ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 16, 2008 Thixotropy in Water-Based Drilling Fluids Ahmadi Tehrani M-I SWACO Research and Technology Centre, Aberdeen, United Kingdom ABSTRACT approximation, the time- and shear- Rheology of drilling fluids affects the dependence of rheological parameters frictional pressure drop and the solids within the range of shear rates encountered carrying capacity of the fluids during the in the drillstring and in the annulus between drilling operation. Drilling fluid rheology is the drillstring and the borehole wall. The commonly controlled by using a variety of model shows potential for use in calculation clay or polymeric materials, depending on of drilling hydraulics. the type of fluid used and the demands of the specific drilling operation. Most drilling INTRODUCTION fluids possess varying degrees of time- and The function of a drilling fluid is to cool shear-dependent thixotropic properties. and lubricate the drill bit, transport cuttings Among them, water-based fluids containing to the surface, and stabilise the wellbore. clays, such as bentonite, exhibit a For the fluid to perform satisfactorily, pronounced thixotropic behaviour. This frictional pressure drop and solids-bearing characteristic can have a significant effect capacity must be maintained at an optimum on the peaks and troughs of pressure that level throughout the drilling operation. The occur in the wellbore when the drillstring or rheological parameters that control these the tool-string is moved in and out, or when properties are the fluid viscosity and the pumping starts after a break in circulation. yield stress. Efficient pumping requires a Major pressure fluctuations can lead to low enough viscosity while adequate yield fracturing of the formation, loss of stress is needed to maintain cuttings in circulation, influx of formation fluids into suspension, particularly during circulation the wellbore, or collapse of the wellbore. breaks. Thus, a practical means of accounting for Drilling fluid rheology is commonly thixotropy in hydraulics calculations would controlled by using a variety of clay or be of great value for maintaining safety as polymeric materials, depending on the type well as the integrity of the wellbore. of fluid used and the demands of the specific As far as can be determined the drilling operation. Of the various types of thixotropic characteristics of drilling fluids drilling fluids, water-based fluids (also have not been quantified for engineering referred to as water-based mud, WBM) applications. In this work, a simple model containing clays such as bentonite (or based on the concept of a structure particulates such as mixed metal oxides or parameter is used to describe drilling fluid hydroxides) exhibit a pronounced thixotropy. Empirical relationships are thixotropic behaviour. The clay-based fluids devised that can predict, with good are suspensions of bentonite in water to which various compounds such as heavier described by a double-exponential function. minerals, polymers and surfactants are More recently, Dolz et al.7 investigated the added to control density, rheology and fluid thixotropic behaviour of water-based loss, and to improve other fluid properties drilling fluids containing bentonite, at 6- such as shale inhibition and lubricity. The 12% (w/w) concentration, and a polymeric suspended clay particles are thin, flat material. They found that the lower platelets that are electrically charged and concentrations of bentonite produced the interact to form a loose house-of-cards greatest thixotropic effect. They obtained an structure that is responsible for the gelling empirical equation that relates shear stress characteristics of the fluid when at rest, and to the concentrations of the thickeners, the its thinning behaviour when sheared.1 This shear rate and the formulation stirring time. structure and the resulting bulk rheological As far as can be determined, the properties are time- and shear-history thixotropic characteristics of drilling fluids dependent and the fluid is said to possess have not been quantified for engineering thixotropy properties. applications. To improve the quality of The rheological properties of the drilling downhole predictions, there is a need for fluid are subject to continuous modification some form of engineering relationship that as the fluid travels around the wellbore. This can describe the time- and shear-history includes changes caused by variations in dependence of rheological properties in shear rate, temperature, pressure, as well as drilling fluids. Currently, some of the more chemical modification of the fluid as it advanced hydraulics modelling calculations contacts various formations on its way to the account for this effect by using the so-called surface. In addition, mechanical and thermal “gel” values that are measured by the API- stresses that lead to the degradation of the standard oilfield viscometric method. In this drilling fluid additives can cause significant method, the shear stress of the fluid is changes in fluid rheology.2 measured after a 10-second, 10-minute, and The shear rates to which the fluid is sometimes 30-minute rest period following subjected can range from around 103 s-1 in a short interval of shearing at a high rate. A the drillstring (as the mud travels down the requirement of this approach has been for well), to ~105 s-1 in the highly turbulent flow the design of fluids with non-progressive as the fluid issues from the drill bit, to those gels, i.e. where the longer-term gels are not prevalent in the annulus, i.e. 0-102 s-1 significantly higher than the 10- or 30- (depending on the eccentricity of the minute gel values. This approach has proved annular space), as it carries the drill cuttings to be adequate for the applications to the surface. encountered to date. However, as drilling The combination of temperature, scenarios become more complex, a need for pressure, composition, time- and shear- a better way of accounting for thixotropy of history dependence of the bulk rheological drilling fluids will inevitably arise. An properties makes a full characterisation of example of this is in depleted-zone drilling the drilling fluid rheology a complex task. A where the operating window for fluid number of works have been reported on the density (mud weight), between the temperature, pressure and composition maximum and minimum dictated by the dependence of water-based drilling fluids,2-5 pore pressure and fracture gradient but only few references exist in the literature considerations, becomes very narrow. In on attempts to quantify the thixotropic such circumstances, more precise behaviour of such fluids. Mercer and knowledge of fluid thixotropy may help to Weymann6 investigated the time better control drilling hydraulics. dependence of viscosity in bentonite-water The work reported here uses a simple suspensions and observed that it can be model based on the concept of a structure parameter to describe drilling fluid shear stress, Fig. 2. The breakdown of thixotropy. Empirical relationships are structure, which occurs as shear rate is devised that can predict, with good increased along the “up” curve, is not fully approximation, the time- and shear- recovered during the “down” curve and the dependence of rheological parameters material completes the cycle with some within the range of shear rates encountered residual broken structure. The area enclosed in the drillstring and the annulus. The model between the “up” and “down” curves (the requires further development but shows hysteresis loop) is an indication of the extent potential for use in drilling hydraulics of thixotropy of the material. calculations. THIXOTROPIC FLOW BEHAVIOUR 140 Shear Rate = 1460 Shear Rate = 461 Shear Rate = 1460 Shear Rate = 146.1 Thixotropic materials are fluids 120 Shear Rate = 1460 Shear Rate = 46 100 Shear Rate = 1460 Shear Rate = 11.6 containing some form of structure as a result Shear Rate = 1460 of formation of flocs or aggregates between 80 suspended particles or moieties. In clay 60 suspensions the formation of structure is (Pa) Stress Shear 40 promoted by increased encounter between 20 suspended particles, which can result from 0 Brownian motion of the particles or from 0.0 10.0 20.0 30.0 40.0 3 the velocity gradient when the bulk of the Time (sec/10 ) material is sheared. Structure breakdown Figure 1. Shear rate step-change tests. can be due to collision of particles and flocs as well as viscous drag exerted by the liquid 120 medium when the material is sheared. On a smaller scale, the Brownian motion of 100 primary particles making up a floc can also 80 cause floc breakup. This means that under 60 certain conditions both Brownian motion 40 and shear can cause structure breakdown. (Pa) Stress Shear Thixotropic behaviour occurs when the 20 buildup effect of Brownian motion is 0 dominant over the breakdown effect of 0 300 600 900 1200 1500 Shear Rate (s-1) shear. When a thixotropic material is sheared, Figure 2. Hysteresis loops demonstrating the buildup and breakdown processes thixotropic behaviour. compete and a dynamic equilibrium eventually results. Since the rates of buildup If the material is now left to rest, the and breakdown of structure are finite, if broken structure will gradually re-form. conditions are displaced from equilibrium However, if it is subjected to successive (eg. by a change in shear rate), the structure ramping cycles until two consecutive loops level will take some time to adjust. The superpose, then there is no further drop in change in structure will be detected by a the level of structure and the loop is called corresponding change in shear stress. This is the equilibrium loop. Another characteristic illustrated in the shear rate step-change of such rheograms is the equilibrium flow experiments of Fig. 1. curve. This is obtained by allowing the Thixotropic behaviour may also be material to reach equilibrium structure level observed by ramping the shear rate up or at each new shear rate such that the “up” down and recording the resulting changes in and “down” curves superpose and there is 1 λ = (4) no hysteresis.