燃爆压裂井井周诱导应力分布规律

Distribution of induced stress by high energy gas fracturing in wellbore

燃爆诱导压裂可在井周形成多条受应力差异控制小的径向裂缝,借助Sneddon线弹性诱导应力模型,结合带压井筒周围径向应力、周向应力模型,通过坐标转化和应力叠加,推导建立预存任意角度多条燃爆诱导裂缝的井周水平地应力分布计算模型,据此分析存在单条或任意多条组合裂缝对水平主应力差的影响,定量评价裂缝长度、裂缝条数及其相位组合、裂缝内净压力、储层原始主应力差异系数对水平主应力分布的影响敏感性。结果表明:由于预存燃爆裂缝的诱导作用,井周水平主应力存在差异反转区、差异减小区和差异增益区3个区域;应力差反转区面积随裂缝长度的增加、裂缝内压力增高、原始水平主应力差异系数的减小而逐渐增大;低角度相位裂缝的诱导应力差异反转发生在裂缝尖端两侧,而高角度相位裂缝应力反转发生在沿裂缝两侧区域,且高角度诱导裂缝的应力干扰强于低角度诱导裂缝,同时多条不同相位裂缝存在应力干扰;综合分析存在平行于最大主应力和最小主应力两条诱导裂缝时对应力差异的平衡作用最好。

High energy gas fracturing（HEGF） can generate several radial fractures with random azimuths,and the principle horizontal in-situ stress distribution will be disturbed by the pre-fracturing,which will promote hydraulic fracturing to activate more natural fissures with various angles.In this study,a new horizontal stress perturbation model with random azimuths of pre-fractures was proposed based on the Sneddon elastic model of induced stress and combined with radial and circumferential stress distribution models in high pressure wellbore.The induced stress distribution with single or multiple fractures during HEGF was analyzed,and the effects of different parameters were investigated,including fracture length,fracture number,fracture azimuths,fracture net pressure and the discrepancy of primitive in-situ stress.The results indicate that,due to the HEGF induced pre-fractures,the discrepancy of the principle stress can occur in three significant areas,including the increasing area,the mitigating area and inversing area.The inversing area will expand with the fracture length prolonging,the fracture net pressure increasing,and the primitive in-situ stress difference decreasing.The stress inversing area will appear in both sides out of the fractures tips with lower angle azimuths,and can occur in both sides along the fractures with higher angle azimuths,which also demonstrate that the jamming capability of the fractures with high angle azimuths is superior to the lower angle azimuths.The multiple fractures will disturb with each other,and the optimal combination is to overlay two pairs of pre-fractures parallel to the minimum and maximum primitive principle in-situ stress.