Synthesis and characterization of lignosulfonate/acrylamide graft copolymers and their application in environmentally friendly water- based drilling fluid

Abstract

Acrylamide (AAm) was grafted onto lignosulfonate (LS) in the aqueous medium using two different H2O2/CaCl2 redox and potassium persulfate (KPS) thermal initiating systems at 30 and 55 °C, respectively. Resulting LS-g-PAAm graft copolymers were characterized by gravimetric, FT-IR and 1H NMR methods, from which initiator type and AAm concentration dependency of the AAm conversion (XAAm), grafting percentage (G(%)), grafting efficiency (GE) and copolymer structure were systematically evaluated. It was found that XAAm and G(%) can be affected significantly by the initiator type and AAm concentration while GE can mostly be affected only by the initiator type. Highest GE of 100% and grafting length of the PAAm chains onto LS was observed in the reaction LA-2 performed using redox initiator at 30 °C and initial weight ratio of AAm to LS of 20 while highest XAAm was observed in the reaction LA-3 performed using KPS initiator at 55 °C. Graft copolymers as well as LS with different amounts were used to prepare bentonite-based tap water (TWF), sea water (SWF) and NaCl salt saturated water (SSWF) fluids. Rheological and fluid loss (FL) properties of the above fluids were evaluated before and after hot rolling at 250 °F for 4 h. It was found that when an amount of 2.45–3.5 g/350 ml water is used; graft copolymers obtained from reactions LA-2 and LA-3 are able to improve rheological properties and to control FL and pH values in the TWF and SSWF, respectively, at low and high temperatures. All fluids showed shear thinning behavior with a gel-like structure under static conditions. Results revealed that thermal stability and salt resistance of the graft copolymers are high enough to be used as an effective additive in the high temperature and high salty water-based drilling fluids (WBDFs). An attempt was made to establish a relationship between the graft copolymer structure and its aqueous solution properties in the various water-based fluids. It was found that simultaneous higher GE (preferentially as high as 100%) and XAAm (as high as possible) in the graft polymers can results in a structure with the best performance in the WBDFs.