The paper presents results of a micro-mechanical Discrete Element Method (DEM) study of the hydraulic fracture initiation and propagation in Enhanced Geothermal System (EGS) performed using the Particle Flow Code (PFC). Hydraulic fracturing is the main means to stimulate and create flow paths to extract heat in hot dry rocks with insufficient permeability to inject and circulate fluids. Hydro-thermo-mechanical coupled modeling is performed to analyze stress and strain changes on fracturing from a wellbore for improving the understanding of the role of thermal stresses on fracture propagation processes and the resulting fracture geometry. Bonded particle model (BPM) that is used for modeling the mechanical response and fracturing of solids was modified for capturing mechanical effects of temperature difference between rock and fracturing fluid infiltration in the propagating fracture. Heat exchange between fluid and rock and fracture is fully coupled processes. As the fracture propagates from the pressurized borehole, both fluid and rock adjacent to a newly formed fracture change temperature. The results show that thermally induced stresses can significantly change the fracture initiation and the fracture propagation pattern. Thermal stresses cause shallow randomly oriented cracks. The study evaluated fracture geometry and orientation with respect to fracturing fluid temperature, viscosity, density, pressure, rock parameters and in-situ stress difference.
This laboratory investigation involved evaluating the potential to enhance drilling Rate of Penetration (ROP) and drilling efficiency by applying axial vibrations on the bit using a pulse cavitation drilling tool in combination with varying levels of axial compliance. The pulse cavitation tool generates cavitation bubbles in the drilling fluid by flowing through a venturi. When the cavitation bubbles produced within the tool are collapsed, pressure pulses are generated which produce pressure pulsations and oscillatory forces on the bit. Drilling results were evaluated on the basis of ROP, Mechanical Specific Energy (MSE), bit loads and bit displacements. The tool was tested both with and without compliance to evaluate the effects of the compliant element. Experimental results show that the pulse cavitation tool starts to cavitate and produce vibrations when a critical flow rate is reached. When the drilling system was rigid (i.e. no compliance was used), the vibration produced did not have any significant effect on the ROP. However, when the drilling system was compliant the vibrations produced by the tool intensified the natural displacement vibration of the compliant element and the ROP was increased and MSE was decreased.
Crushed salt is being considered as a backfilling material to place around nuclear waste within a salt repository environment. In-depth knowledge of salt thermal and mechanical properties as it reconsolidates is critical to thermal and mechanical modeling of the reconsolidation process.
An experimental study was completed to quantitatively evaluate the thermal conductivity of consolidated crushed salt as a function of porosity. Temperature dependence of this thermal conductivity was also determined. Porosities ranged from 1% to 40%, and temperatures ranged from ambient up to 300°C. This range of conditions is expected to more than cover those that might be encountered in a radioactive waste disposal facility. Two different experimental devices were used to measure these values.
The thermal conductivity of reconsolidated crushed salt decreases with increasing porosity or increasing temperature; conversely, salt thermal conductivity increases as the salt consolidates. Thermal conductivity of experimentally deformed bedded salt cores was shown to be related to fracture density, as a type of porosity. Crushed salt for this study came from the Waste Isolation Pilot Plant (WIPP). Salt was observed to dewater during heating, and the weight loss from dewatering was quantified.
A simple mixture theory model is presented to represent the data developed in this study.
In this paper, the use of microseismic data for calibration and modification of wellbore temperature models will be introduced. Moreover, fracturing fluid distribution obtained using the modified temperature numerical model is coupled with the microseismic field data for several Eagle Ford shale wells to improve hydraulic fracture stimulation characterization. By measuring the temperature change along the wellbore, distributed temperature sensing (DTS) data may provide relative fluid distribution. This information may be used to assess the simple geometry of the hydraulic fractures, the fracture initiation points along the wellbore, wellbore integrity issues, and the effectiveness of isolation tools. With recently published wellbore temperature models, quantitative information about which zones receive the stimulation fluid can be numerically solved. However, DTS measurements and fluid distributions calculated using DTS data are restricted to the wellbore and near wellbore environment. For far field diagnostics of hydraulic fracturing stimulation other measurements are needed, specifically microseismic. By combining these two measurements, a new workflow is created which incorporates both the far field and wellbore measurements to characterize hydraulic fractures, both real-time and after the stimulation job. This workflow is especially useful in reservoirs that are naturally fractured or in wellbores were stress shadowing effects are significant, such as multistage fracturing multiple wells that are in close proximity to each other. In these scenarios the path that the fluid travels may be complex, even in the near wellbore environment. Due to this complexity, fluid distributed calculations based on DTS data may provide misleading results. Using information gained from microseismic, the wellbore temperature models may be modified to increase the reliability of the numerically calculated fluid distributions. The purpose of this paper is to propose how microseismic data may be used to modify the wellbore temperature models, and how stimulation fluid placement determined from the modified models may then be coupled with the microseismic to improve hydraulic fracture stimulation characterization.
Medina, R. (University of California) | Elkhoury, J.E. (University of California) | Detwiler, R.L. (University of California) | Morris, J.P. (Schlumberger-Doll Research Center) | Prioul, R. (Schlumberger-Doll Research Center) | Desroches, J. (Services Pétroliers Schlumberger)
We conducted experiments in which a high concentration (50% v/v) of granular solids suspended in a non-Newtonian carrier fluid (0.75% guar gum in water) flowed through a parallel-plate fracture. Digital imaging and particle-imagevelocimetry analysis provided a detailed map of velocities within the fracture. Results demonstrate development of a strongly heterogeneous velocity field within the fracture. We observed the highest velocities along the no-flow boundaries of the fracture and the lowest velocities along the centerline of the fracture. Computational fluid dynamics (CFD) simulations using a recently developed model of the rheology of dense suspensions of mono-disperse solids in Newtonian carrier fluids closely reproduced experimental observations of pressure gradient versus flow rate. Results from additional simulations suggest that small (3%) variations in solid volume fraction within the fracture could lead to significant (factor of two) velocity variations within the fracture with negligible changes in observed pressure gradients. The variations in solid volume fraction persist over the length of the fracture, suggesting that such heterogeneities may play a significant role in the transport of dense suspensions. Our work suggests that a simple average conductivity parameter does not adequately represent the flow of high solid content suspensions in a fracture, as the flow develops strong three-dimensional structure even in a uniform rectangular channel.