The Wettability In Tight Sandstone Formations And Its Effects On Oil Migration And Accumulation

The migration and accumulation processes determine the distribution and scale of oil resources in tight formations.Due to the characteristics of the tight formations,the effects of wettability in the processes of tight oil migration and accumulation are very important and cannot be neglected.By using a series of physical experiments,theoretical calculations and numerical simulations,this study investigated the effects of wettability on tight oil migration and accumulation in Yanchang formation of the Ordos Basin,China.Firstly,the characteristics of wettability and its influencing factors were obtained by using contact angle method under reservoir temperature condition and theoretical analysis calculations.The effects of wettability difference on oil migration starting condition and accumulating condition were then studied through oil charging experiments and relating analyses.Finally,combining the pore network modeling and the boundary layer theory,we built a simulation model of the oil migration and accumulation processes.It shows that(1)the wettability of the Yanchang formations before oil migration and accumulation is mainly water-wet,the contact angle distributed in the range of 20° ~ 60°,showing weakly heterogeneity of the wettability distribution.The wettability of the Yanchang formations after oil migration and accumulation is mainly neutral-wet or oil-wet,the contact angle distributed in the range of 65o~130o,showing strong heterogeneity of the wettability distribution.(2)It shows that the contact angle of the formation decreases with the increasing contents of water-wet minerals,and increases with the increasing contents oil-wet minerals in the formations.Quartz and illite/smectite are the controlling mineralcomposition factors of the wettability performance.(3)The contact angle of the formation increases with the increasing content of the polar components in the crude oil.The range of the contact angle increase is determined by the combined effects of mineral components of the formation and crude oil components in the crude oil.(4)The rock surfaces with different wetting properties show different trends with the change of the temperature.For water-wet mineral surfaces,temperature has a notable impact on the measured contact angles: contact angle decreases with the increasing temperature;but for neutral-wet or oil-wet samples,the influence of temperature on measured contact angles is relatively weak.(5)Seen from the experimental results,the permeability of the rock samples is the most important factor controlling the oil migration and accumulation processes,but wettability is also important for the processes.Under the same condition of permeability,the MMPG(Minimum migration pressure gradient)decreases or the terminal oil saturation increases with the increasing contact angle of the rock surface.(6)Seen from the numerical simulation results,during the oil charging process,the capillary pressure,the water relative permeability,and oil saturation at a certain pressure condition increase with the increasing contact angle,and the water relative permeability decrease with the increasing contact angle.The obtained results and analyses provide valuable insights for understanding and quantitatively analyzing the oil forming mechanism and distribution in tight formations.