Engineers takes deeper look at unconventional oil gas

Engineers take deeper look at unconventional oil and gas

11:44 AM, 25th July 2017
Rice research scientist Philip Singer holds kerogen, a component of oil shale, extracted and compressed for study into a pellet.
Rice research scientist Philip Singer holds kerogen, a component of oil shale, extracted and compressed for study into a pellet.

HOUSTON, US: Understanding how oil and gas molecules, water and rocks interact at the nanoscale will help make extraction of hydrocarbons through hydraulic fracturing more efficient, according to Rice University researchers.

Rice engineers George Hirasaki and Walter Chapman are leading an effort to better characterise the contents of organic shale by combining standard nuclear magnetic resonance (NMR) – the same technology used by hospitals to see inside human bodies – with molecular dynamics simulations.

The work presented this month in the Journal of Magnetic Resonance details their method to analyse shale samples and validate simulations that may help producers determine how much oil and/or gas exist in a formation and how difficult they may be to extract.

Oil and gas drillers use NMR to characterise rock they believe contains hydrocarbons. “NMR instruments are among several tools in the string sent downhole to “log,” or gather information, about a well,” said Hirasaki.

In conventional reservoirs, he said, the NMR log can distinguish gas, oil and water and quantify the amounts of each contained in the pores of the rock from their relaxation times -known as T1 and T2-as well as the diffusivity of the fluids. “If the rock is water-wet, then oil will relax at rates close to that of bulk oil, while water will have a surface-relaxation time that is a function of the pore size,” Hirasaki said.

“In unconventional reservoirs, both T1 and T2 relaxation times of water and oil are short and have considerable overlap,” he said. “Also the T1/T2 ratio can become very large in the smallest pores. The diffusivity is restricted by the nanometer-to-micron size of the pores. Thus it is a challenge to determine if the signal is from gas, oil or water.”

Fluids pumped downhole to fracture a horizontal well contain water, chemicals and sand that keeps the fracture “propped” open after the injection stops. The fluids are then pumped out to make room for the hydrocarbons to flow.

But not all the water sent downhole comes back. Often the chemical composition of the organic component of shale known as kerogen has an affinity that allows water molecules to bind and block the nanoscale pores that would otherwise let oil and gas molecules through.

The Rice project managed by lead author Philip Singer, a research scientist in Hirasaki’s lab, and co-author Dilip Asthagiri, a research scientist in Chapman’s lab, a lecturer and director of Rice’s Professional Master’s in Chemical Engineering programme.

“NMR is very sensitive to fluid-surface interactions,” Singer said. “With shale, the complication we’re dealing with is the nanoscale pores. So to understand what the NMR is telling us in shale, we need to simulate the interactions down to the nanoscale.”

“If we can verify with measurements in the laboratory how fluids in highly confined or viscous systems behave, then we’ll be able to use the same types of models to describe what’s happening in the reservoir itself,” he said.

“Our results challenge approximations in models that have been used for over 50 years to interpret NMR and MRI (magnetic resonance imaging) data. We hope to explain results that have baffled scientists for years,” Chapman said.

Chapman is the William W Akers Professor of Chemical and Biomolecular Engineering and associate dean for energy in the George R Brown School of Engineering. Hirasaki is the A J Hartsook Professor Emeritus of Chemical and Biomolecular Engineering.

The Rice University Consortium on Processes in Porous Media supported the research, with computing resources supplied by the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy and the Texas Advanced Computing Center at the University of Texas at Austin.

© Rice University News


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