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Flow-Back Frac Water Composition – Rock-Fluid Interaction in the Montney Shale Controlled by Faults and Maturity Domain


Extensive sampling of flowback water through time has been performed in the Triassic Montney hybrid shale of the Altares Field, in NE British Columbia. The analysis performed on the data of 79 wells has brought to light new facts and understanding on the interaction between injected water and the rocks penetrated. Our analysis indicated that the injected frac water has interacted with some of the sulfides present in the pore network. Thus evidence of barium sulfate precipitation has been repeatedly seen; the rate of precipitation has been assessed and mapped against some of the known structural elements; XRD analysis indicate that the most likely reacting mineral is marcasite. This poorly stable sulfide can easily react in the presence of injected water and is transformed into as sulfate. From the thin section analysis of the zones with sulfate precipitation, the marcasite has been found to be associated with hydrothermal minerals such as saddle dolomite, barite and sphalerite. These occurrences of marcasite together with barium sulfate precipitation has been solely restricted to the coarser silty upper member of Montney in the field. The contoured maps of the semi-quantitative sulfate precipitation clearly indicates gradual increases towards two distinct subvertical faults, both trending North 10 degrees. A second major finding of our water flowback analysis is the well-behaved change in composition with depth. Each produced water composition being compared to other samples that correspond to the the same volume of water recovered. Thus, in the dry gas domain, i.e. below the isotope reversal at 1.5% Ro, the quantity of every molecular element gradually decreases downward in all of the 79 wells studied. This has been interpreted as a downward reduction in water saturation, a complete opposite trend of the one observed above the isotope reversal. The proposed dehydration mechanism relates to some of the water being absorbed by the desiccated clays in the dry gas domain at a vitrinite reflectance value above or equal to 1.5%Ro to create new methane molecules. The present study firmly supports some previous isotope work that indicated hydrogen from the methane in the dry gas domain having two different isotopic signatures, one of which not in line with the isotope from hydrocarbon molecule. It is thus deducted that some of the hydrogen atoms in the newly generated methane molecules originally belonged to water molecules that have been broken down.