Optimization and Emulsification Mechanism of a High-Density Solids-Free Drill-In Fluid System

Conventional solid-weighted drilling fluids suffer from particle settling under high-temperature and high-pressure (HTHP) conditions and easily induce irreversible reservoir pore damage, limiting safe drilling in deep HTHP reservoirs. To address these drawbacks, two solids-free water-in-oil emulsion drill-in fluid systems based on GTL synthetic oil and saturated potassium formate brine were developed using composite ET/MT emulsifiers. The thermal stability, HTHP filtration, droplet size, interfacial rheology, clay inhibition, and permeability recovery performances were systematically evaluated. The 45:55 oil/water formulation exhibited excellent long-term thermal homogeneity, whereas the 20:80 system achieved higher density. The optimized emulsions presented low HTHP fluid loss, stable interfacial films, prominent clay inhibition, and over 90% core permeability recovery, demonstrating favorable reservoir protection capacity. Molecular dynamics simulations qualitatively clarified the interfacial adsorption behavior of composite emulsifiers and the formation of compact interfacial films. Notably, most tests were single determinations without replicates, blank core controls were absent, and bulk rheological data were unavailable, leading to qualitative mechanistic analysis. This work verifies the feasibility of the proposed solids-free fluid system, while further replicated tests and complete parameter validation are essential for its quantitative performance interpretation and field application.