基于焖井压降曲线的页岩油井压裂缝网特征诊断

Diagnosis of fractured network characteristics of shale oil wells based on the pressure drop curve of post-fracturing shut-in

页岩油压裂井普遍焖井一段时间再投产,焖井期间井口压力呈现不同的递减特征,焖井压降特征与压裂缝网性质存在何种关联缺乏快速有效的诊断分析方法。为此,建立了页岩油压裂水平井焖井压降数值模型,模拟获得的焖井压降导数曲线在双对数坐标下呈现出“W”形的形态特征,可以反映出主、次裂缝的规模、储集能力和导流能力。敏感性模拟结果表明,主裂缝的长度和导流能力对压降导数形态影响微弱,仅影响“W”形曲线的开始时间;次级裂缝的密度、宽度和渗透性对压降导数形态影响较大,决定了两个“V”形的深浅程度、比例和持续时间。储集能力占主导的缝网压降导数曲线呈现“‘W形’凹凸浅、位置偏右上”的特征;导流能力占主导的缝网压降导数曲线呈现“‘W’形凹凸深、位置偏左下”的特征。将模拟结果绘制成压裂缝网诊断图版,并建立基于焖井压降特征曲线的页岩油井压裂缝网诊断分析方法。最后,选取新疆吉木萨尔页岩油12口典型压裂井的焖井压降数据开展单井—平台井、层间—井间压裂效果对比分析,诊断出各井主、次裂缝规模与性质的参数组合,拟合反演出关键缝网参数,从而为综合评价页岩油水平井压裂改造效果提供了理论依据。

It is common to shut in shale oil fractured wells for a period of time before production,during which the wellhead pressure exhibits different declining characteristics;moreover,there is a lack of fast and effective diagnostic and analytic methods for the correlation between the pressure drop characteristics of shut-in wells and the properties of fracture networks.For this reason,a set of shut-in pressure drop models is firstly proposed for shale oil fractured horizontal wells,and the shut-in pressure drop derivative curves obtained from the simulation show the morphological characteristics of W-shape in double logarithmic coordinates,reflecting the scale,storage capacity and flow conductivity of the primary and secondary fractures.The sensitivity simulation results show that the length and flow conductivity of the primary fractures have a weak influence on the pattern of pressure drop derivatives,and only affect the start time of the W-shaped curve;by contrast,the density,width and permeability of the secondary fractures have a significant impact on the pattern of pressure drop derivatives,and determine the depth,proportion,and duration of the two V-shaped curves.On the whole,the pressure drop derivative curve of the fracture network dominated by storage capacity is characterized by“W-shape with shallow concave and convex on the upper right side”,while the pressure drop derivative curve of the fracture network dominated by flow conductivity is characterized by“W-shape with deep concave and convex on the lower left”.In addition,a fracture network diagnosis chart was plotted according to the simulation results,and a diagnostic analysis method for fractured networks of shale oil wells was set up based on the characteristic curve of shut-in pressure drop.Finally,based on the shut-in pressure drop data of 12 typical fractured wells in Jimsar shale oilfield in Xinjiang,we conducted comparative analysis of the fracturing effect of single/platform wells,and interlayers/interwells,found the parameter combination of scale and properties of primary and secondary fractures of the wells,and completed the fitting and inversion of the key fracture network parameters,thus providing a theoretical basis for the comprehensive evaluation of the fracturing effect of horizontal wells in shale oilfield.