自支撑条件页岩裂缝的压缩特性分布及各向异性演化

Compression characteristic distribution and anisotropic evolution of shale fracture with shale self-supporting particles

甲烷原位燃爆压裂技术是利用页岩原位解吸甲烷与氧气协同燃爆产生的高温、高压气体冲击压裂页岩储层,具有广泛的应用前景。燃爆过程中会在压裂裂缝周围产生页岩碎屑颗粒,对裂缝通道形成一定的自支撑作用。其中,页岩碎屑颗粒作用下页岩压裂裂缝的压缩闭合行为及裂缝开度演化是亟待研究的关键问题。以平行层理无支撑(PBU)、平行层理自支撑(PBS)、垂直层理无支撑(VBU)和垂直层理自支撑(VBS)4种试样为研究对象,利用非接触式全场应变测量装置探究了应力加载过程中自支撑颗粒和层理对页岩裂缝压缩变形行为及各向异性演化特征分布的影响。结果表明,所有试样沿着加载方向均可划分为4个特征区域:裂缝区、近基质区、远基质区和无影响区,各区域垂直位移沿加载方向均呈线性下降趋势,且对应下降速率依次减小,在无影响区接近0。自支撑颗粒改善了层理方向对裂缝压缩特征沿裂缝走向分布差异的影响。自支撑裂缝的开度可压缩分量、不可压缩分量及可压缩分量占比分别为无支撑裂缝的47~56倍、11倍和4~5倍。引入层理各向异性系数(I_(BA))定量化分析自支撑颗粒对页岩裂缝变形各向异性演化特征的影响。应力加载过程中,各试样的I_(BA)均依次经历了:I_(BA)下降区、I_(BA)陡增区、I_(BA)震荡区和I_(BA)平稳区。自支撑颗粒缩短了I_(BA)震荡区范围(1~5 MPa),提高了I_(BA)平稳值(5.6),消除了不同层理间的I_(BA)平稳值差异。

The in-situ methane deflagration fracturing technique utilizes the collaborative combustion and explosion of insitu desorbed methane and oxygen to produce high-temperature and high-pressure gases that effectively fracture shale reservoirs.During the fracturing process,some shale debris particles are generated around the fracture,providing a degree of self-support to the fracture channels.Among these,the understanding on the compressive closure behavior and fracture aperture evolution of shale fractures with shale debris particles is a critical research focus.This study focused on four types of specimens:Parallel Bedding Unsupported(PBU),Parallel Bedding Self-Supported(PBS),Vertical Bedding Unsupported(VBU),and Vertical Bedding Self-Supported(VBS).Employing a non-contact full-field strain measurement device,the authors investigated the influence of self-supporting particles and bedding on the distribution of compression behavior and anisotropic characteristics for shale fracture during stress loading.The results revealed that all specimens can be divided into four typical regions along the loading direction:the fracture zone,near-matrix zone,far-matrix zone,and no-impact zone.In each region,the vertical displacement exhibits a linear decreasing trend along the loading direction,with decreasing rates in sequential order and approaching zero in the no-impact zone.Self-supporting particles mitigate the influence of bedding orientation on the distribution of fracture compression characteristics along the fracture direction.The compressible and incompressible components of self-supported fractures are 47-56 times and 11 times that of unsupported fractures,respectively.Meanwhile,the proportion of compressible component of fracture aperture in self-supported fractures is 4-5 times greater than that in unsupported fractures.The Bedding Anisotropy Index(I_(BA)) was introduced to quantitatively analyze the impact of self-supporting particles on the anisotropic evolution characteristics of shale fractures.During the stress loading process,all specimens sequentially experienced four phases:the I_(BA) descent zone,I_(BA) steep increase zone,I_(BA) oscillation zone,and I_(BA) stabilization zone.Self-supporting particles reduced the extent of the I_(BA) oscillation zone to(1-5 MPa) and increased the I_(BA) stabilization value to 5.6,eliminating the differences in I_(BA) stabilization values among different bedding orientations.