Multiscale Investigation of Reactive Fluid Transport Characteristics in Unconventional Shales

Abstract:

The intriguing but challenging interplay of hydraulic fracturing fluids (HFF) with shale minerals and resident formation brines has a major role on the rapid decline of hydrocarbon production after the initial months of fracturing operations but it remains largely unsolved. In this study, the influence of shale-HFF interactions on flow properties were examined by means of representative core-flooding experiments and multiscale imaging tools. Krypton (Kr) flooding tests were conducted under in-situ dynamic X-ray computed tomography (CT) that enables the pre-/post-reaction spatial and temporal evolutions in Kr-accessible porosity distributions within the shales. The samples, cored parallel to bedding planes (1-in diameter and 3-in length), were selected from the economically critical U.S. shale resources, namely Marcellus, Eagle Ford, and Wolfcamp plays with their diverse structural and mineralogical backgrounds. The synthetic brine and HFF solutions were prepared based on the basin-specific recipes. The pre-/post-reaction characterization measurements of mineralogical, structural, and petrophysical properties were performed by X-ray diffraction, X-ray fluorescence (XRF), medical CT, µCT, scanning electron microscopy (SEM)-energy dispersive spectrometry (EDS), and pulse-decay permeability methods. The CT-number distributions under totally vacuum states that are directly proportional to shale density revealed dissolution and scale precipitation paths along the cores. The elemental and composite maps derived from SEM-EDS analysis and XRF of the reacted core surfaces, an iron-bearing precipitate which is not associated with any sulfur or carbonate elements formed around the fracture openings and on the matrix. The reacted clay-rich Marcellus and Wolfcamp samples showed the remarkable interaction between HFF and formation water resulting in barite (BaSO₄) and celestite (SrSO₄) precipitations, respectively. Our experiments indicate that optimization of fracture fluid recipes with a focus on fluid interactions with shale mineralogy is crucial, improves permeabilities of stimulated shale zones, and enhances matrix accessibility. The outcomes gained in this study aid in developing manipulation strategies to enhance flow through shale fractures and matrices.