Greater Depth Of Investigation Research Articles

Response simulation and theoretical calibration of a dual-induction resistivity LWD tool

In this paper, responses of a new dual-induction resistivity logging-while-drilling (LWD) tool in 3D inhomogeneous formation models are simulated by the vector finite element method (VFEM), the influences of the borehole, invaded zone, surrounding strata, and tool eccentricity are analyzed, and calibration loop parameters and calibration coefficients of the LWD tool are discussed. The results show that the tool has a greater depth of investigation than that of the existing electromagnetic propagation LWD tools and is more sensitive to azimuthal conductivity. Both deep and medium induction responses have linear relationships with the formation conductivity, considering optimal calibration loop parameters and calibration coefficients. Due to the different depths of investigation and resolution, deep induction and medium induction are affected differently by the formation model parameters, thereby having different correction factors. The simulation results can provide theoretical references for the research and interpretation of the dual-induction resistivity LWD tools.

Focused-source electromagnetic survey versus standard CSEM: 3D modeling in complex geometries

A novel focused-source electromagnetic (FSEM) method focuses the EM field in the vertical direction to provide deep-reading resistivity data. FSEM offers better spatial resolution and greater depth of investigation than the conventional controlled-source electromagnetic (CSEM) method for land and marine EM surveys. We have proven the high efficiency of FSEM by analyzing 3D models of various complex geologic formations in the presence of seafloor bathymetry, shallow resistive gas-hydrate overburdens, and secondary gas reservoirs formed above deeper oil reservoirs. Combining the power of our focusing technique with the power of our 3D numerical modeling method, we have developed exceptionally challenging test cases to conclude that FSEM automatically cancels unwanted shallow effects and allows simple visual interpretation of deep reservoir responses. In addition, FSEM is insensitive to imperfections in the setup geometry. We achieve these advantages using a proper combination of measurements acquired in the receiver excited by transmitters situated at different space points. The method is promising in anisotropic formations as well.

Focused-source EM survey versus time-domain and frequency-domain CSEM

A new focused-source electromagnetic (FSEM) method exploits the idea of focusing the EM field in a vertical direction to provide deep resistivity data. It allows directing the EM energy into the formation, similar to acoustic beam used in seismic exploration, enabling higher spatial resolution and greater depth of investigation, than the conventional controlled-source EM (CSEM) method.

Transient electromagnetic sounding using a 5 m square loop

Our study of the diffusion of transient electromagnetic (TEM) fields into a two-dimensional (2D) earth, simulated by a finite-difference time-domain (FDTD) algorithm, proves that the depth of investigation depends primarily on the measurement time and the ground resistivity. Sufficient signal strength can be gained by a small central-loop configuration equipped with a high-power transmitter and a large equivalent area receiver coil. When receiver sensitivity is 1 mV, average resistivity of the overlying section is around 50 Ω.m, and the noise level is 3 to 20 nV/m2, a depth of exploration of 150 m can be achieved with a 5 m × 5 m loop with 100 A of transmitter current; for a noise level of 0.1 nV/m2, the depth of exploration is up to 800 m with 1200 A of transmitter current. The results have been compared with results from the electrical resistivity method and from large loop techniques, and verified by drilling. We show that using a small loop can attain not only a greater depth of investigation but also higher resolution. The small loop configuration with a high-power transmitter is a convenient combination, and useful for exploration in mountainous terrains and restricted sites.

Enhancing electrical signals with sensor arrays

Increasingly, electrical surveys employ multiple sensors to make data collection more efficient. The use of multiple sensors can also be used to improve signal fidelity from each sensor, leading to more accurate geological models and greater depth of investigation. New procedures to remove noise in data collected using an array of sensors have been developed and tested. These algorithms attempt to remove two types of noise: harmonic noise associated with the mains power grid and atmospheric transients due to distant lightning strikes and electrical storms. Field tests demonstrate improvements in signal-to-noise by two orders of magnitude. The algorithms are applicable to any type of electrical or EM survey conducted with a sensor array.

Experimental evaluation of hydrocarbon detection with the Long-Offset Time-Domain Electromagnetic Method in the Cretaceous carbonates of the Tampico–Misantla basin, Mexico

In Mexico, an experimental study with a Long-Offset Time-Domain Electromagnetic Method (LOTDEM) was carried out in the Tampico–Misantla basin of northeastern Mexico. The main objective was to evaluate this method for hydrocarbon exploration. The selected area is suitable for LOTDEM experiments because high quality seismic data, geological information and well logging data are available. The results obtained are excellent, and allow us to determine the types of fluids present in the rock pores and fractures of the studied geological structures. Resistivity anomalies correlate in position and depth with the geological structures observed in the seismic sections where the LOTDEM survey was carried out, and with well logging data from the nearby Franco Española oil field. In the studied area, these structures are 800–1200 m deep, in carbonate rocks with high clay content and are invaded by salt water. Consequently, the resistivity and primary porosities are very low. The main reservoirs of oil and gas in this region are a naturally fractured basinal facies, consisting of fine-grained limestones and shaly limestones, that corresponds to the Cretaceous units San Felipe, Agua Nueva, Tamaulipas Superior and Tamaulipas Inferior. The fractured rocks reservoirs are very difficult to detect, even with well logs. However, the results of this survey show the higher resolution, the greater depth of investigation, and the advantages that the LOTDEM method present compared to the traditional frequency domain electric and electromagnetic (FEM) methods.

Use of Permanent Resistivity and Transient-Pressure Measurement for Time-Lapse Saturation Mapping

Summary Optimization of hydrocarbon recovery requires information on the space and time behavior of the saturation of various fluids present in the reservoir. This is particularly true for oil fields under secondary recovery such as waterflooding, where an even reservoir sweep or zones of bypassed oil can be assessed by a proper description of the waterfront advance. Recently, permanent downhole electrodes have been deployed successfully in oil wells. This technology allows the time variation of the electrode potentials to be interpreted in terms of changes in saturation within the formation. However, the depth of investigation of such measurements is limited. Time-lapse pressure transient is an independent source of information with a greater depth of investigation and, therefore, it provides an adequate complement to the permanent resistivity array measurement. In this paper, we propose to use pressure buildup from repeated shut-in in association with the electrical measurements. After recalling analytical analogies in both types of measurements, we propose a quick-look method for interpreting the time-lapse pressure transients. We then compare the physical and practical advantages of each type of measurement and the domain of application of the two measurements with respect to fluid and reservoir properties. Finally, we propose an example showing the benefit obtained by coupling the two techniques.

Evaluation of two-dimensional resistivity methods in a fractured crystalline-rock terrane

A series of two-dimensional resistivity profiles collected in the Blue Ridge Province of southwest Virginia and results from numerical modeling of synthetic data reveal substantial differences in depth of investigation, resolution, and sensitivity using Wenner, Wenner–Schlumberger, dipole–dipole, and pole–pole data collection techniques. Resistivity profiles were collected using short (2-m electrode spacing, 48-m profile length), intermediate (6-m electrode spacing, 144-m profile length), and long (10-m electrode spacing, 240-m profile length) arrays over shallow unconsolidated soils and regolith overlying crystalline bedrock. Pole–pole data were only collected with the short array. Numerical modeling was used to simulate both vertical and horizontal structures similar to subsurface conditions in the study area. All of the apparent resistivity data were inverted into earth models using a computer program that uses an l 1 norm smoothness constrained inversion technique. Earth models generated from both field data and numerical modeling acquired by the dipole–dipole technique consistently indicated more detail and greater depth of investigation than the other techniques. The dipole–dipole method uniquely imaged thin saturated sands and isolated high resistivity bodies beneath the 48-m-length array, significant horizontal and vertical resistivity variations including a thick transitional resistivity zone in the 144-m-length array, and anomalous low resistivity zones in crystalline bedrock in the 240-m-length array. Earth models created from the Wenner and Wenner–Schlumberger apparent resistivity data had shallower depths of investigation and revealed significantly less geologic detail than the profiles generated from the dipole–dipole surveys. The earth model from the pole–pole data had the greatest depth of investigation but low resolution and limited geologic detail when compared to the dipole–dipole survey.

The Application of Electrical Geophysics to Gold Exploration at Mt Wright, North Queensland

The gold resource at Mt Wright, located approximately 10km to the northwest of Ravenswood North Queensland, is uneconomic due to its depth. Definition of a shallower resource of similar grade would have a profound effect on the current Ravenswood operation. Recent exploration at and around Mt Wright has been directed toward this end. Early IP surveys at Mt Wright showed that the mineralised system produced a low resistivity anomaly with moderate chargeability. Trial MIMDAS (MIM Distributed Acquisition System ? MIM owned technology), using the pole-dipole/dipole-pole configuration, and CSAMT surveys proved that both techniques have much greater depth of investigation than conventional IP in the Mt Wright area. More than 25 square kilometres have since been `screened?, using MIMDAS and CSAMT, to >400m depth for systems similar to Mt Wright. Infil surveys around Mt Wright have mapped the extent of the alteration system and highlighted anomalous zones away from Mt Wright itself. Some of these have been drill tested. MIMDAS and CSAMT have proven to be effective exploration tools in the Mt Wright area.

Stratigraphic mapping of sedimentary formations in southern Ontario by ground electromagnetic methods

Multifrequency and transient electromagnetic (EM) soundings were done at several locations in southern Ontario to evaluate the possibility of stratigraphic mapping of gently dipping sedimentary formations by ground EM techniques. The possibility of detecting structural features such as folds, faults, and grabens in the formations was also explored. The two EM techniques were used to map a buried river valley near the village of Copetown, about 75 km southwest of Toronto. The valley, buried under fairly resistive glacial till and dolomite formations was mapped at depths ranging from 100 to 200 m. The interpreted shape and depth of the valley agreed well with information from a high resolution seismic reflection survey and data from wells penetrating the bedrock. Transient EM (TEM) data, using square transmitter loops with side dimensions comparable to the transmitter‐receiver separation for the multifrequency system, was found to be less affected by the presence of shallow conductors and lateral inhomogeneities than the multifrequency data, and had greater depth of investigation. TEM soundings were made on five additional profiles in the area. Lithologic logs from old oil and gas wells located near the profiles provided information for comparison with EM sounding interpretations. The depths to various dolomite, shale, and limestone formations interpreted from EM data agreed well with drillhole information. The EM soundings also detected structures in the Paleozoic formations such as faults and folds that were not previously known. The soundings confirmed the existence of a graben at one site at a depth of 100 m, which was postulated from logs from a cluster of closely spaced drillholes by geologists. The survey results indicated that EM sounding methods can be used for stratigraphic mapping in areas where detailed geological information is unavailable either because the bedrock is concealed by overburden, or when drillholes are sparsely distributed.

Characteristics of inline and broadside transient electromagnetic anomalies

The Transient Electromagnetic (TEM) method is being increasingly used for the detection of metalliferous orebodies, but there are relatively few aids available to assist the design and interpretation of TEM surveys. A first order numerical approximation has been used to investigate the late-time geometric response characteristics of the inline and broadside prospecting techniques, when profiling over a thin finite conductor embedded in a resistive host rock. The study compares the interpretational and operational attributes of the two configurations for differing target-loop geometries, including changes in conductor dip, strike, depth, traverse off-set and loop separation. It is shown that the little-used broadside geometry offers some interpretational advantages, but the inline configuration has a greater depth of investigation, and is more suited to routine applications.

Multifrequency Turam measurements over a sulfide deposit

A six‐frequency electromagnetic (EM) system has been developed and used for a Turam‐type survey over the sulfide deposit of Mailaram copper belt in Andhra Pradesh, India. Turam field strength ratio (FSR) and phase difference (PD) are measured in the range 84 to 2688 Hz. Sulfides in this belt occur as thin concentrated lodes which grade into disseminations. Soil covers a major part of the area. Interpretation of the results is carried out using a thin sheet model in free space. However, an attempt has been made to account for finite resistivity of the normal ground by computing Turam response of a conducting half‐space appropriate for the area and subtracting the same from the observed response. The results of the survey confirm the known advantage of the Turam method, viz., its greater depth of investigation compared to a method like Slingram. The normal ground at Mailaram is only mildly conducting so that the free‐air approximation is valid at low frequencies. A correction for the finite resistivity of the ground is required only at high frequencies. However, the limitation of a single‐body interpretation scheme for the bedrock conductor is well brought out by the results.

FOCUSSED ELECTRICAL RESISTIVITY ARRAYS: SOME THEORETICAL AND PRACTICAL EXPERIMENTS*

AbstractThe “Laterolog 7′’and the “Microlaterolog’ have been studied theoretically for the case of a conducting halfspace containing a single overburden, using a technique based on the method of images. The results have shown these focussed arrays to be more sensitive to the lower medium (i.e., having greater depth of investigation) than unfocussed ones of the same dimensions, when the “correct’ geometric factor is used. The geometric factor of a focussed electrode array is somewhat involved, and is explained with reference to an ideal focussed array; such an array would pass a measuring current of constant intensity into a fixed geometrical shape of conducting material, irrespective of any layering or any other heterogenities that may be present, using auxiliary current sources of variable magnitude. This concept of a constant amount of current flowing in a beam of fixed geometry and current density, is the basis of focussed arrays, and it is shown that the geometric factor, used to calculate the apparent resistivity, is the one derived when considering the homogeneous case. The value of the constant measuring current, alone, is used to convert the measured potential difference into a resistance, the ratio between this value and the combined intensities of the auxiliary current sources, for the homogeneous case, being incorporated into the geometric factor.Surprisingly good agreement was found between the theoretical models and practical experiments using a focussing seabed resistivity probe which is a substantial modification of the “Microlaterolog”. Both show similar deviations from the ideal case which are explained in terms of refraction at the overburden interface. All experiments indicated that focussed arrays have a greater depth of investigation than similar unfocussed ones for a single overburden, whether it be resistive or conductive.

A T-shaped four-electrode configuration for resistivity profiling over thin vertical conducting veins

A new variant of the four-electrode system is proposed for geophysical resistivity investigations, particularly for profiling over conductive bodies. Model tank resistivity profiling experiments with Wenner, Schlumberger and the proposed T-shaped four-electrode arrays were carried out over a thin conducting vein type model to evaluate the efficacy of the new array. The proposed array appears to possess a larger response, a greater depth of investigation and a higher vertical resolution in detecting conducting bodies, as compared to the two-conventional arrays.

DIGITAL LINEAR FILTER FOR COMPUTING TYPE CURVES FOR THE TWO‐ELECTRODE SYSTEM OF RESISTIVITY SOUNDING*

AbstractIn this paper a technique for computing type curves for the two‐electrode resistivity soundings is presented. It is shown that the apparent resistivity due to the system can be represented by a convolution integral. Thus, it is possible to apply the principle of digital linear filtering and compute the desired type‐curves. The filter function required for the purpose is found to be identical with that used to compute the EM sounding curves for the two coplanar horizontal loop system.It is further shown that from the two‐electrode apparent‐resistivity expression one can easily derive the apparent resistivities for other configurations. A comparison of depths of investigation for various systems reveals that the two‐electrode system has greater depth of investigation than other conventional systems. This is also supported by the field example presented in the end, which illustrates the relative performance of the two‐electrode system vis‐a‐vis the Wenner system.