The use of electromagnetic logging tools while drilling to investigate the space in front of the drill-bit and to predict the geologic strata and/or fluid property ahead has been one of the industry-focuses in recent years. The commercial look-ahead tools have two main designs based on the published tool configurations and data-channel definitions. The first type employs combinations of tilted coils to acquire multiple or all components of electromagnetic fields and presents logs defined in novel expressions as functions of these field components. The second kind uses combination of the voltage measurements from orthogonal coils (coils pointing along and perpendicular to tool-axis) and the propagation resistivity logs from coaxial coils of large coil-spacing. This work starts with reviewing the tool physics and the mathematic expressions defining the logs. The look-ahead capabilities are then investigated by considering the case where a borehole is perpendicular to the formation boundaries. It is found that the necessary information for forward detection applications is exclusively from the apparent resistivity logs, not the so-called geo-signals or any other logs. We further explore an ideal scenario, i.e., to detect the presence of a good conductor in front of a high resistivity formation to obtain the asymptotic behaviors for the two measurement types. The asymptotic expressions lay the foundation to understanding the intrinsic differences between the two methods. The insights derived from the theoretical work and the subsequent modeling results for a broad set of models define the response characteristics and the probing capabilities of the look-ahead techniques. It is realized that the look-ahead instrument design can be simplified, such as using only the familiar three-coil coaxial configuration with a suitable long coil-spacing and multiple carefully chosen operating frequencies to achieve a robust detection-distance coverage. It is worth to point out that the commercially available look-ahead services are mainly applicable to vertical and inclined wells so far. For look-ahead applications in horizontal wells, due to the contribution of the geologic and fluid boundaries lateral to the tool and the inhomogeneities ahead of the drill-bit, there is no reliable method yet to simultaneously solve the large number of unknown parameters associated other than some special situations. The look-ahead evaluation in the horizontal wells must be investigated further by understanding the characteristics of exponential attenuation and geometric spreading of the electromagnetic fields in a conductive formation and the inherent relatively poor resolution through combining with other detection physics and the introduction of proper geologic model constraints.
To appraise the look-around and look-ahead technologies used in geosteering, an extension to the sensitivity function is presented by considering jointly with the resolution. The development is of critical importance to the data interpretation of LWD azimuthal electromagnetic tools introduced in the recent years. The extended sensitivity distribution describes the geologic boundary detectability. The spatial distribution of low-frequency electro-magnetic fields in conductive medium is dominated by two factors, namely the exponential decay as function of skin-depth and the geometric spreading which is inversely proportional to the distance cubed. The electromagnetic fields fade away rapidly and consequently so is the associated sensitivity function. The smaller field strength or poorer sensitivity far away from the tool also dictates that the resolution farther away from the tool is much worse than that near the tool. To improve the detectability of geologic target, based on the physics observed, this paper extends the spatial sensitivity function by taking the tool resolution into consideration. The space is divided into cells with the cell-size conforming approximately to the natural resolution, for example, geometric progression. The new sensitivity function is the traditional one multiplied with the cell size to better describe the look-around and look-ahead capabilities, where the emphasis is on the detection of the bed-boundaries as far ahead or around as possible. It is observed that the iso-sensitivity envelope encompassing 50% of cumulative contribution from the tools is within the distance of about one source-receiver coil-separation, L, or less. This is consistent with the depth of investigation for wireline induction logging tools defined through the integrated geometric factor at the same percentage. The observed iso-sensitivity envelope for 90% of cumulative contribution is about 2 to 3 L’s away from the tool, while the iso-sensitivity surface for 99% is observed to be from a volume of about 4L to 5L or slightly more around the tool. For various coil-configurations, it finds that the X-X (likewise Y-Y) coil-combination is more sensitive to look-ahead boundary-detection when the geologic boundary is perpendicular to the sensor. The look-around investigation with sensor parallel to the beddings may benefit relatively more from, for example, the 45-degree tilted-coil configuration.
The depth of detection (DOD), which is an important concept in logging data interpretation, describes the detection capability of the borehole measurements. We extend the definition of DOD for azimuthal information, namely the geosignal delivered by azimuthal resistivity tools, to resistivity logs in logging-while-drilling (LWD) applications. Instead of using the radial geometric factor, the detection thresholds in predicting a geologic boundary is employed to describe the DOD of a measurement. This definition unifies the criteria to evaluate the detectability of different borehole measurements, such as LWD resistivity measurements, and geosignals. It can also be generalized to other kinds of well logging methods in LWD applications. Using the proposed definition, we present the analyses of the detection capability of the LWD resistivity measurements in looking-around and looking-ahead applications. And they provide more tangible descriptions. In vertical or near-vertical wells, the definition provides an indicator to evaluate the capability and reliability of looking-ahead of deep/ultra-deep LWD resistivity tools. The investigations on the influence of the DOD on the distance-to-boundary (DTB) inversion, which can help in developing a robust and accurate inversion scheme, are also presented and discussed
With the increase in the scale of mining in horizontal and highly deviated wells, electromagnetic boundary detection while drilling plays an important role in boundary detection. This paper examines three types of antenna structures commonly used in electromagnetic boundary detection and measurement methods and also performs a numerical simulation of the edge detection capability of the three structures in horizontal wells. The simulation experiment analyzes the influence of formation resistivity contrast, frequency, spacing, and other factors on the capability of edge detection and provides data that supports the design of instrument antenna parameters. The numerical simulation shows that the tilted and orthogonal receiving antennas demonstrate improved performance both in detecting the interface when approaching from high-resistance layers and low-resistance layers. In addition, the capability of boundary detection can be improved by decreasing the frequency and increasing the spacing between the transmitter and receiver.
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