EQUIPMENTANDPROCEDURESFORUATING.doc

EQUIPMENT AND PROCEDURES FOR EVALUATING DRILLING FLUID PERFORMANCEDevising tests that will accurately describe how drilling fluids behave downhole is virtually an impossible task. Most drilling fluids are complex mixtures of interacting components, and their properties change markedly with changes in temperature, shear rate, and shear history. As they circulate through the borehole, drilling fluids are subjected to ever changing conditions-turbulent flow in the drill pipe, intense shearing at the bit, and nominal laminar flow in the annulus at frequently changing shear rates (owing to changes in the gauge of the borehole). The viscous properties of most muds are time-dependent, and they seldom have time to adjust to any one set of conditions. In addition, there is a continuous temperature change as the mud circulates, and a continuous composition change as solids and liquids from the formation are incorporated into the mud by the drilling process. Another problem is that tests at the wellsite must be performed quickly and with simple apparatus. To a lesser extent, this limitation also applies to laboratory tests made in support of field operations. It is not surprising, therefore, that the standard field and laboratory tests which have been accepted by industry are quick and practical, but only approximately reflect downhole behavior. Nevertheless, these tests serve their purpose very well if their limitations are understood and if the data obtained from them are correlated with experience. In this chapter, these tests are briefly described, and their purpose and underlying principles are explained. Detailed laboratory procedures are not given. These will be found in the API publication RP 13B, and it would serve no purpose to repeat them here. Over the years, various tests that more closely simulate downhole conditions have been devised by individual investigators. These tests require more elaborate equipment, are time consuming, and are therefore more suited to research and development. In many cases, the results from these tests are discussed in subsequent chapters; in this chapter, the apparatus and procedures will be briefly described and the appropriate references given. Sample Preparation Since the properties of muds depend so much on shear history and on temperature, it is of the utmost importance that muds to be tested in the laboratory first be subjected to conditions similar to those prevailing in the drilling well. Muds made from dry materials should be given a preliminary mixing, and should then be aged for a day or so to allow the colloids time to hydrate. Then, the mud should be subjected to a high rate of shear until a constant viscosity is obtained, and their properties tested at ambient temperature. If the mud is intended for use in a well with a bottom-hole temperature greater than 212 (100), it should be aged at the temperature of interest, as outlined later in this chapter. Samples brought in from a well will have had time to cool, and, if thixotropic, will have set up a gel structure. Such muds must be sheared at the temperature observed in the flow line until the viscosity corresponds to that measured at the rig. Mixers such as the Hamilton Beach (Figure 3-1) and the Multimixer (Figure 3-2) are used in laboratory tests of mud materials. They do not, however, produce the high rates of shear that exist in circulation of the drilling fluid in wells. High rates of shear are only obtained when there is little clearance between the stator and the rotor, or when the mud is pumped through a small orifice or opening. Food blenders in which the blades rotate in a recessed section at the bottom of container (Figure 3-3) provide high shearing rates, and are suitable for shearing small quantities (about a liter) ofmud. They can only be used for short periods of time because the temperature rises rapidly, with conseqent loss of water by evaporation. For larger amounts of mud, it is best to use a high-shear mixer such as the Eppenbach. This instrument consists essentially of a circulating unit which is mounted on rods extending from the base of the driving motor. This arrangement enables the unit to be lowered into a large vessel (8 liters or so) of mud, and to circulate the mud throughout the vessel. The clearance between the rotor blades and the baffles on the housing is close so that a highrate of shear is maintained in the circulating unit. lternatively, a colloid mill may be used to achieve an extremely fine state of subdivision. Sinha and Kennedy2 obtained excellent reproducibility with an Eppenbach colloid mill, model Q-V-6-3, modified so that the mud wascirculated through a cooling coil and evaporation was prevented. Another excellent way of pre-shearing muds is to pump them around a closed circuit containing a shearing valve or other restriction, and a heat exchanger. A description of such systems is given in the section on multifunctional systems later in this chapter.Properties MeasuredDensity Density, or mud weight, is determined by weighing a precise volume of mud and dividing the weight by the volume. The mud balance (Figure 3 4) provides the most convenient way of obtaining a precise volume. The procedure is to fill the cup with mud, put on the lid, wipe off excess of mud from the lid, move the rider along the arm till a balance is obtained, and read the density at the side of thc rider towards the knife edge. Density is expressed in pounds per gallon (lb/gal), pounds per cubic foot (lb/ft3), grams per cubic centimeter (g/em3), or as a gradient of pressure exerted per unit of depth. Conversion factors are as follows: The mud balance may be calibrated with fresh water. At 70(21) the reading should be 8.33 lb/gal, 62.3 lb/ff3, or 1.0 SG. Densities of flowing slurries can be measured by a gamma ray densimeter.3Viscosity The Marsh Funnel. This instrument is useful on the drilling rig, where it enables the crew to periodically report the consistency of the mud, so that significant changes may be noted by the mud engineer. It consists of a funnel and a measuring cup (see Figure 3-5), and gives an empirical value for the consistency of the mud. The procedure is to fill the funnel to the level of the screen and to then obsve the time (in seconds) of effiux of one quart (946cc). The number obtained depends partly on the effective viscosity at the rate of shear prevailing in the orifice, and partly on the rate of gelation. The time of effiux of fresh water at 70+- 5 (21 +- 3) is 26 +0.5 seconds.V These and other objects of the invention will be obviousto one skilled in the art upon reading this specifcation andclaims. The process can comprise, consist essentially of, or con-sist of the stated steps with the stated materials. The com- :positions can comprise, consist essentially of, or consist ofthe stated materials. DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION l The well drilling and servicing fluids of this inventioncomprise an aqueous liquid having a water soluble polymerhydrated therein and a surfactant. The polymers useful in theELSRV fluids of this invention are such that the ELSRVfluids have a thixotropic index of at least 10, wherein the 1thixotropic index is the ratio of the Brookfield viscosity at0.5 rpm to the Brookfield viscosity at 100 rpm. The thixo-tropic index is indicative of the shear thinning characteristicsof the fluid. The base aqueous fluid in which the low shear rate 2 modifying polymer isJydrated may be any aqueous liquidwhich is compatible with the polymer. Thus the base liquid may be fresh water, or a brine containing soluble salts such as sodium chloride, potassium chloride, calcium chloride, sodium bromide, potassium bromide, calcium bromide, zinc 2 bromide, sodium formate, potassium formate, cesium formate, and the like. The brine may contain one or more soluble salts at any desired concentration up to saturation. The polymers useful in the ELSRV fluids of this invention comprise any water soluble polymer which increases the low 2 shear rate viscosity of the fluid to produce a flu!d exhibiting a high yield stress, shear thinning behavior. Particularly useful are biopolymers produced by the action of bacteria, fungi, or other microorganisms on a suitable substrate. Exemplary. biopolymers are the polysaccharides produced 2 by the action of Xanthomonas compestris bacteria which are known as xanthan gums. These are available commercially from several sources including: Kelco Oil Field Group, Inc., under the trademarks Xanvis and Kelzan; Rhone- Poulenc Chimie Fine, under the trademark Rhodopol 23-p; Pfizer Inc., under the trademark Flocon 4800C; Shell International Chemical Company of London, U.K., under the trademark Shellflo ZA; and Drilling Specialties Company, under the trademark Flowzan. See for example U.S. Pat. No. 4,299,825 and U.S. Pat. No. 4,758,356, each incorporated herein by reference. Other biopolymers useful in the fluids of this invention are the so-called welan gums produced by fermentation with a microorganism of the genus Alcaligenes. See for example U.S. Pat. No. 4,342,866, incorporated herein by reference. Gellan gums am disclosed , in U.S. Pat. No. 4,503,084, incorporated herein by reference. Scleroglucan polysaccharides produced by fungi of the genus sclerotium are disclosed in U.S. Pat. No. 3,301,848, incorporated herein by reference. Commercially available scleroglucan is sold under the trademarks Polytran from , the Pillsbury Company and Actigum CS-Il from CECA S.A. Succinoglycan polysaccharides are produced by culti- vating a slime-forming species of Pesudomonas, Rhizobium, Alcaligenes or Agrobacterium, e.g., Pseudomo- nas sp. NCIB 11264, Pseudomonas sp. NCIB 11592 or Agrobacterium radiobacter NCIB 11883, or mutants thereof, as described in European Patent No. A40445 or A138255. Commercially available succinoglycan biopolymer is sold by Shell International Chemical Company of London, U.K., under the trademark Shellflo-S. The minimum concentration of the polymer required to increase the low shear rate viscosity of the fluid can bedetermined by routine testing. Thus the minimum concen-tration will be an amount sufficient to impart to the fluid thedesired low shear rate viscosity. Generally the fluids willcontain a concentration from about 0.7 kg/m3 (0.25 ppb) toabout 11.4 kg/m3 (4 ppb), preferably from about 1.4 kg/m3(0.5 ppb) to about 7.1 kg/m3 (2.5 ppb). The water base borehole fluids of this invention generallymay contain materials well known in the art to providevarious characteristics or properties to the fluid. Thus thefluids may contain one or more viscosifiers or suspendingagents in addition to the polysaccharide required, weightingagents, corrosion inhibitors, soluble salts, biocides,fungicides, seepage loss control additives, bridging agents,deflocculants, lubricity additives, shale control additives,and other additives as desired. The borehole fluids may contain one or more materialswhich function as encapsulating or fluid loss control addi-tives to further restrict the entry of liquid from the fluid tothe contacted shale. Representative materials known in theart include partially solublized starch, gelatinized starch,starch derivatives, cellulose derivatives, humic acid salts(lignite salts), lignosulfonates, gums, synthetic water solublepolymers, and mixtures thereof The fluids of this invention should have a pH in the rangefrom about 7.0 to about 11, preferably from 8 to about 10.5.The pH can be obtained as is well known in the art by theaddition of bases to the fluid, such as potassium hydroxide,potassium carbonate, potassium humate, sodium hydroxide,sodium carbonate; sodium humate, magnesium oxide, cal-cium hydroxide, zinc oxide, and mixtures thereof. Thepreferred base is magnesium oxide. The suffactants useful in the present invention to createthe aphrons must Lx compatle with the polymers present inthe fluid to create the desired low shear rate viscosity. Thusthe suffactants will generally be non-ionic or anionic. A testprocedure has been devised to determine if a surfactant canbe used in the present invention to generate the aphrons. Theprocedure is as follows: To a low temperature, low pressure APl filtration cell(API Recommended Practice 13 B-i), the cylindrical bodyof which is made from Plexiglas of thickness 0.5 inch (1.3centimeters), is added 200 grams of sand ha