Water-based Mud Research Articles

Inversion-based interpretation of borehole spontaneous potential measurements for accurate estimation of water resistivity in the presence of shoulder-bed and mud-filtrate invasion effects

Electrical resistivity of formation water is a fundamental property used to quantify in situ water quality for human consumption or for assessment of hydrocarbon pore volume. Resistivity interpretation methods commonly used to quantify the electrical resistivity of formation water invoke rock porosity and fitting parameters that require additional and independent core measurements. Alternatively, the spontaneous potential (SP) log can be used to calculate water resistivity without knowledge of rock porosity in wells drilled with water-based mud. In combination with resistivity and gamma-ray logs, SP logs can be used to estimate water quality, apparent volumetric concentration of shale, and for qualitative assessments of permeability. However, SP logs often exhibit shoulder-bed and mud-filtration effects; these effects need to be mitigated before using SP logs for calculation of water resistivity. We have developed a new inversion-based method to simultaneously mitigate shoulder-bed and mud-filtrate invasion effects present in SP logs via fast numerical simulations based on Green functions. The interpretation method was implemented on SP logs acquired across aquifers with various degrees of complexity using noisy synthetic and field measurements to estimate equivalent NaCl concentration, radius of mud-filtrate invasion, and sodium macroscopic transport number. Interpretation results compared well to those obtained from resistivity and nuclear logs, provided estimates of uncertainty, and could incorporate a priori knowledge of aquifer petrophysical properties in the estimation.

Characterization of the mud displacement in an enlarged wellbore: An integrated rock-fluid model

Cement-mud displacement plays a crucial role in the sealability of cement sheaths. Irregular geometric features of a wellbore due to washout can have a negative impact on mud and cement mobilization. An unstable interface between two fluids always leads to mud channeling, interfluid mixing, and cement contamination, degrading the cement quality. Many factors, such as mechanical and rheological properties of fluids, annulus geometry, flow pattern, and flow rate, significantly influence the displacement efficiency. This study investigates the characterization of the mud displacement in an irregular horizontal well using a 3D computational fluid dynamics (CFD) model. Mud is displaced in an enlarged wellbore by geopolymer and neat class G cement. The wellbore geometry is developed based on the caliper log data from an unconventional shale well in the Tuscaloosa Marine Shale (TMS) lithology. The effects of pump rate, density difference, and mud contamination are evaluated by numerical simulations. The results present those residual muds mainly exist in the upper annulus of the enlarged section. Geopolymer has a better sealing performance and can resist more water-based mud (WBM) contaminations than neat class G cement. The scenario with a low mud-cement density difference and high cement injection rate results in a high cement volume fraction, mitigating the gas migration.

Hydrophobic polymer-modified nanosilica as effective shale inhibitor for water-based drilling mud

The oil and gas well drilling operations are primarily dependent on the shale inhibition features of the drilling mud. Our current study describes the consequence of poly-acrylic acid-acrylamide modified nanosilica (PAM-SiO2NPs) on the swelling inhibition of clay and rheological properties of the drilling mud. The unique characteristics of the PAM were combined with nano-silica to get the PAM-SiO2NPs nanocomposite. Large surface area, effective functionalization of SiO2NPs make PAM-SiO2NPs a very appropriate and efficient additive for the water-based mud. The drilling mud was kept under hot-rolling for 16 h at 212 °F and 500 psi, later on, apparent viscosity, plastic viscosity, yield point, and gel strength of the water-based drilling mud were studied. To assess the shale inhibition features of PAM-SiO2NPs the dispersion, shale inhibition durability, and linear swelling tests were conducted. The results demonstrate that PAM-SiO2NPs modified water-based mud (PAM-SiO2NPs-WBM) shows an improvement in the rheological properties. Moreover, the shale cutting treated with PAM-SiO2NPs-WBM displayed the highest % recovery (86.6%) as compared to KCl (49.2%) or the commercial inhibitor modified mud (74.7%) in the dispersion test. The shale inhibition stability of shale treated in PAM-SiO2NPs-WBM demonstrates higher stability for up to 2 days. Additionally, PAM-SiO2NPs prominently control the swelling rate to 33% as compared to 87% in water, 41% in the unmodified drilling mud, and 37.5% in the commercial shale inhibitor. The PAM-SiO2NPs adsorption on the surface of the shale and its inhibition mechanism was explored by different characterization techniques such as FT-IR analysis, TGA, FE-SEM, and EDX. The proposed inhibition mechanism of the PAM-SiO2NPs demonstrates that the PAM-SiO2NPs carry functional groups that can disrupt the hydrogen bonding and plug the nanopores on the shale surface that cause a prominent decrease in the clay swelling. Consequently, the clay surface is being protected against the reactive action of water.

Advanced trends of shale inhibitors for enhanced properties of water-based drilling fluid

Water-based mud (WBM) is an environmentally acceptable drilling fluid. However, it makes shale to be prone to swelling owing to its interaction with active clays. Shale swelling makes drilling to be difficult, so, additives are added to inhibit shale swelling, and improve rheological and filtration properties. The use of conventional additives particularly organic and inorganic compounds are not suitable at extreme conditions of drilling. Other materials like nanomaterials have emerged as promising alternatives used under such conditions. This review aims to highlight in detail the essential types of inhibitors and the evolvement of nanoparticles in enhancing drilling fluid properties.

Experimental analysis of improving water-based drilling fluid properties by using nanoparticles and poly (AN-co-VP)/zeolite composite

• The influence of various concentrations of chosen nanoparticles containing U-MWCNTs, TiO 2 , ZnO, and F-MWCNTs on water-based drilling fluid was evaluated. The rheological properties such as plastic viscosity, yield point and gel strengths of water-based drilling fluid samples were enhanced by using low concentrations of F-MWCNTs that proved desirable effect in the operational condition of the drilling industry. The most reduction of mud filtrate volume was belonged to F-MWCNTs into drilling fluid composition. The filtration results showed that the poly (AN-co-VP) / zeolite composite with F-MWCNTs could help to prohibit the increase of filtration loss of the mud samples, especially under HPHT environments. The excellence thermal characteristics of poly (AN-co-VP) / zeolite composite and enhancement of mud properties of F-MWCNTs were the best characteristic which could progress the drilling efficiency. In this experimental research, the effect of the poly acrylonitrile-co-N-vinyl pyrrolidone /zeolite composite and four types of hydrophilic and hydrophobic of nanoparticles, namely Zinc oxide, titanium dioxide, untreated multi-walled carbon nanotubes, functionalized multi-walled carbon nanotubes on the rheological and filtrate lost attributes of newly formulated bentonite mud were evaluated. This experimental study compares the drilling fluid properties including different concentrations of nanoparticles to select the most favorite agent through several laboratory tests to generate some engineering guidelines. The different concentrations of nanoparticles (0.0028, 0.0143, 0.028, and 0.143 wt%) were used to investigate the effect on attributes of rheological and mud filtration for bentonite mud. Results of experimental tests indicated that yield point property increases up to 250% with the 0.143 wt% of FMWCNTs in bentonite mud with poly acrylonitrile-co-N-vinyl pyrrolidone/zeolite composite. Also, investigating the effects of higher temperature has shown that an improvement in thermal stability can obtain by using this composite and FMWCNTs in drilling mud rheological properties such as plastic viscosity, yield point, mud filtration volume. Evaluating the effects of using novel formulated water-based mud has shown that it can increase the drilling mud rheological properties and reduce mud filtrate volume and decrease formation damage particularly under the HPHT environment.

Effects of Multi-Wall Carbon Nanotubes (MWCNT) on Performance of Water Based Mud (WBM)

The present review article expresses the effectiveness of different types of Multi-Wall Carbon Nanotube(s) (MWCNT) on the rheological properties of Water-Based Mud drilling fluid. Shale formations are troublesome according to their water sensitivity characteristic, low Nano-Darcy permeability with small nanometer-sized pore throats that are not effectively sealed by the solids in conventional drilling fluids. Water invasion into the shale formations causes different problems in the drilling industry. Nanoparticle additives have the potential to be used in drilling operations due to their large surface area and lower kinetic energy than micro-additives. The test results demonstrate that multi-wall carbon nanotube improves the rheological properties of the WaterBased Mud drilling fluid. Acidic-surface modified carbon nanotube in the presence of Polyethylene Glycol (PEG) increases the plastic viscosity and yield point of the sample more than unmodified carbon nanotube and hybrid multiwall carbon nanotube. Test results show that multi-wall carbon nanotubes (MWCNT) improve shale stability and shale recovery, finally leading to more well-bore stability in shale formations.

Modified Corn Starch as an Environmentally Friendly Rheology Enhancer and Fluid Loss Reducer for Water-Based Drilling Mud

SummaryDrilling fluid rheology and fluid loss property are fundamental parameters that dictate the effectiveness and easiness of a drilling operation. Maintaining these parameters under high temperatures is technically challenging and has been an exciting research area for the drilling industry. Nonetheless, the use of drilling mud additives, particularly synthetic polymers, threaten ecological environments. Herein, modified corn starch (MCS) was synthesized, characterized, and investigated as an environmentally friendly rheology enhancer and filtration loss controlling agent for water-based mud (WBM) at high temperatures. The experimental results indicated that MCS exhibits better performance in improving rheological properties and fluid loss controlling ability for WBM than the commonly used mud additives. With the addition of an optimal concentration (0.3 wt%), MCS improved the rheology and fluid loss behavior of WBM formulation at harsh aging temperature (220°C) by practically 4 times and 1.7 times, respectively. The MCS was revealed to perform superbly over polyanionic cellulose (PAC) addition at all investigated temperatures. The better performance of the MCS was ascribed to the improved entanglements in the mud system owing to the additional hydroxyl (OH) groups. Besides, the Herschel-Bulkley model was found to be a constitutive model that described the rheological properties of the investigated muds satisfactorily. Moreover, the MCS was found to exhibit acceptable biodegradability properties.

Evaluation of using waste pinecones as an eco-friendly additive to water-based mud

This study is investigating the possibility of using pinecones as additive materials to develop a water-based drilling mud. Pinecones are environmentally friendly agricultural wastes and have no practical applications. Therefore, this study is of great importance in that it highlights the investment of these natural waste materials in practical fields. The study focused on the effectiveness of pinecone powder in a water-based drilling mud. To this end, an experimental study on five types of pinecones (Austrian pinecones, Black Hills Spruce, Sitka Spruce, Norway Spruce, and White pinecones) was performed and tested to determine their ability to reduce filter loss and also the effect of those eco-friendly materials on the rheological properties and density of the mud. In the study, 26 samples of water-based mud mixed with the different types of pinecones at different concentrations based on the weight of mud sample (1 wt%, 2 wt%, 3 wt%, 4 wt%, and 5 wt%) were used to perform the experiments. The results of the study showed that pinecones have a great ability to reduce filter loss, particularly Norway Spruce cones, which, based on this study, are considered the superior type with a concentration of 3 wt%.

An experimental analysis on nanoparticles role in drilling fluids

The study around nanotechnology has increased in the last decade or so, with rise in applications in many industries. There are now many nanotechnology-based products available like in healthcare, defense, coating industries etc. It has seen rise in usage in oil and gas industry as well. Application of nano particles in water based muds results in reduction of fluid loss and hence increases the wellbore stability and drilling efficiency. In this study, we have examined the fluid loss on water-based mud by adding graphene nanoparticles as additive and estimated the optimum value of nanoparticle composition for having less fluid loss. The results of experimental analysis for filtration loss from water-based mud with addition of graphite nanoparticles was studied and effect of friction factor was analyzed. The nanoparticles concentration added in fluid ranges from 1 to 5 lb/bbl and the size of the particles was observed to be 30–80 nm. Variation of fluid loss volume and friction coefficient under different nanoparticle composition was established.

Physical-mathematical model applied in the study and simulation of dissolution of sodium chloride particles in brines in upward flow in tubes

Drilling trough salt layers has been a challenge for the oil drilling industry. In this way, water-based muds are a friendly environment, although they tend to dissolve generated salt particles along with the wall of the well. Particularly, the particle dissolution may cause changes in the fluid rheology and physicochemical properties. Other problems related to this are the enlarging of the borehole, the excessive torque, the accumulation of salt gravels in the bottom hole, the well controlling issues, and some other operational situations. Thus, the controlling and prediction of the salt dissolution process in drilling fluids are of great interest. The main objective of this work was to study the dissolution of halite (NaCl) particles in brines at different concentrations on upward flow in tubes that simulate the borehole flow. An experimental apparatus was built, capable of reproducing some drilling conditions as the upward fluid flow where the data of NaCl concentration as function of position and time were obtained. A mass conservation mathematical model based on the continuum mechanic was proposed in order to simulate the process. The model was composed of two differential partial equations generated by the conservation law for the mass of salt in two phases: a fluid phase of salt dissolved in water and a solid phase of salt particles. The dissolution data were used to estimate the global mass transfer coefficient in different operational conditions and to validate the mathematical model. The total mean deviations were lower than 5 % for the proposed and validated model.

Characterization of magnesium oxide nanoparticle for water-based drilling mud formulation

This research is based on the synthesis and analysis to determine the efficiency and cost effectiveness of nanoparticle materials used in drilling fluid during oilfield exploration process. The study was carried out in the formulation of a waterbased drilling fluid, in which the prepared additives were treated with magnesium oxide (MgO) nanoparticles. The rheological properties of nanoparticle materials, such as flow time, viscosity, shear rate, shear stress, torque difference, and separation of solids and liquids, have been determined. The study also analyzed the fluid loss with respect to time in a manual filter press for low temperature and low pressure versus high pressure and high temperature using filter press hydraulic dead weight assembly in high temperature and high pressure. The nanoparticle used for the study was characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer and particle size analyzer. Viscosity parameters were optimized using a multi-speed viscometer, lubricity tester and pH meter at effective concentration, as well as contact time, temperature and pressure changes as a function of concentration. The study suggests the use of MgO nanoparticle as an additive for water-based mud exploration in oilfields.

Experimental evaluation of Sarcosine as an eco-friendly green hydrate inhibitor for the drilling of gas hydrate bearing formations

The hydrate inhibitors played a significant role in preventing hydrate formation in the drilling fluid systems. However, the disposal of these inhibitors was the primary concern for the drilling industries, as Thermodynamic Hydrate Inhibitors (THIs) are not eco-friendly, and Kinetic Hydrate Inhibitors (KHIs) are troublesome to dispose of due to their long polymeric structures. Hence, an attempt had been made to evaluate the Sarcosine, an amino acid, as a biodegradable, green hydrate inhibitor for water-based drilling fluid systems. Initial screening of Sarcosine in the THF-water hydrate (structure-II) system showed that it behaved as a zwitterionic, dual-function inhibitor. It had better inhibition characteristics in delaying induction time and improving rheological and filtration properties than PVCap and PVP while used in formulated mud compositions. The water-based mud used in this work was prepared by mixing carboxymethyl cellulose (CMC), polyanionic cellulose (PAC), xanthan gum (XG), and potassium chloride (KCl) with distilled water. For Sarcosine content water-based mud systems, coefficients of determination (R2) values were dependable, while low-temperature (2 C) rheological values were fitted in Herschel-Bulkley model. These experimental investigations showed that the Sarcosine might be used for the drilling of gas hydrate-bearing formations.

Modelling of the cooling effect enhancement in drilling fluid using nanotechnology

Drilling fluid is indispensable to assure the safety and success of a drilling operation. Besides the normal drilling fluid such as water-based mud or oil-based mud, a new kind of drilling fluid has emerged recently, which consisted of the use of nanotechnology. The aim of this paper is to study the cooling effect of nano-drilling fluid used in the petroleum industry. A dynamic model that included a reservoir formation, a well, and a drill string in the drilling process with drilling fluid circulation was built for this objective. Navier-Stoke equation was used for the fluid flow inside the well and the drill string, while Darcy’s equation was used for the flow inside the formation. The rise of temperature due to friction was also accounted for in this model. Two types of drilling fluid were used in the simulation: the normal drilling fluid and the one using nanotechnology. The change of temperature in the wellbore and in the formation over time with these two types of drilling fluid was observed at various positions: at the bottom hole where the drilling bit is constantly in contact with the formation, and at other places further away from the bottom hole. The simulated results showed that, although the temperature fluctuated in the two cases but on average, the nano drilling fluid gave a better cooling effect in comparison with the normal one. This article is the first study about the application of nanoparticles in drilling fluid in Vietnam using an integrated modeling method. The approach proposed in this article can be applied efficiently in practical applications of nano drilling fluid for petroleum drilling in Vietnam. However, it is noted that this research treated typically the technical side of the application of nanotechnology in drilling fluid, while it will be necessary to asset the financial aspect in order to make this technology a real-life application.

Investigation of rheological and filtration properties of water-based drilling mud using commercially available additives

Drilling fluids are an essential component of the drilling system. Conventional copolymers have shown degradation at high temperature. To significantly improve the drilling fluid, the use of nanoparticles (NPs) for advancement of their rheological properties and fluid loss control is introduced. In this study commercially available additives, barite and nano-silica, are taken and in varying concentrations to identify the most optimized fluid that can be used for drilling operations. With this we hope to reiterate the significant difference between the ability of nanomaterials and other additives and verify those claims by performing experimentation. The observation in the past researches and the experimentation concur that nanoparticle enhanced mud has better rheological and improved filtration properties compared to the barite mud.

Multiscale Approaches Including ANN and M5P-Tree with SI and OBJ Assessment Tools to Predict the Shear Thinning of Bentonite Drilling Muds Modified with Clay Nanosize at Various Elevated Temperatures

Improving the flowability behavior of the water-based drilling mud (WBM) using bentonite under high-temperature conditions is significant in the drilling borehole operations. In this study, the influence of bentonite and clay nanoparticle (CNP) contents on the non-Newtonian fluid (rheology) properties such as initial shear stress and fluid’s resistance to flow (viscosity) under high-temperature conditions were tested and quantified. The shear stress and shear strain rate relationships of WBM were simulated using two new rheological models of the vapor pressure and modified Hoerl models, and the modeling outcomes were validated with the Herschel–Bulkley (HB) model. This study also analyzed and formulated more than 268 data sets, including observational data and data obtained from previously published studies ranging with bentonite in the drilling muds from 2% to 8% (%wt. of water), 0% to 1% of CNP (%wt. of bentonite), and heating temperatures were ranging from 25°C to 100°C. The HB rheological model predicted initial shear stress better than the other two rheological models depending on several statistical assessments. The initial shear stress of drilling muds can be predicted well based on bentonite, CNP contents, and temperature using various simulation techniques. According to several statistical tests, the nonlinear regression and artificial neural network (ANN) performed stronger than other model techniques to predict the yield stress of WBM as a function of bentonite, CNP contents, and temperature.

Effect of Cassava Starch and Sodium Carbonate on the Properties of Local Drilling Mud: Beneficiation to Improve the Rheological and Flow Properties of Locally Formulated Mud

Drilling mud is a key component in drilling operations and in accessing oil and gas reservoirs. Bentonite is applied as a viscosifier, fluid loss control agent, and as a weighting material in water-based drilling mud. The type of bentonite used in drilling mud formulation is sodium bentonite due to its high dispersion properties and high swelling capacity. Nigeria has a huge bentonite clay deposit resources which can be evaluated and enhanced in order to be utilized as drilling mud. However, bentonite clay from different parts of Nigeria was investigated and found to be calcium bentonite which is not suitable for drilling mud, because it has low swelling capacity and poor rheological properties. In this study, local bentonite obtained from Afuze, Edo state was used to formulate different samples of drilling mud with each treated using thermo-chemical beneficiation process with sodium carbonate and cassava starch, and then undergo characterization to identify the changes in physical properties and finally, draw comparison with API values for standard drilling mud. The results obtained from this study indicates that, the flow and rheological properties of the beneficiated drilling mud developed through thermo-chemical treatment, showed significant improvement compared to the untreated mud. Therefore, pure calcium bentonite from natural deposits in Nigeria can be modified to sodium bentonite and sufficiently used in drilling mud formulation.

Experimental investigation of water based drilling mud by using graphene

In the oil and gas industry, the need for a particular composition-based drilling mud is characterized by its rheological and fluid loss properties. Enhancing these properties will increase the efficiency of drilling fluid and, hence, the wellbore damage will be controlled. Recent advancements show that the application of nanoparticles in drilling fluids will enhance their efficiency. This research investigates the influence of adding graphene nanoparticles on the performance of water-based drilling fluids. The main objective of this experiment was to investigate the effect of nanoparticles on water-based drilling mud with different concentrations of nanoparticles added to the mud.

Rheological properties of clay-polymer systems: application on water-based drilling mud

Rheological properties of clay-polymer systems: application on water-based drilling mud

Experimental investigation of rheological properties of water-base mud with zinc oxide nanoparticles using response surface methodology

Special and advanced techniques at drilling and production operations are used to drill and produce hydrocarbons from different eccentric reservoirs such as deep-water reservoirs, shale gas, and shale oil necessitates. However, these advanced techniques are considered to be costly and affecting the environment. Therefore, nanotechnology has been implemented in the drilling fluids to improve the rheological properties, hydrogen concentration, and filtration rate. This study is, therefore, aimed at evaluating the properties of zinc oxide nanoparticles and their previous implementation in drilling fluids, examining the rheological properties of water base mud from atmospheric temperature to downhole condition (90 °C). The experiment was designed to investigate the effect of nanoparticles concentration and temperature variation on the rheological properties of water-based mud using response surface methodology. A rotational viscometer was used to measure viscosity, yield point and gel strength. Moreover, a Consistometer was used to heat the fluid sample to different temperatures reaching up to downhole condition (90 °C). It was observed experimentally that addition of zinc oxide nanoparticles impressively improved the rheological properties of water-base mud by 50% for viscosity and gel strength, and 80% for yield point. It also allowed them to maintain their quality at all temperatures. Furthermore, addition of higher concentrations of zinc oxide nanoparticles of more than 1 wt% (6 g) at low temperatures (below 40 °C) gave the highest and most effective results of the rheological properties. • The effects of zinc oxide nanoparticles on rheological properties of water base mud were investigated. • Regression models were developed via Central Composite Design (CCD) method. • Zinc oxide nanoparticles impressively improved viscosity and gel strength by 50% and yield point by 80%. • Addition of more than 1 wt% (6 g) of zinc oxide nanoparticles at low temperatures below 40 °C was found effective. • The developed models' accuracy was affirmed at optimum point.

Nonlinearity in large amplitude oscillatory shear of drilling fluid: A comparative study on the oil and water base muds behavior

A viscoelastic material exhibits both fluid (viscous) and solid (elastic) like properties simultaneously. The rheological behavior of drilling fluids is customarily measured under the assumption of pure viscous flow, yet they exhibit both viscous and elastic characteristics when subjected to deformation. Some imperative properties of drilling fluids such as gel strength, yield stress, and sag tendency at near-zero shear rate region are categorically under the influence of viscoelastic behavior and have not yet been completely understood due to the lack of appropriate measurement methods. Characterizing the complete rheological profile of material requires measuring linear and nonlinear viscoelastic responses at a wide range of stress/strain levels. The viscoelastic nonlinearity can be characterized under Large Amplitude Oscillatory Shear (LAOS) tests. This study presents a large amplitude oscillatory shear rheology investigation of two water-base (Lignosulfonate and KCl/polymer) and two oil-base (low and high temperature) drilling fluids each in three states; unweighted, weighted, and weighted-contaminated. The viscoelastic nonlinearities of four fluid samples were investigated experimentally as a function of strain amplitude and strain-rate space at different temperatures. The results were described using Lissajous-Bowditch (L-B) plots and local nonlinear viscoelastic dynamic moduli through an oscillatory shear cycle. Structural stability of fluid samples was also studied using the creep-recovery test. The nonlinear viscoelastic behavior of drilling fluids was comprehensively compared and distinguished to understand their nonlinear shear mechanism. As a result of the creep-recovery test at 150 °C, high-temperature oil base mud provided higher thermo-structural stability and zero shear viscosity than those of Lignosulfonate, KCl/polymer, and low-temperature oil-base muds. While the elastic nonlinearity was found to be strain/strain rate softening, the nonlinear viscous character was recorded as shear/shear rate thinning at large strain rates for all drilling fluid systems. Quite a different shear mechanism was detected at low and moderate strain rates. Shear rate thinning was found to be the dominant mechanism for all fluids except Lignosulfonate mud samples that demonstrated shear-thickening behavior at moderate strain rates at 25 °C. At high temperatures (100 °C and 150 °C), shear rate thickening followed by shear rate thinning behavior was governing mechanism under the low and moderate strain rates in all mud samples except KCl/polymer muds. An inverse behavior (shear rate thinning followed by shear rate thickening) was observed through low and moderate strain rates in KCl/polymer muds at 100 °C and 150 °C. Therefore, as revealed from the LAOS test results, shear rate thickening behavior at low to moderate strain rates is in contradiction of the commonly known shear thinning behavior for a drilling fluid undergoing viscous deformation. This finding explains the facts underlying some important properties of drilling fluids such as gel structure formation, hole cleaning ability, and sag tendency.