Historically, the well documented diversity and complexity of formations throughout Venezuela have severely complicated the design and engineering of drilling fluid systems capable of maintaining the wellbore stability required to maximize drilling efficiency and reduce formation damage. Producing formations in Eastern Venezuela, are typically low-pressure consolidated sands with elevated bottomhole temperatures and differential pressures ranging from 3,000 and 7,000 psi.
This paper describes the technical development and subsequent application of an ultra-low-invasion drilling fluid additive, designed to deposit a thin, impermeable barrier over the pores and microfractures of weak, under-pressured and otherwise troublesome formations, to maintain wellbore stability and reduce formation damage. Three case studies will be presented to demonstrate the effectiveness of the technology to prevent differential sticking and other wellbore instability issues, minimize fluid-related non-productive time (NPT), increase overall drilling efficiency, and reduce operating costs.
Validated in 14 wells, the operating window of the Naricual Formation was expanded appreciably due to the use of the ultra-low-invasion drilling fluid technology. It was also demonstrated that stability was maintained in the open hole with 7,000 psi of differential pressure at 16,810 ft, which allowed an optimized well design that eliminated one casing section. The wellbore stability was confirmed with wireline pressure-point log measurements. Furthermore, through direct offset comparisons, the authors will detail significant improvements in wellbore stability and the subsequent prevention of losses and differential sticking in a different field, where more than 1,000 bbl of losses had been recorded across the Miocene and mid-Eocene sediments.
Moreover, core tests results will illustrate the efficiency of the thin, but tough filter cake, to prevent the invasion of drilling fluid into the formation matrix, thereby minimizing formation damage down to 4.5% (95.5% retained permeability). Thus, the original structure of the formation is preserved, effectively preventing formation collapse, differential pressure-induced crossflow across open zones, and importantly, pay zone contamination.