TC:
Valhall:
Originally posted by Valhall
loam
Thanks for the list. But you do understand that's a totally different issue, right? Those links are talking about earthquakes induced by drilling,
producing, mining activities and seismic testing. Not about natural earthquakes being able to amplify or transmit better to another portion of the
earth's surface due to the after effects of what oil production does to the reservoir.
Yes, but I think the issue is related, I just didn't have time to fully explain.
The fact that there is such seismic activity, resulting from drilling and mining, stands as direct evidence of a change in the geological
characteristics of an area. The question becomes, do we really know
how it has been changed? In other words, what new physical characteristics
does the resulting geology have and how does it behave in light of soficrow's material?
Originally posted by Valhall
I fully support the idea that drilling, mining and seismic testing could induce earthquakes. In fact, I believe in one of the threads concerning the
Indonesian quake that we actually discussed an article found that talked about some seismic testing that had been taking place in the area just prior
to the big quake.
Agreed. And here is additional material that supports such seismic effects:
Induced Seismicity
Induced seismicity describes earthquakes that in one way or another are related to human activity. They can be divided in two types:
Triggered. This group of earthquakes are caused by tectonic stresses. They would probably have occurred sooner or later, but their proximity to to
human activity in time and space indicates antropogenic activity.
Truly induced. This group of earthquakes are purely antropogenic in that stress buildup can be traced directly to human activity.
For convenience we use the term `induced' for both types.
Earthquakes are mainly induced in three antropogenic settings, (1) in mines, (2) in connection with large water reservoirs and (3) in oil or gas
fields where hydrocarbons are extracted. Furthermore earthquakes are induced in hydrothermal fields, but since this industry is young compared to the
three above less data exist.
more...
Seems clear.
Originally posted by Valhall
What I am arguing against is that an air chamber would act as a better transmitter of a seismic wave than continuous rock. If you place an air
chamber in the path of a wave you are going to decrease the wave transmission to the next layer of "solid media". Every time! It's a baffle!
I think you are right about that, but does it invalidate the broader question of whether the resulting geology- whatever that is- potentially
'amplifies' (lay sense only) the intensity of any seismic activity?
Originally posted by Valhall
Concerning statements made prior about the change in rock properties due to production. You don't actually leave the pores of the rock empty. The
fluids are stacked according to their density, just as they would be in a glass. You've got gas on top, oil in the middle and water below. As you
produce a zone (whether it be gas or oil) the fluids will migrate (unless there is an impermeable barrier between them) up the reservoir. That's why
eventually oil wells start producing water. So you haven't really left the rock empty, you've just changed the filler in areas.
I agree this occurs, but there is no indication that that is uniformly true of all drilling locations. In fact, there is plenty of evidence of
"compaction" (which logically results from 'dehydration' and/or a loss of pressure of the porous material) in drilled oil or gas beds.
Rapid subsidence over oil fields measured by SAR interferometry
Ground subsidence is a major worldwide hazard. One recent estimate placed the annual cost of subsidence damage and mitigation within the U.S. alone at
over $100 million [National Research Council, 1991]. Relatively slow subsidence caused by the natural process of sediment compaction is widespread but
seldom causes problems on human timescales. More rapid subsidence of the ground surface is usually attributable to human activities, such as the
extraction of fluids from beneath the surface. Fast local changes in land elevation and associated surface strains can cause damage to structures that
is costly to replace or repair, and can also greatly increase flooding potential.
Rapid ground subsidence over areas of petroleum and gas extraction has been observed previously [Mayuga and Allen, 1970; Pratt and Johnson, 1926;
Vanhasselt, 1992]. The effects are most noticeable on a coastline where a small elevation decrease may cause inundation, first described over an
oilfield near Houston, Texas [Pratt and Johnson, 1926]. Parts of the city and port of Long Beach, California, suffered major problems due to rapid (up
to 0.75 m yr-1) land subsidence related to extraction of oil from the underlying Wilmington oil field [Mayuga and Allen, 1970]. Problems were caused
both by inundation and by horizontal strains on the sides of the subsidence bowl. Subsidence over petroleum extraction zones can also cause
significant damage to extraction infrastructure itself, including expensive well failures. In this paper, we report subsidence rates as high as 40 mm
in 35 days or an annual rate of > 400 mm yr-1 in two California oilfields...
Conclusions
We have used interferometric analysis of spaceborne ERS SAR to map the subsidence of the surface over oilfields in central California. We measure very
rapid subsidence rates of up to 400 mm yr-1 or >1 mm day-1 (Plate 3), and show the subsidence is largely limited to the petroleum production
properties (Figures 1 and 2). In the Lost Hills oilfield, preliminary elastic strain modelling using an implementation of the Okada [1985]model [Feigl
and Dupré, in press] indicates a net compaction of 1.7 mm day-1 at the center of the subsidence bowl decreasing to 0.6 mm day-1 to the south. That
much compaction over a total area 0.8 x 5 km could account for the observed surface subsidence of the 35-day interferogram (Plates 1 and 3). This
modeling shows that the volume change in the rock units at depth sufficient to cause the observed signal is roughly 1.5x106 m3 yr-1 for the Lost Hills
oilfield. More detailed modelling of the deformation in the fluid reservoirs [e.g., Segall et al., 1994] would require data on pressure changes within
the reservoir from the operating companies...
Here is graphic showing what subsidence looks like.
(Found
here.)
Notice the yellow layer?
While it is doubtful that "an air chamber would act as a better transmitter of a seismic wave than continuous rock," you have to agree (and you
state as much) that the opposite is true in more 'solid' (again, in a lay sense) conditions. Isn't that the resulting physical condition of
subsidence?
Originally posted by Valhall
But let's just say, for the sake of argument, that all your little pores in your rock were left dry. That rock would be less likely to transmit a
wave on upward toward the surface than a rock with pores filled with water or oil. The more "compressive" the fluid filling a pore is, the more it
will absorb the energy in an "echo chamber" type style and not transmit the energy on up. Again, a little bitty baby baffle.
I have to agree, at least with regard to the transmission of sound through porous material. But that assumes it remains in that state. Once that state
changes to a more solid profile (compaction), it seems to support the broader question raised concerning whether drilling has a probable effect on the
intensity, reach, or impact of seismic events.
Now on the issue of sound waves in larger cavernous voids, I would ask whether there is any localized affect caused by these sound waves? Could these
effects contribute to the destabilization of the overall structure of these voids? And, if so, could there be a cascading effect beginning on a
smaller scale and ending on the larger?
(I've run out of time again...
)
[edit on 15-10-2005 by loam]
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