冷冲击作用下干热岩井井筒裂纹扩展数值模拟研究

Numerical simulation of wellbore fracture extension in hot dry rock wells under cold shock effect

摘要:

干热岩（HDR）是一种清洁可再生能源，主要通过增强型地热系统（EGS）进行开发。在EGS工程中，无论是注入井还是生产井的建设都需要采用地热钻井技术，而高温高压钻井过程中地层破裂、井壁坍塌等现象是干热岩钻井施工面临的重要问题。在温差作用下，岩石的矿物颗粒由于其热膨胀、冷收缩特性的差异性，矿物颗粒之间产生温度应力，导致岩体发生热破裂。本文借助RFPA数值模拟软件，对干热岩开采过程中井筒花岗岩冷冲击作用下的裂纹扩展进行研究。结果表明：井筒模型在冷冲击过程中，岩石表面拉应力随着冷冲击时间的增加，先升高至峰值后缓慢下降。裂纹扩展可以大致分为前、中、后期。冷冲击前期，井筒周围出现环形拉应力区，开始出现均匀的微小裂纹。冷冲击中期，随着时间的增加，拉应力区逐渐向井筒外围扩展，裂纹随着拉应力区向外扩展。冷冲击后期，拉应力大小逐渐降低直至小于模型抗拉强度，裂纹扩展速度减缓直至停止扩展。围压、井径、温度对井筒围岩冷冲击时的破坏损伤效果影响显著，其中，温度对冷冲击裂纹扩展起到促进作用，围压对冷冲击裂纹扩展起到抑制作用，井径对冷冲击裂纹扩展起到促进作用。

关键词:干热岩;井筒;裂缝扩展;数值模拟;冷冲击;增强型地热系统

Abstract:

Hot dry rock （HDR） is a clean and renewable energy resource developed mainly through enhanced geothermal systems （EGS）. In EGS engineering， geothermal drilling technology is required for the construction of both injection wells and production wells， and the formation fracture and wellbore collapse during high temperature and high pressure drilling are important problems in hot dry rock drilling construction. Under the action of temperature difference， the temperature stress is generated between mineral particles due to the difference of thermal expansion and cold shrinkage characteristics of mineral particles of rock， which results in thermal fracture of rock mass. In this paper， with the help of RFPA numerical simulation software， the fracture propagation of granite in wellbore under the action of cold shock during hot dry rock mining is studied. The results show that during the cold shock process of the wellbore model， as the increase of the cold shock time， the tensile stress on the rock surface first increases to a peak value and then decreases slowly. The fracture growth can be roughly divided into early， middle and late stages. In the early stage， an annular tensile stress zone appears around the wellbore and uniform micro-cracks begin to appear. In the middle stage， with the increase of time， the tensile stress zone gradually spreads to the periphery of the wellbore and the fracture spreads outward beyond the tensile stress zone. In the late stage， the tensile stress gradually decreases to less than the tensile strength of the model， and the fracture propagation slows down until it stops. The confining pressure， well diameter and temperature have a significant impact on the damage effect of the surrounding rock under cold shock. Among them， the temperature and well diameter promotes the growth of cold shock fracture while the confining pressure inhibits it.

Key words:hot dry rocks (HDR);wellbore;fracture extension;numerical simulation;cold shock;enhanced geothermal system (EGS)