Lu Yunhu

Study on the nonlinear coupling seepage model of shale reservoir with discrete networks

Gas shale is rich in natural fractures, in which the percolation mechanisms are complicated. Consequently, it’s the key scientific problem of accurately evaluating shale gas production to figure out the law of flow in fractured shale reservoirs. In this paper, a multi-continuum/discrete fracture model was built to describe the shale reservoir space after hydraulic fracturing, and a nonlinear coupled seepage flow mathematical model is established based on the adsorbed gas and free gas coupling, including the nonlinear non-equilibrium desorption and surface diffusion of adsorbed gas, viscous flow and Knudsen diffusion mechanism of free gas. The model believes that the surface diffusion of adsorbed gas occurs on the surface of the organic matter, and non-equilibrium desorption occurs between the adsorbed gas and the pores of the inorganic matrix. There are viscous flow and Knudsen diffusion in the pores of the inorganic matrix. But there is only viscous flow in natural and hydraulic fractures. Through the numerical simulation results of a fractured horizontal well, the time-space evolution process of the net desorption rate of the adsorbed gas in the hydraulic fractured shale reservoir was revealed. It was found that the region of the maximum net desorption rate gradually spread from the artificial network to the matrix. Through the analysis of multiple pressure systems, it was found that the diffusion of free gas in natural fractures, free gas in inorganic matrix pores and adsorbed gas in organic matter lagged successively. Through the flow analysis of natural fracture field, it is found that the flow patterns, outside the envelope of discrete fracture network, appear in succession with time: elliptical radial flow, linear flow and non-linear flow caused by primary fracture interference. This study reveals the percolation mechanism of fractured shale reservoirs and provides a scientific basis for the exploitation of the shale gas reservoir.

Experimental study on hydration damage of anisotropic shale under high temperature

The Longmaxi shale samples are used to investigate the anisotropic hydration damage characteristics of shale under high temperature conditions. The FEI Qemscan 650F energy dispersive spectroscopy (EDS)-field emission scanning electron microscope (FESEM) is used to help observe the evolution characteristics of shale clay mineral and micro-structure under different temperature, pressure and soaking time conditions from beddings vertical and parallel directions. It comes to the conclusion that the hydration characteristic induced by microscale anisotropy of Longmaxi shale varies a lot. The damage behavior in the bedding-parallel plane is characterized by dilatation-closure-collapse process of primary fractures which exists in a large amount of nano-scale and micro-scale corrosion pores. While the damage behavior in the bedding-vertical plane is mainly represented as a mass of nano-scale and micro-scale corrosion pores. It’s also found that the shale hydration damage is faster under higher temperature condition when the soaking time and pressure are the same. Besides, under the same temperature and pressure conditions, the shale microstructure hydration process trends to increase at the beginning and be flat at last. The higher temperature shorten the time spending on shale hydration and its inner relationship is required more experiments.

The study on fracture network development in shale fracturing with considering capillary effect

Natural fractures widely exist in shale reservoirs with different scales, ranging from nano scale to micro scale, even to macro and engineering scale. These multi-scale natural fractures have a great influence on the fracture network propagation during a fracturing treatment. Meanwhile, the fracturing fluid flowing into these natural fractures will result in the co-existence of fracturing fluid and natural gas. As a result, the capillary force occurs at the gas-liquid interface in natural fractures. If the sizes of these natural fractures are different, the capillary force in the natural fractures is different as well. In particular, when the natural fracture is small, the capillary force will be very large and even affect the natural fracture initiation and propagation. As a consequence, the fracturing fluid flowing in natural fractures and the fracture network development will be significantly affected by these multiscale natural fractures. In this paper, a model about fracture network propagation was built which considered natural fractures sizes and fracturing fluid wettability. Besides, different methods were adopted to calculate the stress intensity of hydraulic fracture and natural fracture, respectively. In addition, the competitive growth theory of multi fractures is considered to describe the fracture network propagation. The studies found that with long length, small width natural fractures and strong wettability fracturing fluid, natural fractures were more likely to propagate, which resulted in a large fracture network. Besides, when the sizes of natural fractures were different, a more complex fracture network was tended to come out. An experimental study about fracture network propagation in shale was introduced from Suarez-Rivera’s work, in which a “fish bone” shape fracture propagation was observed. By comparing the numerical simulation with Suarez-Rivera’s work, it could be concluded that the simulation was in accordance with shale fracturing experiments results.

Shale gas development: Opportunities and challenges for rock mechanics

Multistage hydraulic stimulations on long-lateral horizontal wells are critical technologies for improving shale gas production in China. With deeper exploration and development in depth, however, more challenges spring up. For example, high risks in drilling and completion, low-efficient stimulation, and rapid production decline, etc. The topics are related with failure and seepage modeling in shale gas reservoirs, for which the nature of the problems is highly focused on rock mechanics. More breakthroughs are excepted in laboratory tests, fracture networks characterization and seepage modeling, multi-field coupling and poroelasticity theories. Although China has achieved initial victory in shale gas development, more challenges remains to be disposed by deepening theories of rock mechanics and industrial innovations. Meanwhile, clean and cost-effective recovery of shale gas is the key role throughout the life-cycle of development. In this paper, we review geomechanics assessments, fracture mechanics, seepage and fluid-solid coupling theories used in shale gas reservoirs. We cover the topics of laboratory tests, wellbore safety and effective development technologies, with discussing the research direction and hotspot in the future.