Multi-Transient ElectroMagnetics

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