Method for reservoir fracture and cross beds detection using tri-axial/multi-component resistivity anisotropy measurements

The invention is a method for inverting reservoir bi-axial anisotropy and identifying complicated fracture/cross-bedding system (104) by using tri-axial induction logs and wellbore survey/image log data. An inversion method is disclosed that uses all nine components that can be measured by tri-axial induction tools (101), plus borehole azimuth and deviation data, to solve for the tri-axial induction response in an arbitrary anisotropic formation due to the non-orthogonal bedding plane and fracture plane (or cross bedding plane). A mathematical formula is provided relating the conductivity tensor in the tool's reference frame to the conductivity tensor in a reference plane associated with the fracture/cross bedding planes (102). This equation is inverted (103) to yield the conductivity tensor components in the fracture/cross bedding coordinate system, along with dip and azimuth angles for both the fracture/cross bedding planes and the tool.

FIELD OF THE INVENTION

This invention relates to the field of hydrocarbon exploration and production, and more particularly to resistivity logging. Specifically, the invention is a method for the inversion of multi-components/tri-axial induction measurements in a fractured reservoir and cross-bed sand to yield characterization of anisotropic resistivity with biaxial symmetry. An inversion scheme and method is disclosed that can use all nine tensor components of induction measurements, borehole azimuth, and deviation data.

BACKGROUND OF THE INVENTION

Resistivity is one of the most important parameters measured in wells for hydrocarbon exploration and production. For many years, conventional induction tools have been built with coils that have magnetic moment along the tool axis (co-axial dipole) that is mainly sensitive to the horizontal resistivity when the formation is horizontal and the well is vertical. Different designs of co-axial multi-coils array induction has appeared in the literature for over 30 years. Although theoretical work on resistivity anisotropy measurements started in 1950's (See Kunz and Moran, “Some effects of formation anisotropy on resistivity measurements in boreholes,”Geophysics23, 770-794 (1958)), and the detailed theoretical derivation on the magnetic moment perpendicular to the tool axis (co-planer dipole) was also published in 1979 (Moran and Gianzero, “Effects of formation anisotropy on resistivity logging measurements,”Geophysics44, 1266-1286 (1979)), the multi-component/tri-axial induction tool was not introduced into commercial service until the first decade of the 21stcentury. First, Baker Atlas commercialized its multi-component tool-3DEX (Schon et. al., “Aspects of Multicomponent resistivity data and macroscopic resistivity anisotropy,” SPE 62909 (2000)), and Schlumberger introduced their version of a tri-axial induction tool-AIT-Z (Rosthal et al., “Field test results of an experimental fully tri-axial induction tool”, SPWLA 44thAnnual Symposium, Paper QQ (2003)); and Barber et. al., “Determining formation resistivity anisotropy in the presence of invasion”, SPE 90526 (2004)). However, these multi-component/tri-axial component tools have been mainly marketed as a thin-bed, low-resistivity-pay tool to invert horizontal and vertical resistivity Rh, and Rv, to be used with assumptions of vertical transverse isotropy (“VTI”) symmetry for (for example) thinly laminated formation, or Horizontal Transverse Isotropy (HTI) symmetry for vertical fractures (Rabinovich et al., “Determination of fracture orientation and length using multi-component and multi-array induction data,” U.S. Patent Application No. 2005/0256645 (2005)). Within the framework of a transverse isotropic model, Rvis assumed to be greater than Rh. (The tool used is designed mainly as a thin-bed tool for laminated formations. Then, in thinly laminated sand-shale sequences with high-low resistivity, the series combination of resistances (Rv) must be greater than the parallel combination (Rh).) Hence, when the inverted Rhis greater than Rv, one solution that has been used to reconcile this conflict is to force the horizontal resistivity to be equal to vertical resistivity, Rh=Rv, i.e., isotropy.

FIG. 1illustrates a case where the inverted Rhis forced to equal Rv. Record11displays the recorded voltage measurements made with a multi-component/tri-axial induction tool. The dashed line is the Vxxdata, the dotted line is Vyyand the solid line is Vzz. The different layers are illustrated at12as a function of depth. Shale layers are denoted by reference number13;14is a thin-bed zone; and15is a layer with crossed beds or fractured beds. In the shale and thin-bed formations, Vxx=Vyy, i.e., these two orthogonal measurements match when rotated to be aligned, and in these formations the consequent inversion for Rhworks as can be seen from record16where the dotted line represents Rhand the solid line Rv. But in the cross-bed or fractured bed formation15, there are physical reasons why Vxx≠Vyy≠Vzzafter azimuthal and dip rotations, and it can be seen in record16that the inversion method has had to force Rhand Rvto be equal in formation15. Such problems can arise, for example, in the Piceance Basin where tight gas sandstone often have natural vertical fractures. Such results indicate that the Rvand Rhinversion is problematic when one cannot rotate the measured 9-component data to match the two orthogonal magnetic moments in the bedding plane, e.g., Hxx≠Hyy(FIG. 1). Record17is a corresponding gamma ray log.

As shown in the schematics ofFIGS. 2A and 2B, the nine components (σij) of the conductivity tensor (conductivity and resistivity are mutually reciprocal quantities) measured by three pairs of the orthogonally orientated magnetic dipoles can be rotated through inversion to find σh, σv, within the framework of a VTI or HTI model. All other non-diagonal terms in the conductivity matrix should vanish when borehole, tool eccentricity, and other effects (such as invasion) are corrected, or become small enough to be ignored. In case the formation has VTI or uniaxial anisotropy, the relationship between the tensor conductivity in bedding coordinate and tri-axial induction measurements in borehole coordinates can be expressed by a rotation matrix (R), in the form {right arrow over (σ)}′={right arrow over (R)}T·{right arrow over (σ)}·{right arrow over (R)}, i.e.,

σ→ij′=R→T·[σxxσxyσxzσyxσyyσyzσzxσzyσzz]·R→⇒[σxx000σyy000σzz]⁢⁢=[σh000σh000σv]⁢⁢where⁢⁢R→=[cos⁢⁢αcosβcos⁢⁢α⁢⁢sin⁢⁢β-sin⁢⁢α-sin⁢⁢βcos⁢⁢β0sin⁢⁢αcos⁢⁢βsin⁢⁢αsin⁢⁢βcos⁢⁢α](1)
In many cases, the above expression cannot be realized by rotation and inversion because the formation simply has higher anisotropic symmetry than VTI system.FIG. 3Ashows an aeolian outcrop section with complicated bedding planes (indicated by solid lines41), cross-bedding planes (indicated by dotted lines42), and fault/fracture planes (indicated by broken lines43). In such a more general anisotropic case, the process expressed in Equation (1) can never be realized by a matrix rotation and inversion process such as is shown inFIG. 1, where the inversion process is limited to an assumption of VTI, or HTI, or in general, uniaxial anisotropic symmetry with arbitrary plane. Note that the thickness of crossbed varies in the range of meters in the large view ofFIG. 3A(see the cartoon people in the lower right for scale), but also includes fine layers in the range of mm to cm as shown in the exploded view ofFIG. 4B(ball-point pen shown as scale indicator). In such more general anisotropic cases, assumptions of the TI symmetry for tri-axial induction measurements and inversion will break down, and dip, azimuth, and resistivity anisotropy will be different if they are inverted from the data taken from transmitter-receiver pairs with different spacing from multi-spacing tools.

What is needed is an inversion method that can use all nine components that can be measured by tri-axial induction tools, plus borehole azimuth and deviation data, to address these issues and solve for the tri-axial induction response in an arbitrary anisotropic formation due to the non-orthogonal bedding plane and fracture plane (or cross bedding plane). The present invention provides such a method.

SUMMARY OF THE INVENTION

In one embodiment of the present inventive method, referring to the flow chart ofFIG. 10, the invention is a method for detecting fault or cross-bedding planes (hereinafter, “fault” planes) in subsurface petroleum reservoir formations with bi-axial anisotropy (one axis perpendicular to the formation's bedding planes, the other perpendicular to the fault planes) from conductivity/resistivity measurements made in a well using a multi-component, tri-axial induction well logging tool, comprising:

(a) expressing a formation conductivity tensor with components as measured by the well logging tool (step101), wherein each component of said tensor is expressed (step102) as a combination of the conductivity components:(i) conductivity normal to bedding plane σnb,(ii) conductivity parallel to bedding plane σpb,(iii) conductivity normal to fault plane σnf,(iv) conductivity parallel to fault plane σpf,

(b) inverting the expression for the formation conductivity tensor to obtain σnb, σpb, σnf, and σpffrom the measured conductivity data (step103); and

(c) detecting one or more fault planes from indications of anisotropy in the inversion results (step104).

In some embodiments of the invention, azimuth and dip angles for the fault planes and azimuth and dip angles for the tri-axial induction well logging tool are also obtained from the data inversion.

The invention will be described in connection with its preferred embodiments. However, to the extent that the following detailed description is specific to a particular embodiment or a particular use of the invention, this is intended to be illustrative only, and is not to be construed as limiting the scope of the invention. On the contrary, it is intended to cover all alternatives, modifications and equivalents that may be included within the spirit and scope of the invention, as defined by the appended claims.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The present invention is a method for inverting reservoir bi-axial anisotropy and identifying complicated fracture/cross-bedding system by using tri-axial induction logs and wellbore survey/image log data. Bi-axial anisotropy is a direct indication of fracture/cross-bedding reservoir rock, and has a significant impact on reservoir characterization, e.g. hydrocarbon pore fluid estimation. The present inventive method will yield profiles of Rxx, Ryyand Rzz, from which it will be possible to ascertain the presence of faults, among other benefits of knowing the resistivity model in its full, anisotropic complexity.

Description of Biaxial Resistivity System

Next, the conductivity equations for the biaxial anisotropic resistivity formation will be developed by considering four, progressively more general, cases.

Uniaxial/VTI Bed and Orthogonal Fault/Fracture System

One example of a biaxial anisotropic formation may be described as laminated beds intercepted by perpendicular fault/fractures (53inFIG. 4), i.e., two orthogonal uniaxial systems (VTI+HTI).FIG. 4is a schematic diagram of a biaxial anisotropic formation composed of a uniaxial/VTI bed with the bedding plane51in the X-Y plane, and a fault/fracture plane52in the Z-Y plane, perpendicular to the bedding plane. This special biaxial anisotropic case can be described by four basic tensor conductivities composed of:

(a). Conductivity normal to bedding plane (σnb);

(b). Conductivity parallel to bedding plane (σpb);

(c). Conductivity normal to fault plane (σnf); and

(d). Conductivity parallel to fault plane (σpf).

Note that VTI is a function of σnband σpb, and HTI is a function of σnfand σpf.

Then, the tensor conductivity {right arrow over (σ)} in the X, Y, and Z directions, is simply the summation of the conductivity in each direction:

σ⇀={σxx=σnf+σpbσyy=σpb+σpfσzz=σnb+σpf(2)
Uniaxial/VTI Bed Intercepted by Fault/Fracture Plane with α° Dip

If the fault/fracture plane62is rotated α° about the Y-axis as shown in the schematic diagram ofFIG. 5, then such an anisotropic formation with bedding planes51aligned with the X-Y plane and fracture plane rotated α° about the Y-axis can still be described by a tensor conductivity {right arrow over (σ)} that is a function of the four basic conductivities, σnb, σpb, σnf, and σpf. In this case, the conductivities in the X, Y, and Z directions, are simply the summation of conductivity in each direction with a fault/fracture dip being a rotation of α° about the Y-axis:

σ⇀={σxx=cos⁢⁢α·σnf+σpbσyy=σpb+σpfσzz=σnb+cos⁢⁢α·σpf(3)
Uniaxial/VTI Bed Intercepted by Fault/Fracture with α° Dip and β° Azimuth/Strike—a General Biaxial Anisotropic Case

If in addition to the rotation shown inFIG. 5, the fault/fracture plane is then rotated β° about the Z-axis to a position72as shown inFIG. 6, then such an anisotropic formation can still be described by a tensor conductivity {right arrow over (σ)} as a function of the four basic conductivities, σnb, σpb, σnf, and σpf. The parallel-to-fault direction is indicated inFIG. 6by74, and the normal-to-fault direction by75. Individual faults are indicated by reference number73.

The conductivity {right arrow over (σ)} in the X, Y and Z directions for such a general biaxial anisotropic case is given by the summation of conductivity in each direction with a fault/fracture dip equal to a rotation of α° and an azimuth/strike of the fault/fracture plane equal to a rotation of β°:

FIG. 6and equation (4) represent a general biaxial system, which however may not be aligned with the tri-axial induction tool measurement system. It will typically not be possible to position the source and receiver(s) in a well (borehole) so that their x-y plane is parallel to the formation bedding planes. The orientation of the bedding planes may not be known, and even if known, the well is not always drilled perpendicular to that plane (e.g., high angle and horizontal wells), and furthermore the tool will rotate in the borehole during the logging. The following expansion of the present inventive method solves these problems.

A general 3-D biaxial anisotropic resistivity model includes tool axis or borehole with arbitrary deviation angle and strike angle penetrating a series of anisotropic or isotropic beds (with arbitrary dip and strike angles).FIG. 7shows some dipping beds cut by faults with arbitrary dips and azimuth.81shows bedding plane orientation, and82indicates the fracture plane. In such a more general anisotropic case, assumptions of the TI symmetry for tri-axial induction measurements and inversion will break down.

Such a generic anisotropic case can be set up using the coordinate system for 3-D resistivity measurement by multi-component/tri-axial tools as shown inFIG. 8. The X-Y-Z axes are oriented such that the Z-axis coincides with the borehole axis. The X′-Y′-Z′ coordinate axes are oriented such that the Z′-axis is normal to the formation plane (e.g., the formation layers' boundary plane). The X″-Y″-Z″ coordinate axes are oriented such that the Z″-axis is normal to the cross-bedding plane or fracture plane. The borehole (Z-axis) deviation (dip) and strike angles are indicated onFIG. 9as (αtool, βtool), respectively, and the cross-bedding or fracture plane (Z″-axis) dip and strike angles are shown onFIG. 9as (α, β), respectively, both being with respect to the formation boundary coordinate axes X′-Y′-Z′. In the X″-Y″-Z″ coordinate system, the biaxial conductivity tensors can be expressed as:

σ⇀=σij″=[σxx000σyy000σzz].(5)
For convenience, the double-prime superscript is dispensed with in writing the individual tensor components. Equation (5) can be expressed in terms of σnb, σpb, σnfand σpfby substituting the relationships (4).

The conductivity tensors in either the cross bedding plane or faults/fracture plane (or planes if there are multiple fractures) in the X″-Y″-Z″ coordinates can be coupled to the borehole axis coordinates X-Y-Z by the following transformation:

In the above treatment, a general biaxial anisotropy system was formed by two simple uniaxial anisotropic systems with rotations of arbitrary dip and azimuth angle in a forward modeling scheme for computer simulation of synthetic magnetic moment or voltage data (from which conductivity can be calculated). For such a general biaxial anisotropic system with arbitrary borehole orientation as shown inFIG. 9and defined by Equation (6), there are seven unknowns: αtool, βtool, α, β, σxx, σyy, σzz. (The nine components of σ are measured by the nine components of the multi-component tool oriented with arbitrary dip and azimuth, and the three non-zero components of σ″, i.e. σxx, σyyand σzz, are the formation biaxial anisotropic resistivities to be inverted; i.e. to be inferred by an inversion process.)

The inversion process can be designed to match and/or minimize the differences between the computer generated synthetic data in the coordinates of the bi-axial anisotropic system and the field data measured by multi-component/tri-axial tools in the form of magnetic moments Hij(see the previously cited paper by Schon, et. al.), or voltage Vij(previously cited papers by Rosthal et al. and Barber et al.) by iteratively updating the borehole and formation parameters. A tri-axial/multi-component resistivity tool may be constructed with paired transmitters and receivers with multi-frequency measurement channels, or multi-spacings of the transmitter and receiver pairs and each pair can also contain three orthogonally oriented transmitters (TX, TYand TZ) and receivers (RX, RYand RZ) with the same spacing L(i). This arrangement is illustrated inFIG. 9and shows how the problem of simulating synthetic H or V data may be set up. The magnetic moments of in-lined and cross-lined coupling between transmitter and receiver pairs can be expressed as

Resistivity logging is based on exciting a source coil with an AC voltage, resulting in a magnetic field generated by the source coil. This magnetic field induces eddy currents in the formation that further induce an AC voltage in a receiver coil, which is measured and recorded. The formation conductivity can be calculated from the known transmitter and receiver parameters and geometry, resulting in a relationship between measured V or H and formation conductivity. This is a tensor relationship in an anisotropic medium. This relationship between the tensor conductivity σijand the tri-axial dipole magnetic moments Hijcan be expressed as:
{right arrow over (σ)}ij=Cij*{right arrow over (H)}ij(8)
where Cijis a coupling matrix, determined by the well logging tool's design. Each geophysical service company will be able to provide these parameters for its tool.

Therefore, equation (8) is substituted into equation (6) for σ, and equation (6) is then inverted to yield σ″. Mathematically, a process for the inversion of Equation (6) can be realized by least-square optimization (see, for example, Zhdanov et al., “Foundation of tensor induction well logging,”Petrophysics42 (2001)), but the invention is not limited to this or any other particular optimization or updating scheme, and a misfit measure, or objective function, can be defined as:
φ(mp)=∥Hij(d)−Hij(c)∥  (9)
where Hij(d) and Hij(c) are, respectively, the measured data from the tool and the predicted data from 3D computer simulation using equation (6) based on the values for the seven unknowns assumed for the present iteration cycle.
1D Modeling in Biaxial Anisotropic System Composed of Two TI Systems

The conductivity tensors in either the cross-bedding plane or fracture plane (or planes if a system of multiple fracture sets) expressed in the X″-Y″-Z″ coordinate system can also be iteratively solved for through Equation (4) defined by the two orthogonal TI system, i.e., using the tensor conductivity {right arrow over (σ)} in Equation (4) composed of the four basic conductivities, σnb, σpb, σnf, and σpf, and the relative dip α and azimuth β between the VTI bedding system and HTI faults/fracture/cross-bedding system. In such a case, the 1D solution described in the Appendix in Anderson et al., “The effect of crossbedding anisotropy on induction tool response”,Petrophysics42, 137-149 (2001), which is incorporated herein by reference, can be reformulated and applied twice orthogonally, and the conductivity tensors {right arrow over (σ)} can be summed as defined by Equation (4). This 1D+1D approach is a less rigorous way of inverting Equation (6) than an approach such as iterative updating using an objective function as outlined above, but it will significantly reduce the forward computation time, and may be the preferred embodiment of the invention from a practical standpoint. However, these two inversion techniques are examples only, and the present inventive method includes any method of inverting Equation (6) or any equivalent relationship.

Constraints from Related Measurements and Rock Physics

The following features can be advantageously incorporated in the invention, depending upon the circumstances, although they are not essential to the invention.

In the step of the initial iteration, the measured borehole deviation survey data can be used to restrain the initial values of the variables αtool, βtool.

Define VTI anisotropic ratio λ=σpb/σnb, and HTI anisotropic ratio μ=σpf/σnf.

In hydrocarbon saturated anisotropic systems, it may be assumed that σpb>>σnb, e.g., λ=σpb/σnb>>1, and, σpf>>σnf, e.g., μ=σpf/σnf>>1. It may be possible to determine these anisotropic ratios from core plug measurements.

In water saturated anisotropic systems, it may be assumed that σpb>σnb, e.g., λ=σpb/σnb≈1-3, and, σpf>σnf, e.g., μ=σpf/σnf>>1-5. It may be possible to determine these anisotropic ratios from core plug measurements.

With these physical and geological/geographical data constraints, the seven unknown parameters αtool, βtool, α, β, σxx, σyy, σzzin Equation (6) can be reduced down to five (α, β, σxx, σyy, σzz), or even fewer, for a more manageable inversion process and more robust results.

The foregoing application is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. All such modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims.