裂缝性储层渗透率返排恢复率的影响因素

Influential Factors on Permeability Recovery during Flowback of Fractured Reservoirs

在裂缝性低渗透储层钻井完井过程中,工作液滤液及固相侵入会对储层造成损害。采用屏蔽暂堵技术形成致密封堵层,可对储层进行有效保护,而渗透率返排恢复率是衡量屏蔽暂堵技术质量的重要指标。使用同一工作液并调整其粒度分布,分别对低渗砂岩裂缝岩样进行了封堵层形成与返排试验,探讨了工作液粒度分布及压力梯度对返排恢复率的影响。试验表明,随着压力梯度增大,返排恢复率呈先增大后减小的趋势。固相粒度与缝宽最优匹配原则不是固定不变的,而是受缝面微凸体高度与缝宽比值的影响。固相侵入浅,返排恢复率高;固相侵入深,返排恢复率低。对于水力学宽度为20～70μm的细砂岩裂缝岩样,最优返排压力梯度为7.8～24.2MPa/m,最优匹配原则为1/3～2/3架桥,且接近于2/3架桥。裂缝性储层返排过程中存在最优返排压力梯度,固相粒度与缝宽匹配程度直接影响固相侵入深度,进而影响渗透率的返排恢复程度。

During drilling and completion in fractured low-permeability reservoirs, filtration and particle invasion would cause damage to reservoir. Shielding temporary plugging is an effective way to protect reservoir by forming tight plugging zone, in which permeability recovery during flowback is an important index in plugging quality evaluation. The experiment of plugged zone buildup and flowback were conducted on fractured sand core samples using the same fluid but different particle size. The impact of particle size and pressure gradient on flowback recovery was studied. Results show that the flowback recovery increases at first then decreases with the pressure gradient increase. The matching between particle size and fracture width directly affects the invasion depth of particles. The recovery is high when solid particle invasion is shallow, while it is low when solid particle invasion is deep. For sand core samples with fracture width of 20 to 70 microns,the best pressure gradient is from 7.8 to 24. 2 MPa/m and the best matching ratio is from 1/3 to 2/3 and close to 2/3 bridge. Optimal pressure gradient exists in the process of flowback in fractured reservoirs. The matching between particle size and fracture width would affect the invasion depth of particles and, in turn, the permeability flowback recovery.