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...
Invert-emulsion fluid (IEF) systems formulated without organophilic clay have successfully addressed issues of barite sag and reservoir productivity. While ‘clay-free’ IEFs provide robust properties, additional materials in the form of low-gravity solids (LGS), like sized calcium carbonate or clay-type materials are required to bolster the rheological properties and suspension character of the system. This increases the plastic viscosity of the fluid, thereby resulting in a lower rate of penetration (ROP) and higher equivalent circulating densities (ECD). In the absence of LGS, the loss of suspension, especially in an inclined well, can lead to sag and cause a density gradient along the fluid column that may fracture the formation. This paper presents a solution towards sag in an IEF that is free of LGS viz. fine sized calcium carbonate or clay-type material, and is formulated with a novel sag control additive to suspend barite. The additive was shown to prevent sag in 9-ppg, 12-ppg, and 16-ppg clay-free IEFs in a static-aging cell held vertical at 90° and then inclined at 45°. The additive provided sag control in different base oils with less than 5-mL oil separation. Extended sag testing with the sag-control additive at 250°F for 24 hours and at 150°F for 60 hours gave minimal oil separation and no barite sag. High temperature-high pressure (HTHP) testing on 9-ppg and 12-ppg demonstrates a flat rheological profile in the presence of this additive. Contamination testing of IEFs containing this additive shows that the suspension characteristics remain unaffected and the mud properties are tolerant to the presence of contaminants. The additive is North Sea compliant with a biodegradability of 71.4% in 42 days and LC50 of >10 g/mL for aquatic organisms. Experimental data demonstrating both the environmental and anti-sag performance of the novel sag control agent is presented and compared to fluids without the sag control agent.
Barite sag and poor hole cleaning are not problems; they are symptoms of well control and stuck pipe problems. Issues with barite sag and hole cleaning are routinely encountered while drilling high pressure high temperature (HPHT) wells. Maintaining optimal mud rheology in HTHP conditions (350°F+) can be very difficult. Adding organo-clays or low gravity solids (LGS) to boost rheology can lead to high equivalent circulating densities (ECD) and low rates of penetration (ROP).The new HPHT organic rheology modifier (ORM) imparts optimal rheological properties to low, medium and high density clay-free invert emulsion fluids (IEF). Clay-free systems have previously demonstrated superior gel strength and rheological profiles over conventional organo-clay and lignite treated fluids. Although significant improvement in these systems seemed unlikely, this was accomplished with the new ORM chemistry.These IEFs exhibit enhanced low shear rheology (even at 9.0 ppg) with lower or similar plastic viscosity (PV) values when compared to IEFs formulated without the new HPHT rheology modifier. When tested at up to 400°F and 18,000 psi, the IEFs formulated with ORM show similar or higher low shear rheology with low PV than under ambient condition. A good low shear rheology implies better hole cleaning and sag control. A low PV improves ECD control.Treatment with ORM imparts fragile gel characteristics to 9.0 to 18.0 ppg clay-free IEFs. The rapid gel-to-flow transition helps to minimize surge and swab pressures and reduce mud losses. The new HPHT rheology modifier with a biodegradability of 67% in 28 days also stabilizes the IEF and provides comparatively low fluid loss values. It also eliminates the need to add LGS to boost rheology. The paper presents experimental data demonstrating both the environmental and rheological performance of the HTHP rheology modifier as well as comparative data from the conventional clay-free fluids without the ORM.
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