Rock damage mechanisms and reservoir protective drilling fluid optimisation for a sandstone block in Dagang Oilfield

The tight sandstone reservoir in a certain block of the Dagang Oilfield exhibits a dual-porosity structure, comprising nanoscale matrix pores (average diameter: 11.99 nm) and natural fractures (50–100 μm). A key challenge facing this formation is severe water sensitivity caused by a mixture of sodium feldspar and montmorillonite clays, which account for 56% of the total clay content. Characterisation of the reservoir indicated that water sensitivity is the primary damage mechanism, accounting for 72.3% of the damage rate. To mitigate this issue, a novel film-forming sealant, CMJ-01, was introduced to establish a composite sealing system that combines physical bridging with chemical film formation. The optimised drilling fluid, to which 1.0 wt% of CMJ-01 was added, demonstrated significantly improved performance: film efficiency (σ) increased from 23.5 to 70.1%, the equilibrium pressure difference (ΔP) reached 0.1034 MPa, and API filtrate loss was reduced to 1.0 mL. Furthermore, the system effectively sealed fractures ranging from 50 to 100 μm in width. Consequently, the permeability loss rate decreased from 61.8 to 35.7%, whilst the permeability recovery rate increased from 58.1 to 91.3%. The film-forming sealant CMJ-01 also demonstrated excellent acid degradability, facilitating efficient post-drilling clean-up. This study validated a synergistic dual-function strategy that simultaneously ensures wellbore stability and excellent reservoir protection in highly water-sensitive, naturally fractured tight reservoirs.