Abstract:
A small diameter tool for measurement of resistivity of geological rock formations, including a complex auxiliary electrode structure which is adjustable in electrode length between deep and shallow mode measurement operations.

Description:
The present invention relates to borehole resistivity measurement apparatus for measurement of resistivity of sub-surface geological rock formations and more particularly to such measurement using a relatively small diameter well logging tool. 
     As a background to the present invention it is known that in the process of drilling a well, the fluids used by the drilling process invade the sub surface geological rock formation for a radial distance around the well, displacing the fluids already there. The electrical resistivity of the fluids in the rock pores is the controlling influence on the measurement of the bulk formation resistivity. The resistivity of the drilling fluids is usually different to that of the original formation fluids giving rise to a “resistivity invasion profile” radiating outwards away from the well. It is necessary, when evaluating a formation for the presence of hydrocarbon, to know the undisturbed formation resistivity. Any measurement made by a logging tool in the borehole will be perturbed by the invasion and by the borehole itself. It is therefore usually necessary to make two or more measurements which have a different sensitivity to the “invaded zone” in order to be able to calculate the resistivity of the undisturbed formation. It is known to use a laterolog logging tool for this purpose. 
     The laterolog logging tool measures a series of electrical currents and potentials from which a resistance can be calculated. An array of electrodes confines measured currents into geometrically defined patterns. A knowledge of these patterns, and of the calculated resistances, enables resistivities to be determined. The electrode array is varied depending on the depth of the measurement required. It is also necessary to make the measurements with alternating current, or switched polarity direct current, to avoid polarisation effects arising from making measurements in an ionic fluid. 
     All the above is well known and described in U.S. Pat. No. 3,772,589. 
     The known apparatus and method is suitable for boreholes having a standard diameter and for logging tools having a relatively large diameter, eg 4 inches (10.0 cm). With such logging tools correction factors are employed using established correction charts which compensate for fluid present in the borehole between the logging tool and the edge of the borehole. 
     Such correction charts are, however, only applicable if the distance between the logging tool and the wall of the borehole is relatively short. As the distance grows then the compensation factor increases steeply and compensation becomes impracticable, thereby rendering any measurement unusable. 
     The present invention relates to relatively narrow diameter logging tools and to improvements in such tools to enable resistivity measurements to be obtained in standard diameter boreholes. Such measurements would not be possible using narrow diameter tools with standard resistivity measurement apparatus, as known from U.S. Pat. No. 3,772 589, because the compensation would be in the upper range of the steep correction curve where the measurement accuracy would not be reliable. 
     The present invention therefore has as its principal object to provide apparatus for measurement of the resistivity of a geological rock formation using a relatively narrow diameter logging tool. 
     The new tool is a small diameter, preferably 2¼ inches and has a novel array which is designed to deliver a similar performance to that provided by a large diameter tool. In order to achieve this, three fundamental criteria are required to be met. 
     Firstly, the level of correction required to correct for the presence of the borehole should be little or no greater than that of the larger conventional tools. 
     Secondly, the array has been developed such that the borehole correction for both measurements (the “Deep” and “Shallow”) is similar, so that the two measurements “track” as the borehole conditions vary. 
     Thirdly, the array has also been designed to ensure that the “vertical resolution” i.e. the vertical distance over which the measurement is made, is the same for the two measurements. 
     The present invention therefore provides logging tool for logging the resistivity of a geological rock information, the logging tool comprising a plurality of electrodes forming part of a switchable circuit that generates currents that are measurable in a said formation for the purpose of determining the resistivity thereof, the plurality of electrodes including a pair of auxiliary electrodes, each said auxiliary electrode being separated into a first electrode portion and a second electrode portion and including switch means that are operable for repeatedly sequentially combining each said first and second electrode portions into a single energizable emitter electrode when said logging tool is used in a deep mode and separating said first and second electrode portions to enable only one of said first or second electrode portions to be energized as an emitter when said logging tool is used in a shallow mode, the operation of the switch means occurring sufficiently rapidly as to provide substantially simultaneous deep and shallow mode measurements. 
     The present invention also provides a method for operating a logging tool, including a plurality of electrodes forming part of a switchable circuit that generates currents that are measurable in a said formation for the purpose of determining the resistivity thereof, to log the resistivity of a geological rock formation, the method comprising the steps of surveying the formation in a first deep mode and then surveying the formation in a shallow mode and comprising the step of changing the length of an auxiliary electrode that is one of the said plurality of electrodes, when said survey in said shallow mode is conducted from the length of the auxiliary electrode when said deep mode survey is conducted, the said changing step occurring sufficiently rapidly as to provide substantially simultaneous deep and shallow mode measurements. 
     The present invention also provides a method for operating a logging tool to log the resistivity of a geological rock formation, comprising the steps of surveying the formation in a first deep mode and then surveying the formation in a shallow mode and comprising the step of changing the length of auxiliary electrode as defined when said survey in said shallow mode is conducted from the length of the auxiliary electrode when said deep mode survey is conducted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 shows a known dual laterolog array in longitudinal cross section, illustrating the various electrodes. 
     FIG. 2 shows a compact laterolog array according to the present invention also in longitudinal cross section. 
     FIG. 3 illustrates the operation of the array of FIG. 2 in deep mode; and 
     FIG. 4 illustrates the operation of the array of FIG. 2 in shallow mode. 
     FIG. 5 illustrates the wave form outputted by the driver of the electrode array. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to FIG. 1, the known array comprises a plurality of electrodes mounted in known manner in al logging tool. 
     The operation of the electrodes will be further explained with reference to FIGS. 3 and 4. 
     The known electrode array shown in FIG. 1 comprises a generally symmetrical array, having a plurality  1  of metal electrodes separated by insulators  16 . The electrodes when energised emit currents into the geological rock formation when the logging tool  10  is lowered down a borehole  18 . 
     In the array a central electrode A 0  is positioned with two electrodes M 1  on each side, followed by two electrodes M 2 , two electrodes A 1  and two large electrodes A 2 . 
     In the arrangement of FIG. 2 according to the present invention the electrodes A 1  are complex, being separated into two electrodes A 1  and A 1 D, but in other respects the arrangement is similar. 
     FIGS. 1 and 2 show exemplary but non-limiting electrode dimensions and spacings. 
     With reference to FIG. 3 an outline of the deep mode operation is as follows: 
     A current  20  is sent out from the electrodes A 1 D, A 1  and A 2 , which in this mode are connected together via closed switch SW 1 . This current flows through the formation to the surface electrode SE and returns to the logging tool  10  via the logging cable  12 . 
     A “measurement current”  22  is sent out from the electrode A 0  and returns the same way. The level of this measurement current, which is measured at IA 0 , is controlled in such a way that the potential difference V 2  between the monitor electrodes M 1  and M 2  is maintained at zero. In this way the measurement current is confined to a known geometric pattern as described earlier and as illustrated. A primary voltage measurement V is made by measuring the potential difference between the M 1  electrode and a remote position, usually the conducting outer armour  24  of the logging cable  12 . This voltage when combined with the current  22  from A 0  is used to calculate the deep resistance and resistivity. A secondary voltage is measured by substituting an electrode (GN) on the insulated part  14  of the logging cable (the “bridle”) for the cable armour  24 . The primary and secondary voltages are measured simultaneously. Comparison of the two measurements can given an indication of anomalous resistivity distributions, producing an effect well known as the “Groningen effect”. 
     The electrodes A 1 +A 1 D are known as auxiliary electrodes, and in the prior art are a single item disposed on each side of A 0 . In the apparatus according to the present invention they are split to enable a different length to be used in the Deep and Shallow modes. 
     With reference to FIG. 4, in the shallow mode, current  26  is sent from the A 1  electrodes and now returned to the A 2  electrodes, instead of the surface, the A 1 D electrodes being disconnected in the present invention to reduce the length of the auxiliary electrode, thereby providing compensation for the use of a narrow diameter logging tool. The current pattern now flares out and penetrates less distance into the formation. In this case, only a primary voltage measurement is made. 
     The invention relates to the switching of the auxiliary electrode lengths between the two modes, giving extra design freedom to enable the three criteria described above to be met with a small diameter tool. 
     In both modes, the measurement current from the relatively small A 0  electrode flares out in the borehole in the immediate proximity of the tool before assuming a path that is approximately normal to the tool. Knowledge of the electrical potential in this region, when combined with the measured current from A 0 , gives a measurement of the borehole mud resistivity which can then be used to better correct the measurements for the perturbing effects of the borehole. This voltage is sensed by the V 1  electrode, situated as shown in the diagrams. 
     A sequence of measurements is initiated by a command to the tool by the surface computer, usually repeated at a regular depth interval as the tool is drawn along the well bore. 
     The deep mode is set and the drive  28  initially sets all the electrodes to zero potential to allow the electrode surfaces to electrochemically stabilise. 
     The drive  28  then switches positive and then negative, see FIG. 5 at  32  and  34 , for similar periods. A series of measurements is then made in the latter half of each period after the electrodes have again stabilised. An average deep measurement is then calculated. This cancels out effects that may arise from electrode polarisation. 
     The shallow mode is then set and a similar sequence undertaken for the shallow measurement, see FIG. 5 at  36 ,  38 , and  40 . 
     After this period, during period  42  in FIG. 5, all the electrodes, with the exception of the surface one are zeroed, during which a measurement of the well&#39;s Spontaneous Potential is made by measuring the potential difference between the SP electrode and the surface electrode. All the above is repeated at the next depth increment. 
     Each period of the measurement cycle has a duration of typically 40 milliseconds which is slow enough to avoid problems due to electrical skin effects. 
     As illustrated, a logging vehicle  44  may be used in deploying the apparatus for the present invention. Referring to FIGS. 3 and 4, R 1 , R 2 , and R 3  designate resistances and SW 1 , SW 2 , and SW 3  designate switches in the apparatus electronics  46 . 
     In summary, although the tool has many unique features, the novel enabling feature allowing such a slim tool to perform with good quality is the splitting of the auxiliary electrodes, and using different lengths for each mode of operation.