Summary

Jurassic in Middle Sichuan was taken as an example to study the driving force characteristics of tight oil secondary migration. Based on the physical analog experiments of core samples and geological data of the oil field, both driving force and resistance of tight oil secondary migration were researched. During secondary migration of tight oil, buoyancy can't overcome the migration resistance in the tight reservoir, while overpressure caused by hydrocarbon generation is able to break through the resistance. According to the physical analog experiment of secondary migration resistance, the magnitude of resistance is about 10MPa, while the maximum value of buoyancy is only 0.09065MPa. On the other hand, the oil field production data suggests that the water content in tight oil production wells is extremely low (water content in 80% wells less than 5%), which indicates the lack of material foundations for the formation of buoyancy. Meanwhile, physical analog experiment of hydrocarbon generation pressurization proved that the maximum overpressure caused by hydrocarbon generation in the study area can reach 38MPa, large enough to break through the secondary migration resistance of tight oil. Moreover, the single well output is positively correlated with formation pressure coefficient of tight oil layers which are directly contacted with the source rock. Such relationship also verifies that overpressure caused by hydrocarbon generation is closely connected with the accumulation of tight oil. Different migration driving forces also caused different migration and accumulation patterns between tight oil and conventional oil. Acquiring these different oil distribution patterns is of great significance during exploration for different kinds of petroleum resources.

Introduction

In traditional petroleum geology theory, buoyancy, hydrodynamic force, or tectonic force all can be the secondary migration driving force of petroleum. However, based on the previous researches, buoyancy and hydrodynamic force are the main driving forces in most petroliferous basins (England W.A. et al., 1987; Bethke C.M. et al., 1991; Li Mingcheng, 2000; Li Duoli et al., 2004). The formation mechanism of these two types of forces is different. Buoyancy is caused by the density difference between petroleum and formation water, hence it will be always exist when there is density difference. Hydrodynamic force is mainly caused by hydraulic pressure in aquifers. Only when the angle of a tilted aquifer is large enough, can the hydrodynamic force be effective secondary migration driving force of petroleum (Davis R.W., 1987; Durand B., 1988; Schowalter T.T., 1979). With the rapid rising production, tight oil is becoming an important petroleum resource, and it is uncertain that whether those two main forces mentioned above are still the main secondary migration driving forces. There are two reasons for the uncertainty. The first one is the extremely low porosity and permeability of tight reservoir, which causes much larger migration resistance than in conventional reservoir. The second one is the absence of necessary geology conditions for the formation of huge buoyancy and hydrodynamic force in tight oil producing regions. The migration resistance and relative buoyancy of the low porosity and permeability reservoir in China has been studied by Li et al. (2010). When the angle of tight reservoir is 5°, migration resistance can be broken through by the buoyancy provided by a continuous oil column of 488.5m or a continuous gas column of 165.5m. If the angle of tight reservoir is smaller, the height of the oil and gas column will be much higher. However, tight oil fields all locate in places with a gentle structure, like Bakken, Eagle Ford in North America, and Yanchang Formation in Ordos Basin in China. The angles and the continuous oil column heights are all lower than the standards mentioned in Li's work (Nordeng S.H., 2009; Zou Caineng et al., 2012; Meissner F.F., 1978; Gao Gang et al., 2010; Cao Feng et al., 2011; Wang Xuejun et al., 2011). Several researches focusing on the mechanic mechanisms of petroleum migration and accumulation in tight reservoir were done by different scholars. By analyzing the continuous accumulation characteristics of petroleum in Bakken, Nordeng S. H. (2009) suggested that not buoyancy but overpressure is the accumulation driving force of petroleum in Bakken. Liu Xiaofeng et al. (2001) studied the relation between the overpressure release and petroleum migration and accumulation and pointed out that the overpressure release can provide driving force for the petroleum migration. Furthermore, about the origin of overpressure, Martinsen R. S. (1995) believed that uncompaction and hydrocarbon generation are the two main origins of overpressure, which is consistent with the viewpoint of Swarbrick R.E.(1998).

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