Influence of gas transport mechanisms on the productivity of multi-stage fractured horizontal wells in shale gas reservoirs

In order to investigate the influence on shale gas well productivity caused by gas transport in nanometer- size pores, a mathematical model of multi-stage fractured horizontal wells in shale gas reservoirs is built, which considers the influence of viscous flow, Knudsen diffusion, surface diffusion, and adsorption layer thickness. A dis- crete-fracture model is used to simplify the fracture mod- cling, and a finite element method is applied to solve the model. The numerical simulation results indicate that with a decrease in the intrinsic matrix permeability, Knudsen diffusion and surface diffusion contributions to production become large and cannot be ignored. The existence of an adsorption layer on the nanopore surfaces reduces the effective pore radius and the effective porosity, resulting in low production from fractured horizontal wells. With a decrease in the pore radius, considering the adsorption layer, the production reduction rate increases. When the pore radius is less than 10 nm, because of the combined impacts of Knudsen diffusion, surface diffusion, and adsorption layers, the production of multi-stage fractured horizontal wells increases with a decrease in the pore pressure. When the pore pressure is lower than 30 MPa, the rate of production increase becomes larger with a decrease in pore pressure.

Numerical analysis of rock fracturing by gas pressure using the extended finite element method

High energy gas fracturing is a simple approach of applying high pressure gas to stimulate wells by gen- erating several radial cracks without creating any other damages to the wells. In this paper, a numerical algorithm is proposed to quantitatively simulate propagation of these fractures around a pressurized hole as a quasi-static phenomenon. The gas flow through the cracks is assumed as a one-dimensional transient flow, governed by equations of conservation of mass and momentum. The fractured medium is modeled with the extended finite element method, and the stress intensity factor is calculated by the simple, though sufficiently accurate, displacement ex- trapolation method. To evaluate the proposed algorithm, two field tests are simulated and the unknown parameters are determined through calibration. Sensitivity analyses are performed on the main effective parameters. Considering that the level of uncertainty is very high in these types of engineering problems, the results show a good agreement with the experimental data. They are also consistent with the theory that the final crack length is mainly determined by the gas pressure rather than the initial crack length produced by the stress waves.

Study on "fracturing-sealing" integration technology based on high-energy gas fracturing in single seam with high gas and low air permeability

To improve the gas extraction efficiency of single seam with high gas and low air permeability,we developed the"fracturing-sealing"integration technology,and carried out the engineering experiment in the3305 Tunliu mine.In the experiment,coal seams can achieve the aim of antireflection effect through the following process:First,project main cracks with the high energy pulse jet.Second,break the coal body by delaying the propellant blasting.Next,destroy the dense structure of the hard coal body,and form loose slit rings around the holes.Finally,seal the boreholes with the"strong-weak-strong"pressurized sealing technology.The results are as follows:The average concentration of gas extraction increases from8.3%to 39.5%.The average discharge of gas extraction increases from 0.02 to 0.10 m^3/min.The tunneling speeds up from 49.5 to 130 m/month.And the permeability of coal seams improves nearly tenfold.Under the same conditions,the technology is much more efficient in depressurization and antireflection than common methods.In other words,it will provide a more effective way for the gas extraction of single seam with high gas and low air permeability.

A Numerical Gas Fracturing Model of Coupled Thermal,Flowing and Mechanical Effects

Gas fracturing,which overcomes the limitation of hydraulic fracturing,is a potential alternative technology for the development of unconventional gas and oil resources.However,the mechanical principle of gas fracturing has not been learned comprehensively when the fluid is injected into the borehole.In this paper,a damage-based model of coupled thermal-flowing-mechanical effects was adopted to illustrate the mechanical principle of gas fracturing.Numerical simulation tools Comsol Multiphysics and Matlab were integrated to simulate the coupled process during the gas fracturing.Besides,the damage evolution of drilling areas under several conditions was fully analyzed.Simulation results indicate that the maximum tensile stress,which occurs in the upper and lower of the injection hole,decreases with the increase of the tectonic stress coefficient(TSC).As the TSC increases,shear fractures increase,a crushed area is gradually formed and the seepage area increases rapidly.The influence of TSC on fracture expansion is concluded as follows:with the decrease of TSC,the relative width of fractures decreases whilst the depth increases.It indicates that thermal stress and pore pressure promote the expansion of tensile fractures but restrain the expansion of shear fractures.Therefore,a relatively lower injection gas pressure is required to obtain the same degree of fracturing with a coupled thermal gradient.

Fractal characteristics of surface crack evolution in the process of gas-containing coal extrusion

In this paper, simulated experiment device of coal and gas outburst was employed to perform the experiment on gas-containing coal extrusion. In the experiment, coal surface cracks were observed with a high-speed camera and then the images were processed by sketch. Based on the above description, the paper studied the fractal dimension values from different positions of coal surface as well as their changing laws with time. The results show that there is a growing parabola trend of crack dimension value in the process of coal extrusion. Accordingly, we drew the conclusion that extruded coal crack evolution is a process of fractal dimension value increase. On the basis of fractal dimension values taken from different parts of coal masses, a fractal dimension of the contour map was drawn. Thus, it is clear that the contour map involves different crack fractal dimension values from different positions. To be specific, where there are complicated force and violent movement in coal mass, there are higher fractal dimension values, i.e., the further the middle of observation surface is from the exit of coal mass, and the lower the fractal dimension value is. In line with fractal geometry and energy theory of coal and gas outburst, this study presents the relation between fractal dimension and energy in the process of extruding. In conclusion, the evolution of crack fractal dimension value can signify that of energy, which has laid a solid foundation for the quantification research on the mechanism of gas-containing coal extrusion.

Technical Breakthrough for Fracturing Gas Reservoir

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High Energy Gas Fracturing Succeeds in Changqing Oilfield

High energy gas fracturing provides a new way for economic and high efficient development of the low permeability reservoir.By the end of 1993,it is tested and used in more:than 50 oil production and water injection wells in Changqing Oilfield with treatment success rate of more than 92%,and treatment effective rate of 86%,with average increase of oil production 3.4 times and somewhat increase.of water iniectivity in the water wells,cumulative increment of oil produc-tion and water injection had reached 11660 tons and 39220 cubic meters respectively.The test was undertaken by 0il Drilling&Production Tech-nology Research Institute in collaboration with Oil Production Plant,both are subordinated to Changqing Petroleum Exploration Bureau.

WATER BREAKTHROUGH SIMULATION IN NATURALLY FRACTURED GAS RESERVOIRS WITH WATER DRIVE

In the fractured water drive reservoirs of China, because of the complex geological conditions, almost all the active water invasions appear to be water breakthrough along fractures, especially along macrofraetures. These seal the path of gas flow, thus the remaining gas in the pores mixes into water, and leads to gas-water interactive distribution in the fractured gas reservoir. These complicated fraetured systems usually generate some abnormal flowing phenomena such as the crestal well produces water while the downdip well in the same gas reservoir produces gas, or the same gas well produces water intermittently. It is very difficult to explain these phenomena using existing fracture models because of their simple handling macrofractures without considering nonlinear flowing in the macrofractures and the low permeability matrix. Therefore, a nonlinear combined-flowing multimedia simulation model was successfully developed in this paper by introducing the equations of macrofractures and considering nonlinear flow in the macrofractures and the matrix. This model was then applied to actual fractured bottom water gas fields. Sensitivity studies of gas produetion by water drainage in fractured gas reservoirs were completed and the effect of different water drainage intensity and ways on actual gas production using this model were calculated. This model has been extensively used to predict the production performance in various fractured gas fields and proven to be reliable.