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Electromagnetic logging while drilling
The electromagnetic-wave resistivity (EWR) tool has become the standard of the logging while drilling (LWD) environment.
Historically the earliest LWD electromagnetic measurements were Toroidal ( The Arps system of the 1960's ) and Short Normal and Laterolog measurements  of the late 1970's Exlog systems. However technical and mechanical survival considerations quickly caused the 2 Mhz systems to dominate, and these were then expanded with other frequencies.The nature of the electromagnetic measurement requires that the tool typically be equipped with a loop antenna that fits around the OD (outer diameter) of the drill collar and emits electromagnetic waves between slots of a steel protective shroud, which enables a robust mechanical design. The waves travel through the immediate wellbore environment, and are detected by a pair of receivers. Two types of wave measurements are performed at the receivers. The attenuation of the wave amplitude as it arrives at the two receivers yields the attenuation ratio. The phase difference in the wave between the two receivers is measured, yielding the phase-difference measurement. Typically, these measurements are then converted back to resistivity values through the use of a conversion derived from computer-modeling or test-tank data.
Resistivity measurement systems
The primary purpose of resistivity-measurement systems is to obtain a value of true formation resistivity (Rt) and to quantify the depth of invasion of the drilling-fluid filtrate into the formation. A critical parameter in measurement while drilling (MWD) measurements is formation exposure time (FET), the time difference between the drillbit disturbing in-situ conditions and sensors measuring the formation. MWD systems have the advantage of measuring Rt after a relatively short FET, typically 30 to 300 minutes. Interpretation difficulties sometimes can be caused by variable FET, and logs should always contain at least one formation exposure curve.
Knowledge of FET does not, however, rule out other effects. Fig. 1 shows a comparison between phase and attenuation resistivity with an FET of less than 15 minutes and a wireline laterolog run several days later. Even the attenuation resistivity has been affected dramatically by invasion, reading about 10 Ω•m, whereas the true resistivity is in the region of 200 Ω•m.
Fig. 1—Comparison of EWR and wireline resistivity in a deeply invaded, high-permeability sandstone; LLD = dual laterolog, LLS = single laterolog, and MSFL = micro spherically focused log. (From Economides, Watters, and Dunn-Norman, Petroleum Well Construction, © 1998; reproduced by permission of John Wiley & Sons Ltd.)
Another example, shown in Fig. 2, illustrates invasion effects in the interval from 2995 to 3025 m. Very deep invasion by conductive muds in the reservoir has caused the 2-MHz tool to read less than 10 Ω•m in a 200-Ω•m zone. Between 3058 and 3070 m, the deep invasion has caused the hydrocarbon-bearing zone to be almost completely obscured. Only by comparison with the overlying, deeply invaded zone from 2995 to 3025 m was this productive interval identified.
Similarly, LWD data density is dependent upon rate of penetration (ROP). Good-quality logs typically have graduations or “tick” marks in each track to give a quick-look indication of measurement-density variations with respect to depth.
Functionality of resistivity systems
Early resistivity systems emphasized the difference between the phase and attenuation curves, and suggested that one curve was a “deep” (radius of investigation) curve and another was a “medium” curve. Difficulties with this interpretation in practice1 led to the development of a generation of tools that derive their differences in investigation depth from additional physical spacings. Identification and presentation of invasion profiles, particularly in horizontal holes, can lead to a greater understanding of reservoir mechanisms. Many of the applications in which LWD logs have replaced wireline logs occur in high-angle wells. This trend leads to an emphasis on LWD for certain specialist-interpretation issues.
The depth of investigation of 2-MHz-wave resistivity devices is dependent on the resistivity of the formation being investigated. The measurement response of a device (both phase and attenuation) with four different receiver spacings is shown in Fig. 3. The region measured by the 25-in. sensor (R25P) is based on a 25-in. diameter of investigation in a formation known to have a resistivity of 1 Ω•m. The phase measurement looks deeper (away from the borehole) and loses vertical resolution as the charts progress to greater resistivities. In contrast, the amplitude ratio at first looks deeper than the phase measurement, and the expected penalty of poorer vertical resolution is paid. In the most resistive case, the attenuation measurement shows a 129-in. diameter of investigation. Many electromagnetic tools transmit at variable frequencies (2MHz, 1MHz, and 400kHz) to capture the benefit of variable depths of investigation and to minimize eccentering effects. Some systems can yield more than 20 resistivity measurements per data point, from which a greater understanding of reservoir characteristics can be derived.
Sources of measurement issues
Dielectric effects are responsible for some discrepancies between phase and attenuation resistivity measurements. Errors are greatest in the most resistive formations. Different approaches to this issue have been taken by vendors, with some opting to assume a set dielectric-constant value such as “10,” whereas others have chosen to vary the dielectric constant as a function of formation resistivity.
Further discrepancies between phase and attenuation resistivity measurements also may be attributed to the effects of formation anisotropy. Anisotropy may also be responsible for the separation of measurements taken at different spacings or at different frequencies. This can be easily misinterpreted as an invasion effect. Anisotropy effects are caused by differences in the resistance of the formation when measured across bedding planes (Rv) or along bedding planes (Rh). An assumption is generally made that Rh is independent of orientation. As borehole inclination increases, the angle between the borehole and formation dip typically increases. When this relative angle exceeds approximately 40°, resultant effects become significant. Anisotropy has the effect of increasing the observed resistivity above Rh. Effects are greater on the phase measurements than the attenuation measurements and greater on longer receiver spacings than short ones. It is important to understand that separation between resistivity curves caused by conductive invasion will result in the deep-resistivity-curve reading less than or equal to the true formation resistivity, whereas resistivity-curve separation caused by anisotropy will lead to measured deep resistivities being greater than true formation resistivity. The importance of trying to resolve these effects has led to a substantial and ongoing effort by the industry to develop robust, fast resistivity-modeling packages.
Toroidal resistivity measurement
Wave resistivity tools are run in most instances in which LWD systems are used, but toroidal resistivity measurements are desirable under some circumstances. Toroidal resistivity tools typically consist of a transmitter that is excited by an alternating current (AC), which induces a current in the bottomhole assembly (BHA). Two receivers are placed below the transmitter, and the amount of current measured exiting the tool to the formation between the receivers is the lateral (or ring) resistivity. The amount of current passing through the lower measuring point is the bit resistivity (Fig. 4). Because of the large number of variables involved, bit resistivity measurements have been difficult to quantify, but measurements from current-generation tools now compare favorably with wireline laterolog measurements. In formations with high resistivities (greater than 100 Ω•m), measurements with a toroidal resistivity tool may be more appropriate than measurements with other tool types. An important side benefit of this technology is its insensitivity to anisotropic effects.
The log example in Fig. 5 shows a case in which 2-MHz measurements have saturated because of the high salinity of the mud. If the drilling fluid is conductive or if conductive invasion is expected, then toroidal resistivity measurement is preferred. If early identification of a coring or casing point is crucial, then bit resistivity measurements give a good first look. In geosteering applications, toroidal bit resistivity measurements are an immediate indicator of a fault crossing.
Imaging while drilling
The first formation images while drilling were acquired through the use of toroidal resistivity tools. When a small-button electrode is placed on the OD of a stabilizer, the current flowing through that electrode can be monitored. The current is proportional to the formation resistivity in the immediate proximity. Effective measurements are best taken in salty muds with resistive formations. Vertical resolution is 2 to 3 in., and azimuthal resolution is less than 1 in. While the tool is rotating at least 30 RPM, internal magnetometer readings are taken, and resistivity values are scanned and stored appropriately. A sample of the data is pulsed to the surface in real time to provide a low-resolution measurement. At the surface, tool memory is dumped, and the data are related to the correct depth. Quality checks are made to ensure that poor microdepth measurements are not affecting the reading.
Imaging while drilling can provide a picture of formation structure, nonconformities, large fractures, and other visible formation features. Azimuthal-density devices may also be processed to provide dip information. Imaging is increasingly used as in geosteering applications. Real-time dip calculations can be carried out in structures with relatively high apparent dips.
- Arps, J. J., 1963, Continuous logging while drilling: A practical reality: SPE Annual Fall Meeting, SPE 710,October 6–9, 1963
- A Focused Current Resistivity Logging System for MWD : Evans, H.B., Brooks, A.G., Meisner, J.E., Squire, R.E., Exploration Logging Inc. SPE Annual Technical Conference and Exhibition, 27-30 September 1987, Dallas, Texas
- Gianzero, S., Foster, M., Chemali, R. et al. 1985. A New Resistivity Tool For Measurement-While-Drilling. Presented at the SPWLA 26th Annual Logging Symposium. SPWLA-1985-A.
- Rosthal, R.A., Young, R.A., Lovell, J.R. et al. 1995. Formation Evaluation and Geological Interpretation from the Resistivity-at-the-Bit Tool. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22–25 October. SPE-30550-MS. http://dx.doi.org/10.2118/30550-MS.
Noteworthy papers in OnePetro
Clark, Brian et al. 1990. Electromagnetic Propagation Logging While Drilling: Theory and Experiment, SPE Formation Evaluation Volume 5, Number 3. 18117-PA. http://dx.doi.org/10.2118/18117-PA
Muqeem, M.A., Jarrett, C.M et al. 2007. The Introduction of Electromagnetic LWD Technology in Saudi Arabia - A Case History and Future Application to Underbalanced Drilling Campaigns, SPE/IADC Drilling Conference, 20-22 February. 105471-MS. http://dx.doi.org/10.2118/105471-MS