Method for determining anisotropic resistivity and dip angle in an earth formation

[0001] 1. Field of the Invention

[0002] The present invention is related to the field of electromagnetic logging of earth formations penetrated by well borehole. More specifically, the invention is related to a method for determining the anisotropic resistivity properties of the earth formation and the dip angle of the borehole in the earth formation.

[0003] 2. Description of the Related Art

[0004] The basic techniques of electromagnetic or induction logging instruments are well known in the art. A sonde, having at least one transmitter coil and at least one receiver coil, is positioned in a well borehole either on the end of a wireline or as part of a logging while drilling (LWD). The axis of the coils is essentially co-linear with the axis of the sonde and borehole. An oscillating signal is transmitted through transmitter coil, which creates a magnetic field in the formation. Eddy currents are induced in the earth formation by the magnetic field, modifying the field characteristics. The magnetic field flows in ground loops essentially perpendicular to the tool axis and is picked up by the receiver coil. The magnetic field induces a voltage in the receiver coil related to the magnitude of the earth formation eddy currents. The voltage signals are directly related to the conductivity of the earth formation, and thereby conversely the formation resistivity. Formation resistivity is of interest in that one may use it to infer the fluid content of the earth formation. Hydrocarbons in the formation, i.e. oil and gas, have a higher resistivity (and lower conductivity) than water or brine.

[0005] However, the formation is often not homogeneous in nature. In sedimentary strata, electric current flows more easily in a direction parallel to the strata or bedding planes as opposed to a perpendicular direction. One reason is that mineral crystals having an elongated shape, such as kaolin or mica, orient themselves parallel to the plane of sedimentation. As a result, an earth formation may posses differing resistivity/conductivity characteristics in the horizontal versus vertical direction. This is generally referred to as formation microscopic anisotropy and is a common occurrence in minerals such as shales. The sedimentary layers are often formed as a series of conductive and non-conductive layers. The induction tool response to this type of formation is a function of the conductive layers where the layers are parallel to the flow of the formation eddy currents. The resistivity of the non-conductive layers is represents a small portion of the received signal and the induction tool responds in a manner . However, as noted above, it is the areas of non-conductivity (high resistivity) that are typically of the greatest interest when exploring for hydrocarbons. Thus, conventional induction techniques may overlook areas of interest.

[0006] The resistivity of such a layered formation in a direction generally parallel to the bedding planes is referred to as the transverse or horizontal resistivity R_h and its inverse, horizontal conductivity _h. The resistivity of the formation in a directive perpendicular to the bedding planes is referred to as the longitudinal or vertical resistivity R_v, with its inverse vertical conductivity _v. The anisotropy coefficient, by definition is: $\begin{array}{cc}=\sqrt{R_H/R_V}=\sqrt{{}_v/{}_h}=\frac{1}{}& \left[1\right]\end{array}$

[0007] Subterranean formations are often made up of a series of relatively thin beds having differing lithological characteristics and resistivities. When the thin individual layers cannot be delineated or resolved by the logging tool, the logging tool responds to the formation as if it were macroscopically anisotropic formation, ignoring the thin layers.

[0008] Where the borehole is substantially perpendicular to the formation bedding planes, the induction tool responds primarily to the horizontal components of the formation resistivity. When the borehole intersects the bedding planes at an angle, often referred to as a deviated borehole, the tool will respond to components of both the vertical and horizontal resistivity. With the increase in directional and horizontal drilling, the angle of incidence to the bedding planes can approach 90. In such instances, the vertical resistivity predominates the tool response. It will be appreciated that since most exploratory wells are drilled vertical to the bedding planes, it may be difficult to correlate induction logging data obtained in highly deviated boreholes with known logging data obtained in vertical holes. This could result in erroneous estimates of formation producibility if the anisotropic effect is not addressed.

[0009] A number of techniques and apparatus have been developed to measure formation anisotropy. These techniques have included providing the induction tool with additional transmitter and receiver coils, where the axes of the additional coils are perpendicular to the axes of the conventional transmitter and receiver coils. An example of this type of tool might include U.S. Pat. No. 3,808,520 to Runge, which proposed three mutually orthogonal receiver coils and a single transmitter coil. Other apparatus include the multiple orthogonal transmitter and receiver coils disclosed in U.S. Pat. No. 5,999,883 to Gupta et al. Still other techniques have utilized multiple axial dipole receiving antennae and a single multi-frequency transmitter, or multiple axial transmitters such as those described in U.S. Pat. No. 5,656,930 to Hagiwara and U.S. Pat. No. 6,218,841 to Wu.

[0010] A new method is provided for determining the anisotropic properties of a subterranean earth formation. The present invention is directed to a method for determining the anisotropic properties of an earth formation utilizing a multi-component induction. Specifically, the present invention contemplates a method for inverting the multi-component induction tool responses to determine anisotropic resistivity of an anisotropic and/or homogeneous formation and determine the tool's orientation with respect to the formation anisotropic direction utilizing both the resistive (R) and reactive (X) portions of the signals from a combination of tool responses.

[0011] In a preferred implementation, an induction logging tool, having multiple mutually orthogonal transmitter coils and receiver coils, is positioned in a borehole and activated. Power is applied to the transmitter coils to induce eddy currents in the formation. These eddy currents then induce currents in the receiver coils. These are processed to generate a preliminary phase shift derived resistivity and attenuation derived resistivity. This information is then compared with a predetermined model that relates phase shift derived resistivity and attenuation derived resistivity, horizontal resistivity, vertical resistivity and the anisotropy coefficient. Utilizing an inversion technique based on the preexisting model, the horizontal resistivity and vertical resistivity for a formation, as well as anisotropy coefficient and deviation angle relative to the formation, may be readily determined from the logging data.

[0012] A better understanding of the present invention may be had by referencing detailed description together with the Figures, in which:

[0013] FIG. 1 is simplified depiction of a logging tool that may be used to practice the present invention deployed in an earth borehole;

[0014] FIG. 2A depicts the orientation of the tool transmitters and receivers;

[0015] FIG. 2B depicts the relationship between the borehole, formation and tool coordinate systems;

[0016] FIG. 3A is a nomograph depicting a means for determining R_H as a function of R_ll;

[0017] FIG. 3B is a nomograph depicting a means for determining R_H as a function of R_ll and X_ll;

[0018] FIG. 3C is a nomograph depicting a means for determining R_V/R_H as a function of R_tt;

[0019] FIG. 3D is a nomograph depicting a means for determining R_V/R_H as a function of the ratio R_tt/R_ll;

[0020] FIG. 4A is a nomograph depicting a means for determining R_H and R_V as a function of R_tt and X_tt;

[0021] FIG. 4B is a nomograph depicting a means for determining R_V/R_H as a function of the ratio R_tt/X_tt;

[0022] FIG. 5A is a nomograph depicting a means for determining R_H as a function R_tt;

[0023] FIG. 5B is a nomograph depicting a means for determining R_H as a function of R_tt and X_tt;

[0024] FIG. 5C is a nomograph depicting a means for determining R_V/R_H as a function of R_ll;

[0025] FIG. 5D is a nomograph depicting a means for determining R_V/R_H as a function of R_uu;

[0026] FIG. 5E is a nomograph depicting a means for determining R_V/R_H as a function of the ratio of R_ll/R_tt;

[0027] FIG. 5F is a nomograph depicting a means for determining R_V/R_H as a function of the ratio of R_uu/R_tt;

[0028] FIG. 6A is a nomograph depicting a means for determining R_H and R_V as a function of R_ll and X_ll;

[0029] FIG. 6B is a nomograph depicting a means for determining R_V/R_H as a function of the ratio of X_ll/R_ll;

[0030] FIG. 6C is a nomograph depicting a means for determining R_H and R_V as a function of R_uu and X_uu;

[0031] FIG. 6D is a nomograph depicting a means for determining R_V/R_H as a function of the ratio of X_uu/R_uu;

[0032] FIG. 7A is a nomograph depicting a means for determining R_H and as a function of R_uu and X_ll; and

[0033] FIG. 7B is a nomograph depicting a means for determining R_H as a function of the ratio of R_ll/X_ll.

[0034] The present invention is intended to be utilized with a multi-component, i.e., multiple mutually orthogonal transmitters and receiver coils. Exemplary of this type of induction tool is that disclosed in U.S. Pat. No. 5,999,883 to Gupta et al., which is incorporated herein by reference. In FIG. 1, an induction tool 10 is disposed in a wellbore 2 drilled through an earth formation 3. The earth formation 3 is shown as having a zone of interest 4. The tool 10 is lowered into the earth formation 3 to the zone of interest 4 on an armored, multi-conductor cable 6. The cable 6 is further part of a surface system (not shown) which might typically consist of a winch, a surface control system, including one or more surface computers, interface equipment, power supplies and recording equipment. The surface systems of this type might include a mobile truck mounted unit or a skid mounted unit for offshore operations. Further, the tool 10 may be transported utilizing other techniques, such as coiled tubing having power and data communications capability or as part of a drilling string in a Logging While Drilling (LWD) suite of tools.

[0035] The tool 10 is comprised of three subsections including an electronics section 14, a coil mandrel unit 8, and a receiver/processing telemetry section 12 that is in communication with the cable 6. The coil mandrel section 8 includes the transmitter coils for inducing an electromagnetic field in the earth formation in the zone of interest 4 upon application of power and the receiving coils for picking up signals created by induced eddy currents characteristic is the zone of interest 4. The electronics section 14 include the signal generator and power systems to apply the current to the transmitter coils. The tool 10 is shown as being disposed adjacent to a zone of interest 4 that is made up of thin formation sections 4A-4E.

[0036] It should be noted that while FIG. 1 depicts the tool 10 as being lowered in a vertical borehole 2, that current drilling techniques commonly result in a borehole which deviates several times along its length from the true vertical position. Accordingly, a borehole may intersect a zone of interest at an angle and could greatly affect the tool's measurement of resistivity characteristics. The preferred method of the present invention is designed to address this problem.

[0037] 1. Relation of Tool, Borehole and Formation Coordinate Systems

[0038] A multi-component induction tool such as that disclosed in U.S. Pat. No. 5,999,883 consists of at least three mutually orthogonal loop antenna transmitters (M_l, M_m, M_n) and at least three mutually orthogonal receiver coils whose responses are proportional to the magnetic field strength vectors (H_l, H_m, H_n), where l, m, and n denote a common coordinate system. It should be noted that the in-phase R-signal is proportional to the imaginary part of the H field. The three transmitters are set at the same position longitudinally along the tool 10 axis, which coincides with the l-axis of the coordinate system. The three receivers are also grouped at a common position spaced away from the transmitters along the l-axis. The two transverse directions are along the m- and n-axes.

[0039] With this arrangement, there exist nine different complex voltage measurements proportional to the magnetic field strength vector at the receiver loop antennas when the transmitters are activated:

[0040] (H_ll, H_ml, H_nl) from transmitter M_l;

[0041] (H_lm, H_mm, H_nm) from transmitter M_m; and

[0042] (H_ln, H_mn, H_nn) from transmitter M_n.

[0043] However, as a matter of reciprocity, H_nlH_ln; H_mnH_nm; and H_mlH_lm. Accordingly, there are six independent measurements, which can be expressed as: $\begin{array}{cc}{H}^{\mathrm{tool}}=\left[\begin{array}{ccc}{H}_{\mathrm{ll}}& H_{lm}& H_{ln}\\ H_{ml}& H_{mm}& H_{\mathrm{mn}}\\ H_{\mathrm{nl}}& H_{nm}& H_{\mathrm{nn}}\end{array}\right]=\left[\begin{array}{ccc}{H}_{\mathrm{ll}}& H_{lm}& H_{ln}\\ H_{lm}& H_{mm}& H_{\mathrm{mn}}\\ H_{ln}& H_{\mathrm{mn}}& H_{\mathrm{nn}}\end{array}\right]& \left[2\right]\end{array}$

[0044] One starts with the assumption that formations 4A-4E of FIG. 1 are layered horizontally, where the true vertical direction is z-axis. A formation will be said to exhibit anisotropy where the resistivity in the vertical direction is different from that in the horizontal direction. The formation conductivity tensor is characterized by two anisotropic conductivity values: $\begin{array}{cc}=\left[\begin{array}{ccc}{}_{\mathrm{zz}}& {}_{\mathrm{xy}}& {}_{\mathrm{zy}}\\ {}_{\mathrm{xz}}& {}_{\mathrm{xx}}& {}_{\mathrm{xy}}\\ {}_{\mathrm{yz}}& {}_{\mathrm{yx}}& {}_{\mathrm{yy}}\end{array}\right]=\left[\begin{array}{ccc}{}_V& 0& 0\\ 0& {}_H& 0\\ 0& 0& {}_H\end{array}\right]& \left[3\right]\end{array}$

[0045] where _H is the horizontal conductivity and _V is the vertical conductivity of the formation. The formation coordinates system in this instance is (z, x, y). Where the coordinate system of the tool (l, m, n) is aligned with the coordinate system of the formation (z, x, y), the magnetic field strength in the formation can be expressed as: $\begin{array}{cc}{H}^{\mathrm{formation}}=\left[\begin{array}{ccc}{H}_{\mathrm{zz}}& H_{\mathrm{zx}}& H_{\mathrm{zy}}\\ H_{\mathrm{xz}}& H_{\mathrm{xx}}& H_{\mathrm{xy}}\\ H_{\mathrm{yz}}& H_{\mathrm{yx}}& H_{\mathrm{yy}}\end{array}\right]& \left[4\right]\end{array}$

[0046] However, as noted above, the borehole is rarely vertical, meaning the coordinate systems rarely align. This represents a deviation angle of . The borehole itself may be considered to have a coordinate system (l, t, u) where the u-axis coincides with the formation's y-axis. See FIG. 3. The borehole coordinate system and the formation coordinate system are related by a rotational operation around the y-axis by an inclination angle about the y-axis: $\begin{array}{cc}\left[\begin{array}{c}\hat{l}\\ \hat{t}\\ \hat{u}\end{array}\right]=\left[\begin{array}{ccc}\cos & \sin & 0\\ -\sin & \cos & 0\\ 0& 0& 1\end{array}\right]\left[\begin{array}{c}\hat{z}\\ \hat{x}\\ \hat{y}\end{array}\right]=R_y\left(\right)\left[\begin{array}{c}\hat{z}\\ \hat{x}\\ \hat{y}\end{array}\right]& \left[5\right]\end{array}$

[0047] where R_y is the rotational operator for angle .

[0048] Presuming that the longitudinal axis of the tool is aligned with the borehole coordinate system, then it may be stated that the antenna coordinate system (l, m, n) is aligned with the borehole coordinate system (l, t, u) and $\begin{array}{cc}{H}^{\mathrm{borehole}}=\left[\begin{array}{ccc}{H}_{\mathrm{ll}}& H_{\mathrm{xy}}& H_{\mathrm{xz}}\\ H_{\mathrm{yx}}& H_{\mathrm{yy}}& H_{\mathrm{yz}}\\ H_{\mathrm{zx}}& H_{\mathrm{zy}}& H_{\mathrm{zz}}\end{array}\right]& \left[6\right]\end{array}$

[0049] Moreover, H^borehole and H^formation are related by the rotational factor: $\begin{array}{cc}\left[\begin{array}{ccc}{H}_{\mathrm{ll}}& H_{\mathrm{lt}}& H_{\mathrm{lu}}\\ H_{\mathrm{tl}}& H_{\mathrm{tt}}& H_{\mathrm{tu}}\\ H_{\mathrm{ul}}& H_{\mathrm{ut}}& H_{\mathrm{uu}}\end{array}\right]=R_y\left(\right)\left[\begin{array}{ccc}{H}_{\mathrm{zz}}& H_{\mathrm{zx}}& H_{\mathrm{zy}}\\ H_{\mathrm{xz}}& H_{\mathrm{xx}}& H_{\mathrm{xy}}\\ H_{\mathrm{yz}}& H_{\mathrm{yx}}& H_{\mathrm{yy}}\end{array}\right]{R_y\left(\right)}^{\mathrm{tr}}& \left[7\right]\end{array}$

[0050] Where R_y()^tr is the transposition of R_y().

[0051] However it is rare that the coordinate system (l, m, n) of the tool antennae is aligned with the borehole coordinate system (l, t, u). Accordingly, the transverse (m, n) tool coordinates are related to the borehole coordinates (t, u) be a rotational operation about the l-axis by an azimuthal angle : $\begin{array}{cc}\left[\begin{array}{c}\hat{l}\\ \hat{m}\\ \hat{n}\end{array}\right]=\left[\begin{array}{ccc}1& 0& 0\\ 0& \cos & \sin \\ 0& -\sin & \cos \end{array}\right]\left[\begin{array}{c}\hat{l}\\ \hat{t}\\ \hat{u}\end{array}\right]=R_l\left(\right)\left[\begin{array}{c}\hat{l}\\ \hat{t}\\ \hat{u}\end{array}\right]& \left[8\right]\end{array}$

[0052] The tool response H^tool is then related to the borehole response H^borehole be the operator $\begin{array}{cc}\left[\begin{array}{ccc}{H}_{\mathrm{ll}}& H_{\mathrm{lt}}& H_{\mathrm{lu}}\\ H_{\mathrm{tl}}& H_{\mathrm{tt}}& H_{\mathrm{tu}}\\ H_{\mathrm{ul}}& H_{\mathrm{ut}}& H_{\mathrm{uu}}\end{array}\right]={R_l\left(\right)}^{\mathrm{tr}}\left[\begin{array}{ccc}{H}_{\mathrm{ll}}& H_{lm}& H_{ln}\\ H_{ml}& H_{mm}& H_{\mathrm{mn}}\\ H_{\mathrm{nl}}& H_{nm}& H_{\mathrm{nn}}\end{array}\right]R_l\left(\right)& \left[9\right]\end{array}$

[0053] where R_l()^tr is defined as the transposition of R_l() .

[0054] 2. Tool Response

[0055] Having defined the coordinate systems for the tool, borehole and formation, the tool response may now be expressed in terms of the formation coordinate system (z, x, y), where the tool (l, m, n) directions are aligned with the formation. At a tool transmitter, the fields in the formation can be described per Moran/Gianzero (J. H. Moran and S. C. Gianzero, Effects of Formation Anisotropy on Resistivity Logging Measurements, Geophysics (1979) 44, p. 1266) as follows: $\begin{array}{cc}{H}_{\mathrm{zz}}=\frac{M_z}{4}\left\{\left(3\left(1-u\right)+u^2\right)\frac{z^2}{r^2}-\left(1-u+u^2\right)\right\}\frac{e^u}{r^3}& \left[10\right]\\ H_{\mathrm{zx}}=\frac{M_z}{4}\left\{\left(3\left(1-u\right)+u^2\right)\frac{\mathrm{xz}}{r^2}\right\}\frac{e^u}{r^3}& \left[11\right]\\ H_{\mathrm{xx}}=\frac{M_x}{4}\left\{\left(3\left(1-u\right)+u^2\right)\frac{x^2}{r^2}-\left(1-u+u^2\right)\right\}\frac{e^u}{r^3}+I_0& \left[12\right]\\ H_{\mathrm{yy}}=\frac{M_y}{4}\left\{-\left(1-u+u^2\right)\right\}\frac{e^u}{r^3}-I_0+I_1& \left[13\right]\end{array}$

[0056] where

[0057] and k_H is the frequency of the magnetic moment induced and r is

[0058] The receiver coils are also presumed to be located relative to the formation coordinate system (z, x, y)(Lcos, Lsin, 0), where is again the deviation angle. The currents induced in the receiver coils may be expressed as: $\begin{array}{cc}{I}_0=\frac{M_z}{4}\left\{\frac{u}{r}\left(e^u-e^u\right)\right\}\frac{1}{{}^2}=\frac{M_z}{4}\frac{e^u}{r^3}\left\{\frac{u}{{\sin }^2}\left(1-e^{u\left(-1\right)}\right)\right\}=\frac{M_z}{4}\frac{e^u}{r^3}{\tilde{I}}_0& \left[17\right]\\ I_1=\frac{M_z}{4}\frac{e^u}{r^3}{u}^2\left(1-\frac{{}^2}{}{e}^{u\left(-1\right)}\right)=\frac{M_z}{4}\frac{e^u}{r^3}{\tilde{I}}_1& \left[18\right]\end{array}$

[0059] where is the inverse of and is the anisotropy factor {square root}{square root over (1(²1)sin²)}, rL, and Lsin. It should be noted that in this instance u is a function of only the horizontal resistivity. Both I₀ and I₁ are dependent on u, and . If all of the transmitters are set at equal transmission power (M₁M_uM_tM₀), then the tool response in the borehole can be written as: $\begin{array}{cc}{H}_{\mathrm{ll}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left(1-u\right)+u\left(1-e^{u\left(-1\right)}\right)\right\}& \left[19\right]\\ H_{\mathrm{tt}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left(1-u+u^2\right)+u\frac{{\cos }^2}{{\sin }^2}\left(1-e^{u\left(-1\right)}\right)\right\}& \left[20\right]\\ H_{\mathrm{lt}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{u\frac{\cos }{\sin }\left(1-e^{u\left(-1\right)}\right)\right\}& \left[21\right]\\ H_{\mathrm{uu}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left(1-u+u^2\right)-\frac{u}{{\sin }^2}\left(1-e^{u\left(-1\right)}\right)+u^2\left(1-\frac{{}^2}{}{e}^{u\left(-1\right)}\right)\right\}& \left[22\right]\end{array}$

[0060] This results in three unknowns that characterize the formations anisotropic resistivity, R_H, R_V, and the deviation angle . Note that H_ll depends only on u (hence R_H) and , only. The remainder of the responses, H_tt, H_lt, H_uu depend on the variables u, , and .

[0061] 3. Azimuthal Correction for Deviated Boreholes

[0062] In deviated boreholes, the azimuthal rotation of the tool in the borehole must be determined. In actual logging, the tool's azimuthal orientation is not known. The directions of two transversally oriented antennas do not coincide with the l- and u-axis directions. The multi-component induction tool measures H^tool that is different from H^borehole.

[0063] In a longitudinally anisotropic formation, the orthogonality condition holds as H_yzH_yx0. This implies H_ulH_ut0. As a result, not all six measurements of H^tool are independent, and (H_lm, H_ln, H_mm, H_nn, H_mn) must satisfy the following consistency condition: $\begin{array}{cc}{H}_{\mathrm{mn}}=\frac{\left(H_{mm}-H_{\mathrm{nn}}\right)H_{lm}{H}_{ln}}{\left(H_{lm}^2-H_{ln}^2\right)}& \left[25\right]\end{array}$

[0064] The azimuthal angle is determined either from (H_lm, H_ln) by, $\begin{array}{cc}\tan =-\frac{H_{ln}}{H_{lm}}& \left[26\right]\\ \cos =\frac{H_{lm}}{\sqrt{H_{lm}^2+H_{ln}^2}}& \left[27\right]\\ \sin =-\frac{H_{ln}}{\sqrt{H_{lm}^2+H_{ln}^2}}& \left[28\right]\end{array}$

[0065] or from (H_mm, H_nn, H_mn) by, $\begin{array}{cc}\tan 2=-\frac{2H_{\mathrm{mn}}}{H_{mm}-H_{\mathrm{nn}}}=\frac{2\tan }{1-{\tan }^2}& \left[29\right]\end{array}$

[0066] If all of the (H_lm, H_ln, H_mm, H_nn, H_mn) measurements are available, the azimuthal angle may be determined by minimizing the error, $\begin{array}{cc}\mathrm{error}={H_{lm}\sin +H_{ln}\cos }^2+{\frac{H_{mm}-H_{\mathrm{nn}}}{2}\sin 2+H_{\mathrm{mn}}\cos 2}^2& \left[30\right]\end{array}$

[0067] The H^borehole is calculated in terms of H^tool by, $\begin{array}{cc}{H}_{\mathrm{lt}}=\sqrt{H_{lm}^2+H_{ln}^2}& \left[31\right]\\ H_{\mathrm{tt}}=\frac{H_{mm}{H}_{lm}^2-H_{\mathrm{nn}}{H}_{ln}^2}{H_{lm}^2-H_{ln}^2}& \left[32\right]\\ H_{\mathrm{uu}}=\frac{H_{\mathrm{nn}}{H}_{lm}^2-H_{mm}{H}_{ln}^2}{H_{lm}^2-H_{ln}^2}& \left[33\right]\end{array}$

[0068] These should be used in determining the R_H, R_V, and , from the multi-component induction tool measurements H^tool.

[0069] 4. Inversion of Multi-Component Induction Resistivity Data

[0070] The present invention utilizes an inversion technique to determine anisotropic resistivity characteristics over a range of deviation angles. Where the tool is placed in vertical borehole (0), there exist two independent measurements, each independent measurement being composed of an H field having both an in-phase and out-phase component. $\begin{array}{cc}{H}_{\mathrm{ll}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left(1-u\right)\right\}& \left[34\right]\\ H_{\mathrm{tt}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left(1-u+u^2{}^2+\frac{1}{2}\right)\right\}& \left[35\right]\end{array}$

[0071] This means that H_ll is a function solely of u, hence the horizontal resistivity. H_ttH_uu which means both are a function of u and anisotropy ²R_H/R_V, i.e., a function of both the horizontal and vertical resistivity.

[0072] FIG. 3A demonstrates the relationship between the relationship between the R-signal and the formation horizontal resistivity R_H. This relationship may be used to invert the conventional R signal to obtain apparent R_H. When using both the resistive R and reactive X components of the signal measured as part of H_ll, the graph set forth in FIG. 3B may be used to determine R_H. This is done by minimizing the model error as follows: $\begin{array}{cc}\mathrm{error}={H_{\mathrm{ll}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left[1-u\right]\right\}}^2& \left[38\right]\end{array}$

[0073] Once R_H is determined from the above, the vertical resistivity R_V can be determined from the H_ll measurement. One means of doing so is to determine the ratio R_H/R_V as a function of the R-signal as set forth in FIG. 3C. An alternative means of doing so would be based on the ratio of the measured H_tt/H_ll, ratio based again on the resistive component of the signal R, i.e., R_tt/R_ll, as demonstrated in FIG. 3D.

[0074] An alternative means of determining horizontal and vertical resistivity may be accomplished utilizing both the resistive and reactive portions of the received signal from H_tt. FIG. 4A is a nomograph showing differing ratios of anisotropic resistivity values R_H, R_V (in this instance, as a function of , the square root of R_H/R_V) . Both R_H and R_V can be determined simultaneously by minimizing the error as follows: $\begin{array}{cc}\mathrm{error}={H_{\mathrm{tt}}^{\mathrm{measured}}+\frac{M_0}{4}\frac{e^u}{r^3}\left[1-u+u^2{}^2+\frac{1}{2}\right]}^2& \left[39\right]\end{array}$

[0075] An alternative means to determine R_H and R_V is shown in nomograph of FIG. 4B, which shows R_V/R_H as a function of both the R and X signals from H_tt. Both R_H and R_V can be determined simultaneously by again minimizing Eq. 29.

[0076] When the tool records R- and X-signals for both H_ll and H_tt, R_H and R_V can be determined simultaneously by minimizing the error: $\begin{array}{cc}\mathrm{error}=|H_{\mathrm{ll}}^{\mathrm{measured}}-\frac{M_0{e}^u}{4}\frac{e^u}{r^3}\left\{2\left[1-u\right]\right\}{|}^2+|H_{\mathrm{tt}}^{\mathrm{measured}}+\frac{M_0}{4}\frac{e^u}{r^3}\left[1-u+u^2{}^2+\frac{1}{1}\right]{|}^2& \left[40\right]\end{array}$

[0077] As noted previously, it is rare in current drilling and logging practice that a well is drilled vertical. At the opposite end of the spectrum is a determination of anisotropic resistivity characteristics when the borehole is essentially horizontal (90). Equations 19-24 reduce to three independent measurements, each with a resistive and reactive component: $\begin{array}{cc}{H}_{\mathrm{ll}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left(1-u\right)+u\left(1-e^{u\left(-1\right)}\right)\right\}& \left[41\right]\\ H_{\mathrm{tt}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left(1-u+u^2\right)\right\}& \left[42\right]\\ H_{\mathrm{uu}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left(1-u+u^2\right)-u\left(1-e^{u\left(-1\right)}\right)+u^2\left(1-e^{u\left(-1\right)}\right)\right\}& \left[43\right]\end{array}$

[0078] Herein, H_tt is a function solely of u, and hence is a function solely of horizontal resistivity R_H. H_tt and H_uu are both a function of variables u and , i.e., u and R_H and R_V.

[0079] FIG. 5A is a nomograph that relates the R signal from H_tt to the horizontal resistivity R_H. This may be used to invert the R signal to the apparent formation resistivity. Where R and X signals are available for H_tt, they may also be used to obtain a formation horizontal resistivity for differing R_H as shown in nomograph 5B. This is accomplished my minimizing the error function: $\begin{array}{cc}\mathrm{error}=|H_{\mathrm{tt}}^{\mathrm{measured}}+\frac{M_0}{4}\frac{e^u}{r^3}\left\{1-u+u^2\right\}{|}^2& \left[45\right]\end{array}$

[0080] Once the R_H is determined by either method above, the anisotropy and the vertical resistivity can be determined from various signals based on the H_ll measurement. The anisotropy R_V/R_H may be obtained from the R signal based on the H_ll measurement as shown in FIG. 5C. Alternatively, the anisotropy R_V/R_H may be obtained as a function of the R signal from H_uu as shown in FIG. 5D. It should be noted that a determination using FIG. 5D is more highly dependent on R_H as opposed to R_V at high ohm-m resistivities. A more accurate determination of R_V/R_H may be made as a ratio of the R signals from H_uu/H_ll as demonstrated in FIG. 5E. Another inversion that is useful only at low resistivities is depicted in FIG. 5E which attempts to derive R_V/R_H as a function of the ratio R signals received an antennae H_uu/H_tt.

[0081] In the preferred method of the present invention, both the R and X signals are used to determine R_H and R_V simultaneously. In the nomograph of FIG. 6A, R_V and R_H are determined as a function of the ratio of the R/X signals at H_ll is obtained for differing anisotropic values. R_H and R_V may be determined by minimizing the error function: $\begin{array}{cc}\mathrm{error}=|H_{\mathrm{tt}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left[1-u\right]+u\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2& \left[46\right]\end{array}$

[0082] However, the ratio of R_V/R_H may be better determined from the ratio X/R signals received at H_ll as demonstrated in FIG. 6B. As in FIG. 6A, one may determined R_V and R_H for differing anisotropy values based on the ratio of the R/X signals received at H_uu, as shown in FIG. 6C and then minimizing the following error function to determine R_V and R_H: $\begin{array}{cc}\mathrm{error}=|H_{\mathrm{uu}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left[1-u+u^2\right]-u\left(1-e^{u\left(-1\right)}\right)+u^2\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2& \left[47\right]\end{array}$

[0083] As with the signals received at H_ll, a better determination of the ratio R_V/R_H may be made with respect to anisotropy values based on the ratio of X/R signals received at H_uu as demonstrated in FIG. 6D and then applying the error function.

[0084] When both R and X signals are available from all antennae locations H_ll, H_uu, and H_tt, R_H and R_V may be determined simultaneously with greater accuracy by minimizing the error function: $\begin{array}{cc}\begin{array}{c}\mathrm{error}=|H_{\mathrm{tt}}^{\mathrm{measured}}+\frac{M_0}{4}\frac{e^u}{r^3}\left\{1-u+u^2\right\}{|}^2+\\ |H_{\mathrm{ll}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left[1-u\right]+u\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2+\\ |H_{\mathrm{uu}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left[1-u+u^2\right]-u\left(1-e^{u\left(-1\right)}\right)+u^2\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2\end{array}& \left[48\right]\end{array}$

[0085] The two above methods address determination of R_H and R_V at the most extreme cases, i.e. 0 or 90. More often than not, the deviation angle for the borehole will be somewhere within this range. The above method for determining the formation vertical and horizontal resistivities may be used where the deviation angle is less than 30. However the effect of the deviation angle becomes significant for values above 30. In which instance, the full form of Eqs. 19-24 must be used: $\begin{array}{c}{H}_{\mathrm{ll}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left[1-u\right]+u\left(1-e^{u\left(-1\right)}\right)\right\}\\ H_{\mathrm{tt}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left[1-u+u^2\right]+u\frac{{\cos }^2}{{\sin }^2}\left(1-e^{u\left(-1\right)}\right)\right\}\\ H_{\mathrm{lt}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{u\frac{\cos }{\sin }\left(1-e^{u\left(-1\right)}\right)\right\}\\ H_{\mathrm{uu}}=\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left[1-u+u^2\right]-\frac{u}{{\sin }^2}\left(1-e^{u\left(-1\right)}\right)+u^2\left(1-\frac{{}^2}{}{e}^{u\left(-1\right)}\right)\right\}\\ H_{\mathrm{ul}}=H_{\mathrm{ut}}=0\end{array}$

[0086] There are three variables that characterize the formation's anisotropic characteristics. H_ll depends only on u (hence, R_H) and , whereas, H_tt, H_lt, and H_uu are all dependent on u, , and . Herein there are four independent measurements, each having an R and X component, for this overly constrained model.

[0087] Using both the R and X signals from H_ll can be used to determine u (or R_H) and , if both (recalling that {square root}{square root over (cos²²sin²)} and as 90, ) and R_V/R_H are large. This relationship is demonstrated in nomograph FIG. 7A, from which one may determining R_H and as a function of the R and X signals from H_ll for varying R_H and . R_H and may also be determined from the R and X signals of H_ll by minimization of the error function: $\begin{array}{cc}\mathrm{error}=|H_{\mathrm{ll}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left[1-u\right]+u\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2& \left[49\right]\end{array}$

[0088] Alternatively, R_H may be determined from the ratio of the R/X signals at H_ll as shown in nomograph FIG. 7A. Upon determining R_H (or u) and , the may be substituted into Eqs. 20-22 to determine the remaining two variables, R_V and . Thus, one can determine the horizontal and vertical resistivities, R_H and R_V, without prior knowledge of the deviation angle , which may be independently determined.

[0089] When R and X signals are available from H_ll, H_tt, H_lt, and H_uu, then R_H and R_V and may be determined by minimizing the error function: $\begin{array}{cc}\begin{array}{c}\mathrm{error}=|H_{\mathrm{ll}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{2\left[1-u\right]+u\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2+\\ H_{\mathrm{tt}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left[1-u+u^2\right]+u\frac{{\cos }^2}{\sin }\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2+\\ H_{\mathrm{lt}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{u\frac{\cos }{\sin }\left(1-e^{u\left(-1\right)}\right)\right\}{|}^2+\\ H_{\mathrm{uu}}^{\mathrm{measured}}-\frac{M_0}{4}\frac{e^u}{r^3}\left\{-\left[1-u+u^2\right]-\frac{u}{{\sin }^2}\left(1-e^{u\left(-1\right)}\right)+u^2\left(1-\frac{{}^2}{}{e}^{u\left(-1\right)}\right)\right\}{|}^2\end{array}& \left[50\right]\end{array}$

[0090] Commercially available computer programs may be used to minimize the error function of Eq. 50. The minimization of the error as defined in Eq. 50 permits the use of measured values, as opposed to values that have been rotated through the azimuth and dip corrections, to simultaneously determine R_H, R_V and .

[0091] Note that if the deviation angle is already known, and is not small, then the any or all of the Eqs. 20-22 may be used to determine R_H and R_V. For instance, both the R and X signals from H_ll can be used to determine R_H and , then R_V from if angle is known.

[0092] Accordingly, the preferred embodiment of the present invention discloses a means for determining earth formation anisotropic resistivity utilizing a multi-component induction tool. Moreover, a method is disclosed for performing inversion techniques to determine the said characteristics utilizing various combinations of R and X signals at varying antenna locations. Moreover, a method of determining formation deviation or dip angle has been disclosed.

[0093] While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and that have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, an alternatives falling within the spirit and scope of the present invention as defined by the appended claims.