Abstract
Supercharging in the vicinity of a borehole is an important factor that affects formation damage and drilling safety, and the filter cake growth process has a significant impact on supercharging in the vicinity of the borehole. However, existing models that predict pore pressure distribution overlook dynamic filter cake growth. Thus, an analytical supercharging model was developed that considers time-dependent filter cake effects, and this model was verified using a two-dimensional numerical model. The influences of filter cake, formation, and filtrate properties on supercharging were investigated systematically. The results indicate that time-dependent filter cake effects have significant influence on supercharging. Supercharging increases in the early stage but decreases over time because of the dynamic growth of filter cake, and the supercharging magnitude decreases along the radial direction. Because of filter cake growth, the magnitude of supercharging falls quickly across the filter cake, and the decreased magnitude of pore pressure caused by the filter cake increases. Supercharging in low-permeability formations is more obvious and the faster rate of filter cake growth, a lower filtrate viscosity and faster reduction rate of filter cake permeability can help to weaken supercharging. The order of importance of influencing factors on supercharging is overbalance pressure > formation permeability > formation porosity ≈ filtrate viscosity > filter cake permeability attenuation coefficient > initial filter cake permeability control ratio > filter cake growth coefficient > filter cake porosity. To alleviate supercharging in the vicinity of the borehole, adopting drilling fluids that allow a filter cake to form quickly, optimizing drilling fluid with a lower filtrate viscosity, keeping a smaller overbalance pressure, and precise operation at the rig site are suggested for low-permeability formations during drilling.
Highlights
Pore pressure, known as formation pressure, is the pressure of the fluid in the pore spaces of porous formations (Ma et al, 2015)
There are many factors that can result in pore pressure variation near the wellbore, and these factors can be classified into physical, mechanical, chemical, and engineering effects (Jia et al, 2019; Khaled and Shokir, 2017; Ma et al, 2016; Meng et al, 2019; Roshan and Rahman, 2010)
The decreased pore pressure caused by the filter cake increased, and a higher filter cake permeability attenuation coefficient was associated with a larger decrease in pore pressure, which contributed to smaller supercharging near the wellbore
Summary
Known as formation pressure, is the pressure of the fluid in the pore spaces of porous formations (Ma et al, 2015). The effects of filter cake buildup and/or filter cake property variation over time on supercharging-related wellbore stability, reservoir damage, and formation testing while drilling have been plaguing the industry, and analytical solutions for supercharging in the vicinity of the borehole involving time-dependent filter cake effects are very limited. To fill this gap, an analytical model of transient response of near-wellbore supercharging during filter cake growth is proposed in this paper, and the influencing factors of supercharging have been systematically investigated.
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