An Experimental Study of Shale-Water Interactions for Applications in Hydraulic Fracturing

• Author / Creator

  Xu, Mingxiang

• The increasing demand for fossil energy and the decreasing trend of conventional resources have shifted the focus of industry towards unconventional resources including shale gas, shale oil, tight gas, tight oil, and coalbed methane. Recent advances in stimulation techniques, including horizontal drilling and multi-stage hydraulic fracturing have led to economic production of previously inaccessible hydrocarbon trapped in ultra-low permeability reservoirs. During a hydraulic fracturing operation, fracturing fluids are pumped into the well to create fractures, which produces a pathway for hydrocarbon flow towards the wellbore. However, a significant fraction of injected fracturing fluid leaks off into natural fractures and shale matrix, which causes the reservoir damage and environmental concerns.Recent experiments show that gas shales which are strongly oil wet based on contact angle measurements might have strong water uptake during imbibition. Clay hydration, microfracture induction, lamination, and osmotic effect may be collectively responsible for the strong water uptake. However, the previous measurements are not sufficient to isolate the above factors nor to explain why the bulk of shale samples can hardly imbibe the oil which completely spreads on their surface. This study aims at interpretation of low flowback efficiency by conducting a series of shale-water interaction experiments. The objective of this research is to investigate the reasons behind low water recovery in flowback process and quantify the impacts of interactions between the lost water and reservoir rock.Extensive imbibition experiments are performed to investigate shale-water interactions at different conditions. Effects of pore network connectivity, salinity of imbibing brine, clay swelling, and air blockage in shale samples are investigated through measuring the imbibed mass, induced strain and stress during imbibition process, and concentrations of dissolved oxygen and ions in the imbibing water. The hypothesis is then tested by 1) measuring imbibed mass of water and oil in crushed shale samples and intact shale samples, 2) measuring soluble/leachable ions in shale samples, 3) measuring hydration-induced strain and stress during water imbibition, 4) conducting imbibition experiments in degassed and oxic conditions, and 5) SEM and EDS imaging of organic and inorganic pores in shale.The comparative study of water imbibition into crushed shale packs and intact shale samples suggests that the connected pore network of the intact samples is water wet while the majority of rock including poorly connected pores is oil wet. In contrast to the artificial pores of crushed rock, the pores of intact rock are already wetted by a film of water and/or covered by precipitated salt. This makes the pores of intact rock prefer to be water-wet than oil-wet. Furthermore, the precipitated salt provides an additional force for water uptake through osmotic effect.By calculating the characteristic thickness of electrostatic double layer (also known as Debye length, ĸ-1) around the charged shale powders, it shows that the imbibition rate is positively correlated to the ĸ-1 values. Electrostatic interactions are part of the disjoining pressure which is not considered in the Young-Laplace equation. A higher ĸ-1 value leads to a higher disjoining pressure value. In turn, this forms a larger hydration shell around the shale powder surface and increases the imbibition rate.The induced strain and stress during water imbibition show a strong correlation among imbibed water mass, induced strain and stress, and clay content of the shale samples. Increasing clay content increases the induced strain and stress caused by water imbibition. For unconfined samples, water imbibition leads to significant pore volume enhancement (up to 0.72% of sample total volume). For partially confined samples, water imbibition induces a relatively low expansive stress. This induced stress can be described as a logarithmic function of confining pressure.Comparative study of water imbibition shows that the imbibition rate and imbibed volume are higher under degassed condition than under oxic condition. These differences are mainly due to the enhanced dissolution of air into water in the shale pore network during water imbibition under degassed conditions. This dissolution will consequently increase the relative permeability of water. The results also suggest that pyrite oxidation from dissolved oxygen produces sulfate ions, iron ions, and iron-compound precipitates. This is supported by SEM/EDS images which show abundant pores in the vicinity of pyrite minerals.

• Subjects / Keywords

  • Clay Hydration
  • Hydraulic Fracturing
  • Gas Shale
  • Water Imbibition
  • Flowback Efficiency

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-a439-6z61

• License

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