Yang Liu, Shi Fukun, Sun Xiaoming, Wang Shuo, Jiang Qingping, Lu Huanyu
State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China.
School of Mechanics and Civil Engineering, China University of Mining & Technology (Beijing), Beijing 100083, China.
ACS Omega. 2020 Oct 9;5(41):26441-26453. doi: 10.1021/acsomega.0c02909. eCollection 2020 Oct 20.
Field experience shows that extending shut-in periods are conducive to increasing tight oil production after fracturing operations. Understanding the regularity of pressure decay is helpful to establish an appropriate shut-in time. However, the characteristics and influencing factors of pressure decay are unclear. This paper studies the porosity, permeability, mineral composition, and pore structure of samples in six different blocks. The pressure decay regularity is tested according to an independently designed indoor shut-in experimental device, and the oil distribution of experimental samples is monitored using nuclear magnetic resonance technology. The results show that the fracturing fluid enters the matrix pores under the action of percolation to slowly drive out the oil, causing the well pressure to decay over time. There are three types of pressure decay characteristics: concave type, fluctuation type, and quadratic type. Compared with conventional sandstone, the pressure decay rate of tight reservoirs is slower, and the pressure decay characteristics are more complicated. Clay mineral-rich reservoirs will swell when exposed to water. As a result, the strength of the framework will be weakened and collapsed. What's more, it will cause blockage of the throat, blocking the flow of oil and the decay of pressure. In addition, the rate of pressure decay is also related to the volume of fracturing fluid, initial borehole pressure, and formation closure stress. At a certain proppant thickness (fracture width), the larger the fracturing fluid volume, the larger the fracture surface area and the faster the pressure decay rate; Moreover, the greater the initial shut-in pressure, the greater the pressure difference and the faster the decay rate; the formation closure stress causes the core porosity and the permeability to decrease, resulting in a decrease in the decay rate. The experimental results are of great significance for establishing a proper shut-in time and enhancing the oil recovery of tight reservoirs.
现场经验表明，延长关井时间有利于提高压裂作业后的致密油产量。了解压力衰减规律有助于确定合适的关井时间。然而，压力衰减的特征及影响因素尚不清楚。本文研究了六个不同区块样品的孔隙度、渗透率、矿物成分和孔隙结构。根据自主设计的室内关井实验装置测试压力衰减规律，并利用核磁共振技术监测实验样品的原油分布。结果表明，压裂液在渗流作用下进入基质孔隙，缓慢驱替出原油，导致井筒压力随时间衰减。压力衰减特征有三种类型：凹型、波动型和二次型。与常规砂岩相比，致密储层的压力衰减速率较慢，压力衰减特征更为复杂。富含黏土矿物的储层遇水会膨胀。结果，骨架强度会被削弱并坍塌。此外，它还会导致喉道堵塞，阻碍油流和压力衰减。另外，压力衰减速率还与压裂液体积、初始井筒压力和地层闭合应力有关。在一定支撑剂厚度（裂缝宽度）下，压裂液体积越大，裂缝表面积越大，压力衰减速率越快；而且，初始关井压力越大，压力差越大，衰减速率越快；地层闭合应力导致岩心孔隙度和渗透率降低，从而使衰减速率降低。实验结果对于确定合适的关井时间和提高致密储层的原油采收率具有重要意义。
