石英砂用于页岩气储层压裂的经济适应性

摘要

四川盆地长宁—威远地区页岩气储层最小主应力介于44～68 MPa, 一直使用可在高闭合压力下保持高导流能力的40～70目陶粒作为主要的支撑剂, 但用量大、成本高.为了进一步降低支撑剂的成本, 在采用气藏数值模拟方法论证储层所需的支撑裂缝导流能力的基础上, 利用页岩气井生产分析结果和人工裂缝模拟结果研究储层作用在支撑剂上的有效应力、有效应力的加载速度和支撑剂的铺置浓度, 提出了适合该区页岩气井压裂支撑剂导流的实验方法, 评价了石英砂的导流能力及其对页岩气产能的影响, 并利用该方法进行了支撑剂的筛选和现场试验.结果表明: (1) 页岩基质渗透率小于6.0×10-4 m D时, 主裂缝导流能力介于0.8～1.0 D·cm、分支裂缝导流能力介于0.05～0.10 D·cm即可满足生产需求; (2) 当主裂缝垂直于最小主应力方向、分支裂缝垂直于主裂缝时, 该地区页岩储层作用在主裂缝内支撑剂上的有效应力最大值为54 MPa, 作用在分支裂缝内支撑剂上的最大有效应力约为69 MPa; (3) 对标准支撑剂导流能力评价实验方法进行了修改——应力加载速度为1.0 MPa/min, 支撑剂铺置浓度为2.5 kg/m2, 最高加载压力设定为70MPa; (4) 优选70/140目石英砂能够满足该区页岩气井压裂需求.在该区2个平台4口井的应用效果表明, 将石英砂比例从30％提高到70％～80％, 单段产气量无明显变化, 单井可以节约支撑剂成本60万元～100万元, 如果石英砂本地化, 则成本可进一步降低.结论认为, 该项成果为在基质渗透率极低的致密油气储层中采用石英砂替代陶粒以降低成本提供了技术支撑.%In the Changning–Weiyuan area of the Sichuan Basin, 40～70 mesh ceramsite that can keep high flow conductivity at high closure pressure is always used as the main proppant in the fracturing of shale gas reservoirs the minimum principal stress of which ranges between 44 and 68 MPa, but its consumption and cost are high. To reduce the cost of proppant, the flow conductivity required to support the fractures in the reservoirs was demonstrated by means of numerical reservoir simulation. Then, the effective stress of reservoir on the proppant, the loading rate of effective stress and the proppant placement concentration were analyzed by using the analysis results of shale gas well production and the simulation results of hydraulic fractures. Finally, the experimental method suitable for the flow conductivity of proppant used in the shale gas well fracturing in the study area was developed and the flow conductivity of quartz sand and its effect on the productivity were evaluated. Furthermore, this method was used for the screening and field test of proppant. The following results were obtained. First, when the matrix permeability of shale is lower than 6.0×10-4 m D, the production requirement can be satisfied as long as the flow capacity of main fractures and branch fractures are 0.8-1 D·cm and 0.05-0.10 D·cm, respectively. Second, when main fractures are vertical to the minimum principal stress and branch fractures are vertical to main fractures, the maximum effective stress of shale reservoirs on the proppant in the main fractures is 54 MPa and that in the branch fractures is about 69 MPa. Third, the experimental method for evaluating standard proppant flow conductivity is modified by setting the stress loading rate at 1.0 MPa/min, the proppant placement concentration at 2.5 kg/m2 and the maximum loading pressure at 70 MPa. And fourth, the 70/140 mesh quartz sands are selected as the optimum proppant for shale gas well fracturing in the study area. The application in 4 wells on two pads in shows that single-section gas production rate is not changed obviously and single-well proppant cost is reduced by CNY 0.6–1 million when the ratio of quartz sand is increased from 30％ to 7080％. And the cost may be cut further if quartz sand is acquired from local markets. In conclusion, the study results provide a technical support for cost reduction by replacing ceramsite with quartz sand in the tight oil/gas reservoirs the matrix permeability of which is extremely low.