IS CAPILLARY POWER A DRIVING FORCE FOR THE PRIMARY MIGRATION OF OIL AND GAS?
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摘要: 一般认为,在油气的二次运移过程中,由于孔隙结构的变化,当连续油(气)相后端曲率半径大于前端时毛细管力是油气运移的阻力,而当连续油(气)相后端曲率半径小于前端时毛细管力是油气运移的动力;毛细管力总是趋向于使非润湿相占据较大的孔隙空间。在浮力、水动力和毛细管力的共同作用下,油(气)呈间歇性运动,这在物理模拟实验中已得到证实。实际上,在毛细管中存在着3种毛细管力。第一、二种方向与毛细管延伸方向平行,第三种垂直于毛细管管壁并指向非润湿相;第三种毛细管力主要起增大非润湿相与孔喉壁之间的摩擦阻力的作用,在孔喉较粗的介质(如储集层)中该类毛细管力常常被忽略。油气要发生运移,则沿前进方向上的动力必须超过该方向上的毛细管阻力和摩擦阻力。现在有一种观点,认为在油气从烃源岩向储层的初次运移过程中,毛细管力是重要的动力,因而在水润湿的条件下,油、气相会在毛细管力作用下自动地由小孔隙和细喉道向较大、较粗的孔隙和喉道内运动。本文从毛细管现象的本质入手对这一问题进行了分析,发现烃源岩和运载层中不同种类的毛细管力的作用有很大区别:在运载层中,因为孔隙喉道较粗,第一、二类毛细管力起主要作用,第三类毛细管力的作用常常被忽略;而在烃源岩中,由于毛细管半径全面缩小,第三类毛细管力与第一、二类相比急剧增大,导致摩擦阻力剧增,从而成为烃源岩排烃的主要阻力。“毛细管力是排烃动力”这一观点是忽视烃源岩中起决定作用的第三种毛细管力而得出的错误认识。通过对泥页岩孔隙结构的研究,作者提出了泥页岩中的孔隙主要是“死孔隙”的观点,而烃源岩的排烃则主要是通过微裂隙的突发涌流,微裂隙的开启或产生及涌流的发生则都与激烈的突发构造运动有关。Abstract: Generally speaking, during the seccodary migration of hydrocarbon, capillary power is the resistance of hydrocarbon migration when the rear-end curvature radius of the continuous oil (gas) phase is greater than the front-end's and is the driving force when the rear-end curvature radius is smaller than the front-end's because of the change of pore structure. Capillary power always tends to making the nonwetting phase occupy larger pore space. Under the joint action of buoyancy, hydrodynamic force and capillary power, oil (gas) moves intermittently. This has been verified in the physical simulation experiment.In fact, there are three types of capillary power. The first and the second types are parallel to the extensional direction of capillary, and the third is perpendicular to the wall of capillary and directs to the nonwetting phase. The third type of capillary power takes effect to increase the frictional resistance between the nonwetting phase and the throat wall, and is usually neglected in the media with wider throats (such as reservoirs). Hydrocarbon can migrate only when the driving force in the advancing direction surpasses the sum of the capillary resistance and the frictional resistance in this direction.A prevailing viewpoint says that capillary power is an important drivivng force during the primary migration of hydrocarbon from source rocks to reservoirs. So under the water-wetting condition, the oil (gas) phase will move spontaneously from small pores and thin throats to larger pores and wider throats. In this paper, the author studied this problem starting with the essence of capillary appearance and found that different types of capillary power act quite differently in source rocks comparing with in carrier beds. Because of the wider throats, the first and the second types of capillary power take main parts and the role of the third type of capillary power is often neglected in carrier beds. And in source rocks, the third type of capillary power increases greatly compared with the first and the second types because the capillary radius reduces all-roundly. This results in the steep rise of frictional resistance and makes it be the main resistance of hydrocarbon expulsion from source rocks. Therefore the viewpoint that "capillary power is a driving force for hydrocarbon expulsion" is a wrong conclusion due to unreasonable neglecting the third type of capillary power which plays a decisive role in source rocks. By studying the pore structure of argillutite, the author suggested that the pores in argillutite should be mainly "dead pores", and the hydrocarbon expulsion of source rocks should be eruptive surging through microfractures whose opening or forming and the happen of surging all have something to do with sudden and fierce tectonic movement.
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Key words:
- capillary power /
- frictional resistance /
- pore structure /
- carrier beds /
- source rocks /
- primary migration
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