Experimental Study On Reconstruction Of Tight Sandstone Reservior Using VES Self-steering Acid

First, this paper summed up two properties that the surfactant used for acidification need to have( two principles of choosing the Surfactant used for acidification). The first principle: The viscosity of surfactant solution is low when the solution pH is low, and then the viscosity increases as pH gets higher. The second principle: The solution viscosity increased significantly with the increase of the calcium and magnesium concentrations based on analyzing the mechanism of reconstructing reservoir using VES self-steering acid. According to these two principles, we select from the four substances(anionic/cationic surfactant mixtures, nonionic surfactants, weakly alkalinic amphoteric surfactant,Ⅰstrongly alkalinic amphoteric surfactant, Ⅰbetaine, Ⅱstrongly alkalinic amphoteric surfactant, Ⅱ betaine,carboxyl betaine)which can became viscoelastic fluid and make the following conclusions: The viscosity of anionic/cationic surfactant mixtures solution increases as pH gets higher, but the solution viscosity decreases significantly when the calcium and magnesium concentrations becomes large enough. This law does not comply with the second principle, so anionic/cationic surfactant mixtures is not used for acidification. When nonionic surfactant solution becomes the viscoelastic fluid, the concentration of nonionic surfactant in solution is very large, so nonionic surfactants mixtures is not used for acidification considering the economy factor. The viscosity of weakly alkalinic surfactant solution is high when the solution pH is pI;if the pH is more than or less than pI the viscosity of weakly alkalinic surfactant solution is low. This law does not comply with the first principle, so weakly alkalinic surfactant is not used for acidification. The solution viscosity of Ⅰbetaine dose not depend on pH, this law does not comply with the first principle, so Ⅰbetaine is not used for acidification. The solution viscosity of Ⅱ betaine is low when the solution pH is low, and then increases as pH gets higher, and tend to be constant in the end. The solution viscosity of Ⅱ betaine increased significantly with the increase of the calcium and magnesium concentration. These two laws reflected by Ⅱ betaine comply with the above two principles, so we identify Ⅱ betaine(carboxyl betaine) as the surfactant used for acidification. We start to study the rheological behaviors of carboxyl betaine solution and making the following conclusions,(1)The solution viscosity of carboxyl betaine increases as the concentration of surfactants increases, and tend to be constant in the end.(2)The solution viscosity increases as pH gets higher, and tend to be constant when pH reachs pI.(3) Inorganic salt cationic and anion play a positive role in increasing the solution viscosity, Different effects on the solution viscosity of surfactants are given by different kinds of ion type, and the order of viscosity is, cationic: Ca2+ ion>Al3+ ion>Mg2+ ion>NH4+ ion>Na+ ion>K+ ion anion: SO42- ion>SO32- ion>F- ion>Cl- ion>Br- ion>I- ion; The solution viscosity of surfactants are not influenced by pressure; The solution viscosity of surfactants increases and then declines as the temperature increases, reaches the maximum when the temperature is 70℃. The solution viscosity of surfactants declines with the increase of shear rate; The solution viscosity of surfactants are not influenced by shearing sequence. Then we got the formulation system of VES self-steering acid based on the geological conditions in research area.(multi-acidizing agent: 12%HF+3%HCl; surfactant used for acidification: carboxyl betaine; 0.5% clay stabilizer: quaternary ammonium clay stabilizer; 0.6%acidulate corrosion inhibitor; 1% citric acid, ferric ion stabilizers) and then evaluate its performance. Then we study the steering performance of VES self-steering acid by the improvement rate of the core with low permeability in paralleled tubes flooding experiments and draw the conclusion that the factors influencing the performances of the steering performance of VES self-steering acid included mineral components of fillings in core, surfactant concentration in acid, temperature, pumping rate of acid injection and permeability differential of high and low permeability core. The effect laws of mineral components of fillings in core is that, the order of the improvement rate of the core with low permeability is C>A≈B>D(A,B,C,D are four different core groups, A: the fillings in high and low permeability core are carbonates; B: the fillings in high and low permeability core are clay minerals; C: the fillings in high permeability core are carbonates, the fillings in low permeability core are clay minerals; D: the fillings in high permeability core are clay minerals, the fillings in low permeability core are carbonates;) The improvement rate of the core with low permeability increases as the concentration of surfactants in acid increases; The improvement rate of the core with low permeability increases with increasing temperature when temperature is lower than 70℃, and remains the same with increasing temperature when temperature is higher than 70℃. The improvement rate of the core with low permeability reduces as displacement velocity increased; The improvement rate of the core with low permeability reduces as permeability differential of high and low permeability core increased.