Kinetics and Equilibria of Calcium Lignosulfonate Adsorption and Desorption onto Limestone

Abstract

Abstract Equilibria and kinetics are two basic ingredients for the proper understanding of adsorption and desorption processes between surfactants and rocks. The adsorption and desorption of calcium lignosulfonate (CLS), commonly used as a sacrificial agent in surfactant-based EOR processes, has been studied. Kinetic results showed that adsorption and desorption are both time-dependent, not instant. Both adsorption and desorption were characterized by a biphasic pattern: a fast step followed by a slow step. Apparent adsorption and desorption rate coefficients were determined by a second-order kinetic model. Desorption is a much slower process than adsorption. Desorption is an un-equilibrium process under normal reservoir flow rate conditions. Equilibrium results show that CLS adsorption and desorption onto limestone can be described well by the Langmuir isotherm over the tested CLS concentration range, and that increasing concentration increases adsorption density. There is significant hysteresis between CLS adsorption and desorption isotherms. Increasing flow rate slightly decreased CLS equilibrium adsorption. Increasing both NaCl and CaCl2 concentrations increased adsorption density; however, CaCl2 had a much greater impact on the adsorption. Increasing pH decreased CLS adsorption onto limestone. The reasons for the effects of the different factors are elucidated in this paper. Introduction Despite the favorable characteristics of CO2 flooding for displacing oil, large density and viscosity contrasts between displacing and displaced fluids in CO2 injection processes result in poor sweep efficiency caused by fingering and gravity override. These are augmented by reservoir heterogeneity. Injecting CO2-foam has proven to be an effective mobility control process for CO2 flooding.1–4 However, costs incurred by the loss of surfactants due to their adsorption onto reservoir rocks exclude this potentially beneficial option for many well operators.5 Earlier studies using lignosulfonate as a sacrificial agent or a cosurfactant demonstrated a significant reduction in the expensive surfactant adsorption.6–16 Lignosulfonates (also called lignin sulfonates and sulfite lignins) are derived from the reaction of wood lignin with bisulfite or sulfate ions during the wood digestion reaction to make pulp.17 Lignin, said to be the most common polymer on Earth,17 is derived from abundant renewable resources. It accounts for approximately 30% of dry wood weight. Commercial lignin is currently produced as a coproduct of the paper industry, separated from wood by a chemical pulping process. Lignosulfonate contains numerous aromatic rings as well as hydroxyl methyl-ether functional groups. Because it is a complex natural polymer with many random couplings, there is no exact chemical structure.18 Lignosulfonate has been tested as a sacrificial agent to reduce surfactant adsorption. The use of lignosulfonate as a sacrificial agent to reduce the primary surfactant adsorption was first reported in a patent by Kalfoglou, et al.9 They found that lignosulfonate reduced the primary surfactant's adsorption on crushed limestone rock samples by 16 to 35%. Hong et al. evaluated lignosulfonate as a sacrificial adsorbate in preparation for a surfactant flooding field test in a Glenn Pool reservoir.10,11 In their laboratory tests, the lignosulfonate reduced surfactant adsorption by 39%. Grigg and Tsau demonstrated that lignosulfonate could reduce the adsorption of the primary foaming agent-CD1045™ by 24–60% in Berea core and 15–29% in Indiana limestone core samples.12–16 In an earlier study the effect of concentration, salinity, temperature and injection rate on the equilibrium adsorption density and the effect of postflush rate, brine concentration and composition, temperature on the calcium lignosulfonate (CLS) desorption process in Berea sandstone cores were performed.19