Research and Students

Students:
Lan Luo, graduated with M.S. - June 2016
Robert Caulk, Ph.D. student
Andre Niquini,  undergraduate, Brazil Scientific Mobility Program (BSMP)
Arthur Ferreira Dos Santos Netto,  undergraduate, Brazil Scientific Mobility Program (BSMP)

Research:

Discrete Element Method (DEM) has been developed and utilized for better understanding convective-conductive heat flow and transport and fracturing in rock. The novel model enables studying hydro-thermo-mechanical behavior of porous and fractured synthetic rock mass using numerical modeling. Fig. 1 shows the development and application of the DEM model, where fluid flow is modeled through a network of fluid parallel-plate channels and reservoirs between bonded DEM particles. Fluid flow and convective heat flow are modeled at each time step, while thermal energy is exchanged between fluid and adjacent particles. In addition, conductive heat flow and transport through bonded particles contacts is enabled. Fig. 2 shows two examples of hydraulic fracturing modeling of synthetic rock in DEM, where the two dimensional rock sample is subjected to deviatoric compressive stresses at boundaries (maximum stress is in vertical direction). Fracture propagation from the bore-hole is shown for the system where fluid and rock have same temperature on the left and for the case where fluid temperature is 50 degrees Celsius while the surrounding initial rock temperature is 250 degrees Celsius on the right. On the Fig. 2 right substantial thermally induced micro-cracking can be observed near the propagating fracture and around the bore-hole. Additionally, the main fracture is wider and shorter compared to the picture on the left.

                        

 Figure 1. Implementation of convective-conductive heat transport in DEM                 Figure 2. Hydraulic fracture propagation without (left) and with (right) thermal stresses

Multi-phase flow of dense suspensions consisting of viscous fluid and small particles present a challenging fundamental problem, which is relevant for many geomechanical applications, such as proppant placement in narrow and rough hydraulic fractures or sand flow, piping failure of dams and even sand liquefaction. At higher particle concentration in suspension, particle-particle interaction forces dominate over hydrodynamic forces in governing particle motion. Particularly, a thin layer of viscous fluid between two mutually approaching particles dissipates their kinetic energy. Fluid lubrication force was implemented in the novel non-linear contact model in DEM coupled with Computational Fluid Dynamics (DEM-CFD). As a result, new knowledge about particles agglomeration mechanism in rough and narrow hydraulic fracture was obtained (Figs. 3,4). 

                                           
Figure 3. Particle flow and transport in a channel in viscous fluid                                    Figure 4. Proppant settling in a narrow and rough fracture

Numerical modeling is currently being supported with experimental work at the UCSD geomechanical lab, where a series of very narrow Plexiglas slots was filled with sand and viscous fluids for observing particles settling. Future efforts are planned in directions of expanding experiments with flowing system and synthetic rough fracture walls manufactured using 3D printing technique based on the experimental hydraulic fracturing rock surfaces obtained from the University of Bochum, Germany. Particle Image Velocimetry (PIV) is being used by master student Lan Luo for video analysis and particle motion tracking (Fig. 5).

Figure 5. Particle Image Velocimetry (PIV) of particles settling in a 2 mm wide slot in 75% volumetric glicerol-water solution (left), cumulative particle velocity vectors for the chosen square patch (right)


    Collaborative research with the researchers from the Applied Geology Department at University of Goettingen in Germany currently focuses on developing across-scale understanding of flow and transport processes in multi-fractured rock mass. The DEM model is coupled with the Lower-dimensional Discrete Fracture Model (LDFM) for better understanding influences of near-fracture zone permeability enhancement in sandstone due to micro-cracking under supercritical CO2 injection (Fig. 6).



Figure 6. Illustration of the coupling scheme between the Discrete Element Model (DEM) and the Lower-dimensional Discrete Fracture Model (LDFM)