Monday, 25 July 2011
Paralana II Fracture Stimulation Success
Last week, Petratherm and its JV partners Beach Energy and TRUenergy advised
that the fracture stimulation undertaken on the Paralana 2 well had been successfully
completed. Petratherm and Geodynamics are the two companies leading the field
of Enhanced Geothermal Systems (EGS) in Australia but pursuing two different
approaches. While Geodynamics is aiming to develop its reservoir in the hot
rock mass itself, Petratherm has chosen a slightly different approach and been
aiming to develop a geothermal reservoir in the insulating layers. These layers
are not producing heat themselves but would have been heated by heat conduction
from the underlying granite rock mass. By aiming to develop a reservoir in the
insulating layer, Petratherm is pursuing a trade-off between lower temperatures
and higher permeability. The insulating layers already have some porosity (albeit
small at those depths) and the company believes that they should be easier to
fracture due to lower mechanical strength.
The results announced by the company show a 500-m stimulated zone extending
about 900 metres in an easterly direction. The fracture events were recorded
by microseismic sensors in six surface and six borehole stations located around
the well. A total of 3.1 million litres of water was injected over five days
at pressures up to 9000 psi (62 MPa) and sustained pumping rates up to 1600
litres/minute.
Pumping 1600 litres/minute at 62 MPa needs about 1800 kW or 2400 hp of pumping
power. The Paralana team was prepared for even harder stimulation as evidenced
by the presence of 8500-hp pumping equipment shown below.
The first successful stimulation of an EGS well in Australia was Habanero 1,
where over 20 million litres of water were injected at maximum flow rates up
to 40 litres/sec (2400 litres/minute) with wellhead pressures sustained at values
as high as 65 MPa and reaching peak values close to 70 MPa. I am sure Petratherm
will publish injection their curves in due time, possibly at the Australian
Geothermal Energy Conference (AGEC) in November.
At this stage, it looks like Paralana reservoir is also overpressured at pressures
similar to Cooper Basin with the wellhead pressure stabilised at 3940 psi or
about 27 MPa. This is not too far below the wellhead pressures observed at the
Cooper basin wells by Geodynamics. . The Geodynamics experience with production
pumping from overpressured reservoirs should be of value to the Paralana JV.
Based on a superficial comparison between the two stimulation events, the numbers
support the original Petratherm hypothesis that insulating layers should be
easier to stimulate. Whether the resultant permeability is high enough to produce
flow rates sufficiently higher than those in Cooper Basin to compensate for
the lower temperatures will be seen after drilling of the Paralana 3 well. The
positioning of the well will require a careful analysis of the stimulation results.
In the graphs included in the ASX release, the directional nature of the stimulated
zone is interesting. The fractures were generated to the east of the well. Based
on the following geological cross section provided by Petratherm on their web
site, it is tempting to speculate that the fractures were generated towards
the vertical fault shown on the east of the well. I am sure we will hear more
from Petratherm in the future as they have time to analyse the results.
Finally, one interesting observation is that in spite of the effort that successfully
stimulated a zone stretching to 900 metres in the plan view, the largest seismic
event recorded on the surface was of magnitude 2.6 on the Richter scale with
98% of the microseismic events detected being below 1.0. According to the USGS,
events below 3.0 magnitude are not felt except by a very few under especially
favorable conditions. For those people who are unnecessarily worried about the
earthquake risks from geothermal development, I should note that the Newcastle
earthquake in 1989 measured 5.6 on the Richter scale and Richter scale is a
logarithmic scale - which means that the largest event measured during the Paralana
stimulation is 3 below (3=5.6-2.6) the Newcastle earthquake on the Richter scale
and therefore one thousand (103)times weaker. Therefore, Paralana
stimulation results prove that the EGS stimulation in stable geological settings
have no earthquake risk.
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