crosswell electromagnetic tomography, Oil & Gas Glossary

What Is Crosswell Electromagnetic Tomography?

Crosswell electromagnetic (EM) tomography is a borehole geophysical technique that measures the electromagnetic field transmitted between two wellbores at low frequencies (1 Hz to 10 kHz) to construct a resistivity image of the formation between the wells, enabling detection of resistivity changes associated with oil-water flood fronts, steam saturation changes, CO2 displacement, and natural fracture networks at interwell distances of 100-1,500 metres with resolution of 5-20 metres.

Key Takeaways

Crosswell EM measures resistivity (ohm-metres) between wells; resistivity is sensitive to pore fluid type (oil = high, brine = low).
Frequency range controls depth penetration: lower frequencies penetrate deeper but have lower resolution.
Source in one well, receivers in adjacent well; multiple source-receiver pairs provide tomographic coverage of the interwell volume.
Oil-to-water saturation change reduces resistivity (conductivity increases); steam and CO2 increase resistivity.
Time-lapse crosswell EM tracks flood front movement for waterflood conformance and EOR injection monitoring.

How Crosswell Electromagnetic Tomography Works

Crosswell EM tomography uses a magnetic dipole source (a small current loop or toroidal coil) lowered into one wellbore to generate a low-frequency oscillating electromagnetic field that penetrates the surrounding formation. In the receiving wellbore 100-1,500 metres away, a set of magnetic field receivers (induction coils or triaxial sensors) measures the amplitude and phase of the transmitted EM signal at multiple depths. The amplitude and phase of the received EM signal are controlled by the electrical conductivity (inverse of resistivity) of the formation between the source and receiver along each source-receiver ray path.

Unlike seismic crosswell tomography (which measures acoustic travel time), EM tomography measures the amplitude decay and phase shift of a diffusive EM wave. The inversion of the multi-frequency, multi-offset data set to a resistivity model requires solving an electromagnetic forward problem — Maxwell's equations in a heterogeneous earth — iteratively to find the resistivity distribution that best matches the measured EM fields. This inversion is computationally intensive and sensitive to starting model assumptions. The resulting resistivity tomogram identifies high-resistivity zones (hydrocarbon-saturated formations) and low-resistivity zones (brine-saturated formations) between the two wells, with spatial resolution typically 5-20 metres depending on the frequency range used, the interwell spacing, and the formation conductivity contrast.

Crosswell EM Tomography Applications Across International Jurisdictions

In Canada, crosswell EM tomography has been investigated for monitoring waterflood conformance in WCSB Cardium and Viking sandstone pools where the contrast between oil-saturated and water-swept zones provides resistivity contrast detectable by crosswell EM. AER-regulated enhanced recovery scheme monitoring programmes for Cardium waterfloods use a combination of injection profiles, tracer tests, and 4D seismic; crosswell EM represents an emerging additional monitoring tool for identifying bypassed oil zones between injection and production well pairs. At Athabasca SAGD operations, the large resistivity contrast between cold unheated bitumen (high resistivity) and steam-heated, partially mobilised bitumen (lower resistivity) provides excellent crosswell EM sensitivity to steam chamber growth.

In the United States, crosswell EM has been tested at several DOE-funded EOR research sites including CO2 injection monitoring in Permian Basin carbonate reservoirs, where the low conductivity of supercritical CO2 creates a strong resistivity contrast relative to the brine-saturated matrix. BSEE does not specifically regulate crosswell EM monitoring technology; the method is used as part of operator-defined monitoring programmes. In Norway, Equinor has evaluated crosswell EM for water saturation monitoring in North Sea chalk fields where the chalk's low porosity and high resistivity contrast with invaded brine zones provides potential EM detectability. In the Middle East, Saudi Aramco's EXPEC ARC research programme has assessed crosswell EM technology for monitoring seawater injection breakthrough in Arab Formation carbonate producers at Ghawar, where early detection of water flooding breakthrough enables intervention before full water breakthrough reaches production wells.

Fast Facts

The penetration depth of crosswell EM is controlled by the skin depth in the formation: δ = 503 × √(ρ/f) metres, where ρ is formation resistivity in ohm-metres and f is frequency in Hz. For a 10 ohm-m sandstone formation at 100 Hz, the skin depth is 503 × √(10/100) = 159 metres — meaning EM energy at 100 Hz can penetrate approximately one skin depth before being attenuated to 37% of its source amplitude. This fundamental relationship explains why lower frequencies must be used for larger interwell spacings: to transmit between wells 500 metres apart in 10 ohm-m formation, frequencies below approximately 10 Hz are needed to maintain adequate signal strength at the receiver.

Crosswell EM Versus Crosswell Seismic for Reservoir Monitoring

Crosswell EM and crosswell seismic both provide interwell imaging with higher resolution than surface methods, but they measure different physical properties with different sensitivities to reservoir fluids. Seismic velocity is sensitive to elastic properties — it detects changes in bulk modulus and density caused by fluid substitution. EM resistivity is sensitive to electrical conductivity — it detects the presence of conductive brine versus resistive oil or gas. For waterflooding in oil reservoirs (replacing resistive oil with conductive brine), crosswell EM provides better fluid saturation sensitivity than crosswell seismic because the resistivity change from oil to brine is large (factor of 10-100) while the velocity change may be small if the oil and brine have similar bulk moduli. For steam flooding (replacing cold oil with steam), crosswell seismic provides better sensitivity because steam dramatically changes the elastic properties; EM provides supplementary saturation information. The two methods are therefore complementary monitoring tools.

Tip: When planning a crosswell EM monitoring survey for waterflood conformance, run a baseline survey before water injection begins and repeat at regular intervals during the flood to establish time-lapse resistivity changes. The baseline provides the reference resistivity model of the undisturbed oil-saturated formation; subsequent surveys show resistivity decreases where water has displaced oil. The signal-to-noise ratio of the time-lapse change (baseline minus monitor) is typically better than interpreting any single survey in absolute resistivity terms because systematic instrument and processing errors cancel in the subtraction. Define the minimum detectable resistivity change for your specific formation and instrumentation before committing to the monitoring programme — if the expected resistivity change from the flood front is below the detection limit, the survey will not provide useful information regardless of data quality.

Crosswell electromagnetic tomography is also referenced as:

Crosswell EM — the standard abbreviation used in research papers and project documentation; shortened from "crosswell electromagnetic tomography" when the context makes the full term clear
Borehole-to-borehole EM — used when explaining the measurement geometry to non-specialist audiences; emphasises the physical setup (a source in one borehole, receivers in another)
Interwell EM tomography — the academic literature terminology; "interwell" emphasises the between-well measurement geometry used for subsurface imaging between existing boreholes

Frequently Asked Questions

What limits the resolution of crosswell EM tomography?

The resolution of crosswell EM tomography is fundamentally limited by the skin depth effect: EM waves in conductive media diffuse rather than propagate as coherent wave fronts, and the diffusion distance determines the minimum feature size that can be resolved. For a given interwell spacing, only frequencies that have sufficient skin depth to reach the receiver well (and therefore carry useful information about the intervening formation) can be used. These lower frequencies have longer wavelengths and correspondingly lower spatial resolution. Practically, the achievable resolution at typical interwell spacings of 200-500 metres in moderate-resistivity (5-50 ohm-m) formations is approximately 5-20 metres. Additionally, the non-uniqueness of EM inversion (multiple resistivity models can fit the same data) and the smoothing inherent in the regularised inversion process reduce the sharpness of detected features. For comparison, crosswell seismic achieves 1-5 metre resolution at the same spacings, but crosswell seismic provides acoustic property images while crosswell EM provides resistivity images — the appropriate tool depends on which physical contrast is more diagnostic for the specific monitoring objective.

Can crosswell EM be run in cased wells?

Crosswell EM can be run in cased wells, but the steel casing substantially attenuates the transmitted EM signal at most frequencies because steel is highly conductive and creates a strong shielding effect for EM fields. To operate through casing, very low frequencies (0.1-10 Hz) must be used because the casing skin depth increases with decreasing frequency, allowing low-frequency EM signals to penetrate through the casing wall and propagate to the receiver well. This casing penetration capability makes crosswell EM potentially useful for monitoring existing fields where all wells are cased, whereas crosswell seismic requires free-wall coupling and works better in open or perforated completion intervals. The reduced frequency range required for through-casing operation further limits the resolution compared to open-hole crosswell EM. Specialized through-casing crosswell EM systems have been developed and tested but are not yet as commercially mature as the open-hole version of the technique.

Why Crosswell EM Tomography Matters in Oil and Gas

Waterflood and EOR monitoring is essential for the efficient management of the trillions of dollars of capital invested in enhanced recovery infrastructure worldwide. When injection fluid bypasses target oil zones and breaks through prematurely to producing wells, the economic loss — both the direct cost of handling produced water and the missed opportunity cost of unswept oil — can be enormous. Crosswell EM provides a direct resistivity measurement between wells that can detect the advancing fluid front before breakthrough occurs at the producing well, enabling adaptive management of injection rates and patterns to improve sweep efficiency. This early warning capability, combined with the tool's sensitivity to oil-water saturation contrasts that are not easily detected by surface seismic, makes crosswell EM an important component of the geophysical monitoring toolkit for EOR and waterflood projects where maximising sweep efficiency determines whether the recovery project achieves its targeted return on investment.

Crosswell Electromagnetic Tomography: Definition, Interwell Resistivity Imaging, and EOR Monitoring