Modeling of Shale-Swelling Behavior in Aqueous Drilling Fluids

Abstract

Shale swelling is a serious issue in water-based muds (WBM) as it may lead to stuck pipe, shale sloughing and decreased rate of penetration. Linear Swell Meter (LSM) testing is one of well-known laboratory tests used to characterize shale swelling. This paper describes how the time-dependent swelling response from LSM is modeled to extract characteristic parameters of swelling behavior. The study was performed for different shales with appreciable variation in smectite, illite and Cation Exchange Capacity (CEC). Swelling of these shales in WBM was investigated using different salt concentrations, mud weights and viscosities of the fluids.Shale swelling as a function of time was modeled in a novel way as %S(t) = A*[f(B*t)+C*g(t)] where %S(t) represents swelling at time t. The equation was derived after combining first order kinetics term as f(B*t) with filtration loss term based as C*g(t). Thus, A represents saturation swelling volume, B represents first-order rate of swelling and C is filtrate loss rate parameter. The model fits well to experimental data with R 2 > 0.97 for all shale-fluid combinations investigated which was used to extract the characteristic parameters.Saturation swelling volume, A, increased with an increase in CEC of the shale. In addition, A decreased linearly till a critical salt concentration in fluid was reached and then flattened out with further increase in salt concentration. Such behavior conforms to osmotic transport of water through shale. It was observed that A was independent of mud weight and fluid viscosity; however B was observed to increase with a decrease in viscosity which may be interpreted as result of diffusion phenomena governing swelling rate. Filtrate loss rate parameter C was estimated to be between 0 and 1 in most of shale-fluid combinations studied. Information obtained from parameters A, B and C can allow us to optimize WBM formulations for shale that can save cost and time. IntroductionTo predict and manage borehole instabilities during drilling, it is important to have a detailed understanding of the effects that a drilling fluid may have on the shale behavior.In the drilling industry, several experimental tests and numerical models have been developed by researchers to describe the interaction between drilling fluids (specifically, water-based muds) and shale. Various experimental devices were demonstrated by Chenevert (1989) that could be used to measure the swelling of shales as they come in contact with waterbased muds. Horsrud (1998) investigated the effect of potassium chloride (KCl) exposure on smectite-rich shales by various experimental techniques, under both atmospheric and simulated downhole conditions. On the computational side, Molenaar (1998) developed constitutive models to describe fundamental understanding of the interaction between the drilling fluid composition and the mechanical behaviour of shale swelling based on transport equations for the fluids and the ions. Huang (1998) probed the effect of physio-chemical shale-fluid interaction on shale...

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