Improved ASP Process Using Organic Alkali

Abstract

Abstract This paper describes the use of a new type of organic alkali that replaces and improves upon traditional inorganic alkalis such as sodium hydroxide and sodium carbonate. The new organic alkali was evaluated in Alkaline Surfactant Polymer (ASP) formulations containing commonly used surfactants and polymers. The effect of organic alkali on IFT, adsorption and viscosity was compared to that of a conventional inorganic alkali in these formulations. The organic alkali was found to be compatible with brines containing high TDS and high divalent cation concentrations. They can be used in brines without the need for softening and in some cases they provide better results than conventional alkali in systems where either can be used. The organic alkalis have the advantage of improved compatibility with unsoftened brines, surfactants and polymers. Their non-toxic, biodegradable properties make them particularly suitable for environmentally sensitive applications such as offshore and inland lakes. Introduction In the ASP process, it is theorized that the alkali reacts with small amounts of acids and esters present in the crude oil to form surfactants in-situ that combine with the injected surfactant to produce synergistic mixtures at the oil/brine interface.[1] The alkali is also claimed to reduce the amount of surfactant adsorption onto the formation, especially in limestone reservoirs. By increasing the pH the iso-electric point of the limestone is exceeded and the surface becomes negatively charged. This increases the electrostatic repulsion between the rock surface and the negatively charged anionic surfactants and thus reduces adsorption. The surfactant reduces the interfacial tension between the brine and residual oil and therefore increases the capillary number. The capillary number (Nc) is used to express the forces acting on an entrapped droplet of oil within a porous media. Nc is a function of the Darcy velocity (?) exerted by the mobile on the trapped phase, the viscosity (µ) of the mobile phase, and the IFT (s) between the mobile and the trapped oil phase. Equation (1) below shows the relationship of the Darcy velocity, viscosity and IFT to the capillary Number. Nc = ? µ/s (1) A capillary number of about 10-6 is found after completion of the typical water flood and this number must be increased by at least two to three orders of magnitude in order to efficiently displace the oil.[2] The IFT between the oil and water during and following water flooding is in the range of 101 to 100 mN/m. The use of the proper surfactant can easily lower the IFT to 10-2 mN/m or less resulting in a corresponding increase in the capillary number by at least two to three orders of magnitude. The relationship between the capillary number and oil increase is shown in Figure 1. Polymer is used to increase the viscosity of the injection fluid for better profile control. The combination of surfactant, alkali and polymer resilts in a process where residual oil can be economically removed from the reservoir. ASP has been evaluated in the laboratory and used widely in the field with great success.[3–23] An early paper by Clark, Pitts and Smith[2] describes the use of surfactant, along with alkali and polymer to recover oil. Papers by Song Wanchao and co-workers,[4] Wang,[7,9,14] French,[10,11] Felber,[18] Miller,[21] and Chang[22] are among those that describe the utility of this technique in field studies to recover appreciable quantities of crude oil even from low acidity crude oils.