Pengcheng Fu

Surrogate-based optimization of hydraulic fracturing in pre-existing fracture networks

Hydraulic fracturing has been used widely to stimulate production of oil, natural gas, and geothermal energy in formations with low natural permeability. Numerical optimization of fracture stimulation often requires a large number of evaluations of objective functions and constraints from forward hydraulic fracturing models, which are computationally expensive and even prohibitive in some situations. Moreover, there are a variety of uncertainties associated with the pre-existing fracture distributions and rock mechanical properties, which affect the optimized decisions for hydraulic fracturing. In this study, a surrogate-based approach is developed for efficient optimization of hydraulic fracturing well design in the presence of natural-system uncertainties. The fractal dimension is derived from the simulated fracturing network as the objective for maximizing energy recovery sweep efficiency. The surrogate model, which is constructed using training data from high-fidelity fracturing models for mapping the relationship between uncertain input parameters and the fractal dimension, provides fast approximation of the objective functions and constraints. A suite of surrogate models constructed using different fitting methods is evaluated and validated for fast predictions. Global sensitivity analysis is conducted to gain insights into the impact of the input variables on the output of interest, and further used for parameter screening. The high efficiency of the surrogate-based approach is demonstrated for three optimization scenarios with different and uncertain ambient conditions. Our results suggest the critical importance of considering uncertain pre-existing fracture networks in optimization studies of hydraulic fracturing.

Thermal Drawdown-Induced Flow Channeling in Fractured Geothermal Reservoirs

We investigate the flow-channeling phenomenon caused by thermal drawdown in fractured geothermal reservoirs. A discrete fracture network-based, fully coupled thermal–hydrological–mechanical simulator is used to study the interactions between fluid flow, temperature change, and the associated rock deformation. The responses of a number of randomly generated 2D fracture networks that represent a variety of reservoir characteristics are simulated with various injection-production well distances. We find that flow channeling, namely flow concentration in cooled zones, is the inevitable fate of all the scenarios evaluated. We also identify a secondary geomechanical mechanism caused by the anisotropy in thermal stress that counteracts the primary mechanism of flow channeling. This new mechanism tends, to some extent, to result in a more diffuse flow distribution, although it is generally not strong enough to completely reverse flow channeling. We find that fracture intensity substantially affects the overall hydraulic impedance of the reservoir but increasing fracture intensity generally does not improve heat production performance. Increasing the injection-production well separation appears to be an effective means to prolong the production life of a reservoir.

Evolution of Various Fabric Tensors for Granular Media toward the Critical State

AbstractThe classical critical state theory of granular mechanics makes no reference to the anisotropy of the material and has thus raised questions over the uniqueness of the critical state when f...

GEOS Code Development Road Map - May, 2013

GEOS is a massively parallel computational framework designed to enable HPC-based simulations of subsurface reservoir stimulation activities with the goal of optimizing current operations and evaluating innovative stimulation methods. GEOS will enable coupling of different solvers associated with the various physical processes occurring during reservoir stimulation in unique and sophisticated ways, adapted to various geologic settings, materials and stimulation methods. The overall architecture of the framework includes consistent data structures and will allow incorporation of additional physical and materials models as demanded by future applications. Along with predicting the initiation, propagation and reactivation of fractures, GEOS will also generate a seismic source term that can be linked with seismic wave propagation codes to generate synthetic microseismicity at surface and downhole arrays. Similarly, the output from GEOS can be linked with existing fluid/thermal transport codes. GEOS can also be linked with existing, non-intrusive uncertainty quantification schemes to constrain uncertainty in its predictions and sensitivity to the various parameters describing the reservoir and stimulation operations. We anticipate that an implicit-explicit 3D version of GEOS, including a preliminary seismic source model, will be available for parametric testing and validation against experimental and field data by Oct. 1, 2013.

Global and Local Mechanical-Fabric Measurements in Granular Assemblies Using DEM

Classical soil mechanics treats a soil sample subjected to laboratory testing as a homogeneous soil element and uses the homogenized strains in constitutive modeling. This method does not appropriately consider the fact that strain localization (i.e. shear banding) is almost an inevitable phenomenon in soils. After shear banding takes place, the continued deformation of the sample localizes in one or few shear bands, whereas the remaining portions experience essentially rigid body motions along the shear bands. In this situation, the mechanical interpretation of homogenized (i.e. average) measurements of soil element behaviors becomes ambiguous, and the effort of constitutive modeling should focus on local measurements within the shear bands whenever applicable. The present paper describes a number of innovative local quantification techniques that we have recently developed or tailored on a numerical platform in order to facilitate characterizing soil behaviors within shear bands. With these new techniques, we compare average mechanical and fabric measurements homogenized over the entire sample, to those retrieved from the shear bands only in the context of numerical simulation with the discrete element method (DEM).