基于孔与裂隙网络模型的平行微裂隙对驱油的影响规律研究

EFFECT OF PARALLEL MICRO-FRACTURES ON FLOODING BASED ON PORE-FRACTURE NETWORK MODEL

认识双重多孔介质中油水两相微观渗流机制是回答形成什么类型的裂隙网络可提高油藏采收率的关键.微裂隙的分布可以提高多孔介质的绝对渗透率,但对于基质孔隙中的流体介质,微裂隙的存在会引起多孔介质中局部流体压力和流场的变化,导致局部流动以微裂隙流动为主,甚至出现窜流现象,降低驱油效率.本文基于孔与裂隙双重网络模型,在网络进口设定两条平行等长且具有一定间隔的微裂隙,分析微裂隙的相对间隔(微裂隙之间距离/喉道长度)和微裂隙相对长度(微裂隙长度/喉道长度)对于微观渗流特征的影响.结果表明:随微裂隙相对长度的增加,出现驱油效率逐渐降低,相对渗透率曲线中的油水共渗区水饱和度和等渗点增加,油水两相的共渗范围减小等现象;随着微裂隙之间相对间隔增大,周围越来越多的基质孔穴间的压力差减小,在毛管压力的限制下,驱替相绕过这些区域,而导致水窜现象.

It is of great importance to understand the micro-seepage mechanism of water flooding in dual pore-fracture media, for the improvement of the recovery efficiency. The existence of micro-fractures can on the one hand increase the absolute permeability, on the other hand change the local fluid pressure distribution and flow in the porous media.The fracture flow prevails, the surrounding oil cannot be displaced, reducing the displacement efficiency. In this paper, a pore-fracture network model is used in the analysis, two parallel micro-fractures with equal length are set at the inlet. The effects of the relative interval（micro-fracture gap length/throat length） and the relative length（micro-fracture length/throat length） of the micro-fractures on micro-seepage are investigated. The results show that with the increase in relative length of micro-fractures, the displacement efficiency decreases, while the water saturation at the co-permeable zone and at the intersection of the relative permeability curve increase. With the increase of the relative interval between the two fractures, the pressure of the surrounding pores is approximately equivalent, and the oil displacement will not occur due to the capillary pressure, leading to water channeling and the decrease of the oil recovery.