基于声学数据反演定量评价致密砂岩储层微裂隙应力敏感性新方法

A new method for quantitative evaluation of microcrack stress sensitivity of tight sandstone reservoir based on inversion of acoustic data

设计试验方法流程对塔中地区深层致密砂岩储层岩样进行不同围压条件下声学测试,提出基于声学数据反演定量研究致密砂岩储层微裂隙应力敏感性的评价方法。通过Biot相洽理论和DEM理论组合模型分别对岩石裂隙密度?和孔隙纵横比?进行反演,探讨了不同围压条件下岩石?、?的变化规律,并对微裂隙应力敏感性进行评价。结果表明:?随围压的增加而降低,?随围压的增加而增加,这主要是由于微裂隙的闭合引起的;根据不同围压条件下?值降低幅度可以确定发生闭合的裂隙或依然保持张开的裂隙所占的百分比;在较高围压条件下依然具有较大?值的样品应力敏感性较弱,反之,应力敏感性较强;定义试验中岩石内部孔裂隙性质发生急剧变化的点为转折点,发现该点对应的?值相差不大,该值可视为该致密砂岩储层的裂隙共性特征,并可作为裂隙相对发育程度的对比指标;分别探讨了?值(围压65 MPa)及所定义转折压力与岩石孔隙度间的相关性,结果表明,两者正相关性均非常好,对于该地区深层低渗致密砂岩储层而言,存在低应力敏感性的有利类型裂隙受岩石物理性质影响程度最大。

Acoustics tests are conducted on deep tight sandstone reservoir samples in Tazhong area. A new method is developed for quantitatively evaluating the microcrack stress sensitivity of tight sandstone reservoir using inversion of acoustic data. The fracture density ？ and the aspect ratio ？ are inversed by using the Biot consistent model and the DEM model, and the variations of ？ and ？ are discussed under different confining pressures. The stress sensitivity of microfractures are also evaluated. The results show that, with the increase of confining pressure, ？ decreases and ？ increases, mainly due to the closing of the microfracture. According to the reducing amplitude of ？ under different confining pressures, the percentage of fractures that close or open can be determined. Samples with a larger ？ under high confining pressure represent low stress sensitivity. Otherwise, the stress sensitivity is higher. This paper defines the point, where the pore-fracture in the rock changes greatly, as the turning point. The values of ？ corresponding to the turning points of different samples are similar. This phenomenon can be seen as the common characteristics of tight sandstone reservoir fractures, and it can also act as the contrast indicator of the fracture development degree. The correlations of ？（with a confining pressure of 65 MPa） and turning pressure with rock porosity are discussed, which shows that they both have a very good positive correlation. For the deep low permeability sandstone reservoir in this region, the favorable fracture that has weak stress sensitivity is influenced mostly by physical properties of rock.