Methods of Solving Remain High Crude Oil After Primary and Secondary Recovery in Oil Reservoir<br />By Saja Haider Mohammed<br />With decreasing conventional light oil reserves, unconventional resources have becoming more and more important for meeting the increasing demand of crude oil. Heavy oil resources as one of the most important unconventional resources are widely distributed in several countries such as China, Canada, and Venezuela. The viscosity of heavy oil typically ranges from 50 to 50000mPa·s at reservoir temperature and pressure. Water flooding is recognized as one of the most common secondary oil recovery techniques after primary production period, which has been widely applied for the recovery of light oil. However, for heavy oil reservoirs, due to adverse mobility ratio of injection water and crude oil, the fingering effect and water breakthrough occur. It has been reported that the recovery of heavy oil reservoirs developed by conventional water flooding is only about 5%~10% of the original oil in place. Hence, water flooding is more suitable for light or less viscous oil flooding than for heavy oil. In order to recover more heavy oil after conventional water flooding, different enhanced oil recovery (EOR) techniques including thermal oil recovery techniques and nonthermal techniques have been investigated . The EOR processes focus on the reduction of oil viscosity or improvement of injection water viscosity, which<br />can improve the mobility ratio of injection water and crude oil and enlarge sweep efficiency. On the other hand, the EOR processes focus on the reduction of oil-water interfacial tension and thereby reduction of the residual oil saturation, which can improve oil displacement efficiency.<br />Thermal oil recovery techniques including steam injection, steam huff and puff, and steam-assisted gravity drainage (SAGD) have been usually employed and proven to be efficient in many heavy oil reservoirs . Compared with other thermal recovery methods, the SAGD provides more efficient recovery of unconventional oil resources, such as heavy oil and bitumen. However, for the deep and thin reservoirs, the SAGD or other thermal methods are economically non profitable. The severe heat loss during injection and high consumption of energy and water to generate steam can lead to non profitable and environmental concerns. Therefore, nonthermal recovery methods such as chemical enhanced oil recovery (EOR) methods are more promising methods for recovery of heavy oil in recent years .<br />Chemical enhanced oil recovery (EOR) methods are usually categorized as polymer flooding, surfactant-polymer (SP) flooding, and alkali-surfactant-polymer (ASP) flooding. By increasing the viscosity of the injection water and improving the mobility ratio, polymer flooding can improve sweep efficiency and thereby enhance oil recovery. Polymer flooding technology has been considered and implemented in many heavy oil reservoirs . Although ASP flooding process has been implemented in fewer heavy oil reservoirs, the incremental oil recovery of ASP flooding is higher than that of polymer flooding. The surfactant can reduce the oil water interfacial tension and thereby reduce the residual oil saturation and improve oil displacement efficiency. Adding an alkaline agent such as NaOH can promote the formation of in situ surfactant and reduce oil-water interfacial tension. Moreover, adding alkali can contribute to in situ formation of emulsion, which can improve mobility ratio due to the resistance factor during transport in porous media. However, the alkali in the ASP flooding system can cause some problems, which can restrict the application of ASP flooding. The alkali can react with rock mineral to cause severe scaling problems and promote the formation of water-in-oil (W/O) emulsion in the produced liquids that is too stable to be broken . In addition, the high consumption of alkali for the heavy oil with low acid number, which will lead to salt sensitive effect and reduce the viscosity. Thus, due to the existence problems of severe scale formation and disposal of produced emulsions, surfactant-polymer (SP) flooding has gained more interests for enhancing oil recovery in recent years . In fact, SP flooding pilots have been performed and proven to be efficient in many light oil reservoirs.<br />While for heavy oil reservoirs, SP flooding has received much less attention. The EOR mechanism of SP flooding for light oil reservoirs includes mobility ratio improvement, ultralow interfacial tension, emulsification, and wettability alteration. Similarly, the mechanism of ultralow interfacial tension has also been suggested for the recovery of heavy oil. In recent years, the mechanism of in situ formation of oil-in-water emulsion has been more focused and investigated to recover heavy oil. It is believed that viscous oil is difficult to displace and the adverse mobility ratio of water and oil is the dominant reason that governs the low incremental oil recovery. It has been reported that the in situ formation of emulsion can improve the mobility ratio and enlarge the sweep efficiency. Firstly, the in situ formation of oil-in-water (O/W) emulsion can reduce the viscosity of heavy oil and make it easier to flow in the reservoir. Secondly, the formed O/W emulsion can plug rock pores and improve sweep efficiency.