NO324919B1 - Source logging device for use in determining resistivity at different radial distances from the borehole using multiple transmitters and one receiver pair - Google Patents

Description

The present invention relates to a device for measuring formation resistivity about a borehole which extends through a formation comprising a tubular housing which has a longitudinal axis and is threaded at each end for threaded engagement in a drill string over a drill bit,

means for sending electromagnetic energy into the formation surrounding the house, the transmitting means including first and second transmitters having a first common transmission frequency in the range 0.5-4.0 MHz, and a receiver pair for receiving electromagnetic energy from the formation, where the transmitters and receivers are placed on the house at distances from each other along the longitudinal axis of the house,

where the first transmitter is located at a first distance from a first of the receivers and at a second distance from the second of the receivers,

where the second transmitter is located at a third distance from the first of the receivers (Ri) and at a fourth distance from the second of the receivers (R2),

where at least one further transmitter is located at a further distance from the first of the receivers and at a second further distance from the second of the receivers, and where the first distance is less than the second distance, the third distance being greater than the second distance , but less than the fourth distance and the further distance being greater than the fourth distance, where n > 3, and means for converting received electromagnetic energy into a logging of formation resistivity having different radial survey depths from the borehole. Thus, the present invention relates to logging tools for use down the borehole where electromagnetic energy is used to carry out measurements of the formation or the borehole's parameters.

It is desirable for many reasons to send electrical signals through the earth as a medium and to receive the signals at a location that is at a distance from the transmitter. A signal system of this kind is used, for example, both to determine various parameters linked to the medium and for communication purposes. These systems are often used for investigations of the environment surrounding a borehole and especially the surrounding formations. Different types of systems for logging boreholes are available for carrying out these investigations. One class of these systems makes use of electromagnetic field phenomena to generate data from the environment surrounding the borehole. A type of previously known logging is electrode logging which makes use of an electric field in the surrounding formation to provide a measure of the formation's conductivity. A conductive mud is required for proper use of this system, which makes the system unusable with oil-based mud. Inductive logging is another type of previously known electromagnetic logging that uses a time-varying magnetic field in the formation to produce a secondary current in it. The secondary current sets up a second magnetic field which induces current in receiving coils located in the borehole, where the induced current in the receiving coil or coils is proportional to the secondary current flowing in the formation and thus directly proportional to conductivity or inversely proportional to the resistivity of the surrounding formation . The use of electromagnetic energy to investigate the surroundings around a borehole is the object of the invention in this case.

From the area of well logging, it is well known that it is desirable to be able to measure parameters of interest, for example the resistivity of the formation at different radial distances from the borehole. This is usually referred to as performing measurements at different survey depths.

In, for example, US patent no. 3,453,530 there is a general treatment of induction logging and of the need for survey measurements at several depths, at least as long as they apply to cable operated loggers. To show this, here is a description of the simultaneous production of resistivity measurements in three radially different formation zones. Furthermore, the patent states that when relatively thin formations are encountered, more than one type of formation can enter the measurement being carried out at any given moment, and this is the same problem faced at the boundary between two different formations. From this description, professionals in this area will be aware that it is highly desirable that all measurements can be made with as fine layer resolution as possible and that the layer resolution for all the sensors must be carefully adapted to each other. It is also clear that this system shows the use of the same transmitters for different receiver systems.

US patent no. 3,893,020 describes the use of two transmitters with different frequencies and a single receiver, but here frequencies are used which are significantly higher, perhaps an order of magnitude higher than those used according to the present invention. In this publication, moreover, the signals from the two transmitters are received simultaneously at the receiver coil. In the preferred embodiment in this publication, different frequencies and different transmitter distances are used to concentrate the electromagnetic field at the same depth in the formation in order to be able to calculate the dielectric constant and conductivity in a part of the formation. Measurements at two frequencies, but at the same depth in the formation, are necessary since all measurements in this publication are based solely on the amplitude of the received signal.

US patent no. 4,319,192, US patent no. 4,107,597, US patent no. 3,551,797 and US patent no. 4,209,747 are typical of many solutions, some where several frequencies are used, and where the use of multiple transmitters with multiple receiver pairs to perform surveys at multiple depths.

For further clarification of the prior art, reference is made to US Patent No. 4,609,873 which describes a dielectric well logging system for determining dielectric constant and resistivity in water-invaded formations to determine oil and water saturations in the formations. Multiple transmitters are activated in a time-division multiplexed manner and sensed by receiver coils. The amplitude ratio and the phase difference between the signals sensed by the receiver coils are then used to determine the dielectric constant and the resistivity, which can then be used to determine the invasion characteristics. US patent no. 4,451,789 describes a cable logging system for measuring formation resistivity, and where the system has an elongated housing with three transmitters and three receivers arranged at measured distances on the housing. The three transmitters operate on three different frequencies, and each receiver receives all three frequencies. It is stated that it is desirable to be able to carry out a number of different radial measurements at a specific vertical depth, but it is not described how this objective is to be achieved.

US patent no. 4,968,940 describes well logging techniques for e.g. determine formation resistivity at various radial depths to be investigated and with high accuracy.

US patent no. 4,810,970 is related to electromagnetic energy well logging, where electromagnetic energy is sent mainly at a frequency equal to 4 MHz into the soil formation from a borehole that traverses the soil formation.

US Patent No. 4,651,101 relates to the use of a system said to be an improvement in logging with electromagnetic wave propagation associated with what the patentees believe is a "quasi-static electromagnetic field". This patent incorrectly states that only when preferred frequencies between about 10 and 400 KHz and not above 400 KHz are used, displacement currents are negligible. When displacement currents are significant, the output signal responds not only to a conductivity of the formation, but also to its dielectric constant, which would be undesirable for the purposes of the present invention. The publication further states that the upper limit of the frequency range should be around 200 KHz and that a preferred lower limit of the frequency range would be around 20 KHz.

US Patent No. 4,818,946 uses a special antenna array and digital processing techniques in an attempt to improve the resolution of an induction type logging tool. It is said here that the resolutions of sensors for several depths in normal logging equipment are different, as depth-measuring sensors have a poorer resolution than shallow-measuring sensors. The method described in the patent requires at least one transmitter and at least two receivers.

US patent no. 4,837,517 shows the use of one or more transmitters together with two or more receivers.

US Patent No. 4,873,488 is another example of a system that uses at least one transmitter with at least two receivers.

US patent no. 4,899,112 describes a system for determining the resistivity in a formation at both shallow and deeper investigation depths, but this system is based on phenomena that have been known for a long time regarding the measurement of phase shift between a pair of receivers, which which should provide a survey depth that is different from measuring amplitude damping between the same pair of receivers, which are essential differences from the present invention where different survey depths can be achieved by using either amplitude damping or phase shift alone to achieve the different survey depths. However, although this publication shows a series of transmitters, they are placed equidistant from the pair of receivers on opposite sides of the receivers.

In the publication entitled "Invasion Profile from the Digital Induction Log" presented by P.A.S. Elkington and H.K. Patel at the SPWLA 26th Annual Logging Symposium 17-20. June 1985, there is a general discussion of the use of one transmitter and four receivers. The publication addresses the desirability of producing logs from different survey depths with the same vertical sensitivity and resolution and provides evidence of achieving this goal using a combination of digital processing and mutual inductance equalization.

In the publication entitled "Induction to the High Resolution Induction Tool" by R. Strickland, P. Sinclair, J. Harber and J. DeBrecht, presented at the SPWLA 28th Annual Logging Symposium June 29 - July 2, 1987, it is described that "These prototypes are of entirely new mechanical construction in which every turn of every coil is positioned precisely along a mandrel made of highly temperature-stable materials. The position of each turn was calculated to precisely zero the mutual inductance with no extra adjustment". This statement implies a plurality of receiving antennas, as discussed in the technique found in the above-mentioned patents where mutual inductance is treated. There seems to be a discrepancy in this publication because a later piece of information states that "All three measurements are made at the same point so that depth shifting errors are eliminated". What has been said above regarding mutual inductance is apparently incompatible with the claim that all three measurements are made at the same point since the use of several receivers means that measurements cannot be made simultaneously at the same depth. It may be that digital signal processing techniques used in conjunction with previously known high-resolution induction sensors make it possible for all practical purposes to claim that the measurements are made at the same depth, but this is only a guess. According to the way the measurements are carried out, they cannot be taken at the same point. It should be pointed out that the publication does not directly state that the measurements were taken simultaneously at the same point, but there can otherwise be no other reason to deal with this topic since most tools can take measurements at the same point, but at different times.

Another approach makes use of multiple frequencies, as for example in "Resistivity profiling with a Multi Frequency Induction Probe" by David F. Allen and Scott J. Jacobsen, presented at the SPWLA 28th Annual Logging Symposium June 29 - July 2, 1987 .A variation on this subject is the use of a pulsed induction logging tool, such as that manufactured by MPI, Inc., 4174 Technology Drive, Freemont, California 94538. This sensor simultaneously emits a wide range of frequencies.

In the publication entitled " Applications of the High Resolution Deep Investi<g>ation Resistivity Instrument ", presented by R.A. Khokhar, T.D. Lawrence and W.H. Fertle at SPWLA 12th French Section (SAID) Int. Formation Evaluation Symposium, Transaction Paper No. K, 1989, a transverse pad contact type device is discussed where it is said that a layer resolution of 12.5 mm can be achieved with their tool, while resistivity measurement can be achieved in layers 25.4 mm thick or better. This is an example of the difference between resolution and full layer sensitivity when using two distinctly different tools to perform measurements at multiple depths. The publication does not clearly indicate a need to obtain measurements at several depths and with a common layer resolution that is as high as is possible in practice.

In the publication entitled "Field Test Results of the High Resolution Induction", submitted by M.W. Alberty and D.S. Epps at the SPWLA 29th Annual Logging Symposium 5 - 8 June 1988, it is clarified that it is highly desirable to separate the vertical and horizontal layer sensitivities from each other.

In yet another publication entitled "Advances in High Resolution Logging" published in Technical Review, Volume 36 No. 2 on pages 4-14, there is a description of the phasor induction tool. In particular, the publication shows that high resolution is achieved for the sensors, not directly with the depth-measuring part of the sensors, but with the shallow-measuring part of this. High-resolution information that emerges in this way is used to synthetically improve the resolution of the depth-measuring part of the sensor.

In the publication entitled "Vertical Enhancement by Combination and Transformation of Associated Responses". which was presented by P.A.S. Elkington, J.R. Samworth and M.C. Enstone at the SPWLA 31st Annual Logging Symposium 24 - 27 June 1990, it appears that there is a rather sharp difference between layer resolution and layer sensitivity. This difference is in accordance with the comments made above regarding US Patent No. 4,818,946.1 this publication relates to a general method for improving the vertical sensitivity of a wide class of sensors. The point of this publication is that there are often visible features in a log (uncalculated) that are not found in the log anywhere near their true value (fully developed). It is thus clear that there are visible features of a sensor at a short distance that are not found in a sensor that is at a greater distance since, with the types of sensors in question here, the layer resolution decreases as the depth of investigation increases.

In the publication entitled "Theory of Microinduction Measurements" presented by W.C. Chew and R.L. Kleinberg in IEEE Transactions on Geoscience and Remote Sensing, Volume 26 No. 6, November 1988, pages 707-719, there is a discussion of the use of induction type measurements made with a very shallow depth of investigation. This publication shows that with extremely small dimensions of the sensor it is possible to make a rough approximation of the sensor's sensitivity using geometric factor theory, but that the sensitivity is best understood expressed with what the authors describe as a "full wave" theory. The induction frequency for the microinduction sensor is 25 MHz and the plane of the transmitting frame antenna is parallel to the borehole wall. Although considerable column space is used in the publication for the use of the geometric factor theory in relation to a "full wave" theory, in reality the publication leads away from the present invention. This is quite clear from a statement on page 708 of the publication in the three paragraphs preceding Chapter II.

Finally, US patent no. 4,940,943 deals with the use of a single transmitter together with a pair of receivers where the transmitter's frame antenna is placed in a first cutout in the conductive housing and the receiver's antennas are placed in further cutouts in the conductive housing, such cutouts in the conductive housing is used to influence the patterns of the electromagnetic energy from the transmitter to the receiver.

As a summary, in the main part of the previously known publications, when it comes to measuring the resistivity in a formation at different investigation depths, a plurality of transmitters together with a plurality of receivers standing at different distances have been used. In measurement-while-drilling (MWD) where the sensors are carried in the drill string, a plurality of receivers having different distances can cause serious problems due to the effect of "invasion", i.e. time-dependent movement of borehole fluid into the formation. If, for example, the hole is drilled at a rate of drilling (ROP) of 1.5 m per hour, and a series of receiver pairs stand at a distance of 75 cm, the time that elapses between measurements taken by a first pair (Ri, R2) and the other pair (R3/R4) at the same vertical depth in the borehole be 30 minutes, and during this time fluid in the formation can move over a considerable distance and thus affect the measurement of resistivity. Furthermore, the previous publications deal with the use of a plurality of transmitters standing at different distances from each other with a single pair of receivers, and for example in US patent no. 3,893,020 discussed above, this is used to concentrate the electromagnetic field at the same radial depth of investigation, which is a significant departure from the present invention which applies to different depths of investigation.

It is therefore a main purpose of the present invention to come up with a new and improved device for logging the resistivity in formations that surround a borehole with different depths in terms of radial distance from the borehole.

It is also a purpose of the invention to arrive at such measurements at mainly fixed vertical depths.

Furthermore, it is a purpose of the invention to arrive at largely the same layer resolution for each of these measurements.

Another purpose of the invention is to enable more investigation depths with a reduced number of antennas, so that a tool design is achieved that is shorter and more reliable.

Further purposes of the invention are to utilize the geometric factor theory when logging formations that surround a borehole in the earth using electromagnetic energy, as well as to ensure optimal operation under varying borehole conditions.

The initially stated device is characterized according to the invention in that the further transmitters have a second transmission frequency in the range 0.5-4 MHz, but which is different from the first transmission frequency.

According to further embodiments of the device, the first distance in the area is approx. 15-30 cm. Furthermore, the second distance can be in the area of approx. 30-45 cm, the third distance can be approx. 61 cm, and the fourth distance can be approx. 76 cm.

According to another embodiment of the device, the second transmission frequency is 1.0 MHz.

According to yet another further embodiment of the device, the first frequency can be 2.0 MHz and the second frequency can be 1.0 MHz.

It would also be advantageous, according to the invention, that said receivers and said transmitters are placed on the housing at a distance from each other, and where the second of the receivers is located deepest in the borehole and where the first receiver, the first transmitter, the second transmitter and said additional transmitter are located successively higher in the borehole.

The invention enables sequential pulsing of the plurality of transmitters in order to achieve a time separation between the bursts of electromagnetic energy emitted by the transmitters.

The invention is otherwise characterized by the features reproduced in the claims, and the purpose and advantages of the invention will be described in more detail in the following with reference to the drawings, where: Fig. 1 shows, seen from the side, an MWD tool (tool for measuring during drilling ) according to the present invention suspended in a vertical borehole as part of a drill string, Fig. 2 shows, seen from the side, a well logging instrument according to the invention, and a block diagram is also shown for the circuits used in operating the transmitters and for processing the signals received at one receiver pair,

Fig. 3 is a pulse diagram showing the time sequence for pulse control of the transmitter, and

Fig. 4 is a pulse diagram showing a time sequence which is an alternative to that shown in fig. 3.

Reference should now be made to fig. 1 which reproduces the preferred embodiment of a logging tool 10 according to the present invention while hanging in the vertical section of a borehole 12 in the earth in a drill string pipe 14 from the ground surface, and such a drill string includes one or more weight pipes 11. A transmitter section comprising transmitters 16, 18 and 20 stand along the length of the logging tool 10 at a distance from a receiver section 22 which includes a pair of receivers which in the following are sometimes referred to as R1 and R2. When using transmitter frequencies that are different, for example 2 MHz and 1 MHz, one can, if desired, use a pair of coils in each receiver, one tuned to 2 MHz and one tuned to 1 MHz. Each pair of such coils in a receiver can, if desired, be placed side by side around the circumference of the tool 10 or they can be stacked concentrically. The transmitters 16, 18 and 20, which are sometimes referred to in the following as transmitters T1, T2 and T3, are covered with a non-conductive material, which is well known in this area. Likewise, the mott aker section 22 has the receivers Ri and R2 covered with a non-conductive material. The transmitters and receivers can be manufactured according to the description in US patent no. 4,940,943. It should be pointed out that the main part of the tool 10 is preferably made of steel to prevent the tool 10 from becoming a weak link in the drill string 14. Usually, and in a manner well known in this field, one or more weight tubes 24 are screwed onto the lower end of the logging tool 10 while the drill bit (not shown) is screwed into the weight tubes.

It should be pointed out that the logging tool 10 also has the necessary electronic circuits (shown in Fig. 2) for the processing of the signals received by the receiver section 22 according to the present invention, thereby converting the received signals into a log or another indication of the formation's resistivity. It should also be pointed out that the processed signals can be registered in the electronics section of the tool 10 or can be fed with a conventional telemetry system (not shown) to the surface for simultaneous processing and reading. A typical example of a well-known telemetry system is one which produces sludge pulses which can be detected at the surface of the earth and which indicate the processed signals.

In fig. 2, the well logging instrument 10 is shown with a series of transmitters Ti, T2, T3 T„. Although in the preferred embodiment only three such transmitters (T1-T3) are intended, Tn is reproduced to show that further transmitters can be used if desired. It should be pointed out that Ti, T2, T3 Tn are placed in order at a distance from the receiver pair Ri and R2. The distance between the coils used for Ri and R2 is usually 15 cm along the longitudinal axis of the instrument 10. The distance between the receiver pair and the successive transmitters which are spaced apart will be different for some applications which are discussed in more detail below. It is sufficient at this stage to say that a typical embodiment may have a distance between Ti and R1/R2 of 30 cm/45 cm; the distance between T2 and R1/R2 of 60 cm/76 cm and a distance between T3 and Ri/R2 of 91 cm/106 cm, and it should be pointed out that the number "30/45" as an example indicates that the distance between Ti and Ri is 30 cm and that the distance between Ti and R2 is 45 cm on the condition that Ri and R2 are 15 cm apart.

A number of amplifiers Ai, A2, A3.... A„ are connected to the transmitter coils Ti, T2, T3.... Tn. These amplifiers are in turn driven by associated oscillators Fi, F2, F3... Fn. The

it should be pointed out that the frequency is preferably of the order of 2 MHz. Due to the power attenuation at greater survey depths, as will be the case with transmitters that are further apart, it is advantageous if the frequency Fi > F2 > F3 > .... Fn. In the preferred embodiment of the present invention, it is intended that Fi will be 2 MHz, F2 will be 2 MHz and F3 will be 1 MHz. Based on various attempts that have been

implemented, however, the invention works quite well in the range from about 0.5 MHz up to about 4 MHz. The oscillators Fi, F2, F3....Fn are controlled by the transmitter control 30 which itself has an interface to a microprocessor 32, while this in turn has an interface to a communication interface circuit 34 and an analog to digital converter 36. The communication interface circuit 34 is common and forms a interface between computers, an internal memory, mud pulsator, microprocessor and the operators or computers at the surface of the earth after the tool 10 is brought up to the surface of the earth.

The differential receiver pair Ri and R2 are connected respectively to amplifiers 40 and 42 which are connected to mixing circuits 44 and 46.

A series of oscillators Fi' and F2\ F3' ... Fn' are connected through an oscillator selector circuit 48 whose output is supplied to the associated inputs of the mixer circuits 44 and 46. The oscillator selector circuit 48 receives its inputs from the microprocessor circuit 32.

The respective outputs of the mixer circuits 44 and 46 drive low-pass filters 50 and 52. The outputs of the filters 50 and 52 respectively drive the amplitude meter circuits 54, 56. The outputs of the circuits 54 and 56 are connected to the multiplexer circuit 60. The outputs of the low-pass filters 50 and 52 are also connected to the inputs of the circuit 62 for relative phase measurement.

During use of the device and the circuits shown in fig. 2, one must be aware that it is desirable to process intermediate frequency signals instead of the nominal 2 MHz signals received by the receiver pair Ri and R2. For that reason, the oscillators Fi', F2', F3'....Fn' are chosen so that they lie very close to the corresponding frequencies Fi, F2, F3....Fn. For example, Fn can be set to a frequency of 1.998 MHz and thus give an intermediate frequency coming from the mixer circuit 44 or the circuit 46 of 0.002 MHz (2 KHz). In a similar way, F2' and F3' can be set to 1.998 MHz and 0.998 MHz. Thus, the only signals that pass the low-pass filters 50 and 52 will be the intermediate frequencies that arise by mixing the frequencies Fi, F2, F3....Fn with the frequencies Fi', F2', F3'....F„'. It should be pointed out that the amplitude measuring circuit 54 gives a measure of the amplitude of the signal received by Ri, while the circuit 56 measures the amplitude of the incoming signals received by the receiver R2. In a similar way, the circuit 62 for measuring relative phase will give a measure of the phase difference between the signals received at the receiver Ri and signals received at the receiver R2. As is well known in this area, the amplitude measurements and the measurements of relative phase together will indicate the resistivity of the formation which, by time multiplexing, gives any combination that is desired for the amplitude and phase measurements.

It should be pointed out that the frequencies Fi, F2, F3.... F„ could have the same frequency apart from the practical considerations linked to the power loss in the formation due to the increased distance the signals have to travel through the formation. However, the usual multiplexer circuits used in this system enable time separation between the sequential pulsing of the transmitters Ti, T2, T3... Tn. For example, as shown in FIG. 3, the transmitter Ti can be pulsed for one second followed by a pause of one second, followed by a pulse of the transmitter T2 for one second followed by a pause of one second, and followed by a pulse of the transmitter T3 for one second etc, etc. It should be clear that the duration of pulsing of each transmitter can be varied, which also applies to the duration of the intermediate pauses, for example as shown in fig. 4. It should be pointed out that the term "time separation" between the pulses includes the preferred embodiment where a pulse begins immediately at the end of the immediately preceding pulse. As desired, the duration of pulses controlling Ti may differ from the duration of pulses for T2, which in turn may differ from the duration of pulses for transmitter T3 to produce a signature for the received pulses at receivers Ri and R2 for better identification of the transmitters and thus the depth of investigation for the particular pulses that are received. Thus, measurements are made to different depths in the formation by activating each transmitter at a separate time so that only one transmitter is active at a time and by recording or telemetry transmission of the received phase difference and/or amplitudes/(amplitude ratio) corresponding to each transmitted signal. As an alternative, the transmitters Ti, T2, T3.... T„ could all be operated at different frequencies, and they could be pulsed simultaneously where separation of the signals is a function of frequency difference instead of difference in time separation according to the preferred embodiment of the invention. However, it should be quite clear that transmitting all transmitter signals simultaneously will usually require the use of additional filters and processing circuits to enable the instrument to properly discriminate between the different frequencies.

An important feature of the present invention is the achievement of the sensitivity of an induction-type sensor when it comes to the shallowest measurement, even if the sensor works with radio frequencies (around 2 MHz). This is contrary to common knowledge in this area (see for example Rodney, P.F. et al.: "The Electromagnetic Wave Resistivity MWD Tool", publication SPE 12167 presented at the 1983 SPE Annual Technical Conference and Exhibition, San Francisco, 5.- October 8, 1983, and Dayev, D.S., Hieh-Frequency Electromagnetic Techniques of Well Logging. "Nedra" Publishing House, Moscow (1974) 94-127, which will cause the transmitter for the shallow measurement to be placed further from the receiver than necessary or desirable. The close proximity between the shallow transmitter and receiver will effectively bring the receiver within the induction zone of the shallow transmitter. Thus, propagation effects will be largely negligible for the shallow receiver. This simplifies borehole corrections required for this sensor. Since it is extremely important that borehole corrections are carried out correctly based on shallow measurements (since these are those that are affected by the borehole) the approximate validity for geometric f actor theory at this distance a clear advantage. It is also more important that since the shallow measurement takes place in the induction zone, well-established techniques for focusing the field of an induction sensor can be used for the shallow measurement to reduce the influence from the borehole and still achieve a shallow measurement in the formation (see for example Fundamentals of Formation Evaluation .Donald P. Helander, OGCI Publications, Oil

and Gas Consultants International, Inc., Tulsa, 1983, page 110).

It has also been fairly well established that there is a geometric limit to the depth of investigation of an electromagnetic resistivity sensor, given (diametrically) as the sum of the distance between the near and far antennas from the transmitter. This applies to measurements that are based on phase difference, while amplitude-based measurements are approximately 50% deeper. However, all of these are rules of thumb. It has been established that the investigation depth is a function of the formation, the borehole, resistivity in the relevant zone and only approaches this limitation at high resistivities in mud and formation. In a borehole with 0.03 ohm meter mud, the penetration depth is 6.17 cm at 2 MHz. Under these conditions, one can generally expect the real depth of investigation to fall shorter than the maximum depth of investigation. However, it has been rather unexpectedly found that this depends on the location of the receiving antennas, as the closer the receiving antennas are to the transmitter, the better their survey depth will approach the ideal value under all conditions. For example, the survey depth for a receiver pair of 60 cm/76 cm under these conditions is approximately 55 cm, and according to common knowledge one can then expect that a 23 cm/38 cm sensor has a survey depth of

where 55 cm indicates the survey depth for a 60 cm/76 cm sensor, while 136 cm corresponds to 60 cm + 76 cm which is the maximum survey depth for the 60 cm/76 cm sensor and 60 cm comes from the 23 cm/38 cm sensor. The real depth of investigation was calculated to be 50cm for the 23cm/38cm sensor and this was confirmed by experiments, or more strikingly with a 15cm/30cm antenna spacing. As the receivers approach the transmitter, the survey depth will thus approach the ideal survey depth. In summary, it has been determined that the preferred distance between the nearest transmitter and the nearest receiver is 15 cm - 30 cm. The preferred distance range from the nearest transmitter to the farthest receiver is 30 cm - 45 cm, preferably with a 15 cm distance between the receivers.

This can be understood theoretically as follows. The fields for an induction sensor or sensor with wave propagation are fairly well approximated to the fields for a magnetic dipole. The field of an oscillating magnetic dipole with its moment oriented along the z-axis is well known and described in (Electromagnetic Theory by Julius Adams Stratton, McGraw-Hill Book Company, New York, 1941, page 437).

It should be pointed out that when the product of the number of waves and the distance from the nearby transmitter is small, the static term and the induction term will tend to dominate over the radiation term. In other words, the sensor has a tendency to act as an induction sensor. It is for this reason that sensitivity of the induction type is observed when the transmitter is very close to the receivers. The radiation term is then negligible. Although this may seem simple in retrospect, it was not in any way close until we had built a sensor with a 30 cm/45 cm distance between the receiver pair and the transmitter. This sensor measured far deeper in the formation than we expected.

For this reason, one is forced to set the transmitter for shallow measurements much closer to the receiver than one might expect, but one also achieves a 2 MHz measurement where geometric factor theory can be applied (at least as an approximation). This is a great advantage as it simplifies the borehole correction function and enables accurate corrections to be made in terms of borehole effects. However, it should be pointed out that in boreholes with a large diameter, the outer boundaries of the borehole and the formation are again in the radiation zone, so that geometric factor theory cannot be applied to boreholes with a large diameter. For boreholes with normal dimensions, the fact that the measurement is carried out in the induction zone will nevertheless mean that the method traditionally used to align an induction signal can be applied to shallow measurements if desired to reduce effects from the borehole. As described in the above-mentioned publication by Helander, this per se known setting is achieved by adding two to four additional antennas with currents which are directed opposite to the currents in the main transmitter antenna.

We have thus concluded that with the correct placement of the shallowest transmitter, an induction-like sensitivity is achieved. Therefore, geometric factor theory can be applied to the shallowest measurement. This is a great advantage since the shallowest measurement, due to its nature, is the one most affected by conditions in the borehole. The application of geometric factor theory means that tuning techniques usually practiced at 20 KHz (a frequency two orders of magnitude lower than for the present device) can be used to tune the shallow sensor and almost eliminate effects from the borehole.

It is important to understand that if the instrument follows the geometric factor, the depth of investigation will be virtually independent of resistivity. This means, quite unexpectedly, that distance defines depth in a peculiar way. Application of this finding involves the following methods where Li is the distance between Ti and R2, and L2 is the distance between Ti and R2: (1) Li is calculated to be Rborehuii + 25.4 cm which leads to the main part of the signals being a measure the resistivity in a 2.54 cm ring around the borehole. This virtually ensures an Rxo measurement. It is clear that Li will be different for boreholes of different sizes and thus for different tools. (2) If Li is sufficiently shallow, differences between readings taken using the shallow probe and those taken using the deeper probes could provide an estimate of shale hydration. This in turn indicates swelling and subsequent desquamation (in some cases). Logging of elapsed time will show how these conditions progress, particularly in connection with density measurements. Another application could be that if it is known that shale does not contain filtrate, density can be used as an indication of pore pressure. In addition, a good Rxo can be used to estimate porosity. If Rw is known from sand with clean water

where Sw is water saturation, F is formation factor, M is the Archie exponent and Rmf is the resistivity of the sludge filtration.

In a fully flooded zone Sw = 1, Ri = Rxq

The purpose of a multi-depth resistivity tool is to find resistivities at three or more survey depths to identify the invasion profile. As a depth is generally defined as the distance at which 50% of the signal is obtained, very different depths are necessary to be useful. In an oil-bearing formation with resistivity, we have:

(Approximate examination depth in cm)

This shows a good distribution and can be adapted to modeling invasion. However, the greatest importance of a tool of this nature will probably be in the estimation of invasion in water zones. This is because in most cases the water zone under oil contact is far more susceptible to being invaded than the oil zone and yet both will have the same permeability. A typical water zone will have resistivities in the vicinity of 0.2 ohm meters.

During the development of this project, it was surprisingly found that 30 cm/45 cm distances measured a depth too close up to 60 cm/76 cm distance in conductive formations. Ideally . one would also like the shallow measurement shown Rxo, so that a depth of 60 cm is too great. The consequence of what has been found here results in a tool design that requires a closer, shorter distance. A distance that is too short will increase borehole corrections, especially with saline mud, so the preferred short distance in the tool is between 15 cm/30 cm and 30 cm/45 cm. Naturally, the distance between the two receivers can also be adjusted from the preferred 15 cm shown without adversely affecting the logic. The borehole corrections mentioned above will be more prominent at the higher resistivities where the differences in investigation depths can be accepted even at 30 cm/45 cm. To reduce these corrections, common knowledge would have convinced us to use a short distance of between 30 cm/45 cm and, for example, 50 cm/65 cm (at expected depths in the range from 17.5 cm to 52 cm) . The new discoveries force us to use shorter distances in a way that is not obvious.

A further advantage of the present invention is achieved by using a single pair of receiving antennas. The stability (mechanical, thermal and electrical) of the receiver electronics is far more important than the stability of transmitters (which can result in a change in power level or a modest change in operating frequency). A drill string valve can be designed with a variety of transmitters and more than are actually needed for a particular application without any particular additional cost. The transmitters needed for a particular application can be selected, for example, by turning the transmitter insert into a number of preset patterns that link the oscillators with the associated transmitter antennas. This creates a sensor that has minimal power consumption and can be used in both large diameter boreholes (where one can expect to find it necessary to choose a relatively large diameter for the shallowest investigation) and boreholes with a smaller diameter (where it may be necessary to select a transmitter antenna that is very close to the receivers to perform measurements in flooded or invaded zones of the formation). The invention largely extends to the idea of selectable movement of the transmitter's position as adaptation to the area of application where one embodiment is to be able to turn the insert.

Although the preferred embodiment contemplates the use of a series of sequentially spaced transmitters, the invention also encompasses the use of a single electromagnetic energy transmitter operating in a frequency range of approximately 0.5 MHz - 4.0 MHz together with a single pair of receivers standing in the area of 15 cm/30 cm-

30 cm/45 cm from such a simple transmitter.

Claims (9)

1. Device for measuring formation resistivity about a wellbore extending through a formation, comprehensive a tubular casing which has a longitudinal axis and is threaded at each end for threaded engagement in a drill string over a drill bit, means for transmitting electromagnetic energy into the formation surrounding the house, the transmitting means comprising first and second transmitters (Ti, T2) having a first, common transmission frequency in the range of 0.5-4.0 MHz, and a pair of receivers (Ri, R2 ) for receiving electromagnetic energy from the formation, where the transmitters and receivers are placed on the housing at distances from each other along the longitudinal axis of the housing, where the first transmitter (Ti) is located at a first distance (Di) from a first of the receivers (Ri) and at a second distance (D2) from the second of the receivers (R2), where the second transmitter (T2) is located at a third distance (D3) from the first of the receivers (Ri) and at a fourth distance (D4) from the second of the receivers (R2, where at least one further transmitter (Tn) is located at a further distance (D2n-i) from the first of the receivers (Ri) and at a second further distance (D21O from the second of the receivers (R2), and where the first distance (Di) is less than the second distance (D2), the third distance (D3) being greater than the second distance (D2) but less than the fourth distance (D4) and the further distance (D211 -1) is greater than the fourth distance (D4), where n > 3, and means for transforming received electromagnetic energy into a logging of formation resistivity having different radial survey depths from the wellbore, characterized in that the additional transmitters have a second transmission frequency in the range of 0.5 - 4 MHz, but which is different from the first transmission frequency. 2. Device as stated in claim 1, characterized in that the first distance (Di) is in the area approx. 15-30cm. 3. Device as specified in claim 1 or 2, characterized in that the second distance (D2) is in the area approx. 30-45 cm. 4. Device as stated in claim 1,2 or 3, characterized in that the third distance (D3) is approx. 61 cm. 5. Device as stated in any of claims 1-4, characterized in that the fourth distance (D4) is approx. 76 cm. 6. Device as stated in any one of claims 1-5, characterized in that the second transmission frequency is 1.0 MHz. 7. Device as stated in any one of claims 1-5, characterized in that the first frequency is 2.0 MHz and the second frequency is 1.0 MHz. 8. Device as stated in any one of claims 1-7, characterized by atn=3. 9. Device as specified in any of claims 1-8, characterized in that said receivers and said transmitters are placed on the housing at a distance from each other, and where the second of the receivers is located deepest in the borehole and where the first receiver, the first transmitter, the second transmitter and said additional transmitter (E) are located successively higher in the wellbore.