term “wireline” as used hereinafter includes a cable, a wireline, as well as a slickline. The tool system  100  can include an instrument suite comprising one or more modules  102   a, b , each of which has a tool or a plurality of tools  104   a, b,  adapted to perform one or more downhole tasks. The term “module” should be understood to be a device such as a sonde or sub that is suited to enclose, house, or otherwise support a device that is to be deployed into a borehole. While two proximally positioned modules  102   a, b  and two associated tools  104   a, b , are shown, it should be understood that a greater or fewer number may be used. 
   In one embodiment, the tool  104   a  is a formation evaluation sensor adapted to measure one or more parameters of interest relating to the formation or borehole. It should be understood that the term formation evaluation sensor encompasses measurement devices, sensors, and other like devices that, actively or passively, collect data about the various characteristics of the formation, directional sensors for providing information about the tool orientation and direction of movement, formation testing sensors for providing information about the characteristics of the reservoir fluid and for evaluating the reservoir conditions. The formation evaluation sensors may include resistivity sensors for determining the formation resistivity, dielectric constant and the presence or absence of hydrocarbons, acoustic sensors for determining the acoustic porosity of the formation and the bed boundary in formation, nuclear sensors for determining the formation density, nuclear porosity and certain rock characteristics, nuclear magnetic resonance sensors for determining the porosity and other petrophysical characteristics of the formation. The direction and position sensors preferably include a combination of one or more accelerometers and one or more gyroscopes or magnetometers. The accelerometers preferably provide measurements along three axes. The formation testing sensors collect formation fluid samples and determine the properties of the formation fluid, which include physical properties and chemical properties. Pressure measurements of the formation provide information about the reservoir characteristics. 
   The tool system  100  can include telemetry equipment  150 , a local or downhole controller  152  and a downhole power supply  154 . The telemetry equipment  150  provides two-way communication for exchanging data signals between a surface controller  112  and the tool system  100  as well as for transmitting control signals from the surface processor  112  to the tool system  100 . 
   In an exemplary arrangement, and not by way of limitation, a first module  102   a  includes a tool  104   a  configured to measure a first parameter of interest and a second module  102   b  includes a tool  104   b  that is configured to measure a second parameter of interest that is either the same as or different from the first parameter of interest. In order to execute their assigned tasks, tools  104   a  and  104   b  may need to be in different positions. The positions can be with reference to an object such as a borehole, borehole wall, and/or other proximally positioned tooling. Also, the term “position” is meant to encompass a radial position, inclination, and azimuthal orientation. Merely for convenience, the longitudinal axis of the borehole (“borehole axis”) will be used as a reference axis to describe the relative radial positioning of the tools  104   a, b . Other objects or points can also be used as a reference frame against which movement or position can be described. Moreover, in certain instances, the tasks of the tools  104   a, b  can change during a borehole-related operation. Generally speaking, tool  104   a  can be adapted to execute a selected task based on one or more selected factors. These factors can include, but not limited to, depth, time, changes in formation characteristics, and the changes in tasks of other tools. 
   Modules  102   a  and  102   b  may each be provided with positioning devices  140   a ,  140   b , respectively. The positioning device  140  is configured to maintain a module  102  at a selected radial position relative to a reference position (e.g., borehole axis). The position device  140  also adjusts the radial position of module  102  upon receiving a surface command signal and/or automatically in a closed-loop type manner. This selected radial position is maintained or adjusted independently of the radial position(s) of an adjacent downhole device (e.g., measurement tools, sonde, module, sub, or other like equipment). An articulated member, such a flexible joint  156  which couples the module  102  to the tool system  100  provides a degree of bending or pivoting to accommodate the radial positioning differences between adjacent modules and/or other equipment (for example a processor sonde or other equipment). In other embodiments, one or more of the positioning devices has fixed positioning members. 
   The positioning device  140  may include a body  142  having a plurality of positioning members  144 ( a, b, c ) circumferentially disposed in a space-apart relation around the body  142 . The members  144 ( a, b, c ) are adapted to independently move between an extended position and a retracted position. The extended position can be either a fixed distance or an adjustable distance. Suitable positioning members  144 ( a, b, c ) include ribs, pads, pistons, cams, inflatable bladders or other devices adapted to engage a surface such as a borehole wall or casing interior. In certain embodiments, the positioning members  144 ( a, b, c ) can be configured to temporarily lock or anchor the tool in a fixed position relative to the borehole and/or allow the tool to move along the borehole. 
   Drive assemblies  146 ( a, b, c ) are used to move the members  144 ( a, b, c ). Exemplary embodiments of drive assemblies  146 ( a, b, c ) include an electro-mechanical system (e.g., an electric motor coupled to a mechanical linkage), a hydraulically-driven system (e.g., a piston-cylinder arrangement fed with pressurized fluid), or other suitable system for moving the members  144 ( a, b, c ) between the extended and retracted positions. The drive assemblies  146 ( a, b, c ) and the members  144 ( a, b, c ) can be configured to provide a fixed or adjustable amount of force against the borehole wall. For instance, in a positioning mode, actuation of the drive assemblies  146 ( a, b, c ) can position the tool in a selected radial alignment or position. The force applied to the borehole wall, however, is not so great as to prevent the tool from being moved along the borehole. In a locking mode, actuation of the drive assembly  146 ( a, b, c ) can produce a sufficiently high frictional force between the members  144 ( a, b, c ) and the borehole wall as to prevent substantial relative movement. In certain embodiments, a biasing member (not shown) can be used to maintain the positioning members  144 ( a, b, c ) in a pre-determined reference position. In one exemplary configuration, the biasing member (not shown) maintains the positioning member  144 ( a, b, c ) in the extended position, which would provide centralized positioning for the module. In this configuration, energizing the drive assembly overcomes the biasing force of the biasing member and moves one or more of the positioning members into a specified radial position, which would provide decentralized positioning for the module. In another exemplary configuration, the biasing member can maintain the positioning members in a retracted state within the housing of the positioning device. It will be seen that such an arrangement will reduce the cross sectional profile of the module and, for example, lower the risk that the module gets stuck in a restriction in the borehole. 
   The positioning device  140  and drive assembly  146 ( a, b, c ) can be energized by a downhole power supply (e.g., a battery or closed-loop hydraulic fluid supply) or a surface power source that transmits an energy stream (e.g., electricity or pressurized fluid) via a suitable conduit, such as the umbilical  120 . Further, while one drive assembly (e.g., drive assembly  146   a ) is shown paired with one positioning member  144  (e.g., position member  144   a ), other embodiments can use one drive assembly to move two or more positioning members. 
   Referring now to  FIG. 4  there is shown an exemplary formation evaluation tool system  200  disposed in an open hole section  11 . The tool system  200  includes a plurality of modules or subs for measuring parameters of interest. An exemplary module  202  is shown coupled to an upper tool section  204  and a lower tool section  206  by a flexible member  156 . In one exemplary embodiment, the module  202  supports an NMR tool  208 . As discussed in U.S. Pat. No. 6,525,535 to Reiderman et al., depending upon the size of the borehole, the NMR tool may be operated in either a centralized manner or in an eccentric manner. In the open hole  18 , the acoustic tool  208  may be set in a decentralized position (i.e., radially eccentric position) by actuating the positioning members  140   a  and  140   b . This decentralized or radially offset position is substantially independent of the radial positions of the downhole device (e.g., measurement devices and sensors) along or in the upper/lower tool string section  204  and  206 . That is, the upper or tool string section  204  and  206  can have formation evaluation sensors and measurement devices that are in a radial position that is different from that of the module  202 . In this decentralized or radially offset position, the NMR tool can be used to gather data in large diameter boreholes. In a small diameter borehole, the NMR tool may be operated in a central position of the borehole. It should be appreciated that such motion can be accomplished by sequentially varying the distance of extension/retraction of the positioning members. 
   Referring next to  FIG. 5 , a flow chart generally illustrating the method of the present invention is shown. The downhole tool system is operating at an initial logging speed  301 . The initial logging speed may be determined based on prior knowledge of the expected geologic formations and fluids. Measurements are made with a plurality of formation evaluation sensors. To simplify the illustration, only two such FE sensors depicted by  303  and  305  are shown. In actual practice, there may be more than two FE sensors in the logging system. As will be discussed later, the signals measured by the sensor  305  are analyzed by a processor, preferably the downhole processor, to see if the data quality are good enough to permit a speedup  323  of logging. The processing may also be done by a surface processor, or by both a surface processor and a downhole processor. Similarly, the signals measured by sensor  303  are analyzed by a processor to see if the data quality are good enough to permit a speedup  333  of logging. The specific nature of the check is discussed below with reference to individual sensor types. Similarly, the measurements of sensor  303  are checked to see if a speedup is warranted  333 . If all the sensors provide a speedup signal  329 , a speedup signal is provided  321 . Not shown in  FIG. 4  is a check to make sure that all the speedup signals are valid. Possible situations in which a speedup signal may not be valid are discussed below. 
   Checks are also made to see if the sensor  305  would require a slower logging speed  327  with a similar check to see if the sensor  303  would require a slower logging speed  335 . The specific nature of the check for slower logging speed is discussed later. If at least one of the sensors provide a valid slowdown signal  337 , then logging is slowed down  331 . It should further be noted that the order of performing the slowdown and speedup evaluation in  FIG. 4  is for illustrative purposes only, and the evaluation could be performed in the opposite order. An important feature of the invention is summarized in the following test:
     1. If all the sensors provide a valid speedup signal, then logging speed is increased;   2. If one or more of the sensors provide a valid slowdown signal, then logging speed is decreased; and   3. If neither (1) nor (2) occurs, the logging speed is maintained.   

   The decision as to whether to speed up or slow down the logging may be based on a comparison measurements made over several time intervals. With the natural gamma ray tool, a typical sampling rate is 10 ms. With a logging speed of 1200 ft/hr, 100 samples are obtained every second, which corresponds to a distance of four inches. If the average and/or the variance of measurements over say 1 second is substantially the same as the average of measurements over 2 seconds, it is an indication that logging speed may be increased without loss of resolution of loss of precision. With the HDIL tool, for example, transmitter and receiver coils are configured to operate with different depths of investigation by operating at several frequencies and/or by using data from several transmitter-receiver spacings. There is a high degree of redundancy in the data. Again, by comparing averages over different time intervals, an indication can be obtained as to whether logging speed may be increased, or, conversely, whether logging speed should be decreased. 
   With nuclear sensors such as used for neutron porosity or gamma ray density logs, the count rates are subject to statistical fluctuations. This is due to the fact that over short time intervals, the source emits radiation that may fluctuate, and furthermore, the interaction of the source radiation with nuclei in the formation is also governed by statistical processes. In order to make a meaningful determination of porosity and/or density, it becomes important to make sure that the actual number of accumulated counts has a minimum value for all the detectors used in making the nuclear measurements. 
   Another point to note with respect to nuclear sensors is that some compensation is applied to account for offset of the detectors from the borehole wall. For example, if ρ ss  and ρ ls  are measurements made by short spaced (SS) and long spaced (LS) sensors, a density correction is applied to give a corrected density according to the relation:
 
Δρ=ρ−ρ LS   =f (ρ LS −ρ SS )  (1)
 
This is called the “spine and rib” correction. In situations where there are washouts, is possible that the individual sensor measurements may pass the tests described above regarding the count rate and the statistical fluctuations. Nevertheless, corrected density measurements may still be invalid due to the large washout. This is an example of the requirement that the speedup or slowdown indicator (discussed above) be a valid one.
 
   An acoustic sensor may also be part of the instrument suite. This is generally used at least for measurements of compressional (P-wave) velocities of earth formations. To make measurements of P-wave velocities, the transmitter(s) and receivers that comprise the acoustic sensor are operated in a monopole mode. Energy generated by the transmitter travels through the formation as a refracted P-wave and from traveltime measurements at the array of receivers, the P-wave velocity can be determined. The semblance of the received signal is an indication of the quality of the data, and if the semblance is sufficiently high, then it is possible to increase the logging speed without detriment. It should be noted that if the formation shear wave (S-wave) velocity is greater than the velocity of sound in the drilling mud, it is possible to determine formation S-velocities using a monopole excitation. 
   The acoustic tool may also be operated in a dipole mode in which shear waves (S-waves) are excited in the formation. Again, the semblance of the received signals may be used as an indicator for possible speedup of logging speed. It should be noted that acoustic measurements take several milliseconds to make, compared to the few microseconds needed for nuclear and resistivity measurements. The acoustic tool may also be operated in the so-called cross-dipole mode wherein the transmitter is activated in a first dipole mode and then activated in a second dipole mode orthogonal to the first dipole mode. The cross-dipole mode is useful in determining azimuthal anisotropy in the earth formation, an indication of possible fracturing. It should be clear that with respect to the acoustic tool, the speedup or slowdown indicator is based on all of the modes of acquisition that are desired. 
   For logging in laminated reservoirs (that have transverse isotropy in both resistivity and acoustic properties), a multicomponent resistivity sensor may be included in the instrument suite. Service with such a sensor is provided by Baker Hughes Incorporated under the mark 3DEX SM  and a tool suitable for the purpose is disclosed in U.S. Pat. No. 6,147,496 to Strack et al. Using such a device, it is possible to determine resistivity anisotropy in the earth formation that is a better indication of reservoir quality than the HDIL measurements (that are sensitive only to horizontal resistivity). In anisotropic reservoirs, it may also be desirable to operate the acoustic sensor in the dipole mode. The natural gamma ray sensors do not necessarily have the resolution to be able to identify laminations at the scale at which resistivity and/or acoustic anisotropy may occur. Consequently, it may be desirable to include a high resolution resistivity sensor, such as a microlaterolog, as part of the instrument suite. Signals from such a sensor could be used to start acquisition with the 3DEX device and/or to switch the acoustic sensor to joint monopole/dipole operation. 
   In one embodiment of the invention, a processor including an expert system analyzes the output of the FE sensors. The processor that does the analysis may be a downhole processor or a surface processor. The expert system determines the lithology of the formation being logged and a speedup or slowdown signal may be provided based on the determine lithology. For example, in shale formations where high resolution and detailed NMR spin relaxation information is not necessary and the NMR spin-lattice relaxation time is short, a speedup signal may be provided. If a microresistivity log indicates a laminated formation, a slowdown signal may be provided to enable accurate 3DEX measurements. 
   U.S. Pat. No. 7,193,414 to Kruspe et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discloses the use of an expert system as part of a processor that uses the output of FE sensors to determine lithology. 
   Specifically, gamma rays measurements may be used to determine the shale content of the formation at the depth of the NMR sensors. In a shaly interval, short pulse sequences and small values of ΔTE are sufficient. The presence of hydrocarbons in the formation is diagnosed from resistivity measurements. The presence of gas in the formation may be indicated by acoustic log measurements. As would be known to those versed in the art, even a small amount of gas in the formation significantly lowers the P-wave velocity in a porous sand formation and additional changes in P-wave velocity changes are only slightly affected by the amount of gas present. In such a situation, it is desirable to use a dual wait time acquisition and processing to determine the gas saturation. 
   The Expert System is preferably implemented using neural networks (NNs). In one embodiment of the invention, more than one NN is used. A first NN is used for determination of lithology and formation fluid type from FE measurements. A second NN may be used for modifying the acquisition and processing parameters based upon the knowledge of the lithology and fluid type and the drilling conditions. 
   In one embodiment of the present invention, acquisition of data using an NMR tool is dynamically altered. U.S. Pat. No. 5,309,098 to Coates teaches an arrangement in which an initial test is performed to determine the relaxation characteristics of the formation being tested. This information obtained from this initial test is then used to select a sampling interval which optimizes the collection of data for the particular pore structure of the formation being tested. The NMR tool disclosed in Coates is a single frequency tool such as that described in U.S. Pat. No. 3,213,357 to Brown et al. 
   One of the problems with NMR tools is the possibility of the sensitive volume extending into the borehole. In such a situation, the NMR signals from the borehole fluid will dominate the signals from the formation, rendering the results incorrect. In one embodiment of the present invention, a test is performed at intervals that may be determined in many ways. For example, standoff measurements made by a caliper may be used to detect washouts. From the standoff measurements, U.S. Pat. No. 6,603,310 to Georgi et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches the determination of the fraction of the sensitive volume that includes borehole fluid and correcting the NMR signals for the contamination. In the present invention, the sensitive volume is altered on the basis of the standoff measurements. This may be done in many ways. 
   In one embodiment of the invention, the NMR tool may be used as part of the logging string of Frost discussed above with reference to  FIGS. 2 and 4 . The standoff measurements may then be used by a processor to control the movement of an eccentered tool towards or away from the borehole wall. This is illustrated with reference to  FIGS. 6   a  and  6   b . Shown in  FIG. 6   a  is a borehole  351  with a logging tool denoted by  353  inside it. The sensitive volume is denoted by  355 . The direction of the static magnetic field is given by  357 . For the case shown in  FIG. 6   a , the sensitive volume extends into the borehole. By moving the tool to the position indicated by  353 ′ in  FIG. 6   b , the sensitive volume  355 ′ is completely within the formation. 
   In another embodiment of the invention, a field shifting electromagnet may be used to alter the distance from the tool for a given frequency of operation. When the field of the field-shifting magnet reinforces the static magnetic field of the permanent magnet, then for a given frequency of operation, the sensitive region is moved away from the tool. When the field of the field-shifting magnet opposes the static magnetic field of the permanent magnet, then for a given frequency of operation, the sensitive region is moved closer to the tool. Shown in  FIG. 7   a  is a case where the sensitive region  405  of the tool  403  extends into the borehole  401 . The direction of the static magnetic field is given by  407 . By increasing the static magnetic field strength using the field shifting magnet, the sensitive volume  405 ′ in  FIG. 7   b  does not extend into the borehole  401 ′ for the same position of the tool  403 ′. 
   Instead of using a field shifting magnet, the sensitive region can be changed by altering the frequency of operation. Lowering the frequency moves the sensitive region away from the tool while increasing the frequency moves the sensitive region towards the tool. The principle for this is the same as for the field shifting magnet shown in  FIGS. 7   a  and  7   b . It should be noted that the example shown in  FIGS. 7   a  and  7   b  are for an eccentered tool having an arcuate region of examination. This is not a limitation of the invention and the method could also be used for a centered tool within the borehole. 
   The discussion above with reference to  FIGS. 6   a ,  6   b ,  7   a  and  7   b  dealt with a examples in which a single sensitive region was altered. The method of dynamic alteration of the region of examination may also be used with a multifrequency logging tool such as that disclosed in Reiderman. U.S. Pat. No. 7,227,355 to Chen et al. having the same assignee as the present invention and the contents of which are incorporated herein by reference discloses a method for determining the fraction of each of a plurality of sensitive volumes that may be affected by the presence of borehole fluid. This may be done without the use of standoff measurements. Chen also teaches a method of correcting the data to correct for the effects of borehole fluids. In the present invention, instead of correcting the data, alteration of the plurality of sensitive volumes may be done using the methods discussed above. Alternatively, data from some of the sensitive volumes may be discarded. 
   The processing of the measurements made in wireline applications may be done by the surface processor, the downhole processor or a remote location. The data acquisition may be controlled at least in part by the downhole electronics. Implicit in the control and processing of the data is the use of a computer program on a suitable machine readable medium that enables the processors to perform the control and processing. The machine readable medium may include ROMs, EPROMs, EEPROMs, Flash Memories and Optical disks. 
   As would be known to those versed in the art, wireline logging operations are typically conducted with the instrument suite being pulled up the borehole. This is preferable to logging with the instrument suite being lowered into the borehole since in the latter situation, there may be sticking of the instrument suite into the borehole with the result that the logging depth as determined at the surface location may not correspond to the actual depth of the instruments. With the variations in logging speed that are possible during practice of the present invention, due to the elasticity of the wireline, there may be “yo-yoing” of the instrument suite at the bottom of the wireline. U.S. Pat. Nos. 6,154,704 and 6,256,587 to Jericevic et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference, present methods for correcting the measurements for the effects of the yo-yoing. In the context of the present invention, this correction may be made after the logging operations are conducted on the stored data at the surface. Alternatively, a correction may be applied downhole prior to any determination to speed up or slow down logging speed. 
   The present invention has been described in the context of a wireline device. The method of the present invention is equally applicable on slickline conveyed device wherein a downhole processor is used for providing a signal to speed up or slow down the speed. The slickline may optionally be conveyed inside a drilling tubular. The FE sensor measurements are stored in a downhole memory for subsequent retrieval. 
   While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope of the appended claims be embraced by the foregoing disclosure.