A conceptually new geosteering tool is discussed. The tool uses conventional coil transmitters but measures the electric field unlike any conventional resistivity tool that measures the magnetic field. The detection sensitivity is defined for each type of electromagnetic field measurement. Examination of both electric and magnetic field responses to a distant conductivity anomaly from magnetic dipole transmitters showed that the electric field responses are significantly more sensitive to the anomaly. With a transverse magnetic dipole transmitter, the orthogonal and transverse electric field measurement is sensitive to detect a conductivity anomaly far ahead of the tool. While the coplanar magnetic field measurements in conventional geosteering tools are known to be sensitive to the conductivity anomaly around the tool, the axial electric field measurement using the transverse magnetic dipole transmitter and the transverse electric field measurement using magnetic dipole transmitter are much more sensitive to the anomaly around the tool than any magnetic field measurements. This deep looking capability is achieved with relatively short source-receiver spacing.
Presentation Date: Tuesday, October 16, 2018
Start Time: 8:30:00 AM
Location: 212A (Anaheim Convention Center)
Presentation Type: Oral
We examined the electric field strength response to tri-axial electric dipole sources in VTI anisotropic formations. The tri-axial electric field strength data are used to measure not only the anisotropic resistivity of the formation but also the relative dip angle of the tool with respect to the formation.
When a tri-axial electric dipole tool (frequency-domain tool) is run at multiple frequencies, the multi-frequency focusing method can be applied to the tri-axial electric field responses. Then, both the effective dip of the formation and the anisotropy of the formation are algebraically determined by the frequency-focused tri-axial electric field responses at one measurement depth.
Such a tri-axial electric dipole tool can be also run using a step-on or step-off current dipole source and measuring the transient electric field responses (time-domain tool). When the transient responses are measured, both the transient apparent dip and the transient apparent anisotropy of the formation are algebraically defined from the transient tri-axial electric field responses. The transient apparent dip and the transient apparent anisotropy approach the true dip and the true anisotropy, respectively, at later time in a step-off mode at one measurement depth.
Presentation Date: Wednesday, October 19, 2016
Start Time: 1:30:00 PM
Presentation Type: ORAL
The permeability anisotropy is related to the electric resistivity anisotropy and the pore throat cross-section anisotropy as, (equation), where the subscript H and V indicate the conduction in the horizontal and vertical direction, respectively. RH and RV are the horizontal resistivity and vertical resistivity of brine filled rock formation. FH and FV are the formation factor measured in the horizontal and vertical directions. Similarly, (equation) and (equation) are the average pore throat cross-sections to the horizontal and vertical conductions, respectively.
The permeability anisotropy is an important parameter in evaluating reservoir performance. The above relation will help to identify permeability-anisotropic formation from the electrical anisotropy measurements. If the formation is electrically anisotropic and RV≠RH (FV≠FH) when it is wet, then the formation is also permeability-anisotropic and KV≠KH. By examining the macroscopic anisotropy of the permeability and electric conductivity in thinly laminated formations, we show that the KH/KV is bound by the RV/RH of the wet formation: KH _ (equation) On the other hand, the permeability is anisotropic even if the formation is electrically isotropic and RV=RH (FV=FH) when it is wet: (equation)
To estimate the permeability anisotropy, the pore throat cross-section anisotropy (equation), has to be known. Although the (equation) and hence, (equation), can be measured petrographically when proper core samples are available, these are generally unknown. When the cores are not available, they may be estimated from pore size distribution measurements using NMR log data.
Sealing sands in some reservoirs may be interpreted as an example of laminated anisotropic rocks. The permeability-resistivity relation may be used to identify such sealing sands from log measurement of resistivity anisotropy.
The relation between permeability (flow conductivity) and electric conductivity has been known if the electric conduction through the rock surface is ignored or negligible (Purcell, 1949). The relation relates the permeability k to the formation factor F as,
The CSEM data are grouped into the data sets with constant transmitter-receiver offsets. Symmetrized data set with a constant offset may be used to identify a resistivity anomaly with the depth of investigation approximately of the transmitter-receiver offset. Symmetrized data sets from different offsets with multiple depths of investigation may help to determine the location and the resistivity of a resistivity anomaly. An anti-symmetrized data set (differential amplitude) with the constant offset may be used to easily detect the lateral boundary of the resistivity anomaly. The method is also applicable to the streamer EM data where the data are collected with constant offsets to begin with.
Layered structure generates macroscopic anisotropy whether each constituting layers are isotropic or anisotropic. The macroscopic anisotropy is different from the anisotropy of constituting layers. The macroscopically anisotropic resistivity is determined in the directions parallel and normal to the layers, when the resistivity of individual layers and their fractions are known, with the assumption that the layered structure extends laterally sufficiently enough. When the layered formation consists of two types of layers, the resistivity or the fraction of the layers can be determined from the anisotropic macroscopic resistivity with some additional data. The number of data required to determine the resistivity of one type of layers depends on whether the constituting layers are isotropic or anisotropic. The resistivity estimate can be significantly mistaken if the constituting layers are anisotropic but the estimate is made with the assumption of isotropic layers.
Several recently published studies discuss the concept of inductiveresistivity-logging devices with oblique transmitting and/or receiving coils.Both wireline induction and logging-while-drilling (LWD) propagationresistivity-tool concepts have been considered. Directional resistivitymeasurements and improved anisotropy measurements are among the benefitspromised by this type of device. Analyses based on point-magnetic dipoleantennas were used to illustrate these potential benefits.The effects ofa metallic mandrel, borehole, and invasion were not considered because of theabsence of a suitable forward model.
This paper characterizes mandrel, borehole, and invasion effects for avariety of candidate tilt-coil devices with antenna array parameters similar tothose of the previous studies. The characterization is based on calculationsfrom a new forward model that includes tilted transmitting and receiving coilsof finite diameter embedded in a concentric cylindrical structure.
Important details of the forward model used in the calculations are alsoprovided.
Conventional propagation resistivity devices are routinely used forgeosteering applications. Because data from these devices have essentially noazimuthal sensitivity, the LWD engineer is greatly aided by a prioriinformation regarding the proximity of the target bed relative to othergeologic features such as shales and water-bearing zones. Suitable a prioriinformation is often available from offset logs. In cases in which offset logsare not fully useful because of changing depositional environments or differenttectonic settings, azimuthally sensitive resistivity data would improve thequality of the geosteering effort.
One way to achieve azimuthal sensitivity to benefit geo-steering (and to useit for imaging) is to construct a tool similar to a conventional propagationresistivity device, but with the transmitters and/or receivers tilted withrespect to the axis of the drill collar. In fact, directional resistivity tools(DRTs) have been proposed in the literature for this pur-pose.1-3 To theknowledge of the authors, DRTs have only been analyzed with point-dipolemodels, which ignore both the drill collar and the finite size of the antennas.For apparent lack of a suitable forward model, mandrel, borehole, and invasioneffects have not been considered in the literature. A model has been developedthat accounts for tilted transmitters and receivers embedded in arbitrarylayers of a concentric cylindrical structure. Many important details of thismodel are discussed in Appendix A.
The term mandrel effect is used here to denote the difference between valuescalculated with a point-dipole model and the model that accounts for themandrel encompassed by the antennas. Mandrel effects on DRT measurements willbe grouped into three categories:
1. Absolute effects where the mandrel primarily attenuates the signalsbecause of a reduction in the magnetic moment of the antennas.
2. Residual effects that remain after an air-hang calibration is applied tothe data.
3. Perturbations to the azimuthal sensitivity of the measurement caused bythe finite size of the antennas and the drill collar.
Algorithms that transform raw tool measurements to resistivity values can bebased on computationally simple point-dipole solutions without significantlydegrading the accuracy of the results if mandrel effects associated withcategories 1 and 2 can be suppressed. For conventional LWD propagationresistivity measurements, mandrel effects of type 1 are addressed by air-hangcalibration. Algorithms that suppress type 2 mandrel effects are discussed inthe literature.4 Type 3 mandrel effects are not discussed here.
de Kock, A.J. (Shell U.K.) | Johnson, T.J. (Halliburton Energy Services) | Hagiwara, Teruhiko (Halliburton Energy Services) | Zea, H.A. (Halliburton Technology Center) | Santa, Fernando (Halliburton Energy Services)
This paper (SPE 52891) was revised for publication from paper SPE 39130, first presented at the 1997 Offshore Technology Conference held in Houston, 5-8 May. Original manuscript received for review 30 May 1997. Revised manuscript received 20 February 1998. Paper peer approved 12 March 1998.
Formation compaction in unconsolidated geopressured turbidite reservoirs in the Gulf of Mexico (GOM) is an issue of great uncertainty and concern. When compaction occurs, it changes the porosity and permeability properties of the reservoir rock and can affect recovery efficiency and well productivity. It can deform well tubulars, creating operational problems and shortening well life. If compaction is significant, especially when multiple stacked reservoirs are involved (as in the case in this field example), then compaction can create a subsidence bowl at the ocean floor. In GOM offshore operations, this could cause platforms to subside deeper into the water and create potentially severe safety problems; therefore, failure to properly address issues of compaction and subsequent subsidence during the design and development phases of these capital intensive, deepwater projects could lead to severe financial setbacks.
It is crucial to have a compaction monitoring program in place because of the broad impact of compaction and seafloor subsidence. As a result, we developed a new formation-compaction monitoring tool (FCMT) and new methods of measurement and interpretation. The FCMT is a wireline device that uses multiple gamma ray detectors to determine locations of and precise distance between radioactive (RA) markers. Compaction of the formation can be measured by changes in the distance between the markers. For precise estimation of the vertical distance between a pair of markers, the new method uses an array of three or four detectors.
By examining the tool response to a marker, we developed a new method to determine the exact vertical and lateral location of the marker by using a Lorentzian response model; consequently, not only the vertical compaction but also lateral displacement of markers can be monitored with the new method.
The accuracy of the tool was established in the test facility where gamma ray sources were placed at precisely known intervals. The tools were run centralized at three logging speeds (5, 10, and 15 ft/min.), and data were collected at 0.1-in. intervals. The vertical distances between a pair of RA markers spaced 30 ft apart were measured accurately to within 0.1 in.
The first baseline logs were collected successfully in the four wells in a GOM deepwater development. High consistency among measurements from different logging passes proved that the FCMT can provide precise distance measurements with newly developed methods.
We examine induction log responses to layered, dipping, and anisotropic formations analytically. The analytical model is especially helpful in understanding induction log responses to thinly laminated binary formations, such as sand/shale sequences, that exhibit macroscopically anisotropic resistivity. Two applications of the analytical model are discussed.
In one application we examine special induction log shoulder-bed corrections for use when thin anisotropic beds are encountered. It is known that thinly laminated sand/shale sequences act as macroscopically anisotropic formations. Hydrocarbon-bearing formations also act as macroscopically anisotropic formations when they consist of alternating layers of different grain-size distributions. When such formations are thick, induction logs accurately read the macroscopic conductivity, from which the hydrocarbon saturation in the formations can be computed. When the laminated formations are not thick, proper shoulder-bed corrections (or thin-bed corrections) should be applied to obtain the true macroscopic formation conductivity and to estimate the hydrocarbon saturation more accurately.
The analytical model is used to calculate the thin-bed effect and to evaluate the shoulder-bed corrections. We will show that the formation resistivity and hence the hydrocarbon saturation are greatly overestimated when the anisotropy effect is not accounted for and conventional shoulder-bed corrections are applied to the log responses from such laminated formations
In another application, we examine the effect of shale anisotropy in thinly laminated sand/shale sequences. It is known that the macroscopic conductivity of a laminated formation is determined uniquely by the sand and shale laminae conductivity and the sand/shale ratio. Conversely, the sand-laminae conductivity is estimated from the macroscopic formation conductivity, and the hydrocarbon saturation in the sand laminae is computed from the sand conductivity.
The shale-laminae conductivity itself may be anisotropic in such laminated sequences. How do induction logs respond to such laminated formations when shale laminae are anisotropic? How accurate are the estimates of the sand-laminae resistivity and of the hydrocarbon saturation in these sand laminae? To answer these questions we used the analytical model to examine the effect of shale lamina anisotropy on induction log responses to thinly laminated formations. We learned that the macroscopic formation resistivity and hence the sand-lamina resistivity can be greatly overestimated if the shale anisotropy is not accounted for in interpreting induction log data from laminated formations. On the other hand, the estimate of the net-to-gross ratio is insensitive to shale anisotropy except for low sand-laminae resistivity (Rsd/Rsh < 5).
Tilted, multilayered formations are not uncommon, and the induction log response to such formations is complex, especially when the layers are only a few feet thick. Deviated boreholes are also common, and the log response from such a borehole is complicated when the bedding planes are intersected at an oblique angle. The induction log response in tilted and layered formations or from a deviated borehole can be modeled analytically. Such analytical tool models are very important in analyzing and interpreting induction tool responses.