18 controlled source electromagnetic resonance
Reservoir characterisation:
microseismics now a key tool
William Wills looks at the increasing
changes are taking place, and to map any fracture pattern
use of passive seismic and microseismic
or other rock property changes.
As in all seismic survey applications the most accurate
monitoring within the oil and gas results are achieved by measuring higher frequencies and
industry.
having a high signal to noise ratio. Hence the evolution
of geophone arrays being placed closer to the source
P
assive seismic monitoring has been used of micro-seismic events in much quieter ambient noise
on a global scale for many years to predict locations, ie deep underground. Here an added advantage
potential earthquake or volcanic eruption comes from being able to more readily identify and
activities. Today, passive seismic and utilise the shear wave energy generated by the micro
microseismic (MS) applications are now being widely earthquakes.
used within the oil and gas industry. With a current
industry trend towards more long term deployments of
Hydro-fracturing techniques
geophones to complement traditional surface source Arrays of three component seismic receivers locked into
driven seismic techniques to map reservoir dynamics, nearby monitoring wells are now in common use for both
there is an increasing demand for acquisition hardware passive and induced MS monitoring.
to be developed parallel with the rapidly improving MS induced events are epitomised by the monitoring
microseismic processing speeds, visualisation, and of oil or gas bearing rocks being treated with hydro-
analytical techniques. fracturing techniques used to enhance hydrocarbon
recovery, particularly from ‘tight’ zones or reservoirs
Changing reservoir dynamics
where production has significantly slowed. Known as EOR
Microseismic events can be described as small magnitude (Enhanced Oil Recovery), ‘fraccing’ is just one of several
microearthquakes (microseismicity) associated with local methods utilised, others examples are, among many,
stress changes in and around a reservoir. steam and water flood.
These stress changes can occur when removing fluids The primary principle is to use the most appropriate
from existing geological structure (micro-subsidence) methods to stimulate production. Fluids are pumped
or be induced by pumping fluids back into reservoirs. into the reservoirs, often at high pressure. The fluid
These changing reservoir dynamics will generate the used for such recovery depends on the process needed
microseismic events which can then be employed to to make the hydrocarbons flow from the host rock.
pinpoint where and to what extent reservoir geology Such applications include heat for low viscosity oil and
pressure to persuade migration of hydrocarbon fluids
towards where the producer needs them, ultimately the
surface.
Additionally, these fluids can be employed to fracture
rocks to produce less restrictive travel paths and contain
a proppant for propping open existing and induced rock
cracks and fractures.
Hedging against future shortages
There is a modern trend to store natural gas back
underground to create reserves, taking advantage of
periods where there is a commercial surplus or when
hedging against future supply shortages.
Quite often carbon gas is used for stimulating EOR.
Fig. 1. Microseismic monitoring employing a non-destructive source This has naturally a commercial benefit as well as
for cross-well imaging during CO
2
sequestration. Using a repeatable capturing undesirable carbon. It is anticipated that future
high energy source permits a high S/N ratio and accurate receiver carbon sequestration will involve pumping waste CO
2
into
orientation. This is a unique approach for the monitoring of the suitable subsurface structures and chambers. One such
integrity of the reservoir rock subject to changes in local stresses and formation intended for sequestration is located above the
pore pressure induced by injection of compressed CO
2
. Sleipner field, one of the larger natural gas producers in
Image: Applied Seismology Consultants Ltd. the North Sea.
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