This research was designed to develop an efficient micellar/polymer process for tertiary oil recovery at Lao Jun Miao (Yumen fleld, China), a reservoir with a high clay content, mainly smectites.
Preliminary laboratory tests indicated poor final oil recovery Preliminary laboratory tests indicated poor final oil recovery due to high retention of the sulfonate attributed to adsorption and precipitation by in-situ generated divalent cations as a result of ion exchange.
Alkaline additives were chosen for improving the process efficiency. This paper describes a number of preflush compositions and their effect on surfactant retention. With this strategy in view, a phase behavior study was carried out to formulate an efficient micellar slug for the tertiary recovery of Lao Jun Miao crude.
Additional insights into the implementation of the process are given. In particular, the design of the preflush composition was improved to prevent adverse effects such as day swelling. The results demonstrate the technical feasability of micellar/alkaline/polymer flooding at Lao Jun Miao.
Micellar flooding has received much attention in recent years. Previous studies have shown that variables such as reservoir brine Previous studies have shown that variables such as reservoir brine salinity, days and divalent cations present on the rock surfaces are the most detrimental to the oil recovery process. On contact with the micellar solution these divalent metal cations are exchanged with monovalent cations. Once released the divalent cations may affect the efficiency of the micellar solution by increasing the retention of the surfactant, by leading to the formation of gels or by modifying the phase behavior of the formulation. Different ways of improving the process efficiency have been proposed: some of them recommend the use of a preflush to condition the reservoir. High salinity water is the most efficient way to elute divalent cations from the clays. Unfortunately, an increase of salinity is detrimental to the micellar process. Moreover, the divalent cations released by ion exchange can mix with the following micellar slug, and a salinity buffer must be implemented for the protection of the surfactant solution. On the contrary, alkaline additives have been shown to be beneficial. Holm and Robertson indicated that an alkali preflush improves the efficiency of a micellar flood, whereas Nelson improved alkaline flood performance by adding a low concentration of cosurfactant to a large alkaline slug. There is evidence that several mechanisms may be responsible for the improved performance of the micellar process in an alkaline environment. Besides the formation of surfactant with acidic crude, alkaline chemicals confer a negative charge to the day surface that leads to a reduction of sulfonate loss by adsorption. Another factor that contributes to the improvement of the process is the removal of divalent cations, both from the day and from the solution by precipitation; thus the micellar solution is protected from the armful divalent ions.
The case Lao Jun Miao reservoir is an example where divalent cations are present in significant amount. Lao Jun Miao reservoir has a high clay content, mainly smectites with a high cation exchange capacity. The reservoir and the injected waters are calcium brines, and ion exchange is expected. The objective of the paper is to show the technical feasability of the injection of a micellar solution at Lao Jun Miao. The study investigates the use of an alkaline environment to implement the process and compares the efficiency of different alkaline chemicals.
The paper is organized in four major sections. The first section gives the results of preliminary experiments where the micellar flood was designed as usual and demonstrates the importance of rock/fluid reactions. The second part outlines the improvement achieved by the use of a preflush and examines different additives: neutral salt, polyphosphate, silicate and carbonates are compared. The problem of long term alkaline consumption from rock dissolution was not considered in this study.