Study On Wettability Alteration Of Rock Surface And Its Eor Induced By Surfactants

Reservoir wettability has a great impact on oil recovery and surfactants can effectively change the wettability of solid surface.Thus,the studies of surfactant-induced wettability alteration and it's mechanisms for enhanced oil recovery have great significance.Quartz was used to simulate sandstone surface and sessile drop method was utilized to determine contact angles in this paper.Other experimental methods including ? potential measurement,IR,AFM and SEM were used to study micro-control mechanisms for wettability alteration of originally oil-wet and water-wet sandstone surface induced by the cationic surfactant(Cetyltrimethylammonium Bromide,CTAB),nonionic surfactant(TritonX-100,TX-100)and anionic surfactant(Alcohol ethoxylate sulfated sodium,AES-1).When quartz sand is initially water-wet,with the increase of cationic and nonionic surfactant concentrations,contact angles first increase and gradually decrease.Due to electrostatic attraction or polarity-dipolar interaction,hydrophilic head groups of surfactants face toward solid surface and hydrophobic groups face toward aqueous solution,making solid surface oil-wet and contact angles increase.When the concentration of surfactants continues to increase,due to hydrophobic interaction,hydrophobic groups of surfactants previously adsorbed on the surface attract hydrophobic groups of surfactants laterly adsorbed on the solid surface,forming a bilayer adsorption structure.In this case,the solid surface becomes water-wet and contact angles decrease.There are two platform regions in the ? potential curves for water-wet quartz powder at different concentrations of CTAB solutions.The concentration corresponding to the occurrence of the first platform is nearly equal to the concentration of the monomolecular adsorbed layer formation.Following the first platform,values of ? potential sharply increase,indicating the formation of the molecular structure of closly arranged molecules of double layer at solid-liquid interface.Due to the strong electrostatic repulsion effect caused by AES-1 and the negatively charged surface of sandstone,the weak van der Waals forces or hydrogen make the adsorption amount for anionic surfactants small.Thus,anionic surfactants have little effect on wettability alteration of water-wet sandstone surface.In infrared spectrum for the oil-wet quartz powder treated with CTAB,many absorption peaks disappear,including stretching vibration peak of C-H of paraffin chain and stretching vibration peak of C=O group of crude oil.CTAB molecules and carboxylic acid groups of crude oil interact to form ion pairs,which desorb from sandstone surface and are solubilized into micelles formed by CTAB,resulting in the disappearance of the above absorption peaks.In infrared spectrum of the oil-wet quartz powder treated with TX-100 and AES-1,there are still stretching vibration peaks of the above groups,indicating that adsorption was responsible for mechanisms for wettability alteration of originally oil-wet sandstone surface caused by TX-100 and AES-1.With the increase of CTAB concentration,values of ? potential for the oil wet quartz powder increase.Desorption of ion pairs makes values of ? potential reduce and eventually reach equilibrium.AFM was used to scan microscopic morphology of particles of mica surface.Results show that desorption of ion pairs makes many particles disappear and mean roughness of the oil-wet mica surface reduce after the oil-wet mica surface is treated with CTAB.TX-100 and AES-1 molecules are adsorbed on the oil-wet mica surface,making many parts of oil-wet mica surface become relatively flat and mean roughness decrease.Results of SEM experiment show that: Due to desorption of ion pairs,the amount of adsorbed asphaltenes greatly reduced after the spontaneous imbibition of CTAB solutions into oil-wet cores.A large number of asphaltenes were still adsorbed on the core surface after the spontaneous imbibition of TX-100 and AES-1 solutions into oil-wet cores.In this paper,artificial sandstone pillar cores were employed for spontaneous imbibition tests and influcing factors of the spontaneous imbibition for low permeability cores were investigated.Results show that the higher the permeability of the core,the higher the ultimate oil recovery by spontaneous imbibition.In addition,oil recoveries by spontaneous imbibition decrease with the increase of core length and the short cores have higher imbibition rate.Oil recoveries by spontaneous imbibition for the cores in the CTAB solution are higher than that for the cores in the TX-100 and AES-1 solutions at room temperature.Based on the driving mechanisms of the fluid during spontaneous imbibition,the imbibition process is divided into three stages:(1)Distribution of oil and water is controlled by capillary force.(2)Distribution of oil and water is controlled by capillary force and gravity.(3)Distribution of oil and water is controlled by gravity.Furthermore,the stronger the water-wetness of the core surface,the higher the ultimate oil recovery by spontaneous imbibition.Macro-displacement and micro-displacement were used to study the effect of surfactant-induced wettability alteration on waterflooding.Based on the formula of capillary number and capillary parallel model of double pores,mechanisms of enhanced oil recovery by surfactant-induced wettability alteration were analyzed in this paper.During waterflooding,water uniformly moves forward and the contact angle between the oil-water interface and the pore surface is close to 90° for the weakly water-wet system,making oil recovery increase.Static and dynamic Young equations were used to analyze mechanisms of spontaneous detachment of oil drops from sandstone surface.The process of spontaneous detachment of oil drops is divided into three stages:(1)Due to reservoir wettability change,the contact area of oil droplets and solid surface decreases rapidly.(2)Due to the balance of oil-water interfacial tension,surfactant molecules slowly penetrate and spread along oil-solid interface and the shape of oil droplets has not changed much.(3)Oil droplets are gradually lengthened and are pulled off by buoyancy until oil droplets strip from the solid surface.