SA Mines and Energy Journal : June-July 2010
JUNE/JULY 2010 SA MINES & ENERGY JOURNAL 22 INNOVATION The upside of deep stress CSIRO team's new method for estimating in situ rock stress at depth will aid geothermal energy extraction. Australia's potential mineral wealth is well documented but what is only just emerging is the potential value of its geothermal energy resources. Research is being done across the country to investigate their worth and feasibility, and the positive impact their use could have on the environment. One piece of recent work has developed a numerical method for estimating the stresses at work on in situ rocks: largely undisturbed rock masses usually at considerable depths (more than 3500m). Whereas shallow drilling can successfully use conventional methods such as hydraulic fracturing, stress determination in deep drilling is a far tougher prospect. The method was devised by a team from CSIRO's Exploration & Mining Division (now part of CSIRO Earth Science and Resource Engineering (CESRE), led by Dr Baotang Shen, the principal research scientist with the sustainable mining systems team. "A good understanding of the stress regime within in situ rocks is essential for the efficient extraction of geothermal resources because it is imperative to determine the position and shape of the fracture 'cloud' created by hydraulic fracturing in the hot rock, "Dr Shen explained. "The cloud becomes the geothermal heat exchange reservoir and stresses affect whether the cloud is vertically or horizontally aligned. The ratio of the horizontal stresses to vertical stress is extremely important to the performance of the heat exchange reservoir as it influences the direction of fluid flow." A horizontally aligned fracture cloud is preferred because it provides a much better drilling target and extends the reservoir across a narrow temperature interval. The CSIRO team undertook the study in deep granite at Habanero No.1 in South Australia's Cooper Basin, the first hot-fractured-rock site in Australia, and one owned and operated by Geodynamics Limited. The study was conducted using a fracture mechanics numerical code called FRACOD, which simulates the rock fracture initiation and propagation. The stimulation involved injecting water at high pressure into the rock formation for two months, a process which opens up existing fractures or creates new ones in the vicinity of the borehole, creating microseismic events and high permeability zones. "The stress estimate work was done before the stimulation tests, which means the results from the stimulation tests provided a very good validation after wards, "Dr Shen. said "This study demonstrates that it is possible to use the borehole breakout data to estimate the magnitude of the principal horizontal stress if a proper numerical model that captures the true rock fracturing process is used. "The key results showed that when a reservoir develops sub-horizontally (at a low angle less than 30 degrees from the horizontal direction), there is a high efficiency in thermal exchange and low liquid leakage but when it develops subvertically (at an angle less than 30 degrees from the vertical direction), the result is low efficiency in thermal exchange and high liquid leakage, " Dr Shen said. The team said it was aware of the general trend of the high horizontal stress regime in the Cooper Basin based on previous petroleum drilling but this new numerical model is the first to quantitatively estimate the magnitude of the stresses. The new method offers the industry an alternative and inexpensive way to study in Deep heat: A seismic cloud recorded during hydraulic fracturing operation by injecting high pressure water into the rock formation.