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Multi-Transient ElectroMagneticsMulti-Transient EM
Aug 2007
The need for an EM solution
• Seismic methods determine structure
• The structure may reveal a potential reservoir (in general a porous rock bounded by a cap)
• The porous rock contains a fluid (water or hydrocarbons, or a mixture) but seismics cannot usually determine the nature of that fluid –drill!
• Resistivity – a physical rock property determined through EM – can make that distinction –drill in the right place!
Resistivities of Rocks
Diagram courtesy Henri Brasse http://userpage.fu-berlin.de~hbrasse
Fluid saturation and rock properties
P-wave velocity is affected only slightly by hydrocarbon saturation in a porous rock.
Resistivity can vary by more than an order of magnitude.
Picture redrawn from Wilt M. and Alumbaugh D. 1998, The Leading Edge 17, 487-492.
Seismic tracesSeismic amplitudes and attributesLogging measurementsLogging before drillingMTEM Resistivity
Direct Resistivity Indicator
MTEM for resistivity
• MTEM is sensitive to transverse resistance. • Red is more resistive. • Stronger resistivity comes from:
– Thicker interval– Higher saturation– Higher porosity– Less clay and shale
Cases• Gas storage monitoring• Imaging Heavy Oil • Imaging in structurally complex subsurface• Imaging thrust belt. • Imaging where seismic and drilling is very expensive• Locating steam flood front in SAGD production monitoring • Identifying channel sands with low acoustic impedance
contrast• Proving up reserves to increase reserves-production ratio• De-risking drilling location decisions.
MTEM Challenges to date
Gas storage monitoring
resistivity scale
Shallow Heavy Oil
Thrust Belt
Thrust belt
Onshore
Offshore
MTEM Global Operations
Land acquisition
• Surface conditions successfully operated in– Farmland– Desert– Jungle– Plantation– Urban– Rice paddies– Muskeg– Open prairie– Beach / Littoral
MTEM Hardware
Receiver station and battery
Receiver box and battery in the line. Box with acquisition and telemetry electronics. Grounded with earth electrode.
Source Buggy and Unit
Source buggy with generator and source unit. Source engineer with Zonge and control electronics.
Source Electrodes
An array of 1 to 40 electrodes hammered into 80-100cm into the ground. Fenced off (3m) and manned when source cables are connected.
Source Safety system
Source activation and emergency shut off at the source electrode array.
Emergency shutoff cuts power to the entire system.
Source electrode transport
Source electrode transport buggy. Low ground pressure buggy to transport the 40 electrodes per source pole location.
Land Operations
Input Recording(combination of Earth and System Responses)
A Step Change in the use of EM
Airwave
Earth response
Impulse response - land
Offset = 1 km
Air Wave
Earth Impulse Response
Source1A.m Source Receiver (V)
x = 1000m
500m 20 Ohm.m 500 Ohm.m
Dx
25m
1D model
Impulse responseEffect of fluid saturation
Offset = 1 km
Impulse response – landConvolution with a system response
Air Wave
Full Waveform
Impulse response measurements
Amplitudes
Travel Time
Multi-Transient
Theoretical PRBSIn red
Measured SignalIn black
Deconvolution
Input amps at source Output volts at receiver
Deconvolved impulse response (ohms/m2/s)
Deconvolved Data – common offset
Air Wave
High peak amplitude and early arrival time indicate a resistor.
Data Processing
Apparent resistivityStep Response
Integration Peak picking
Impulse Response
Inversion model and responseOffset 1000m
Inversion model and responseOffset 2000m
Inversion model and responseOffset 2500m
Inversion model and responseOffset 3000m
Inversion model and responseMulti-offsets 2000m, 2500m and 3000m
What does MTEM provide
• Feasibility study – desktop• Feasibility study – field assessment• Survey design• Acquisition• Processing• Inversion• Technical/interpretation support
– 3D modeling, advanced inversions
Well Logs
1D Modeling
• % difference in impulse response.
• We take a collaborative approach with the client• Problem solving, asset-based • Oilfield services contractor• Support asset team with their interpretation and decisions
Top Reservoir from seismic
How do we work?
Multi-Transient EM
Inversion model and responseOffset 1500m
Inversion model and responseOffset 3500m