The 23rd International Symposium on Recent Advances in Exploration Geophysics (RAEG 2019)

One of the most attractive topics in geothermal development is the realization of enhanced geothermal systems in ductile zones. Thermal energy extraction from hot dry rock may require hydraulic fracturing through rocks in ductile zones, where rocks are supposed to deform in a ductile way due to the high pressure and high temperature (HPHT) conditions. One of the key factors in its realization is, therefore, to understand mechanical behavior of ductile rocks and hydraulically created fractures under the HPHT conditions, which have not been well investigated in the past. Numerical simulations are widely accepted as the effective approach to understand the mechanism of hydraulic fracturing. Numerical studies taking cooling effects from injected fluid on hydraulic fracturing into consideration have been conducted. Though distinct element methods (DEM) are frequently used to understand brittle failure or ductile deformation mechanism of rock, hydraulic fracturing simulations including both ductile behaviors and cooling effects due to injected fluid in the HPHT environment has not been fully investigated yet. Since the mechanical response to the fluid injection shows drastic changes at the brittle-ductile transitional condition, incorporation of the transitional behavior of granite into DEM is an essential step. In this study, we demonstrated hydraulic fracturing simulations with degradation approaches for the bond properties in DEM, i.e. degradation model and bi-linear approximation model, to replicate semi-brittle or ductile behavior at the HPHT condition. The numerical simulation results showed that results from bi-linear approximation model, which is suitable for replicating ductile behavior of granite, were consistent with laboratory experiment results under the HPHT condition. We can observe the cooling of rock mass due to injected fluid, while a considerable interaction between solid and fluid cannot be observed due to the shortness of injection time.

Ground Penetrating Rader (GPR) is a nondestructive testing method to visualize shallow subsurface using reflection of down-going electromagnetic (EM) waves generated on the surface. Travel-time of reflection signals tells us the position and the depth of objects in the subsurface. Since we can survey wide area in an expeditious way, GPR has been widely used in many engineering fields. On the other hand, great demand for quantitative estimations, i.e. physical properties of buried targets, has been increasing in recent years. Since it is difficult for conventional GPR survey to estimate physical properties in the subsurface, a novel approach for investigating them has been waited for. In this study, we propose a methodology for estimating physical properties in the subsurface only using reflection of EM waves. Since the estimated values by our method show good agreement with the true values, the proposed method can tell us not only the position and depth of the targets but also physical properties of them.

Interstitial fluid flow in reservoir rocks for oil, gas, and unconventional resources has a great influence to the production efficiency. In the present study, we simulate fluid flow in reservoir rocks and analyze the relationship between the permeability and the grain size distribution. We used a smoothed particle hydrodynamics (SPH) method, which is one of the particles methods, to simulate fluid flow in reservoir rocks. It is known that the grain size distribution of reservoir rocks obeys the Weibull distribution in many cases. The probability density function of the Weibull distribution is determined by the shape parameter and the mean value. We modeled plural three-dimensional digital specimens of poroelastic medium composed of matrix and pore space. The grain size distribution of the specimens has the Weibull distribution with different shape parameters. We estimated the permeability of each digital specimen using fluid flow simulations. The numerical results showed that the permeability of reservoir rocks is affected by the shape parameter and the mean value, i.e., two major parameter of the Weibull distribution. From this result, it is suggested that the permeability of reservoir rocks can be estimated from the shape parameter and the mean grain diameter of the grain size distribution, and the accuracy of the simulation of flow in reservoir rocks could be enhanced in the future.

Hydraulic fracturing technique has been widely used in the development of unconventional oil reservoirs or of enhanced geothermal systems. To prevent the induced fractures from closing, supporting materials (proppants) are pumped into the induced hydraulic fractures. The ultimate goal of hydraulic fracturing is to keep high conductive flow paths from the surrounding formation to the wellbore. In the past decade, some new techniques have been proposed to improve the fracture conductivity (e.g. surface modification agent). Among these techniques, making open channels throughout the induced fracture is one of the most effective options due to its drastic enhancement of conductivity, thereby production. In this technique, fluid with and without proppant is alternately pumped into the well. This treatment creates discontinuous proppant pillars in a hydraulic fracture, and then, the fracture conductivity could be improved significantly. However, proppant slurry behavior inside the fracture still remains poorly understood. In the present study, we applied a smoothed particle hydrodynamics (SPH) method to the fluid-solid interaction analysis in order to investigate proppant behavior inside the fracture. Our final goal is to establish analysis method for slurry behavior in hydraulic fractures. As a preliminary step toward the final goal, we simulate the Couette flow between coaxial cylinders to investigate the accuracy of the coupled simulation with the SPH method. We evaluate the L2-norm error as a function of the number of particles along the diameter of the inner cylinder. As a result, about 15 and 20 particles are required to achieve less than 15 % and 10 % error, respectively. Based on this result, at least 20 particles along the diameter of proppant grains should be used. In the future study, many effects (viscosity of fluid, grain shape, fracture roughness) on the efficiency of creating open channels will be investigated by using the proposed method.