Resistivity anisotropy in both laminated shale-sand and clean sand formations is well documented. Tools that are sensitive to formation anisotropy are also well documented, and the leading contender for this type of measurement is the transverse induction array. Such an array, whose transmitter generates formation currents in the plane of the borehole axis, has a good sensitivity to the vertical resistivity of the formation, Rv. Invasion of mud filtrate into permeable formations has long complicated wireline log analysis. Interpretation of anisotropic formations will be no different. In most drilling environments, these formations will be invaded as readily as isotropic formations. Although we expect that invading mud filtrate from water-based mud will reduce the anisotropy, we also expect that invading oil-based mud (OBM) filtrate will increase it. Thus the anisotropic properties of each zone must be determined separately. A single-spacing induction array (of any orientation) cannot, by itself, separate uninvaded zone properties from those of the invaded zone. Multiple-spacing tools have been used for many years to make this separation. A multiarray triaxial induction tool is in field test, and invasion interpretation algorithms are under development. A fast analytic algorithm corrects shoulder effect on all nine triaxial couplings of each array, allowing Rv and the horizontal resistivity Rh to be determined at several radial depths in the formation. Rigorous 2D and 3D inversions are also used to evaluate beds on the order of 1 m thick in both vertical and deviated wells. Examples in both modeled formations and in actual formations demonstrate the methods. Introduction Induction tools (along with laterolog tools) have been the standard resistivity devices for borehole geophysics for about 50 years. The nearly geometric response of the induction array has made it easier to interpret than the non-geometric response of the laterolog tools. Conventional induction tools are built with coils that have their magnetic moments along the tool axis. The resulting sensitivity to the formation is in a direction perpendicular to the wellbore axis. When the beds are dipping, or when the well is deviated, the response is more complicated, but not in a way that allows determination of the dip angle.

A new crosswell electromagnetic (EM) tool, now in field test, provides unique means to remotely sense the resistivity between two wells without having to drill an intermediate well. The wells can be open hole or cased with fiberglass, nonmagnetic chromium steel, or ordinary magnetic carbon steel. This tool allows one to determine the resistivity distribution between wells spaced up to 1000 m apart. It is an induction measurement with a large magnetic field transmitter in one well and sensitive low-noise receivers in a second well. The transmitter generates a time varying field in the frequency range of 5 Hz to 1000 Hz. Since the transmitter and the receivers are not at a fixed position, the primary field cannot be cancelled as in conventional induction tools. Instead, the receiver senses the primary field of the transmitter combined with the secondary field produced by the formation between the wells. This secondary field increases with frequency of operation, formation conductivity and spacing between wells. In general, a high-frequency measurement with large formation effect is combined with a low-frequency effect to provide a geometry correction for the wells. Data from multiple transmitter and receiver positions are processed through a two-dimensional non-linear inversion algorithm to produce a resistivity image of the formation between the wells. A priori information about the formation as well as conventional well logs are used to constrain or improve the image. Although the performance of this technique is strongly dependent upon the specific problem, the resulting image can be used for detecting bypassed oil, or in a time-lapse application, it can be used to monitor the movement of a reservoir or of injected fluid. This paper presents the unique tool features, the survey planning process, and factors important for success as well as modeling and measurement examples. In addition, sensitivity to frequency and the starting model used as the initial step in processing the data to an image are examined. Introduction Traditional resistivity logging tools measure the formation resistivity at a distance of a few inches to perhaps ten feet from the wellbore. This is generally sufficient to determine the properties of the formation beyond the invaded zone and to characterize the formation near the wellbores. There are no commercial tools that directly measure the resistivity at large distances from the wells. The crosswell EM tool is designed to meet that need, providing resistivity measurements at reservoir level. This capability is important as in mature fields fluid movement typically occurs over periods of years or decades. Whether the field is undergoing primary production or enhanced production, it is important to be able to identify the movement of fluids at distances of hundreds or thousands of meters from the wells. Although crosswell EM logging has existed since the mid 1990s (e.g., Alumbaugh and Morrison, 1995a; Wilt et al., 1995) recent industrial interest coupled with advances in technology have enabled the development of a next-generation system that we have named Cross-well EM Resistivity. This system provides significant advances in terms of measurement accuracy, resolution, logging speed, field operations, modeling, and processing tools.

Hunka, J.F., and Barber, T.D.,* Schlumberger Well Services;Rosthal, R.A., Logging While Drilling; Minerbo, G.N., Schlumberger-Doll Research; Head, E.A., *Howard Jr., A.Q., and Hazen, G.A., Schlumberger Well Services; and Chandler, R.N., Etudes and Productions Schlumberger Abstract A new induction logging system has been developed that represents a fundamental departure from the technology and application of previous induction-based resistivity tools. The AIT* Array Induction Imager tool abandons the concept of fixed-focused sensors and is constructed of several independent arrays with main coil spacings ranging from a few inches to several feet. The AIT tool is operated simultaneously at several frequencies; in-phase and quadrature signals are acquired from every array at the frequencies suitable for that array length. The presented log curves range in median depth of investigation from 10 in. to 90 in. Each log uses all the measured channels, combined with a nonlinear processing algorithm, to virtually eliminate environmental effects such as cave effect, shoulder effect, and skin effect. Reliable logs can be obtained even in difficult cases of bad borehole and extreme invasion. These logs are available at resolution widths of 6 ft and 2 ft. Because of the large number of measurements made by the AIT tool, deep two-dimensional quantitative imaging of formation resistivity is possible. These images expose bedding and invasion features in a clear and quantitative manner. The resistivity in the part of the formation undisturbed by fluid invasion is accurately obtained without making any prior assumptions about the invasion profile. New invasion description parameters conveying more profile. New invasion description parameters conveying more meaningful information about the presence of transition zones and annuli are a result of this imaging capability. Using established interpretation principles, this quantitative information about the invasion can be converted into a two-dimensional image of water saturation. Introduction In recent years, the most troublesome environmental features of induction tools have been corrected by tools such as the Phasor* Induction tool. Vertical resolution has been improved from 8 ft to 2 ft; automatic correction for borehole effect is available at the wellsite; and inversion of the three measurements (deep induction, medium induction, and Spherically Focussed (SFL*) log or laterolog) into R, estimates has been made automatic. With the elimination of the grosser environmental distortions, some remaining effects that can introduce errors in difficult logging situations have received more attention. The most troublesome of these are: Cave effect in irregular boreholes, andDetermining Rt in the presence of invasion transition zones. Cave effect is produced when an induction toolen counters a washout or cave in a borehole with high formation resistivity to mud resistivity contrast. Induction arrays normally have response peaks very close to the tool that are very sensitive to conductivity. These responses are cancelled in smooth boreholes, but they do not cancel in rugose holes. Large excursions on the logs can occur when one of these sensitive areas encounters a washout or other irregularity. These "hot spots" are present in all previous induction tools. An invasion transition zone is any radial resistivity profile other than a simple step profile. Transition zones profile other than a simple step profile. Transition zones can be produced by any fingering or mixing of mud filtrate with connate fluids over some radial distance or by the formation of an annulus. Dual induction tools can detect some transition zones, and recent field log evidence suggests that transition zones may be more common than previously realized. previously realized. P. 295

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Over 30 years ago Poupon proposed using several simple induction arrays, each measured separately, and combining them to produce an induction log with improved characteristics. The response of each simple array becomes a "basis function" that can be combined with the others without introducing unwanted interactions. This approach has been used in the Array Induction Imager Tool (AIT*). The AIT tool consists of eight three-coil arrays, six of which are operated simultaneously at two frequencies. By treating the response of each measurement as a basis function, an optimization technique has been developed to combine all the raw measurements into logs with vertical, radial, and two-dimensional (2-D) response properties that are not achievable with conventional fixed-focus induction arrays. Using this approach, a set of logs has been developed with a 90% vertical resolution width of 1 ft and a reasonable rejection of borehole rugosity effects. Additional log sets are available that have different resolution-cave effect tradeoffs. A log set with a vertical resolution of 2 ft has less cave effect than conventional induction logs with 8-ft resolution, and a 4-ft resolution set has virtually no cave effect. The effectiveness of this response shaping technique has been demonstrated in case studies with both computed and field logs over a wide range of environmental conditions.