Using MT Technologies For Rapid, Cost Effective and Accurate Hydrocarbon Resource Detection
EM Complements Reflection Seismic and Reduces Risk
Oil and gas exploration companies have used the reflection seismic technique for years and rely on its strengths while recognizing its limitations. Factors in the geologic structure of the prospective area can form a high-velocity barrier, degrading resolution below the barrier and limiting the depth of investigation. Volcanic rocks near the surface or in the section, dense, highly-indurated limestone, and crystalline basement thrust over sediments are common obstacles to definitive modelling with seismic. In addition, rugged terrain, jungle vegetation, and geographic remoteness can make seismic difficult, costly, or even impossible to use.
Many oil companies world-wide have used electromagnetic (EM) methods as a cost-effective, useful complement to seismic techniques and as a substitute where seismic cannot be used at all. Both natural-source methods (MT) and man-made, controlled-source methods (IP and CSEM) have made valuable contribtions in refining geologic models, reducing exploration risk, and improving production monitoring. Deep gas discovery in Romania based on 3-D MT.
MT Provides More Detail for Structural and Statigraphic Interpretation
Magnetotellurics (MT) is the most commonly used technique. From the natural signal of the earth's magnetic field, MT derives a resistivity-vs-depth image of the subsurface. At a basic level of interpretation, resistivity is correlated with different rock types. High-velocity layers are typically highly resistive, whereas sediments - porous and permeable - are much less resistive.
While high-velocity layers are an acoustic barrier, their electrical resistivity means the magnetic signal passes through almost unimpeded, allowing MT to see deep beneath these layers.
MT Sees Clearly Below 1 km Seismic Barrier
MT data provides both dimensionality and directionality indicators, adding more detail to structural and stratigraphic interpretation.
Patterns of Resistivity are Direct Hydrocarbon Indicators
The presence of hydrocarbons in rocks is also associated with characteristic resistivity patterns revealed by MT - so-called " Direct Hydrocarbon Indicators " - as demonstrated by the international Project Paleorift in Uzbekistan in 2002.
PROVEN DRILLING SUCCESS RATE OF 73% WITH IP
Certain mineral-fluid interfaces undergo a charge separation when an electric current is made to flow through them, so the interface becomes polarized and a capacitance effect occurs. The Induced Polarization (IP) method identifies anomalies by the characteristic phase lag between transmitted and received signals caused by the capacitance. Using IP, China National Petroleum Corp. (CNPC) has achieved a drilling success rate of 73% (based on 65 wells).
Reservoir Mapping, Secondary Recovery Monitoring With CSEM
Controlled-Source Electromagnetics (CSEM) methods measuring polarization and resistivity have proven successful in mapping hydrocarbon reservoirs. The intrusion of water and/or steam in secondary recovery operations is evident from the resulting resistivity changes, as demonstrated recently by Brazil's PETROBRAS.
Deep Rapid Reconnaissance and Detailed Follow-Up
From near-surface down to any practical drilling depth and beyond, MT allows rapid reconnaissance of areas as large as tens of square kilometers, while detecting conductive zones to 2000 m and deeper.
Closely spaced stations along lines or nets provide data redundancy, high lateral resolution, and a continuous picture of the subsurface resistivity structure.
In a two-pass methodology, station and line spacing are as wide as possible in the first pass to keep cost to a minimum. Once areas of interest have been identified, a second pass with more stations at closer spacing increases resolution. The result is rapid, accurate, and cost-effective identification of conductive zones.
Rapid, Cost-Effective Mapping Of Conductive Zones
Equipment weighs only 30 kg per site, so it's portable by backpack, ATV, snowmobile, or helicopter, from tundra to jungle, in any season. This logistic simplicity reduces cost and increases productivity.
The small footprint and environmentally benign installation make the technique practical almost anywhere.
Flexible site location and offline sensitivity allow meaningful profiles to be constructed without the rigid grid of methods like seismic and IP.
Induction vectors indicate the direction and relative strength of offline conductors and are especially useful where the surface is resistive or frozen (no electrodes required).
MT/AMT sees through the thick conductive clay (impenetrable by airborne or other surface techniques) that covers many prospective areas.