Study on Filter Cake Removal Fluid of EZFLOW Weak Gel Drilling Fluid.

EZFLOW weak gel drilling fluid, a drilling fluid system with distinctive internal architecture, has been extensively implemented in horizontal well drilling operations at the Western South China Sea oilfields. Its unique internal structure causes specific functional mechanisms. The rheological mechanism was investigated through microstructural characterization, revealing that the microstructure comprises a reversible network structure with sol particles either encapsulated within the network or embedded at nodal points. This distinctive spatial network configuration endows the system with exceptional rheological properties. The plugging mechanism was elucidated via pre- and post-PPA test characterization of sand disc surface morphology. Experimental results demonstrate that the rheology modifier EZVIS forms deformable aggregates and films through intermolecular or intramolecular association in aqueous solutions, effectively plugging micro-nano pores/throats and microfractures to inhibit drilling fluid filtrate invasion. Concurrently, the rigid plugging material EZCARB establishes physical barriers at micro-nano pores/throats through bridging mechanisms. Notably, the dense filter cake formed by EZFLOW weak gel drilling fluid exhibits poor flowback characteristics, potentially inducing reservoir damage. Based on mechanistic analyses of rheological behavior, plugging performance, and filter cake composition, a filter cake removal fluid formulation was developed through: (1) creation of retarded acid HWCP to degrade polymer EZVIS and dissolve temporary plugging agent EZCARB; (2) development of corrosion inhibitor HWCI to mitigate corrosion rates. Laboratory evaluations demonstrated effective filter cake elimination and reservoir protection capabilities. Post-treatment analysis of EZFLOW-contaminated reservoir cores showed complete filter cake removal at core end faces with permeability recovery values exceeding 95%, indicating superior filter cake dissolution capacity and reservoir protection performance that significantly reduces formation damage.