In shale plays, on the opposite spectrum from the “factory drilling” approach is the “engineered well,” where science plays a role in determining the best spots to land the lateral and perforate the well. Range Resources has perfected this technique in the Marcellus and Utica shales, first using focused ion-beam scanning electron microscope technology but later relying on nuclear magnetic resonance (NMR) logs to correctly land its laterals.

Now Rice University is extending that research, according to a recent press release. Researchers George Hirasaki and Walter Chapman are combining NMR measurements with molecular dynamics simulation to better define the contents of shale. Their findings have recently been summarized in the Journal of Magnetic Resonance.

NMR has long been kind of the outsider of logging measurements because of its cost and relatively newness. But because of its ability to identify the type of molecule present in a rock, it can provide valuable information relating to liquid distribution as well as pore size. Essentially the tool uses radio-frequency electromagnetic pulses to manipulate hydrogen atoms and then measures their relaxation time to determine the molecular mix.

In the press release, Hirasaki explained that in conventional reservoirs a water-wet rock will see the oil’s relaxation time replicate that of bulk oil, while the water’s relaxation time is a function of the pore size. In an unconventional reservoir, the relaxation times of both oil and water are short and overlap. “The diffusivity is restricted by the nanometer- to-micron size of the pores,” he said. “Thus, it is a challenge to determine if the signal is from gas, oil or water.”

The first goal of the study is to determine whether the rapid relaxation times are the result of paramagnetic sites and asphaltene aggregates on the mineral surfaces and/or the pore size restriction on the ability of the molecules to move.

Another focus of the study is the effect of water, the main ingredient in fracturing fluid, on the kerogen found in unconventional rocks. Water molecules tend to bind with kerogen and block the pore spaces containing hydrocarbons.

The study is applying NMR measurements to kerogen samples and comparing the output to computer models simulating the interaction of the substances. This is particularly applicable to the rock’s wettability. The goal is to better interpret the NMR results in these tight rocks and also could help manufacturers develop fracture fluids that are less likely to interact with the kerogen.

“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 in the press release.

Another goal is to incorporate the simulations into Chapman’s inhomogeneous statistical associating fluid theory, which simulates the free energy landscape of complex materials.

“Our results challenge approximations in models that have been used for over 50 years to interpret NMR and MRI [magnetic resonance imaging] data,” Chapman said in the press release. “Now that we have established the approach, we hope to explain results that have baffled scientists for years.”

For more information, visit http://news.rice.edu.

Contact the author Rhonda Duey at rduey@hartenergy.com.

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