Experimental Research On Damage Characteristics Of Rocks After Perforation

At present,more and more attention has been paid to unconventional energy in the world,and the development of shale gas has become a hot spot in recent years.China's shale gas resources are abundant and the mining potential is huge.However,the low porosity and permeability of shale gas determine that its development must adopt hydraulic fracturing and other stimulation measures.Perforating completion is generally used in hydraulic fracturing wells.Perforation passes through casing,cement sheath and formation to form a channel connecting wellbore and production zone,which provides support for fracturing operation.Perforation not only penetrates the formation to form a channel,but also causes some damage to the rock around the channel,which has a non-negligible impact on the fracturing effect.In recent years,many scholars at home and abroad have studied the penetration depth of perforation and the compaction zone at the edge of the channel,but there are few studies on the damage characteristics and crack distribution around the rock channel.In order to deeply study the damage characteristics of surrounding rock caused by perforation,this paper designs a penetration experiment of cement sample to simulate the perforation operation of underground rock.In order to eliminate the boundary effect,the diameter of cement sample is expanded to 400 mm.Based on the similar fracture mechanics properties of PMMA and rock under dynamic load,the penetration experiment of PMMA plate is designed,which compares with cement sample.The combination of prefabricated cross section and longitudinal section is used to explore the damage characteristics of perforating operations to surrounding rocks.There are mainly five conclusions after the study.Firstly,the damage zone of cement samples with boundary constraints can be divided into compaction zone,severe tensile damage zone and tensile damage propagation zone from the beginning of the pore.Secondly,the compression-shear damage and tensile damage of the samples decrease with the increase of perforation depth.Thirdly,the prefabricated longitudinal section has a significant effect on the cracking of the surrounding rock of perforation,and the stress wave relieves pressure at the longitudinal section,so that the damage of the half section of the tunnel is greater than that of the non-channel section.Fourthly,the damage area at the bottom of the channel can be divided into compaction zone,compression-shear zone,transition zone and tensile zone from the beginning of the channel.Lastly,the damage characteristics of PMMA plate and cement sample are similar under the penetration stress field.The compression-shear zone and the number of radial tensile cracks of cement sample is larger,and the peak position of the maximum number of radial cracks is further from the center of the tunnel.