ABSTRACT: Drilling fluid invasion into shales is one reason for instabilities while drilling. Invaded drilling fluid affects near wellbore stresses, rock strength, and overbalance wellbore pressure. The fluid invasion is a coupled-transport phenomena mainly due to hydraulic drive and chemical potential drive. The invaded fluid will increase near wellbore pore pressure and reduce effective stresses, therefore the likelihood of wellbore instabilities arise. The flow of fluid through shales’ pores and micro fractures should be mitigated using an effective additives in a water-based drilling fluid system. This paper will experimentally evaluate using of Combusted Carbon Residuals (CCRs) as a shale inhibitor additive. Combusted Carbon Residuals were mechanically grinded. Pressure transient testing was used to evaluate CCRs in a water-based drilling fluid system for controlling fluid invasion into Catoosa shale samples. Also, two chemically made nano silica, AEROSIL & AERODISP were tested in comparison to fine grained CCRs. The testing results shows the positive impact of using fine grained CCRs in controlling fluid invasion rate compared to the conventional water based drilling fluid and the two other nano products were tested.
Time-dependent drilling fluid invasion in shales causes wellbore instabilities [1-7]. Fluid invasion in shales is believed to be a physiochemical process mainly due to hydraulic potential drive and chemical potential drive [6, 8]. The Darcy flow of water is driven by hydraulic potential gradients (pressure imbalance), and diffusion of solutes are driven by chemical potential gradients (chemical imbalance) between the drilling fluid and the shales’ pore fluid. Increase of near wellbore pore pressure, reduction of near wellbore rock strength, increase of hydration stress in pore space, and shales swelling or wellbore size shrinkage are the main consequences of shale hydration [3, 9, 10]. Invaded drilling fluid increases pore pressure since shales have a low permeability and cannot dissipate excess pore pressure [1, 2, 6, and 7]. The shale hydration causes differential micro-strains and weakens the cohesive bonds between clay platelets which results in strength reduction . There are different theories that have been developed to describe the shales swelling process, such as hydraulic pressure balance, capillary suction (surface hydration), and osmosis pressure [6, 8, 12]. However, shales swelling phenomena is not well-understood and there is no agreement as to which mechanism is dominant in the shale hydration.