CHARACTERISTICS OF HIGH QUALITY FRAC SAND, AND WHERE TO FIND IT IN THE UPPER MIDWEST

Geologists define sand as any sediment particle from 1/16^th millimeter to 2 millimeters in size. However, there is a narrower definition of sand that would be ideal for use as a “proppant” in hydraulic fracturing operations and a host of other characteristics that relate to the economic value of a frac-sand deposit. Hydraulic fracturing involves injecting water, proppant, and chemicals into bedrock formations under pressures great enough to induce fractures. The sand grains “prop” open the fractures so that hydraulic conductivity is maintained. Fine grained rocks such as shale can be developed for oil and/or gas production using hydraulic fracturing techniques.

Characteristics of grain size, shape, and hardness are important attributes for frac sand. The optimum grain size for frac sand is in the range of 0.212 millimeters to 0.850 millimeters which correspond to ASTM sieves sized 70 to 20. Frac sand that is well rounded provides the best packing arrangement and porosity, thus the best hydraulic conductivity. Finally, the harder the sand grains the better for hydraulic fracturing. In natural sand deposits hardness usually equates to pure quartz arenites.

Drilling and testing of various sandstones in Minnesota and Wisconsin have identified at least three strata containing high quality frac sand. The Ordovician St. Peter Sandstone, the Van Oser beds in the Cambrian Jordan Sandstone and the Cambrian Wonewoc Sandstone contain beach sands that meet the requirements for frac sand. The extensive distribution of these strata in the driftless areas of Minnesota and Wisconsin provide a variety of mining opportunities based on thickness of overburden, depth to groundwater, proximity to transportation and other factors that influence the economic viability of extraction.