Formation Of Colloidal Polymer Associates From Hydroxyethyl Cellulose (HEC) And Their Role To Achieve Fluid Loss Control In Oil Well Cement

摘要

The working mechanism of hydroxyethyl cellulose (HEC, M_w ～ 3.8·10~5 g/mol) as fluid loss additive (FLA) in oil well cement was investigated. For this purpose, characteristic properties of HEC such as specific anionic charge amount, intrinsic viscosity in cement pore solution, and hydrocolloidal behavior were determined. Fluid loss performance of HEC was probed through static filtration of cement slurries at 70 bar differential pressure. It was found that HEC achieves fluid loss control by reducing cement filter cake permeability. No influence of HEC on the filter cake structure (pore size) was observed. Zeta potential measurements and a filtration test which used filtrate from a preceding filtration test as mixing water for cement indicated that HEC does not adsorb on cement particles. Environmental scanning electron microscopy images revealed that in a wet environment, HEC swells to a multiple of its initial particle size, thus confirming its enormous water-binding capacity of HEC. Moreover, concentration dependant measurement of the hydrodynamic radii of HEC molecules dissolved in cement pore solution show that, beginning at an HEC concentration of 10 g/L, the cellulose ether molecules form large associates with diameters up to 3 μm. These colloidal associates physically obstruct the pores of the cement filter cake which are ～ 1 μm in diameter. Further, it was found that addition of sulfonated melamine formaldehyde (SMF, M_w ～ 2.0·10~5 g/mol) dispersant to cement slurries containing HEC greatly improves fluid loss control. Maximum effectiveness was observed for a combination of HEC and SMF at a ratio of < 1.3 (wt. / wt.). Dynamic viscosity and zeta potential measurements indicate that SMF instigates HEC association to occur at lower HEC concentrations, and that mixed associates containing both HEC and SMF are formed. Thus, the synergistic effect of the combination relies on this specific interaction between the two additives.