A Sharper Eye on Underground Fluids
“Enhanced Subsurface Fluid Characterization Using Joint Hydrological and Geophysical Imaging,” awarded to Michael Commer, Stefan Finsterle, Michael Kowalsky, and Gregory Newman
Recovering the last drop of oil, gauging geothermal resources, disposing of toxic and nuclear waste, sequestering captured carbon dioxide – needs like these grow in urgency day by day. To meet them will require better imaging of subsurface structures and the fluids in their pore space.
“Presently, subsurface imaging depends on two different approaches,” says Michael Commer of the Earth Sciences Division’s (ESD’s) Geophysics Department, principal investigator of the Discovery project. “The hydrological method builds a model using data from direct sampling, with often widely separated boreholes. The geophysical approach doesn’t sample fluids directly but measures properties like electrical resistivity, which are related to fluids.”
Michael Kowalsky of ESD’s Hydrogeology Department notes that “since 1987 ESD has developed a powerful set of codes called TOUGH to model fluid and heat flows in complex geological circumstances. TOUGH codes are used all over the world. But since the modeling is based on averaging the direct data, there can be large uncertainties.”
On the geophysics side, a program called EMGeo has been developed by Gregory Newman, head of the Geophysics Department, working with Commer. EMGeo images the subsurface using electrical and electromagnetic data collected remotely.
“The motive is to identify fluids like oil and gas over wide areas, and indeed major oil and gas companies use EMGeo for that purpose,” Newman says. “But we don’t image the fluids directly, and we can’t predict where the fluids are going to go.”
The Discovery team, which includes hydrogeologist Stefan Finsterle, principal developer of the TOUGH code known as iTOUGH2, will investigate strategies for combining hydrological and geophysical approaches using maps of electrical conductivity, which is affected by salinity, temperature, and pressure, among other fluid properties, and by porosity and other properties of the rock.
Commer says, “We’ll start with a 3-D image of electrical conductivity and use it as data to build a hydrological model. We’ll also build conductivity maps from two separate models, one geophysical and one hydrological, and see how they compare.”
Finally, says Newman, “we’ll couple data from both methods and link them through a petrophysical model” – a model of rock formations common to both. Although this approach is risky because of many unknowns, “if we can achieve a picture of flow consistent with the rock physics – one that matches observations in the field – we will have achieved our goal.”
EMGeo and the TOUGH codes have complementary strengths and weaknesses. Hydrology removes ambiguity about what’s being looked at, and geophysics sees far beyond a few limited samples.
“If we can combine the technologies, it will be like adding two plus two and getting five,” Newman says.
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