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Effective control
of shale swelling and lost circulation using
drilling fluid is considered the dominant strategy for maintaining
borehole stability, especially drilling operations in deep oil and
gas wells. In this work, a hybrid silicate that contains lithium silicate
and potassium methyl silicate (PMS) was employed as a film-forming
additive to reduce shale hydration and filtration loss in the high-temperature
drilling fluid. Scanning electron microscopy (SEM) results revealed
that a dense quartz crystal film coating on the shale can be formed
in a hybrid silicate solution when the temperature exceeds 150 °C.
The in situ-formed film on the shale surface with a thickness of 60–130
μm was composed of fibrous crystalline silica. Furthermore,
the aqueous hybrid silicate exhibited enhanced hydration inhibition
ability by preventing water invasion of the formation. Aqueous hybrid
silicate with a concentration of 0.5–3 wt % lithium silicate
and 0.1–0.2 mol/L PMS was first chosen to obtain the optimum
concentration according to the hydration inhibition ability and film
formation characteristics. The hybrid silicate was added into a drilling
fluid formulation applicable in high-pressure and high-temperature
conditions, and the rheological characteristics and filtration properties
were investigated. The results confirmed that drilling fluids with
the addition of hybrid silicate can mitigate variation of viscosity
and yield point before and after aging at 180 and 220 °C. Besides,
the filtration behavior was also improved by adding hybrid silicate
into the drilling fluid. A lower filtration loss was observed at the
concentration of 1.0 wt % lithium silicate and 0.2 mol/L potassium
methyl silicate, which showed 63 and 50% HPHT fluid loss reduction
for unweighted and weighted formulations at 205 °C and 3.5 MPa,
respectively. In addition, the drilling fluid featured stable rheological
and filtration properties and excellent shale hydration inhibition
characteristics when exposed to high temperatures, making it a promising
candidate for drilling in deep oil and gas wells.
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