Predicting fluid flow in shale and tight gas formations is one of the many challenges associated with unconventional oil and gas exploration. Now a team of scientists from Purdue University may have found a way to quickly and remotely evaluate fluid flow in subsurface fractures, having found a nearly universal scaling relationship between fracture stiffness and fluid flow that applies to low porosity rock—roughly more than 50 percent of all rock on Earth.
Laura Pyrak-Nolte and David Nolte, both professors of physics at Purdue used a mathematical scaling relationship to create a tool that reveals a fracture’s potential fluid flow rate, which can then be used to predict flow path and evaluate the hydraulic integrity of a site.
Fractures in rock provide the dominant conductive pathways for fluids to move through the Earth’s crust, influencing a wide range of subsurface human activities including the extraction of hydrocarbons, the sequestration of green house gases and the protection of aquifers. However, fractures are intrinsically heterogeneous and easily modified by natural and human processes. They may be altered geochemically and deformed under stress, affecting fluid flow rates that can vary across many orders of magnitude.
“In nonporous media, you have fractures that have a high heterogeneity, meaning lots of different fractured geometries and fracture scales,” Nolte explained. “You have fractures that can extend for hundreds of meters or you can have small fractures only a few inches in length.”
However, the Purdue researchers have managed to demonstrate that a scaling relationship exists that accounts for spatial correlations in fracture distributions and also captures the behaviour of flow within a fracture.
Although it might seem that a wide variety of fractures types would be different with different fluid flow rates, Pyrak-Nolte said, “Now we have found the single underlying physical principle that explains them all.”
This relationship can be used as a guide for predicting fracture behaviour in models of the subsurface where stresses vary with depth. This relationship also provides a basis for developing remote monitoring of fluid flow among fractures in the Earth’s subsurface.
Another variable that the scientists measured was the stiffness of a fracture, measured with high-frequency waves. Pyrak-Nolte explained that this technology, when paired with the new scaling law, allows for a remote scan of a fracture to reveal the potential fluid flow at a particular site, as well as to monitor potential changes in fluid flow at a site over time.
In order to better understand the research, we have to understand that for hydrologic purposes, a fracture may be viewed as a quasi-two-dimensional network of void spaces through which fluids flow.
Volumetric flow rates are controlled by the size and spatial distribution of the apertures of the void space. On the other hand, the mechanical properties of a fracture are controlled by the asperities, which are the discrete points of contact between the two fracture surfaces.
Apertures and asperities are complementary aspects of the same fracture geometry and each represents an influence network. The connected apertures define a percolation network, whereas the discrete points of contact are connected through the rock matrix as a separate stress network that controls the mechanical deformation of a fracture.
It would appear that the more the asperities – the discrete points between the two fracture surfaces – the more stable (or stiff) the fracture. According to Nolte, it was possible to link the two because both characteristics depended on a shared geometry.
Speaking to The Bakken Magazine, David Nolte said that the research findings have not yet been tested in the field, although the researchers have ran more than 3,600 simulations for each fracture type using Purdue’s Rosen Center for Advanced Computing.
He nevertheless believes that they could be applied to the Bakken and other tight oil shale plays, both for naturally occurring fractures and induced fractures from hydraulic fracturing.
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