Influence Mechanism Of Proppant Porous Media Conductivity In Hydraulic Fractures

The conductivity of proppant-filled fractures is one of the key factors affecting the productivity of CBM wells.Studying the influence mechanism of the conductivity of proppant porous media in hydraulic fractures can better understand the conductivity evolution of proppant-filled fractures,and then better predict the production of coalbed methane.In this paper,theoretical analysis,physical experiment,mathematical modeling,program development and numerical calculation were used to study the influence mechanism of proppant porous media conductivity in hydraulic fractures.The main results were as follows:The void ratios of the porous media formed by proppant particles under different compaction states were different,and the conductivity of the supporting fracture porous media was related to the void ratio of the fracture.The void ratio was related to the movement of particles,the contact relationship between particles and the degree of connectivity between the voids.Affected by the closure stress from the upper part of the propped fracture,the proppant particles moved in the fracture,and the proppant particles and particles squeezed each other,the contact force between the proppant particles generally increased,the voids between the proppant particles were closed/opened,and the coordination number of the particles generally increased,the skeleton structure was compacted,and the void ratio of the particles decreased.The experimental system for simulating fluid flow in fractures was used to carry out experimental research on the change of fracture conductivity under different fracture widths and different proppant particle sizes.Experimental results of proppant porous media conductivity in hydraulic fractures showed that: when the initial fracture width was the same,with the closure stress increasing,the fracture conductivity first increased and then continued to decrease.This was because that the porous media structure formed by proppant particles was different due to being affected by the closure stress,and the conductivity of supporting fractures also changed.The better the porous media structure of supporting fractures,the easier the fluid can pass through,and the better the fracture conductivity.In addition,under the same initial fracture width and closure stress conditions,the larger the proppant particle size,the better the conductivity of the fracture;under the same closure stress and proppant particle size,the larger the initial fracture width,the better the conductivity of the fracture.Under the same closure stress condition,the force was transferred from the top layer(stress transfer zone)to the middle layer,and some top layer particles entered the middle layer.The middle layer(stress bearing zone)formed a strong force bearing structure,the proppant particles were denser,the coordination number increased,and the void ratio in the middle layer was the smallest.The proppant particles in the stress-bearing zone had a weaker ability to transmit force to the bottom of the fracture.This led to a sudden decrease in the stress that the proppant in the bottom layer(stress attenuation zone)bore,the force chain contacts between proppant particles was relatively weak and the void ratio was the largest.Under the same closure stress conditions,the total compressive force of the top and bottom proppant particles was 0.607 to 0.695 times that of the middle layer,the coordination number was 0.760 to 0.905 times that of the middle layer,and the void ratio was 2.93 times to 4.61 times that of the middle layer.In addition,with the increase of the closure stress,the total compressive force and coordination number of the three-layer proppant particles continued to increase,while the void ratio gradually decreased.The fluid pressure value showed a linear decreasing trend from the fluid inlet to the outlet,and the fluid pressure value changed very little along the direction of the gravity.This was because the fluid was mainly driven by the pressure difference between the inlet and the outlet to move along the direction of differential pressure drive in the proppant porous medium,and then flowed along the gravity direction in the proppant porous medium under the influence of gravity.Therefore,the fractional velocity of the fluid along the direction of differential pressure drive was generally greater than the fractional velocity along the gravity direction.At the same inlet flow rate,affected by the structure of the proppant porous media channel,the larger the average equivalent pore size of the proppant porous media channel cross-sectional area under the closure stress,the larger the fluid pressure in the proppant porous media,the smaller the fluid flow rate,the longer the fluid streamline time,the proppant porous media channel was the optimal porous media channel,which can hold more fluids,and the proppant porous media had the best flow conductivity,and the fluid was easier to discharge under this condition.