关中盆地沣西地区地热对井采灌开发模式的数值模拟

热储回灌是地热资源可持续利用、避免环境污染的有效措施。在进行地热开发之前科学合理的规划采灌井井位布局,有利于延长地热井的使用年限。关中盆地是新生代拉张性断陷盆地,地热资源开采利用历史悠久。沣西地区位于关中盆地腹地,地下5000m深度范围内有5个含水热储层,自上而下分别为第四系下更新统三门组(600~960m)、新近系上新统张家坡组(960~1920m)、新近系上新统蓝田-灞河组(1920~2900m)、新近系中新统高陵群(2900~3800m)和古近系渐新统白鹿塬组(3800~5000m)。本文依据水文地质、地球物理等资料,建立了研究区三维地质模型,采用有限元模拟软件FEFLOW对目前开采最多的蓝田-灞河组热储层在不同对井采灌开发模式下同层采灌时渗流场及温度场的变化进行了模拟计算。结果表明:1)以30年为系统设计寿命,为确保抽水井和回灌井有水力联系又不发生明显的热突破,抽、灌井井间距以500~600m为宜;2)采灌对井的布置以抽水井位于天然径流流场的上游方向、回灌井位于天然径流流场的下游方向为佳;3)如果设计合理,采灌过程中回灌尾水温度对抽水井出水温度的影响可以得到有效控制;4)恰当实施地热尾水回灌,同时按照实际需求调节抽水井与回灌井的采灌量,可实现地热资源的可持续开采利用。上述结果为合理开发利用研究区地热资源提供了依据。

NUMERICAL MODELING OF DOUBLET WELL SYSTEM FOR EXTRACTING HEAT FROM SANDSTONE GEOTHERMAL RESERVOIR: A CASE STUDY OF FENGXI AREA,THE GUANZHONG BASIN,NW CHINA

Reinjection is effective for prolonging the lifespan of geothermal system and avoiding environmental pollution. Scientific planning of reinjection scheme is critical for exploitation and management of geothermal fields. The Guanzhong Basin, located in NW China, is a Cenozoic extensional graben basin with a long history of utilization of geothermal water. Fengxi Area is located in the hinterland of the Guanzhong Basin. In this area, five sandstone strata, namely the Sanmen Group (600~960 m), the Zhangjiapo Group (960~1920 m), the Lantian-Bahe Group (1920~2900 m), the Gaoling Group (2900~3800 m) and the Bailuyuan Group (3800~5000 m), within the depth of 5000 m have been identified as hydrothermal reservoir bearing strata. The Fengxi hydrothermal system acquired little fluid recharge in the natural state. Reinjection is, therefore, crucial to the sustainability of geothermal development in this area. To investigate the impacts of different reinjection schemes on the performance of geothermal reservoirs, this paper firstly established a three-dimensional geological model of the Fengxi hydrothermal system based on existing hydrogeological and geophysical data. In our numerical experiment, the Lantian-Bahe Gruop is taken as the target stratum of geothermal fluid extraction and reinjection using a doublet well system. The numerical simulation software FEFLOW was then used to study the variations of temperature and hydraulic head of reservoir under different doublet well layouts and operating settings. The modeling results show that, over 30 years of operation, a distance of 500~600 m between the production and reinjection wells is optimal for maintaining reservoir hydraulic pressure and avoiding thermal breakthrough. And it is more productive to deploy the reinjection well at the downstream of the natural flow field with the production well at the upstream. Furthermore, the impact of the low temperature of injected fluid on the temperature of production zone can be minimized with a better configuration of doublet well layout. The simulation of different flow rates of production well shows that a greater flow rate would result in an earlier thermal breakthrough. Therefore, adjustment of flow rate according to actual demand can promote sustainable exploitation of geothermal resources. The results of this study not only provide reference for rational exploitation of geothermal resources in the Fengxi Area, but enrich the research in geothermal reinjection.