Including highly conductive steel infrastructure into electromagnetic (EM) earth modeling is motivated by the fact that long metal-cased boreholes have the potential to be used as boosting antennas that enable larger source dipole moments and greater signal penetration depths. Unfortunately, geophysical algorithms designed to simulate EM responses over rather regional scales are complicated by material property contrasts and structure geometries that are more typical for EM engineering applications. Hence, the great majority of earth-modeling algorithms that consider EM responses from steel-cased boreholes use integral-equation methods. To be able to model complex casing scenarios, we revisited the finite-difference time-domain (FDTD) method to advance the modeling of transient-EM field responses from steel-cased boreholes. A time-dependent function that allows for larger FDTD time steps in the DuFort-Frankel method was developed, alleviating the generally large computational overhead. We compared our method against three different kinds of benchmark solutions to demonstrate the reliability of the FDTD field solutions. These test cases were carried out to check the feasibility of a final hydraulic fracturing study. Images of the electric current distribution in a sheetlike rock fracture were calculated for the cases with and without the presence of a connecting borehole casing, demonstrating the casing’s potential of illuminating deep target zones.

Transient-electromagnetic finite-difference time-domain earth modeling over steel infrastructure

To advance and optimize secondary and tertiary oil recovery techniques, it is essential to know the areal propagation and distribution of the injected fluids in the subsurface. We investigate the applicability of controlled-source electromagnetic methods to monitor fluid movements in a German oilfield (Bockstedt, onshore Northwest Germany) as injected brines (highly saline formation water) have much lower electrical resistivity than the oil within the reservoir. The main focus of this study is on controlled-source electromagnetic simulations to test the sensitivity of various source–receiver configurations. The background model for the simulations is based on two-dimensional inversion of magnetotelluric data gathered across the oil field and calibrated with resistivity logs. Three-dimensional modelling results suggest that controlled-source electromagnetic methods are sensitive to resistivity changes at reservoir depths, but the effect is difficult to resolve with surface measurements only. Resolution increases significantly if sensors or transmitters can be placed in observation wells closer to the reservoir. In particular, observation of the vertical electric field component in shallow boreholes and/or use of source configurations consisting of combinations of vertical and horizontal dipoles are promising. Preliminary results from a borehole-to-surface controlled-source electromagnetic field survey carried out in spring 2014 are in good agreement with the modelling studies.

Controlled‐source electromagnetic monitoring of reservoir oil saturation using a novel borehole‐to‐surface configuration

In recent years, three-dimensional (3-D) inversion of electromagnetic (EM) data gained a lot of attention and the number of 3-D EM case studies has increased. Many publications discuss the challenges and advancements of 3-D inversion with respect to the numerical aspects and often show synthetic studies to prove their assumptions. On the other hand, field data have other/additional demands than synthetic data sets. There are challenges and requirements to fulfil along the entire sequence from survey planning to interpretation. To obtain a meaningful and reliable interpretation it is not sufficient to only be aware of and address issues with respect to one step along this sequence. Ideally one should be concerned with all or at least most of them, because many of these challenges are related to or even consequences of each other. Not all issues when dealing with field data can be solved, but one should at least be aware of the consequences of unavoidable shortcomings as this knowledge may be crucial for interpretation. With the intention to raise awareness, this review comprises a variety of difficulties related to the data acquisition, the numerical part—preparation for and performance of 3-D inversion—and the interpretation itself, when dealing with field data sets. The majority of published work on 3-D EM inversion of field data is related to magnetotellurics; nevertheless, there are also aspects discussed that are specific to other EM methods or illustrate different ideas to deal with challenges (e.g., airborne and controlled-source EM).

Application of 3-D Electromagnetic Inversion in Practice: Challenges, Pitfalls and Solution Approaches

Steel well casings in or near a hydrocarbon reservoir can be used as source electrodes
in time-lapse monitoring using grounded line electromagnetic methods. A requisite
component of carrying out such monitoring is the capability to numerically model
the electromagnetic response of a set of source electrodes of finite length. We present
a modelling algorithm using the finite-element method for calculating the electromagnetic
response of a three-dimensional conductivity model excited using a vertical
steel-cased borehole as a source. The method is based on a combination of the method
of moments and the Coulomb-gauged primary–secondary potential formulation. Using
the method of moments, we obtain the primary field in a half-space due to an
energized vertical steel casing by dividing the casing into a set of segments, each assumed
to carry a piecewise constant alternating current density. The primary field is
then substituted into the primary–secondary potential finite-element formulation of
the three-dimensional problem to obtain the secondary field. To validate the algorithm,
we compare our numerical results with: (i) the analytical solution for an infinite
length casing in a whole space, excited by a line source, and (ii) a three-layered Earth
model without a casing. The agreement between the numerical and analytical solutions
demonstrates the effectiveness of our algorithm. As an illustration, we also
present the time-lapse electromagnetic response of a synthetic model representing a
gas reservoir undergoing water flooding.

Three-dimensional controlled-source electromagnetic modelling with a well casing as a grounded source: a hybrid method of moments and finite element scheme

As motivation for considering new electromagnetic techniques for hydraulic fracture monitoring, we develop a simple financial model for the net present value offered by geophysical characterization to reduce the error in stimulated reservoir volume calculations. This model shows that even a 5% improvement in stimulated reservoir volume for a 1 billion barrel (bbl.) field results in over 1 billion U.S. dollars (US$) in net present value over 24 years for US$100/bbl. oil and US$0.5 billion for US$50/bbl.
oil. The application of conductivity upscaling, often used in electromagnetic modeling to reduce mesh size and thus simulation runtimes, is shown to be inaccurate for the high electrical contrasts needed to represent steel-cased wells in the earth. Finescale finite-difference modeling with 12.22-mm cells to capture the steel casing and fractures shows that the steel casing provides a direct current pathway to a created fracture that significantly enhances the response compared with neglecting the steel
casing. We consider conductively enhanced proppant, such as coke-breeze-coated sand, and a highly saline brine solution to produce electrically conductive fractures. For a relatively small frac job at a depth of 3 km, involving 5,000 bbl. of slurry and a source midpoint to receiver separation of 50m, the models show that the conductively enhanced proppant produces a 15% increase in the electric field strength (in-line with
the transmitter) in a 10-m background. In a 100-m background, the response due to the proppant increases to 213%. Replacing the conductive proppant by brine with a concentration of 100,000-ppm NaCl, the field strength is increased by 23% in the 100-m background and by 2.3% in the 10-m background. All but the 100,000- ppm NaCl brine in a 10-m background produce calculated fracture-induced electric
field increases that are significantly above 2%, a value that has been demonstrated to be observable in field measurements.

/pdf/hydro-frac-monitoring-using-ground-time-domain-3p4kaq4djs.pdf

Hydro-frac monitoring using ground time-domain electromagnetics

Modern hydrocarbon development techniques make extensive use of highly deviated and horizontal wells to maximize production and reduce drilling costs. However, many of the existing logging tools and associated interpretation procedures were originally designed for operation in vertical wells. Therefore, it becomes imperative that the response of conventional tools be properly understood when logging through highly deviated wells. In like manner, interpretation procedures need to be adapted for their accurate and reliable use in non-vertical wells. This paper undertakes the numerical simulation of dual-laterolog measurements acquired in 3D logging environments to appraise the reliability of measurement and interpretation procedures in non-conventional wells. Specifically, we consider the following borehole measurement environments: (1) dipping beds penetrated by vertical wells, (2) dipping anisotropic formations, and (3) multiple dipping anisotropic and mud-filtrate invaded formations. It is found that dual-laterolog measurements are chiefly sensitive to the horizontal resistivity even in the case of highly deviated wells. An increase of dip angle causes an increase of apparent resistivity due to the influence of vertical resistivity. In all cases, laterolog apparent resistivity readings will fluctuate between the horizontal resistivity and the geometric mean of horizontal and vertical resistivity. Shoulder-bed effects considerable increase with an increase of dip angle.

Numerical simulation of dual-laterolog measurements in the presence of dipping, anisotropic, and invaded rock formations

Weusenumericalsimulationstoinvestigatethepossibility of enabling steel-cased wells as galvanic sources to detect and quantify spatial variations of electrical conductivity in the subsurface.The study assumes a vertical steel-cased well that penetrates electrically anisotropic horizontal layers. Simulations include a steel-cased vertical well with a finitelength thin wire of piecewise-constant electric conductivity and magnetic permeability.The steel-cased well is energized atthesurfaceorwithintheboreholeatanarbitrarydepthwith an electrode connected to a current source of variable frequency. Electromagnetic EM fields excited by the energized steel-cased well are simulated with an integral-equation approach. Results confirm the accuracy of the simulationswhenbenchmarkedagainstthewhole-spacesolutionof EM fields excited by a vertical electric dipole. Additional simulationsconsiderawiderangeoffrequenciesandsubsurfaceconductivityvaluesforseveraltransmitter-receiverconfigurations, including borehole-to-surface and crosswell. The distribution of electric current along the steel-cased well is sensitive to vertical variations of electric conductivity in thehostrock.Inaddition,numericalsimulationsindicatethat crosswell and borehole-to-surface receiver configurations couldreliablyestimateverticalvariationsofelectricconductivity within radial distances of up to 500 m for frequencies below 100 Hz and for average host rock electric conductivitiesbelow1 S /m.

1D subsurface electromagnetic fields excited by energized steel casing

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Numerical simulation of dual-laterolog measurements in the presence of dipping, anisotropic, and invaded rock formations

1D subsurface electromagnetic fields excited by energized steel casing