深层致密储层大斜度井压裂裂缝扩展机制研究

Experimental study on fracture propagation morphology of deviated well in tight reservoir

冀东摊海复杂断块油藏致密砂岩储层高效的开采方式为大斜度丛式井体积压裂。斜井射孔水力裂缝起裂后沿水平最大主应力方向和垂向双向扭曲导致裂缝形态复杂,扭曲交界面最易形成砂堵点,因此明确斜井裂缝扩展规律对解决砂堵难题具有重要意义。基于室内真三轴水力压裂物理模拟试验(300 mm^(3)),探究水平应力差、井斜角、方位角和射孔相位角对斜井压裂裂缝起裂特征和扩展规律的影响。试验结果表明:(1)斜井压裂后裂缝面形态分为单一裂缝面、双翼平行裂缝面、多次级缝裂缝面和复杂裂缝面。(2)低水平应力差异系数下,孔眼起裂数量多;高水平应力差异系数下,与水平最大主应力方向夹角较小的孔眼起裂。(3)20°或者80°井斜角下射孔孔眼起裂点少,40°或者60°井斜角下射孔孔眼起裂点多;裂缝扭曲程度随井斜角增大呈先增后减趋势。(4)方位角大于80°时,裂缝起裂压力增大,裂缝形态以多次级缝裂缝面和复杂裂缝面为主。(5)射孔相位角增大,起裂难度增加,孔眼间距增加导致连通作用变弱,易产生次级扩展缝。建议水平应力差大的地层井眼轨迹设计时应该采用大井斜角、小相位角和小方位角施工,可以有效降低裂缝起裂压力和扭曲程度,解决现场施工砂堵难题。

The efficient production method of tight sandstone reservoir in Tanhai block of Jidong oilfield is volume fracturing of deviated wells.The bidirectional deformation along the horizontal maximum principal stress direction and the vertical stress direction leads to the complex fracture morphology.The distortion interface is easy to form sand blocking phenomenon.Therefore,it is of great significance to clarify the law of fracture propagation in deviated wells.Based on the large-scale true triaxial hydraulic fracturing physical simulation experiment,this paper studies the influence law of horizontal stress difference,well inclination angle,azimuth angle and phase angle on the fracture initiation characteristics and propagation morphology of deviated well.The results showed that:(1)the fracture morphology of deviated well after fracturing can be divided into single fracture surface,double wing parallel fracture surface,multiple fracture surface and complex fracture surface.(2)Under the low horizontal stress difference coefficient,the number of perforation hole initiation is more.On the contrary,only the perforation hole with a small angle to the direction of the horizontal maximum principal stress starts to crack.(3)There are fewer fracture initiation points in 20°or 80°wells,while there are more fracture initiation points in 40°or 60°wells.The degree of fracture distortion increases first and then decreases with the increase of inclination angle.(4)When the azimuth angle is greater than 80°,the fracture initiation pressure increases,and the fracture morphology is mainly multiple fracture surface and complex fracture surface.(5)The increase of perforation phase angle leads to the increase of fracture initiation pressure.With the increase of hole spacing,the communication between holes becomes weaker,leading to the generation of secondary spreading fractures.It is suggested that the well trajectory design of formation with large horizontal stress difference should adopt large well inclination angle,small phase angle and small azimuth angle.It can effectively reduce the fracture initiation pressure and distortion degree,so as to solve the problem of sand blocking in site construction.