Investigation Of New Cationic Viscoelastic Surfactant Fracturing Fluid

In recent years, with the large-scale exploitation of the global oil and gas fields, the number of the oil wells with large reserves and easiness to exploit has decreased rapidly. The main way to increase the production of oil and gas reservoirs is secondary fracturing of original oil and gas field. The secondary fracturing required a higher fracturing technology, the key of which lies in the fracturing fluid. The traditional hydraulic fracturing fluid has some shortcomings, such as the low utilization efficiency, difficult to break gel, not easy to flow-back and serious reservoir damage and so on. With the advantages of simple preparation process, easiness to flow-back, small stratum damage, etc, viscoelastic surfactant fracturing fluid has become the focus of the fracturing fluid research at present.As regard the drawback of inadequate temperature-resistance of the present viscoelastic surfactant fracturing fluid, the paper selects organic salt4-styrenesulfonate (SSS) with conjugated structure and rigid benzene group, organic salt of sodium salicylate and inorganic salt of potassium nitrate as well as cationic surfactant cetyltrimethylammonium bromide (CTAB) to prepare CTAB-based viscoelastic surfactant fracturing fluid. Performance and affecting factors of the viscoelastic surfactant fracturing fluid such as rheological behavior, temperature-and shear-resistance, sand carrying and gel-breaking were systematically investigated. The viscoelastic surfactant fracturing fluid was characterized by UV-VIS spectra, AFM and viscoelasticity. A preliminary discuss about the possible formation of the CTAB/SSS viscoelastic surfactant was also investigated. We have got the viscoelastic surfactant fracturing fluid with good shear-resistance, sand carrying and gel-breaking and temperature-resistance above80℃. The main research results are as follows:(1) As regard CTAB/sodium salicylatefracturing fluid, its apparent viscosity firstly increased and then decreased with increasing sodium salicylate amount. Its apparent viscosity decreases with increasing temperatures, while decrease in viscosity slows down in higher temperatures. Apparent viscosity of2%CTAB/4%sodium salicylatefracturing fluid decrease from280mPa.s to50mPa.s with temperature increasing from30℃to80℃. Its apparent viscosity decreases with increasing shear rate. CTAB/sodium salicylatefracturing fluid shows shear thickening phenomena with increasing shear time. Its sand carrying performance is related to its apparent viscosity. Gel structure of CTAB/sodium salicylatefracturing fluid can be broken with oil instead of water.(2)As regard CTAB/SSSfracturing fluid, its apparent viscosity firstly increased and then decreased with increasing SSS amount, while increased with increasing CTAB amount. Its apparent viscosity changes little in low shear rates while indicates shear thinning phenomena in higher shear rates, which is characteristic of wormlike micelles rheological behavior.3%CTAB/2%SSSfracturing fluid shows shear thickening phenomena with increasing shear time. Its apparent viscosity increases to351mPa-s after shearing for60min.Its apparent viscosity gradually decreases with increasing temperature. Apparent viscosity of3%CTAB/2%SSSfracturing fluid does not change greatly below70℃, while decreases rapidly above70℃3%CTAB/2%SSSfracturing fluid has excellent sand carrying with glass beads settlement rate as low as0.026mm/s. Gel structure of CTAB/SSSfracturing fluid can be broken with oil rapidly within15min while a little bit longer with water, especially for3%CTAB/2%SSSfracturing fluid whose gel-breaking time is more than3hours with water. CTAB/SSSfracturing fluid is not sensitive to pH, whose apparent viscosity, sand carrying and gel-breaking performances do not essentially change in the different pH condition. UV-VIS spectra show red shift of maximum absorption peak of CTAB/SSSfracturing fluid, a preliminary indicative of formation of compound between CTAB and SSS. AFM image further indicates wormlike network of3%CTAB/2%SSSfracturing fluid.(3)As regard CTAB/SSS/KNO3fracturing fluid, its apparent viscosity firstly increased and then decreased with increasing KNO3amount, with maximum apparent viscosity of200mPa.s at0.5%KNO3.Its apparent viscosity decreases gradually with increasing temperatures, while decrease rapidly above60-70℃.Its sand carrying becomes worse with increasing KNO3amount. CTAB/SSS/KNO3fracturing fluid shows shear-thickening phenomena with increasing shear time. Apparent viscosity of CTAB/SSS/KNO3fracturing fluid with0.5%KNO3increases from280mPa.s to500mPa.s after shearing for20min.Gel structure of CTAB/SSSfracturing fluid can be broken with oil rapidly within lOmin while much longer with water. Dynamic rheological behavior of2%CTAB/1%SSS/0.5%KNO3fracturing fluid shows that storage modulus (G’)and loss modulus (G") fit into Mexwell equation in low frequency region while deviate from Mexwell equation in high frequency region. liquid-like behavior (G’<G") is observed in the low-frequency region, and solid-like behavior (G’>G") is observed in the high-frequency region. This is the typical viscoelastic behavior shown by wormlike micellar solutions.AFM image further indicates formation of wormlike network of2%CTAB/1%SSS/0.5%KNO3fracturing fluid.