INTRODUCTIONAlthough a great deal has been written about the pressure and temperature behaviour of the viscosity of simple non-ABSTRACT Newtonian fluids, and an understanding of this behaviour at The theological behaviour of invert emulsion muds has been the molecular level is emerging, no consensus exists on how studied at pressures up to 1000 bar and temperatures up to to deal with concentrated suspensions. This can easily be 240°C. Theological parameters were calculated for the Bingunderstood, considering the widely different nature of nonham, Herschel-Bulkley and Cssson theological models. The Newtonian fluids, Invert emulsion muds are suspensions of Iierachel-Bulkley and Cesson modeb both give good fits to the solids and emulsions at the same time, and as there is no experimental rheograms. The Cesson model is more reliable generally accepted theological model that can be applied to for extrapolation purposes than the Herschel-Bulkley model. emulsions and suspensions, the engineering aspects of invert A pair of two similar exponential expressions were found to emulsion muds are not always based on very sound scientific be able to model the pressure and temperature behaviour of principles, Thus, while it is known that at pressures and temthe two parameters of the Casson model. The expressions, peratures encountered in the wellbore the rheology of the mud which are baaed on the relation for pure liquids derived theoretically by Eyring, contain temperature dependent pressure will be different from that measured at the surface, lack of coefficients. The simplifications inherent in the temperature the ability to quantify the effects involved has perpetuated the and pressure model are dkcussed in the light of the tempera-field practice of using theological parameters measured at atture and pressure behaviour of the viscosity of common base mospheric pressure. Traditionally the mud industry has, with oils and their constituent hydrocarbons. Field application of a few exceptions, adhered to the uee of the Bingham and power the model requires measurement of the rheology of the mud at law theological models, which have the advantage that hytwo or more temperatures and knowledge of the pressure co-draulics calculations are available for fluids obeying these modefficients relating the behaviour of the plastic viscos~ty to that els. Hence, it is not surprising that the existing techniques for of the yield point, or the Casson high shear visccgity to that prediction of downhole rheology are based on these models. of the Casson yield stress. Pressure meaauremer !S or other A number of recent publications have dealt with the problem. information are then not required. Applications can be baaed Combs and Whitmire (1) showed that the change in the viscoson Caason or Bingham theological meesurements. The rela-ity of the continuous phase is the main factor in controlling the tionships between the parameters of the Casson and Bingham change in the viscosity of the mud with pressure. Both yield models are disussed. point and plas...
Previous work on shale mechanical properties has focused on the slow deformation rates appropriate to wellbore deformation. Deformation of shale under a drill bit occurs at a very high rate, and the failure properties of the rock under these conditions are crucial in determining bit performance and in extracting lithology and pore-pressure information from drilling parameters. Triaxial tests were performed on two nonswelling shales under a wide range of strain rates and confining and pore pressures. At low strain rates, when fluid is relatively free to move within the shale, shale deformation and failure are governed by effective stress or pressure (Le., total confining pressure minus pore pressure), as is the case for ordinary rock. If the pore pressure in the shale is high, increasing the strain rate beyond about 0.1 %/sec causes large increases in the strength and ductility of the shale. Total pressure begins to influence the strength. At high strain rates, the influence of effective pressure decreases, except when it is very low (Le., when pore pressure is very high); ductility then rises rapidly. This behavior is opposite that expected in ordinary rocks. This paper briefly discusses the reasons for these phenomena and their impact on wellbore and drilling problems.
Summary Four carboxymethyl cellulose (CMC) polymers were characterized by molecular weight, degree of substitution, and intrinsic viscosity. These polymers were used to make simple water-based muds with various polymer and bentonite contents. API fluid loss and high shear viscosity were determined for each mud. Fluid loss is independent of polymer molecular weight at low ionic strength. The high shear viscosity of muds and polymer solutions is related to the product of the intrinsic viscosity of the polymer and its concentration. Introduction Growing concern amongst government and environmental agencies over the environmental impact of oil-based drilling fluids1,2 has increased the drilling industry's reliance on water-based muds. An important aspect of water-based muds is the design and testing of water-soluble polymers to control the main mud functions: rheology, fluid loss, and shale stabilization. Both naturally occurring and synthetic polymers ranging from low-molecular-weight dispersants (e.g., lignosulphonates3,4) to high-molecular-weight polymers for shale stabilization (e.g., partially hydrolyzed polyacrylamides5–7) have been used extensively in water-based muds.3,8 Several recent papers9–11 describe the range of water-soluble polymers and their functions in water-based drilling fluids. The most common naturally occurring polymers are the polysaccharides, which include CMC, starches, xanthan gum, and guar gum. CMC polymers are probably the most common and are used routinely both to control fluid loss and to increase viscosity. Interest has increased in synthetic polymers that extend temperature and salinity/hardness limits of naturally occurring polymers. A recent example is the development of a sulfonated copolymer12 for fluid-loss control in drilling fluids subject to high temperatures and high calcium concentrations. Despite the growing potential and extensive use of polymers in water-based muds, they often are characterized poorly in terms of their basic compositions, average molecular weight, molecular-weight distribution, charge density (and charge distribution), and, particularly in the case of high-molecular-weight polymers, the frequency of long-chain branching. Polymers have long been used sucessfully in water-based muds; however, the influence of polymer Composition on mud properties often is unclear. For example, understanding of the mechanisms that allow shales to be stabilized by both high-5–7 and low-13 molecular-weight polymers has been inadequate. Another problem has been the paucity of reliable techniques to monitor polymer concentration and molecular-weight degradation in the field. The increasing use of polymers is expected to emphasize these problems. The objective of this paper is to establish the influence of molecular weight and charge density (degree of substitution) of a number of well-characterized CMC samples on the rheology and fluid loss of bentonite-based drilling fluids. CMC Characterization Composition and Impurities. Fig. 1 shows the chemical structure of cellulose and. CMC polymers. Cellulose is composed of repeating units of D-glucopyranosyl with a 1,4 glycoside linkage. Cellulose is modified to form the sodium salt of CMC by reaction with monochloroacetic acid in the presence of caustic soda 14:R(OH)3+ClCH2CO2H++2NaOHR(OH)2OCH2CO2Na+NaCl+H20 Each repeating D-glucopyranosyl unit contains three hydroxyl groups capable of etherification, to give a maximum charge density of three sodium ions per monomer unit (i.e., a degree of substitution of three). Ho and Klosiewicz15 used nuclear-magnetic-resonance (NMR) spectroscopy to demonstrate that the three hydroxide sites are not equally active in the etherification reaction; the order of reactivity is 2 > 6 > 3 (see Fig. 1). The addition of ionic groups to cellulose produces a water-soluble polymer that is used widely in many industries.16 CMC is water-soluble when the degree of substitution is greater than 0.414; the most common degree of substitution range for industrial CMC is 0.4 to 0.8.14,16 The most common form of CMC is the sodium form, where the carboxylate anion is balanced by a sodium counter-ion. A recent European patent application17 described the use of potassium CMC in drilling fluids. Table 1 shows the analysis of several CMC products, including a "polyanionic cellulose" (PAC). The CMC was dissolved in water (10 g/L) and acidified with an equal volume of 0.25 M sulfuric acid (providing a 10-fold excess of protons in solution over sodium from the polymers) to generate the acid form of the polymer and to release the sodium into solution. Some of the polymer precipitated and was. removed by centrifugation. The anion and cation content of the supernatant was determined by ion chromatography, and the ion concentrations were reported as moles per gram of polymer. The technical oilfield-grade CMC samples generally were characterized by high (up to 20 wt %) NaCl content. The NaCl, a byproduct of manufacturing, therefore has been only partially removed. In contrast, the laboratory-grade samples are almost free of NaCl. The pH's of the various CMC solutions (Table 1) indicated that residual acids and alkalis also may have been contaminants; again, the technical oilfield-grade solutions showed the greatest degree of contamination. Also, CMC is hygroscopic, and a significant fraction of its weight may be water. Measurement of weight loss after drying indicated that the water content ranged from 5 wt % to 16 wt %; equilibrium moisture content from 20 wt% to 30 wt% for CMC polymers with degrees of substitution of 0.7 to 1.2 at 80% relative humidity have been reported.16 The measured degree of substitution for the oilfield CMC products ranged from 0.80 to 0.96, compared with reported values ranging from 0.7 to 0.917,18; our PAC sample, with a 1.00 degree of substitution, was in agreement with the range of 0.9 to 1.5 that has been reported.17–19 The laboratory reagent-grade samples generally have lower degrees of substitution (0.71 to 0.83) than the oilfield grade samples but contain lower impurity levels. Two samples of potassium CMC, prepared as in Ref. 17, also were analyzed; the measured degree of substitution for these polymers was in good agreement with published values.18 Composition and Impurities. Fig. 1 shows the chemical structure of cellulose and. CMC polymers. Cellulose is composed of repeating units of D-glucopyranosyl with a 1,4 glycoside linkage. Cellulose is modified to form the sodium salt of CMC by reaction with monochloroacetic acid in the presence of caustic soda 14:R(OH)3+ClCH2CO2H++2NaOHR(OH)2OCH2CO2Na+NaCl+H20 Each repeating D-glucopyranosyl unit contains three hydroxyl groups capable of etherification, to give a maximum charge density of three sodium ions per monomer unit (i.e., a degree of substitution of three). Ho and Klosiewicz15 used nuclear-magnetic-resonance (NMR) spectroscopy to demonstrate that the three hydroxide sites are not equally active in the etherification reaction; the order of reactivity is 2 > 6 > 3 (see Fig. 1). The addition of ionic groups to cellulose produces a water-soluble polymer that is used widely in many industries.16 CMC is water-soluble when the degree of substitution is greater than 0.414; the most common degree of substitution range for industrial CMC is 0.4 to 0.8.14,16 The most common form of CMC is the sodium form, where the carboxylate anion is balanced by a sodium counter-ion. A recent European patent application17 described the use of potassium CMC in drilling fluids. Table 1 shows the analysis of several CMC products, including a "polyanionic cellulose" (PAC). The CMC was dissolved in water (10 g/L) and acidified with an equal volume of 0.25 M sulfuric acid (providing a 10-fold excess of protons in solution over sodium from the polymers) to generate the acid form of the polymer and to release the sodium into solution. Some of the polymer precipitated and was. removed by centrifugation. The anion and cation content of the supernatant was determined by ion chromatography, and the ion concentrations were reported as moles per gram of polymer. The technical oilfield-grade CMC samples generally were characterized by high (up to 20 wt %) NaCl content. The NaCl, a byproduct of manufacturing, therefore has been only partially removed. In contrast, the laboratory-grade samples are almost free of NaCl. The pH's of the various CMC solutions (Table 1) indicated that residual acids and alkalis also may have been contaminants; again, the technical oilfield-grade solutions showed the greatest degree of contamination. Also, CMC is hygroscopic, and a significant fraction of its weight may be water. Measurement of weight loss after drying indicated that the water content ranged from 5 wt % to 16 wt %; equilibrium moisture content from 20 wt% to 30 wt% for CMC polymers with degrees of substitution of 0.7 to 1.2 at 80% relative humidity have been reported.16 The measured degree of substitution for the oilfield CMC products ranged from 0.80 to 0.96, compared with reported values ranging from 0.7 to 0.917,18; our PAC sample, with a 1.00 degree of substitution, was in agreement with the range of 0.9 to 1.5 that has been reported.17–19 The laboratory reagent-grade samples generally have lower degrees of substitution (0.71 to 0.83) than the oilfield grade samples but contain lower impurity levels. Two samples of potassium CMC, prepared as in Ref. 17, also were analyzed; the measured degree of substitution for these polymers was in good agreement with published values.18
The issue of drilling depleted zones is increasing in importance as more wells are drilled in mature fields. These zones are typically produced or producing reservoirs overlaid and interbedded with shale layers. Pressure overbalances have been reported as high as 13000 psi but are more typically of the order of a few thousand psi. Wellbore stability problems associated with drilling in these zones can be linked with drilling-induced and pre-existing fractures. We describe an approach that links a fracture-fluid-flow model with fluid rheology over a wide range of flow rates and flow behavior in a fracture generation apparatus. The understanding gained is used to develop guidelines for minimising losses into fractures. A numerical fracture simulation scheme with Perkins-Kern-Nordgren (PKN) geometry and flexible rheology of the invading fluid predicts fluid volume lost as a function of time. The drilling environment - differential pressure, fracture gradient, pore pressure and rock properties - can be varied. The effect of fluid rheology on fluid loss rate is demonstrated under various combinations of the parameters relevant to depleted zone drilling. Drilling fluid rheology was investigated in shear flow over the shear rate range 0.001 - 1000 s−1, and in transient flow. Most fluids exhibited shear-thinning and thixotropic behavior that could not be described in terms of PV and yield point (YP) alone. Constitutive rheological models were used to describe the data for input to the simulation model. A wide range in transient behavior was found, and it forms the basis of an experimental test to rank and select fluids to minimize losses in fractures. The fracture generation apparatus enables a fracture to be initiated in a rock core, closed and then re-opened. We evaluated a suite of water-based and oil-based fluids and lost circulation materials, some of which show unexpected increases in the reopening pressure. Introduction The issue of drilling depleted zones is increasing in importance as more fields mature. These zones are typically produced or producing reservoirs overlain and interbedded with shale layers. Pressure overbalances have been reported as high as 13000 psi in the Gulf of Mexico1 but more typically are on the order of a few thousand psi. Drilling problems in these zones can be broadly categorised into three main areas: Wellbore StabilityThe presence of normally pressured shales means a higher mud weight is required to prevent collapse even when drilling in the depleted zone.The drilling profile with regard to bedding must be considered both with regard to the overlying shales and weakening of the reservoir rock itself that can result from the depletion process.There can be an issue with mechanical sticking from creeping shales if mud weights are not maintained high enough. Proper levels of inhibition need to be maintained to prevent chemical swelling of shales.If salt structures are being drilled, high salt concentrations are needed to prevent dissolution; therefore it is difficult to lower the mud weight for the depleted zone. Lost Circulation into Pre-Existing and Drilling-Induced fracturesA high mud weight can result in fracturing of rock already weakened by the depletion process.The loss of fluids into fractures is costly and may lead to well control problems.There could be loss of productivity with blocked fractures.Fractures can increase overall wellbore stability problems
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