Dynamics of changes in static secondary filtration of drilling muds depending on the content of acrylic reagents

Abstract

The cause of hydration and swelling of clay minerals in rock mass most often is the filtrate of water-based drilling fluids. The volume of filtrate depends on the pressure drop in the well, the porosity and permeability of the rock. In such cases, there is a need to create a dense impermeable barrier on the walls of the well, and the filtration crust can serve as such a barrier if its permeability to filtrate is lower than the permeability of the rock mass. During the circulation of the drilling fluid, favourable conditions arise for the formation of a filtration screen, and to ensure its low hydrodynamic characteristics, reagents are added to the drilling fluid to reduce the filtration index. The article discusses the effect of acrylic polymer-based reagents that serve as flocculants, rheological parameter regulators, and stabilisers of filtration and thixotropic properties, in particular POLYPLUS, SYPAN, and POLYGUM. Laboratory studies were conducted to determine the dependence of the filtration index under static conditions and the thickness of the filtration crust on the content of these reagents, and it was found that SYPAN best reduces the filtration index at a concentration of 0.8–1.0% to 3 cm3/30 min, with the addition of POLYGUM at a concentration of 0.8–1.0% and POLYPLUS at a concentration of 0.07–0.1%, the filtration index decreased only to 4–7 cm3/30 min. To study the influence of the studied polymer chemical reagents on the formation of filtration crust properties, in particular, thickness, permeability, porosity, and density, experimental studies were carried out on models of fresh, mineralised, and salt-saturated drilling fluid filtrate, during which the effectiveness of the reagents was evaluated by the secondary filtration index of the crust. The addition of POLYPLUS contributed to a reduction in secondary filtration by approximately 1.5 times and, characteristically, for the salt-saturated solution model, this indicator is almost three times lower compared to the models of fresh and mineralised solutions. Similar laboratory studies of the effect of SYPAN and POLYGUM confirmed the regularity of the previous experiments. Based on the obtained values of secondary filtration, calculations of the permeability of the filtration crust were performed, which allow selecting a reagent that ensures the formation of its quality indicators and significantly reduces the intensity of the filtration process.