Laboratory Evaluation Of Lignosulfonates As Sacrificial Adsorbates In Surfactant Flooding

Abstract

Abstract One of the most difficult problems facing the field application of surfactant flooding is tile large loss of surfactants in a reservoir during a flood. Several physic-chemical mechanisms may cause surfactant retention, the most important being: surfactant adsorption at the solid-liquid interface, surfactant trapping in the immobile oil phase due to unfavourable phase behaviour, and surfactant precipitation due to poor surfactant solubility in reservoir brines. This paper considers surfactant loss due to adsorption and investigates one possible way of reducing it by pre-adsorbing lignosuljonates on reservoir rock surfaces. The main objective of this paper is to develop a better understanding of lignosulfonate contribution to enhancement of tertiary oil recovery by surfactant flooding. It has been shown previously that there are two possible ways for lignosulfonates to improve oil recovery. First, they could reduce surfactant adsorption by being pre-adsorbed on reservoir rock surfaces, and second, they could act together with the surfactant at the liquid-liquid interface and further reduce interfacial tension between oil and brine. The paper describes a series of surfactant floods with and without a lignosulfonate preflush, designed in a way which allows suppression of the role of lignosulfonates at the liquid-liquid interface in order to isolate the lignosulfonate action as sacrificial adsorbate. Experimental results include dependence of oil recovery on the size of the lignosulfonate preflush and detailed data on the reduction in surfactant retention. It is shown that this reduction in surfactant retention by itself does not contribute to the improvement in oil recovery. Therefore, it is concluded that the improvement in oil recovery is due to the role of lignosulfonates as beneficial cosurfactants rather than due to their role as sacrificial adsorbates. Introduction One of the most obvious means of improving the economic performance of surfactant floods is to reduce the adsorptive capacity of reservoir surfaces by pre-adsorbing waste materials or some other inexpensive chemicals on active surface sites of reservoir rocks. Lignosulfonate materials have been suggested for this purpose(1) and, more recently, silicates have been Keywords: Reservoir engineering, Lignosulfonates, Surfactant flooding, Enhanced oil recovery, Adsorption tested in a similar capacity(6). This paper considers lignosulfonates, which are 50 to 75% cheaper than the petroleum sulfonates suitable for enhanced oil recovery, thus meeting the basic economic criterion for potential sacrificial adsorbates. It has been proven previously that lignosulfonates may be used as additives to surfactants to enhance the efficiency of surfactant floods. Initially, lignosulfonates were assumed to act as sacrificial agents adsorbing irreversibly on the surfaces of reservoir rocks, thus reducing the number of sites available for surfactant adsorption(1). More recent studies have attributed the enhancement of oil recovery to the interfacial activity of lignosulfonates, and it has been suggested that beneficial synergistic effects exist between surfactants and lignosulfonates(1). The main objective of this paper was to design a series of laboratory experiments which would help to elucidate the relative importance of these two different mechanisms which may be contributing to the improved surfactant flood performance in the presence of lignosulfonates.