Dynamic Temporary Plugging Mechanism Of Multi-cluster Fracturing In Horizontal Wells

Staged and muilti-cluster fracturing of horizontal well is the main fracturing technology for unconventional oil and gas reservoir development.Among them,temporary plugging and diverting fracturing technology is an important technical means to achieve cost reduction and quality improvement and efficiency.Its technical idea is to seal the opened fractures or perforations by diverters,and then,force the understimulated or unstimulated area to creat a new fracture.Although this technology has been widely used at home and abroad,some field monitoring results show that temporary plugging fails to effectively improve the non-uniform expansion of multiple fractures.In particular,for horizontal wells with multiple clusters fracturing,the plugging position and controlling mechanism of diverter are still unclear,which limits the further development of temporary plugging fracturing technology.To this end,this paper investigates the transport law of diverter at the perforation and the sealing law with the rough fractures by conbining indoor experiments and numerical simulations,and the main results are as follows:(1)To address the problem of diverter transport at the perforation,two sets of diverter transport experimental systems are established in this paper,one of which has the characteristics of large size,high pumping ability and high pressure resistance,and can be used to analyze the transport law of individual diverter under complex flow conditions;the other experimental device has the characteristics of visualization,and can be used to visually observe the transport behavior of diverter in the horizontal wellbore,and thus study the transport efficiency of diverter at the perforation.The effects of injection rate,perforation flow ratio,perforation angle,diverter size and gravity on diverter transport were systematically analyzed,and the field data were verified according to the experimental results.The results show that the perforation flow ratio needs to exceed the critical to ensure the diverter enter the perforation,and the transport efficiency is linearly related to the perforation flow ratio.The critical perforation flow ratio decreases and then increases with the increase of the injection rate,and at the same time,the diverter transport efficiency increases and then decreases with the increase of injection rate.Increasing the viscosity of the carrier fluid and decreasing the perforation angle can both increase the diverter transport efficiency.(2)A numerical model of diverter transport was established based on the coupled computational fluid dynamics–discrete elment method(CFD-DEM)to reveal the effect of interference between perforations on diverter transport during multi-cluster fracturing of horizontal wells,and a dynamic control method of diverter transport by fluid viscosity and pumping rate was developed.The results show that the downward spiral perforating method is more favorable than the upward spiral perforating method for the diverter to enter the perforation because of the perforation interference,while the effect of fixed-plane perforating is between the two.The diverter is preferred to enter the toe-side perforation cluster under the high pumping rate(4 m~3/min),while the opposite is true under the low pumping rate(2 m~3/min).In addition,increasing the viscosity of the carrier fluid can promote the plugging efficiency of heel-side perforation clusters,and the dynamic temporary plugging can be realized by alternately injecting slug of variable viscosity.(3)For temporary plugging of the rough fracture,a 3D printed rough fracture model is established,the characterization parameters of the fracture surface morphology are proposed,and the permeability model of the particle and fiber mixture is established based on the particle accumulation theory,and the permeability change of the plugging zone is inverted by the pressure curve,and the fitting relationship between the length of the temporary plugging zone and the plugging pressure was obtained by combining theoretical derivation with the experimental data.The results show that the plugging pressure curve has a two-stage characteristic and the pressure inflection point is about0.4 MPa.There is a characteristic plugging position in the rough fracture,which often overlaps with the narrow area in the fracture.There is a strong correlation between the newly proposed fracture surface morphology characterization parameter and the plugging efficiency.Temporary plugging is a process in which the plugging zone is extended and the permeability is reduced.Based on the target plugging pressure,the required length of temporary plugging zone and the amount of diverter can be predicted.The research results of this paper are of great theoretical significance for the dynamic control of the temporary plugging process and the design of diverter dosage in the multi-culster fracturing of horizontal wells.