The Aggregation Of Surfactants In High Saline Of Ca²⁺ And The Removing Of Oil-based Mud

Oil-based mud (OBM) drilling fluid is a kind of drilling fluid that oil is the continuous phase. According to the content of the water phase, OBM drilling fluid can be divided into all oil-based drilling fluid (water content< 10%) and water-in-oil (W/O) emulsion drilling fluid (water content 10-60%). The advantage of OBM drilling fluid is that it has resistance to high temperature and erosion of Ca2+, good lubricity for oil and small damage to gas reservoir. OBM drilling fluid is widely used in high temperature deep well and complicated stratum.After drilling, OBM remained on the casing and the borehole wall will become thick filter cakes, removing the residual OBM rapidly will be conducive to the following operation. Cleaning fluid as an effective method to removing the residual OBM and the thick filter cakes should have the following features:(1) The cleaning fluid must be able to change the wettability of the interface effectively, and it have good compatibility to OBM and slurry; (2) The cleaning fluid must have certain rheological and suspension properties; (3) The density of cleaning fluid should be controllable.The aggregation behavior of surfactants were systematically studied in this work, a new type of microemulsion that has resistance to high temperature and erosion of Ca2+ had been prepared, and possible factors that have an effect on the efficiency of removing OBM had been studied. (1) The aggregation behavior of surfactants under the high Ca2+ concentration.AES/BL-n/CAB-35 has been chosen as the most suitable system of cleaning fluid under high Ca2+ concentration, by changing the number of polyoxyethylene group (EO) in the system, the rheological properties and the density of aqueous solution can be effectively controlled.In AES/BL-n/C AB-35 system, different types of surfactant aggregates such as Li, Lα,can be got by changing the numbers of EO. Adding the mineral oil to different numbers of EO system, mineral oil can be emulsified by the method of phase inversion temperature (PIT). when n is small, mineral oil can’t be emulsified; when n = 4, microemulsion will be spontaneously formed as the amount of mineral oil is small, a large number of mineral oil joined can form nanoemulsion; when n is bigger, mineral oil can be emulsified, but emulsification efficiency is poor. This is because HLB value of this system is the most suitable for the formation of oil-in-water (O/W) emulsion when n= 4.Focused on AES/BL-4/CAB-35 system, it is insensitive to the change of temperature within the scope of 25-70℃, the appearance of system has no obvious change, its inner structure is also stabled characterized by Rheometer, Small Angle X-ray Scattering, Polarize Microscope. This is due to the critical packing parameter of this mixed surfactants system is about 1, under the joint action of the hydrophobic effect between the surfactant molecules, electrostatic interactions and hydrogen bonding interaction of Ca2+, the bilayer of surfactant molecules will be the parallel assigned. Under this condition, the viscosity of the system is very high, elastic modulus G’is greater than the viscous modulus G". This relatively stable structure will help counteract the effects of changing temperature, the original structure continues to maintain the stable. (2) Using Winsor IV microemulsion to remove washing OBMUnder the high Ca2+ concentration, Winsor IV microemulsion is formed by AES/BL-9/n-butanol/mineral oil in 40 wt% CaCl2 as OBM cleaning fluid, which can effectively remove OBM and change the wettability of the interface. The influence of washing temperature and the dosage of surfactant agent in the removing process of oil-base mud is studied.The interaction between AES and BL-9 is studied in this chapter, the influence of Na+, Ca2+ on the microemulsion phase behavior are comparative studied in order to get the optimal mix proportion. At same concentration, the influence of the Ca2+ in microemulsion phase is much more obvious than Na+. Microemulsion, formed with Ca2+ is much less sensitive to temperature. Finally, the phase diagrams were prepared in the corresponding conditions.The actual production of the removing process has been imitated in the laboratory. In this process, the increase content of surfactant is beneficial to reduce the amount of co-surfactant, and it also improve solubilization ability of microemulsion; The removing efficiency of microemulsion is high, the maximum removing efficiency can be achieved in a relatively short time; under the specific composition, the removing efficiency will not increase as temperature changed.