Numerical Simulation for Interaction between the Fracture Surface and the Proppant Column after HiWay Fracturing

HiWay fracturing uses impulse sand to achieve the proppant column supporting the rock wall surface, creating an open flow channel that is more conducive to improving the stimulation effect. The deformation law of the proppant column under the compression of the fracture wall surface is directly related to the effectiveness of the high conductivity fracture. However, the related laboratory physical simulation experiments are challenging to conduct, which restricts the study of the interaction between the fracture surface and the proppant column. In this paper, first, establish the finite element model of the interaction between the fracturing crack and the proppant column, and the variation law of stress-deformation of the crack wall and displacement of the crack wall under the closure stress is analyzed. Furthermore, this paper carries out the quantitative characterization of the active support area of the proppant column. Finally, the accuracy of the simulation results is verified by applying the experimental results of indoor proppant column rock plate compression. The numerical simulation method proposed in this paper can effectively realize the simulation of the interaction between the fracture surface and the proppant column in HiWay fracturing based on real engineering parameters, which lays theoretical and simulation experimental support for improving the effect of channel fracturing stimulation.

Growth model for large branched three-dimensional hydraulic crack system in gas or oil shale

Recent analysis of gas outflow histories at wellheads shows that the hydraulic crack spacing must be of the order of 0.1 m (rather than 1 m or 10 m). Consequently, the existing models, limited to one or several cracks, are unrealistic. The reality is 10 5 –10 6 almost vertical hydraulic cracks per fracking stage. Here, we study the growth of two intersecting near-orthogonal systems of parallel hydraulic cracks spaced at 0.1 m, preferably following pre-existing rock joints. One key idea is that, to model lateral cracks branching from a primary crack wall, crack pressurization, by viscous Poiseuille-type flow, of compressible (proppant-laden) frac water must be complemented with the pressurization of a sufficient volume of micropores and microcracks by Darcy-type water diffusion into the shale, to generate tension along existing crack walls, overcoming the strength limit of the cohesive-crack or crack-band model. A second key idea is that enforcing the equilibrium of stresses in cracks, pores and water, with the generation of tension in the solid phase, requires a new three-phase medium concept, which is transitional between Biot’s two-phase medium and Terzaghi’s effective stress and introduces the loading of the solid by pressure gradients of diffusing pore water. A computer program, combining finite elements for deformation and fracture with volume elements for water flow, is developed to validate the new model. This article is part of the themed issue ‘Energy and the subsurface’.

The Flow Resistance of Experimental Models of Naturally Occurring Cracks

Naturally occurring cracks have rough surfaces which mate in such a fashion as to close the crack completely when the surfaces are pressed together. Experimental work shows that friction factors are given by a Nikuradse type of equation when the crack surfaces are widely spaced. The equation remains applicable as the crack closes until roughness elements from opposing surfaces start to overlap and then an upper limit is achieved. Further reduction in the crack wall separation causes a reduction in the friction factor, which may fall to the level applicable to a smooth-walled tortuous channel. These observations are in accord with theoretical concepts.

On the Nature of the Occluded Cell in the Stress Corrosion Cracking of AA 7075-T651—Effect of Potential, Composition, Morphology

The SCC of AA 7075-T651 has been investigated in various electrolytes at different applied potentials. The rate of crack growth in the precracked, double cantilever beam, specimen was measured and related to the aluminum ion concentration and pH within the crack as well as the morphology of the crack wall surface as viewed with a scanning electron microscope. The rate of crack growth, the composition of the solution within the crack, and the morphology of the crack wall are determined mainly by the anion in solution and this in terms of the aluminum-anion complexes formed during the reaction. In sodium chloride solution, the pH at the crack tip was acidic at the open circuit and in the anodic potential range while, in the cathodic range, it was basic. At anodic potentials, the aluminum ion concentration reached 0.4M within the crack. In Na2SO4 solution, crack propagation was very slow at the open circuit and at anodic potentials even though the pH at the crack tip was acidic. However, when the potential was shifted into a cathodic range, the crack began to propagate at an appreciable rate. In NaNO3 solutions, crack propagation rate increased in the cathodic range due to the formation of ammonia within the crack. Very slow crack growth was observed in the anodic range. The analysis of the solution within a simulated crevice indicated that the composition of the solution in a simulated crevice and an actual stress corrosion crack were quite similar.