The Development and Function Mechanisms of an Zwitterionic Polymer/Polyol Borehole Wall Strengthening Additive

A compound borehole wall strengthening agent WSC-1 was developed to deal with the borehole wall instability frequently encountered in shale oil development with water based drilling fluids. The WSC-1 additive was synthesized through esterification reaction with a zwitterionic polymer AM-AA-DMDAAC (acrylamide-acrylic acid-dimethyl diallyl ammonium chloride) and polyvinyl alcohol. WSC-1 possesses cationic groups and poly hydroxy groups which make it easy for the WSC-1 to be adsorbed onto the surfaces of the borehole walls. In laboratory evaluation, the point load strength of an artificial core was increased by 26.90% with the help of WSC-1, and the cohesion of the core after soaking was maintained at 71.3%. The linear expansion of shale cores can be reduced by 63.22% tested with a 3% WSC-1 solution. In hot rolling test, the percent recovery of shale cuttings after hot rolling at 150 ℃ was 87.2%, a recovery higher than the results tested with commonly used borehole wall stabilizers such as polyetheramine and polyglycol. WSC-1 also showed some filtration control capacity in the test. Using IR and SEM analyses, it was found that WSC-1 can be adsorbed on the surfaces of the rocks through ionic bonding and hydrogen bonding, thereby inhibiting the hydration and dispersion of clay minerals. WSC-1 can also form an adsorption film through the “multi-point adsorption” of the polyvinyl alcohol, the film can seal off the fractures in the formation rocks, thus minimizing the filtration of free water into the formation and strengthening the borehole walls.