Development of Aluminum-Based Dissolvable Alloys for Hydraulic Fracturing Applications
Creators: Ahmed, Ezz Q
Publisher: name: University of Alberta Library
Year: 2022

<p>The application of dissolvable materials is becoming widely used in hydraulic fracturing, especially in low permeability reservoirs. To date, most of the DAs are based on Al alloys (AAs) or Mg alloys, which offer desirable dissolvability, yet limited mechanical properties. This work aims to develop a series of new Al-based DAs with desirable dissolution and mechanical strength. <br>
Given the enormous possibilities in tuning the structures and compositions of potential DAs, a literature review was initially done to summarize the effects of various alloying elements on AAs. It was found that the abundant electrochemical data in the literature were reported in NaCl solutions, while the downhole environments contained KCl. Therefore, electrochemical studies were performed to analyze the cation type effects (K+ vs. Na+) on AAs. It was concluded that the associated cation type had insignificant effects on the corrosion behavior of the selected AAs, and the electrochemical data obtained from the literature in NaCl environment could be used for the selection of the alloying elements for the design of the new Al-based DAs. The selected alloying elements are Ga, In, Sn, (GIS), and Ag, which are expected to give the desired dissolvability, as well as Cr and Zr to enhance the mechanical properties. <br>
The base alloy of Al-Zn-Cu-Mg was used in this study. After the selection of the alloying elements, Taguchi’s method (L27) was utilized to change the chemistry of the base alloy to facilitate the design and further investigate the elemental selection. Scheil-Gulliver model was used to predict the phases formed as well as their fractions through ThermoCalc simulations. The results predicted the formation of 6 main phases: α-Al, η-MgZn2, Mg2Zn11, Mg2Sn, θ-Al2Cu, Al3Zr, and Al45Cr7 in all the alloys. Based on the simulation results, DA13, DA18, DA25, and DA27 were targeted as they had considerable fractions of desired phases (i.e., η-MgZn2, Mg2Zn11, and θ-Al2Cu) and were anticipated to potentially yield high mechanical properties and active corrosion behavior.<br>
The four selected DAs were further studied to evaluate the microstructure, corrosion behavior, and mechanical properties. The alloys were cast and machined for sample preparation. The microstructures of the DAs were analyzed using X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). Immersion corrosion tests were conducted to evaluate the corrosion behavior at 90 °C in 0.59 M KCl. For mechanical properties, hardness and tensile testing were performed on the as-cast alloys. The results presented the formation of the main predicted phases distributed in the as-cast microstructure, namely, η-MgZn2, Mg2Sn, θ-Al2Cu, Al3Zr, and Al45Cr7. The results also suggested there existed a trade-off between dissolvability and mechanical properties. The presence of low melting point intermetallic phases was crucial for the dissolvability of the alloys. DA18 (containing the highest GIS content of 3 wt.%) corroded intensely (177.76 mg/cm2 h) compared to the alloys with less GIS content, yet its mechanical properties were at their minimum (46.27 MPa for UTS and 113 HV for the micro-hardness). All the DAs were reported to be brittle with limited elongation (maximum elongation was 0.053% for DA27). It was concluded that In-containing phases are the main source of Al degradation. In addition, the presence of coarse phases tended to reduce the strength of the alloy (besides the liquid embrittlement caused by GIS phases).</p>