Abstract:
An apparatus and method for estimating a parameter of interest of an earth formation, particularly relating to borehole logging methods and apparatuses for estimating electrical resistivity properties at multiple depths of investigation. The apparatus may include two or more transmitters for introducing electrical current to the earth formation. The apparatus may include a controller configured to deliver an electrical signal to the two or more transmitters either simultaneously or sequentially. The controller may deliver an electrical signal to two or more transmitters at the same frequency for estimating depth of investigation. The apparatus may include one or more receivers responsive to electric signals from the earth formation at one or more frequencies to provide data from one or more depths of investigation. The method may include steps for using the apparatus to obtain data that may be used to estimate the parameter of interest.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/313,907, filed on 15 Mar. 2010, and U.S. Provisional Patent Application Ser. No. 61/379,647, filed on 2 Sep. 2010, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     In one aspect, this disclosure generally relates to borehole logging methods and apparatuses for estimating at least one parameter of an earth formation. More particularly, this disclosure relates to estimating electrical resistivity properties of the earth formation using toroids. 
     BACKGROUND OF THE DISCLOSURE 
     Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging an electric current signal may be imparted into the earth formation for the purpose of estimating the resistivity of the earth formation. The magnetic or electric current source(s) and receiver(s) sensitive to magnetic and/or electric signals may be conveyed into the borehole and used to determine one or more parameters of interest of the formation. A rigid or non-rigid carrier is often used to convey the magnetic or electric current source(s) and receiver(s), often as part of a tool or set of tools, and the carrier may also provide communication channels for sending information up to the surface. 
     SUMMARY OF THE DISCLOSURE 
     In aspects, the present disclosure is related to apparatuses and methods of estimating a parameter of interest of a formation using one or more transverse toroids to receive electric signals from an earth formation. 
     One embodiment according to the present disclosure is an apparatus for estimating a parameter of interest of an earth formation, comprising: a carrier; a primary transmitter on the carrier; and a receiver toroid on the carrier, the receiver toroid being positioned transversely on the carrier and including a single coil antenna. 
     Another embodiment according to the present disclosure is a method of estimating a parameter of interest of an earth formation comprising: positioning a logging tool in a borehole in the earth formation; using a transverse receiver toroid on a carrier on the logging tool, wherein the transverse receiver toroid includes a single coil antenna; and producing a signal responsive to an electrical signal produced by a primary transmitter. 
     Another embodiment according to the present disclosure is an apparatus for estimating a parameter of interest of an earth formation, comprising: a carrier; a first transmitter on the carrier; a second transmitter on the carrier; and a controller ( 125 ,  FIG. 3 ) in electrical communication with the first transmitter and the second transmitter, the controller being: configured to deliver an electrical signal to the first transmitter at a first frequency for a first depth of investigation, configured to deliver an electrical signal to the second transmitter at a third frequency for a third depth of investigation, and configured to deliver electrical signals to the first transmitter and the second transmitter at a second frequency for the second depth of investigation. 
     Another embodiment according to the present disclosure is a method of estimating a parameter of interest of an earth formation comprising: estimating the parameter of interest using signals responsive to electrical signals produced by a first transmitter and a second transmitter for at least three different depths of investigation, wherein a first signal is produced by the first transmitter at a first frequency for a first depth of investigation, a third signal is produced by the second transmitter at a third frequency for a third depth of investigation, and a second signal is produced by the combination of the first transmitter and the second transmitter at a second frequency for a second depth of investigation. 
     Examples of the more important features of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood and in order that the contributions they represent to the art may be appreciated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
         FIG. 1  shows a schematic of a downhole tool deployed in a wellbore along a drill string according to one embodiment of the present disclosure; 
         FIG. 2  shows a graphical illustration of a receiver toroid according to one embodiment according to the present disclosure; 
         FIG. 3A  graphically illustrates of the flow of current relative to the apparatus for the first depth of investigation according to one embodiment of the method according to the present disclosure; 
         FIG. 3B  graphically illustrates of the flow of current relative to the apparatus for the second depth of investigation; 
         FIG. 3C  graphically illustrates of the flow of current relative to the apparatus for the third depth of investigation; 
         FIG. 4  shows a flow chart of an estimation method according to the present disclosure; 
         FIG. 5A  shows a schematic of one embodiment according to the present disclosure using three transmitter toroid pairs and the corresponding potential distribution curves; 
         FIG. 5B  graphically illustrates the pseudogeometric factor curves for the three depths of investigation using the embodiment of  FIG. 5A ; 
         FIG. 6A  shows a schematic of one embodiment according to the present disclosure using two transmitter toroid pairs and the corresponding potential distribution curves; 
         FIG. 6B  graphically illustrates the pseudogeometric factor curves for the three depths of investigation using the embodiment of  FIG. 6A ; 
         FIG. 7A  graphically illustrates the pseudogeometric factor curves for six depths of investigation for a range amplitude contributions for one embodiment according to the present disclosure; and 
         FIG. 7B  shows a table of amplitude contributions corresponding to the curves for  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to borehole logging methods and apparatuses for estimating at least one parameter of interest of an earth formation. More particularly, this disclosure relates to estimating electrical resistivity properties of the earth formation using at least one transverse toroid. 
     In the toroid concept, a coil wound around a toroid core may act as a receiver of electric current. Toroids may be well suited for logging while drilling (LWD) applications because toroids do not require the electrical isolation of components of the drill collar. Herein, the toroid core refers to a magnetic core with sufficient permeability to be used to confine and guide magnetic fields, such as iron and other ferromagnetic compounds. A toroid may include a toroid core and at least one coil wound a substantial distance around the toroid core (more than 50%). A toroid core may be generally circular or polygonal (such as rectangular or semi-rectangular). A toroid core may be continuous or have an air gap present. As one of skill in the art will understand, the borehole environment may be hostile, especially during drilling. One advantage of toroids may be their robustness when exposed to a hostile drilling environment. Nonetheless, embodiments according to this disclosure may also be implemented in less hostile borehole environments such as on post-drilling wireline tools. 
     In some embodiments, multiple transmitter toroid pairs may operate at one frequency, where the toroid pairs may be energized separately. The amplitude, frequency, and distance of the toroids from a receiver may determine the depth of investigation of the apparatus. In some embodiments, transmitter toroid pairs may operate at two or more frequencies. In some embodiments, the transmitter toroid pairs may operate simultaneously. For example, if a first toroid pair simultaneously operates at frequency f 1  with amplitude A 1  and at frequency f 2  with amplitude ½A 2 , and a second toroid pair simultaneously operates at frequency f 2  with amplitude ½A 2  and at frequency, f 3  with amplitude A 3 , then the signals of frequencies f 1 , f 2  and f 3  may be used to estimate electrical resistivity properties of the earth formation at three depth of investigation. Varying the proportion of amplitude A 2  in the two toroid pairs may provide depths of investigation along any points between the depths on investigation bracketed by the first toroid pair and the second toroid pair. 
     Hence, in a three toroid configuration, the middle toroid pair may be removed and substituted by combining two frequencies in the two other toroid pairs. The toroids pairs may be placed on the mandrel at a distance from each other to create the deepest and the shallowest depth of investigation desired. Any number of curves with a depth of investigation between the two extremes can be measured by driving the two transmitter pairs with a linear combination of the source signal without any additional hardware. 
       FIG. 1  schematically represents one embodiment according to the present disclosure wherein a subterranean formation  10  is intersected by a borehole  12 . Suspended within the borehole  12  near the bottom end of a carrier  14 , such as a drill string or wireline, is a downhole tool  100 . The carrier  14  may be carried over a pulley (not shown) and/or supported by a derrick  20 . The carrier  14  may be a drill string, coiled tubing, a slickline, an e-line, a wireline, etc. Downhole tool  100  may be coupled or combined with additional tools. In some embodiments, the borehole  12  may be utilized to recover hydrocarbons. In other embodiments, the borehole  12  may be used for geothermal applications or other uses. 
     Downhole tool  100  may include a primary (first) transmitter  110  and a secondary (second) transmitter  120 . Each transmitter  110 ,  120  may include a pair of opposing toroids  110 A,  110 B,  120 A,  120 B. The use of two opposing toroid pairs as transmitters  110 ,  120  is illustrative and exemplary only, as embodiments according to the present disclosure may be implemented with non-toroidal transmitters, such as current electrodes, and embodiments may be implemented with only one transmitter electrode pair, one transmitter toroidal pair, or multiple electrode and/or toroidal transmitters. Downhole tool  100  may also include a receiver toroid  150 . The receiver toroid  150  may be positioned along the downhole tool  100  such that the receiver toroid  150  is transverse or substantially transverse to the longitudinal axis of the downhole tool  100 . The receiver toroid  150  may include a coil  155  ( FIG. 2 ) that may partially or completely surround the receiver toroid  150 . In some embodiments, the transmitter pairs  110 ,  120  may be positioned such that each transmitter toroid  110 A,  110 B,  120 A,  120 B of a transmitter pair  110 ,  120  may be equidistant from the receiver toroid  150 . 
     In some embodiments, multiple receiver toroids  150  may be arrayed around the circumference of the downhole tool  100 . In operation, the downhole tool  100  may positioned in borehole  12  in proximity to an earth formation  10 . During drilling operations, the downhole tool  100  may travel along a segment of the borehole  12 . Electric currents from the transmitters  110 ,  120  may penetrate the borehole wall  12 . Interaction of the electric currents with formation  10  may produce responsive electric signals that may be detected by the receiver toroid  150 . The electric signals may include electric currents and/or voltages. Typically, the electric signals are in the form of electric currents or electric voltages. These detected signals may be used to estimate at least one parameter of interest of the earth formation  10 , such as resistivity properties. Herein, resistivity properties include, but are not limited to, resistance, conductivity, permittivity, and dielectric constant. Additional receiver toroids  150  may provide more extensive azimuthal coverage or improved resolution of the responsive electric signals than a single receiver toroid  150 . Additionally, multiple receiver toroids  150  may provide continuous coverage in along multiple azimuthal directions during occasions where the drilling tool  100  slides within the borehole  12 , whereas a single receiver toroid  150  would only provide coverage in a single direction. 
     In another embodiment, electrical current may be introduced into the earth formation  10  from electrodes (not shown). By providing a constant potential on the surface of downhole tool  100  over a desired length, electric current may be introduced into the formation and a responsive electric current return at the one or more receiver toroids  150 . In this embodiment, the electrodes may need to be electrically isolated from the one or more receiver toroids  150  and from the tool body. Using pairs of electrodes and connecting voltage sources operating at different frequencies between the pairs, multiple depths of investigation may be achieved. 
       FIG. 2  shows one embodiment, according to the present disclosure, of the receiver toroid  150 , which includes coil  155  that partially surrounds a semi-rectangular core  140 . The use of semi-rectangular core is illustrative and exemplary only, as other shapes may be used as desired. Unlike receiver toroids that may require multiple coils; embodiments according to this disclosure may be implemented with one or more coils. When a single coil arrangement is used, the receiver toroid  150  may have many of the properties of a closed loop antenna. 
       FIGS. 3A-C  shows one embodiment according to the present disclosure, wherein multiple transmitter toroid pairs  110 ,  120 ,  130  transmit electric currents  300  into a formation  10 . The responsive electric signals from the formation  10  may be detected by the one or more receiver toroids  150  positioned within the multiple transmitter toroid pairs  110 ,  120 ,  130 . The multiple transmitter toroid pairs  110 ,  120 ,  130  may be activated simultaneously or sequentially. Depth of investigation regarding the formation  10  may be controlled by altering the spacing of one or more of the transmitter pairs  110 ,  120 ,  130 , thus, due to the different spacing of the multiple transmitter toroids  110 ,  120 ,  130 , the electric currents  300  may penetrate the formation  10  to different depths simultaneously. A large distance between opposing toroids of a particular transmitter toroid pair may result in a large depth of investigation. The different electric currents  300  may be seen in  FIGS. 3A-C , as  FIG. 3A  shows the electric current  300  produced when transmitter toroid pair  130  is energized;  FIG. 3B  shows the electric current  300  produced when transmitter toroid pair  120  is energized; and  FIG. 3C  shows the electric current  300  produced when transmitter toroid pair  110  is energized. The responsive electric signals received by the receiver toroids  150  may provide information regarding the resistivity properties of the formation  10  at different depths. The multiple transmitter toroids  110 ,  120 ,  130  may transmit signals sequentially at identical frequencies, which may allow the depth of investigation to be varied at a particular frequency at different times, or simultaneously at different frequencies, which may allow formation information to be gathered from multiple depths of investigation simultaneously. 
     As shown in  FIG. 4 , method  400  is a method for estimating at least one parameter of interest of an earth formation. Method  400  may include step  410 , where a downhole tool  100  may be positioned in a borehole  12  in proximity to an earth formation  10 . In step  420 , one or more transmitters  110 ,  120 ,  130  introduces an electric current into the earth formation  10  resulting a responsive electric signal due to interactions between the electric current and the earth formation  10 . The electric currents may be introduced simultaneously at two or more frequencies or sequentially at one or more frequencies. In step  430 , a transverse receiver toroid  150  detects the responsive electric signal from the earth formation  10 . In step  440 , a parameter of interest of the formation may be estimated using the detected electric signal. 
       FIG. 5A  shows a schematic of one embodiment according to the present disclosure, wherein three transmitter toroid pairs  110 ,  120 ,  130  introduce an electric current at three different frequencies and a graph that illustrates the potential distribution.  FIG. 5B  is a graph that illustrates the pseudogeometrical factors  510 ,  520 ,  530  related to the depths of investigation provided by the three transmitter toroid pairs  110 ,  120 ,  130 . 
       FIG. 6A  shows a schematic of another embodiment according to the present disclosure, wherein two outer transmitter toroid pairs  110 ,  130  are used to independently provide two depths of investigation and combined to achieve a third depth of investigation. It also shows the potential distribution of this embodiment.  FIG. 6B  is the corresponding graph of pseudogeometric factors that illustrates the two independent depths of investigation and  510 ,  530  and the combined depth of investigation  620 . It may be apparent that the combination of the two outer transmitter toroid pairs realizes an intermediate depth of investigation  620  that is similar or exactly the same as the depth of investigation  520  realized by transmitter toroid pair  120 . 
     Varying the amplitude of frequencies used by the outer transmitter toroids  110 ,  130  may realize any potential distribution for depths of investigation between the bounds of the outer transmitter toroids  110 ,  130 , as may be seen in  FIG. 7A , which corresponds with the  FIG. 7B , when operating as follows:
 
f n : α n *T 1 *sin(ω n t)+(1−α n )*T 3 *sin(ω n t)  (1)
 
where f n  is the frequency for the desired depth of investigation, α n  is the amplitude contribution from the outer transmitter toroid pair, ω n  is the angular frequency used for the desired depth of investigation, α n *T 1  is amplitude of the outer transmitter toroid pair, and (1−α n )*T 3  is the amplitude of the inner transmitter toroid pair. By driving the two toroid pairs in a different ratio at different frequencies, a multitude of curves with different depth of investigation can be created. This is not software focusing, but instead, a unique potential distribution is created which results in an independent measurement. This does not mean that the information is independent, but it is a separate new measurement.
 
     For example, if a 50% amplitude contribution is used for the outer T 1  and inner T 3  toroid pairs, then the formulas for the three depths of investigation from a two toroid system would be as follows:
 
f 1 : 1.0*T 1 *sin(ω 1 t)+0.0*T 3 *sin(ω 1 t)
 
f 2 : 0.5*T 1 *sin(ω 2 t)+0.5*T 3 *sin(ω 2 t)
 
f 3 : 0.0*T 1 *sin(ω 3 t)+1.0*T 3 *sin(ω 3 t)  (2)
 
where the outer toroid pair operates at angular frequencies, ω 1  and ω 2 , the inner toroid pair operates at angular frequencies ω 2  and ω 3 .
 
       FIG. 7A  shows depths of investigation  710 ,  720 ,  730 ,  740 ,  750 ,  760 , which have amplitude contributions  715 ,  725 ,  735 ,  745 ,  755 ,  765  ( FIG. 7B ) from transmitter toroid pairs T 1 , T 3 . It may be observed that depth of investigation curves  710  and  750  correspond to the normal, uncombined operation of transmitter toroid pairs T 1  and T 3 , respectively. Curves  720 ,  730 , and  740  represent intermediate depths of investigation consistent with formula (1). 
     In some embodiments, the combined depth of investigation may be outside of the bounds of the inner and outer transmitter pairs. The amplitude contributions of the toroid pairs may be subtracted instead of added. For example, as shown in curve  760  and line  765 , the net amplitude contribution is still 100%, however, one toroid pairs&#39; amplitude contribution is negative while the other toroid pairs&#39; contribution exceeds 100%. This condition may be called “overfocusing”. 
     While the foregoing disclosure is directed to the one mode embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all variations be embraced by the foregoing disclosure.