|
Seismic
exploration methods
are the most
successful methods for exploring hydrocarbon deposits, but also in mining
and engineering exploration. Usually seismic works with artificial sources
as explosions or vibrators. Recorded data are rearranged to produce a 2 or
3-dimensional image of the subsurface. To image time dependent effects, as
fluid flow in reservoirs, seismic surveys have to be repeated what is
usually named 4D- seismic or time laps seismic. It will deliver images in
longer time intervals. Surveys, continuous in time have been developed but
are still rarely used.
Passive seismic
methods,
methods using natural sources of seismic energy, are historically older and
are used mainly in earthquake seismology. Observation of smaller
seismological events, possibly men made, are also observed routinely for
instance in the mining industry. Methods developed for earthquakes have been
modified and used in this field successfully. Principally with passive
seismic methods everything producing seismic energy in the subsurface can be
studied. Observing small mining produced seismic event has been called
seismic monitoring. Observing still smaller (usually high frequent)
events in non destructive testing was also named acoustic emission (AE).
In the
hydrocarbon business seismic events in the reservoir are produced during any
kind of production. Events produced from fluid flow but also from internal
subsidence have been successfully recorded and used to study fluid flow in
time and space.
Much larger
events in oil/gas reservoirs are generated during stimulation with
artificial hydro-fracs. Monitoring the development of those fracs is usually
called fracture monitoring. It is actually the only routinely
available method to follow the development of a fracture in time and space.
Online processing and interpretation allows feedback to the fracturing team.
There are
possibilities to record the fracture generated events in the same well, but
usually the noise generated within the well is a showstopper. Normally for
the recording nearby observation wells are used. A typical distance is
several hundreds of meters, but also in more than 1000m observations have
been successful. Principally a singe 3-component geophone allows
locating the event. The direction is derived from a hodogram-analysis of the
P-wave arrival and the distance from the P-S travel time difference. Using
more than one geophone gives redundancy and thus better results. Using a
geophone array, i.e. several 3C-geophones on different levels allows
location of the events from P-wave arrivals only and avoids the often
problematic and inexact reading of S-wave onsets. Drill hole and array
geometry have to be modelled in advance to estimate the location accuracies.
Observing
production induced events in a reservoir needs continuous recording over a
longer time period. Permanent installations are the best way to
record continuously. They furthermore have superior coupling compared to
temporary installations. There are techniques available to install geophone
arrays behind the casing or on tubing in still used wells but
also techniques to install geophones permanently in abandoned wells.
For frac-monitoring
recording is only needed for several hours or days. Service companies offer
multi-level instrumentation on wireline for this type of surveys. The
instrumentation is close to those used for VSP measurements, but should
allow continuous recording.
As a feedback
to the fracture team is needed, the instrument should allow an online
real time processing and interpretation. The software should pick events
from the continuous data flow. This part of the processing is usually called
triggering. A frequently used method is a shorttime/longtime quotient
that can be used on a single channel or multi channel basis, where the multi
channel version may be called coincidence trigger. A second
processing step, after cutting out the events is the event location. This
can be done using hodogram-analysis and P-S differences but also by
triangulation of P-arrivals or by a combination of both. Assuming a
homogeneous isotropic background is often insufficient and priory knowledge
of a 3D-velocity background has to be imported.
Using adequate
online software the event locations can be observer real time on the
monitor. This gives the reservoir engineers an on line control of the
fracture development.
After the
survey is finished further seismological interpretation of the data
is possible. This includes the estimation of parameters as: fracture length,
fracture area, seismic moment, stress drop, stress directions and more.
Those parameters are essential for the reservoir engineering and extremely
helpful for planning future fracture activities in the same field.
What does
HarbourDom offer?
HarbourDom is a
technology transfer company. It participated in the development of passive
seismic methods for decades. HarbourDom’s international contact to
instrument/software manufacturers and to service companies allows offering
oil-companies help by introducing novel technologies as fracture monitoring.
We can help with permanent and temporary installation. We choose for you the
best available instrumentation and the best service suppliers. HarbourDom
will supervise all activities and make sure to gain optimal results.
|
Downhole
Geophone array
(Geospace)
Six channel recording
unit for continuous registration
(ISSI)
Tri-axial
borehole geophone
(ISSI)
Visualisation of event
location and intensity (ISSI)
Geophone installations behind casing and on tubing (Bathellier&Czernichow
1997)
Hodogram analysis (Hardage)
Seismic events during injection in a geothermal well (Fehler)
|
