覆压对致密砂岩人工闭合裂缝渗透率的影响

EFFECT OF OVERBURDEN PRESSURE ON PERMEABILITY OF MAN-MADE SEALED FRACTURES IN TIGHT SANDSTONE

目前低渗透致密油气藏开发主要采用体积压裂技术,形成的大规模缝网中有相当部分次级裂缝未被填砂,在地层受力不均的情况下会发生闭合或者剪切滑移,对储层渗透率改善具有重大影响。采用实验方法研究致密砂岩岩心人工闭合裂缝渗透率随覆压增大的变化规律,结果表明:人工裂缝即使在完全闭合的情况下仍然可以比岩心基质渗透率平均增大950.4倍;裂缝错位后的渗透率也比闭合时的渗透率有所增加,其增大程度与错位程度和裂缝壁面的粗糙度有关,错位程度和粗糙度越大,渗透率增加幅度就越大。实验还表明,覆压增大对人工裂缝闭合和错开时的岩心渗透率影响均不大,在6%范围以内。

At present, a technology is mainly used to develop low-permeability and tight reservoirs. In many large-scale fracture networks generated after a fracturing, there are lots of secondary fractures which are not filled by sand. Moreover, some fractures are sealed, or even occur a shear slip under the condition of uneven formation forcing. These two aspects can improve reservoir permeability. In addition, we carry out an experimental study on some cores from tight sandstone. And for man-made sealed fractures, the permeability changes with an increase of overburden pressure. These change laws are studied in lab. Results show that(1)even if the man-made fractures are under a sealed situation, their permeability may enlarge by 950.4-time on average compared with core matrix permeability;(2)the permeability of misplaced fractures is much bigger than one under the sealed situation and(3) the degree of both enlargement and misplacement is relative to roughness of fracture surface, i.e., more misplacement degree and roughness is, more enlarging is the permeability. Meanwhile, these experiments illustrate that overburden-pressure enlargement has a little effect on closure of man-made fractures and core permeability under misplacement, within a range of 6%.