HYDRAULIC FRACTURING "SWEET SPOTS" IDENTIFIED BY RADIOGENIC NOBLE GASES

Within the past twenty years, the growing energy demands in the United States have been satisfied by the increased production of natural gas through hydraulic fracturing. The technology reactivates existing and induces new fractures, which then serve as pathways for hydrocarbons to migrate from the shale to the well borehole. Although this method is now the dominant technique of extracting natural gas, less than 30% of wells and lateral intervals recover economic abundance of natural gas. This is largely due to the uncertainty surrounding how fluids flow in a relatively impermeable rock, the inability to accurately model existing fractures and the heterogeneous composition of shale. A potential solution to this issue is to use stable noble gas isotopes to identify the hydraulic fracturing “sweet spot(s)”. We define “sweet spot” as a region of shale containing a high amount of naturally occurring fractures with accumulations of extractable hydrocarbons. Noble gases can be utilized on a wide array of problems because their original gas composition is preserved independent of geologic conditions and microbial processes. For this study, the distribution of radiogenic and nucleogenic crustal noble gas isotopes (e.g. ⁴He, ²¹Ne*, ⁴⁰Ar) and their associated ratios (e.g. ⁴He/²¹Ne*) will be used as geochemical tracers of fluid migration.

The ⁴He/²¹Ne ratio of drill cuttings is particularly useful to find fluid accumulations due to the fixed production rate of each isotope in mineral grains (⁴He/²¹Ne* = 22 x 10⁶) and their differing interactions with the grain (quartz). Both isotopes can diffuse out of the crystal and equilibrate with surrounding pore fluids when temperatures are above the closure temperature. However, ⁴He is released relative to ²¹Ne* at temperatures below 80 degrees Celsius, causing mass dependent fractionation and an elevated ⁴He/²¹Ne* ratio in zones of gas accumulation. Here, we will present ⁴He/²¹Ne* data obtained from cuttings recovered from Appalachian and Permian Basin wells. The analytical results for noble gases will be compared against gas production data from each fracking stage of various wells in both basins.