ASSESSING BIOGEOCHEMICAL INTERACTIONS IN THE RESERVOIR AT MARCELLUS SHALE ENERGY AND ENVIRONMENT LABORATORY

The introduction of hydraulic fracturing fluids into the deep subsurface can initiate a variety of water-rock-microbe interactions within the formation. Isotopic characterization of produced waters and gases can play an important role in determining biogeochemical interactions over time in these reservoirs. The Marcellus Shale Energy and Environment Laboratory (MSEEL) site in Monongalia County, West Virginia has allowed for unprecedented access to samples from all stages of drilling and production to test for these water-rock-microbial interactions. The MIP#3H and #5H wells near Morgantown were hydraulically fractured in December 2015 to initiate production. The injected fluid consisted of a mix of fresh water, chemicals, and silica sand. All fluids injected and returned from the wellbore, as well as produced gas, were analyzed for geochemical and isotopic signatures at both wells. During the initial three day post-injection period, termed “flowback”, samples were taken at six-hour intervals. Following the flowback period, sample collection transitioned to daily for two weeks and is now scheduled monthly for the duration of the project. The carbon isotopic signature of dissolved inorganic carbon (δ¹³C_DIC) in the injected freshwater and fracturing slurry have values of -8.7 and -8.2‰ V-PDB respectively. However, ¹³C_DIC produced water from these wells ranged from +10 to +31‰ V-PDB. The δ¹³C_DIC reveals a sharp initial increase in value during initial flowback period. This is likely attributed to initial dissolution of ¹³C enriched carbonate minerals deposited during an older phase of biogenic methanogenesis. A δ¹³C value of +14.6‰ V-PDB was obtained for a carbonate vein from the same stratigraphic interval in another core. We hypothesize that the following slow increase in δ¹³C_DIC could possibly reflect microbial processes that utilize lighter ¹²C for their metabolic activity. The δ ¹⁸O and δ ²H are also being analyzed from injected and produced water and will be used to discern water-rock interactions such as dissolution and gas exchange within the reservoir. In addition, we will be collaborating with a team of microbiologists who will utilize genomic tools to fingerprint the microbial populations in the produced water.