缝洞型定容油藏多周期注氮替油加大注气量开发研究论证

STUDY ON ENHANCE OIL RECOVERY BY INJECTING MULTI-CYCLE NITROGEN ON CONSTANT FRACTURE-VUGGY RESERVOIR

缝洞型碳酸盐岩油藏以大型溶洞、溶蚀孔洞及裂缝为主要储集空间,非均质性极强,储集体可初步分为定容型储集体和非定容型水驱储集体[1]两类。对于定容型储集体,油井前期主要依靠注水替油开发,待注水失效生产高含水后,通过注氮气驱替顶部阁楼油进行挖潜。在多轮次注气驱替过程中,发现随着注气轮次的增加,方气换油率存在逐步下降的现象。本文从氮气驱替机理出发,结合现场实践,通过数学理论推导的方法发现,在地层条件下,氮气的压缩系数同原油及地层水差异较大。随着注气替油轮次的逐步增加,储集体中的氮气逐渐累积,导致油藏流体综合压缩系数的大幅度上升,造成了氮气驱油方气换油率逐步下降。因此,定容型储集体油井多轮次注气需不断加大注气量,以减缓氮气累积造成的方气换油率下降的问题。

The storage media of fracture-vuggy carbonate reservoir, with high heterogeneity, mostly are the largekarst cave, solution pore and cavity, and fissure. Storage media could be categorized into constant-volume andnon-constant-volume by water flooding. For constant-volume oil reservoir, water injection-production was theprimary development technology during early reservoir development. Nitrogen injection was implemented to dis-place attic oil while production performance declines with high water yield. During the process of multi-cycle onnitrogen injection, oil replacement ratio was steadily declined along with multi-cycle. As is known to all that un-der certain geological setting, the compressibility component of nitrogen has the great difference with compress-ibility to water and oil. This study From the mechanism of nitrogen flooding, derivation of mathematical theoryand Deposit in practice. The declining of oil replacement ratio due to the greatly rising of comprehensive com-pressibility during the increasing of accumulation of nitrogen in the reservoir after serval injection cycles. Thus,in order to mitigate the oil replacement ratio dropdown during the nitrogen of multi-cycle-injection, maximizingthe injection volume per cycle is necessary.