非常规油气藏体积改造技术核心理论与优化设计关键

The core theories and key optimization designs of volume stimulation technology for unconventional reservoirs

北美页岩气藏在储层渗透率低至纳达西的情况下仍能实现有效开发,其核心是增大储层改造体积,用技术体系来表征即为“体积改造技术”.“体积改造技术”强调“打碎”储层,使裂缝壁面与储层基质的接触面积最大,在三维方向实现对储层的“立体”改造.针对页岩和致密油气储层的不同特点,界定了“狭义”和“广义”体积改造技术的异同：“狭义”体积改造技术源于对象、技术和验证3个要素（页岩、“水平井钻井＋水平井分段压裂”、微地震裂缝诊断）;“广义”体积改造技术是针对致密油气储层提出的水平井多段和直井多层压裂技术方法.两种技术针对的储层对象有所不同,但最终目标是一致的.体积改造技术的核心理论为：①“打碎”储层,形成复杂缝网,“人造”渗透率;②基质中的流体沿裂缝“最短距离”渗流;③大幅度降低基质中油气流动所需驱动压差.进一步提出了满足体积改造技术理论的核心条件为：储层具有明显脆性,天然裂缝与层理发育,最大最小应力差较小.其中脆性指数是岩石发生破裂前的瞬态变化快慢（难易）程度的表征,而体积改造技术优化设计的关键是“逆向设计”方法,以及分簇射孔模式、最优孔数及裂缝间距优化.现场实际研究表明：分簇射孔确保各簇有效开启的最优孔数为40～50个,并可获得最优孔数与排量的关系,以及最优缝间距越小越易实现裂缝转向;同时还给出了孔眼优化、实现应力干扰的最佳裂缝间距、细分切割基质的理论模型与计算结果.体积改造技术对提高非常规油气藏的改造效果有着重要的指导作用.

The key for effective development of shale gas reservoirs with nano-darey permeability in North America is maximizing stimulated reservoir volume, which in terms of technology system, is called ＂volume stimulation technology＂. It emphasizes ＂breaking up＂ reservoir to get the largest contact area between fracture surface and reservoir matrix, further achieving three di mensions stimulation of the reservoir. In this study, volume stimulation technology is defined in ＂narrow＂ and ＂broad＂ sense ac- cording to different characteristics of shale and tight gas reservoirs. The ＂narrow-sense＂ definition is derived from three elements including object（shale） , technology（horizontal well drilling ＆ fracturing） , and verification（ micro seismic fracture diagnosis）. The ＂broad-sense＂ definition refers to multi-interval horizontal well and multi layer vertical well fracturing techniques for tight oil and gas reservoirs. Despite their different objects of reservoir, two techniques have the same ultimate goal. The core theory o[ vol ume stimulation technique is proposed as follows： ＂breaking up＂ the reservoirs to form complex fracture networks for artificial permeation, enabling porous flow from the matrix to the fracture in the shortest distance, and maximally reducing the driving pressure of oil and gas flow in the matrix. Further, the core reservoir conditions for volume stimulation are put forward, including obvious brittleness, development of natural fractures and bedding, and small difference between the maximum and minimum stresses. Herethe brittleness index measures the speed（or difficulty） of transient changes（i, e. , fracture formation and expanmon） prior to the breaking of the rocks. The key optimization design for volume stimulation includes ＇reversal design＇ , multi-cluster perforation, and optimization of hole number and fracture spacing. Field test demonstrates that multi-cluster perforation ensures the effective opening of each cluster with an optimal hole number of 40 to 50. The relationship between optimal hole number and output volume can be ob- tained, and the smaller fracture spacing, the easier fracture reorientation. Additionally, theoretical models and calculation results of perforation optimization, optimal fracture spacing for achieving stress interference, and matrix segmentation are presented. The a- bove theory and optimization design of volume stimulation technology have great guidance value for improving the post-fracturing performance of unconventional reservoirs.