Innovative Method for Connate Water Determination in Core Material Identifies Formation Heterogeneity and Potential Damage Mechanisms

摘要

The continued evolution of hydraulic fracturing technology has enabled hydrocarbon production in tight, unconventional reservoirs. This usually requires large amounts of water, injected deep underground, which interacts with formations through different pathways. One of the rock-fluid interactions that may occur is salt leaching and mineral dissolution, which mobilizes large amounts of potentially reactive ionic species within the fracture network. Liberation of salt from the reservoir can have a number of consequences, ranging from high salinity flowback water treatment issues to production of geochemical scale that can damage fracture conductivity. An advanced salt analysis method has been designed to characterize the ionic species present in interstitial waters confined within core material, to analyze soluble mineral species that can be mobilized during hydraulic fracturing, and to quantify the scaling potential of reactive ionic species present during treatment and initial production. This method determines connate water chemistry by utilizing a novel solvent extraction procedure that characterizes free and clay bound ions, along with related rock properties (mineralogy, water saturation, and cation exchange capacity). This combined dataset is used to determine the thermodynamically stable composition of the connate water at reservoir conditions, and examine the heterogeneity in chemical composition from the core scale to the reservoir scale. Connate water composition obtained by this method can be utilized for a number of applications, including resistivity log correlation and chemical fingerprinting that can be used with flowback water chemistry to identify fracture propagation. Furthermore, potential chemical incompatibility issues such as scale formation within the target interval or adjacent zones can be identified so that hydraulic fracturing fluid chemistries can be properly treated to eliminate potential incompatibilities. A case study on connate water chemistry in the Montney and Duvernay shale formations is presented using this detailed characterization strategy. The results demonstrate that the presence of key ions, like vanadium and molybdenum in the Montney, can be used to identify the extent of fracture propagation. Because this methodology was also able to identify zones that could produce geochemical scale (e.g. BaS04), hydraulic fracturing fluid chemistries could be tailored to minimize its effect on production.