Summary Large-scale testing of the settling behavior of proppants in fracturingfluids was conducted with a slot configuration to model realistically theconditions observed in a hydraulic fracture. The test apparatus consists of a1/2 × 8-in. [1.3 × 20.3-cm] rectangular slot 14 1/2 ft [4.4 m] high, faced with Plexiglas (TM) and equipped with pressure taps at 1-ft [0.3-m] intervals. Thisconfiguration allows both pressure taps at 1-ft [0.3-m] intervals. Thisconfiguration allows both qualitative visual observations and quantitativedensity measurements for calculation of proppant concentrations and settlingvelocities. We examined uncrosslinked hydroxypropyl guar (HPG) andhydroxyethylcellulose (HEC) fluids, as well as crosslinked guar, HPG, andcarboxymethyl HPG (CMHPG) systems. Sand loadings of 2 to 15 lbm/gal [240 to1797 kg/m3] (8 to 40 vol% of solids) were tested. Experimental results werecompared with the predictions of existing particle-settling models for a40-lbm/1,000-gal predictions of existing particle-settling models for a40-lbm/1,000-gal [4.8-kg/m3] HPG fluid system. Introduction The problem of particle or proppant settling in hydraulic fracturingoperations is of great importance. Knowledge of proppant-settling velocity isnecessary for prediction of the proppant-settling velocity is necessary forprediction of the final proppant distribution in the fracture. A thoroughunderstanding of proppant transport aids in designing fracturing treatments toobtain more effective proppant distribution. During a hydraulic fracturingprocess, some fluid is lost to the formation, thus increasing the proppantconcentration of the slurry pumped. Proppant settling in slurries is a verycomplex phenomenon compared with single-particle settling in a fluid.phenomenon compared with single-particle settling in a fluid. Single-particlesettling in a non-Newtonian fluid in the fracture has been studied extensively. In contrast, very little literature exists that explains the complexhindered-settling effects of particles in non-Newtonian fracturing gels. Without adequate particles in non-Newtonian fracturing gels. Without adequatetheory for description of this phenomenon, most fracturing-design simulatorstoday use correlations for single-particle-settling velocities in fracturinggels, and from the theory of Newtonian suspensions or slurries, correct thesevelocities to account for the hindered settling effects. For example, from hisexperimental work with water and 35-cp [35-mPa] oil, Novotny suggested using Richardson and Zaki's Newtonian-fluid correlations. Novotny demonstrated fromlimited experimental data that the correction for Newtonian-fluid hindranceeffects was also reasonably good for non-Newtonian fracturing gels. Novotnyalso reported that in some cases proppants agglomerated or clustered duringmeasurements of settling velocities of proppant-laden slurries flowing throughfractures. Kirkby and Rockefeller conducted static sedimentation experiments toevaluate the settling behavior of nonflowing slurries with thedifferential-pressure technique for measuring proppant concentration andsettling velocity in concentrated slurries. A proppant clustering phenomenonoccurs in all fracturing gels at proppant clustering phenomenon occurs in allfracturing gels at typical proppant concentrations, resulting in average staticslurry-settling velocities that are considerably greater than those of singleparticles. Dunand and Soucemarianadin used viscometric measurements to studysedimentation of single particles and suspensions in static HPG fracturingfluids. They showed that the viscosity function of these fluids can bepresented by the Ellis model and that theoretical predictions andexperimentally measured velocities of single particles agree well. Using gammaray attenuation measurements, Dunand and Soucemarianadin studied the settlingbehavior of concentrated proppant suspensions. They claimed that, in fluidswith identical single particle-settling velocities, the average settling rateof a concentrated suspension in a static non-Newtonian fluid is two to threetimes higher than in a corresponding Newtonian fluid. These two studies triedto answer some of the questions related to proppant settling in fracturing gelsin a multiparticle environment. Despite the contributions of these and otherauthors, however, further investigation is needed to understand this complexphenomenon fully. Therefore, an experimental study was undertaketo phenomenonfully. Therefore, an experimental study was undertaketo explain the complexphenomenon of particle settling in fracturing slurries. The objectives of thisinvestigation are two-fold:to design and build a large-scale testingapparatus for simulating the behavior of proppant-laden fluids in a fracture andto report observations of proppant settling in the presence of multipleparticles in controlled test conditions. Preliminary findings of particles incontrolled test conditions. Preliminary findings of sedimentation experimentswith various fluid systems also are reported. Theory When a particle settles in a given fluid and its fall is not affected by thecontainer boundaries or by the presence of other surrounding particles, theprocess is referred to as "free settling." When the particles, theprocess is referred to as "free settling." When the particles are neareach other, the motion of a particle is impeded particles are near each other, the motion of a particle is impeded by other particles and the process iscalled "hindered settling." Steinour proposed the following equationfrom his experimental study on the sedimentation of Newtonian suspensions: vH/vt = =, ..........................(1) where a = 1.82. Ramakrishna and Rao have used this equation extensively. Since Steinour's pioneering work, many investigators have derived the functionf() in Eq. 1. In the absence of wall effects, Richardson and Zaki proposed thefollowing empirical equation for N Rep less than 0.2: vH/vt = 4.65.........................................(2) They also give equations for other NRep ranges. Barnea and Mizrahi developeda generalized correlation: vH/vt =, ................................... (3) where f()={1+(1−)1/3 exp[5(1-)/3]}-1 ................. (4) Barnea and Mizrahi tested this correlation, which holds over the entirerange of applicability of the drag-coefficient curve, to NRep =3,500. Slatteryshowed theoretically that for creeping flow past a sphere in a non-Newtonianpower-law fluid, CD = 24X(n')/NRep,......................................(5) where X(n') is a function of power-law flow-behavior index, n'. Thus, X(n')can be considered a "correction factor" to the Newtonian equation. Using the definitions of CD and NRep, we can reduce Eq. 5 to v = gd (p-P)/18K'X(n'),..........................(6) Assuming that Stokes' law (Eq. 6) is modified by a factor f() when otherparticles are present, then v = [gd (Pp-Pf) /18K'X(n')]f ...................(7) SPEPE P. 305
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper discusses the tests conducted to determine the mechanical properties and the integrity of the cement sheath when subjected to cyclic loads. In addition, lower-density cement systems are discussed in this paper, along with how the density was lowered by incorporating conventional additives such as:• Pozzolanic beads (cenosphere)• Hollow glass beads • Gas bubbles • Water-binding additives • Silica fume • Fly ash Primary cementing compositions for oil-well applications are becoming increasingly complex and challenging because of the extremes encountered in well-operating conditions. The stresses exerted on the cement sheath during well operations could be severe enough to damage the cement sheath and negatively affect the safety and the economics of the well.The results discussed in this paper should help operators design a cement sheath that can withstand the stresses from well operations and thus help improve the safety and economics of the wells.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCement integrity preservation during completion, stimulation, production, and even, during well abandonment is of critical importance for an operator from long-term economic, productivity, and safety perspectives. Traditionally, compressive strengths have been considered indicators of cement integrity. However, numerous squeeze cementing jobs regularly performed on completed wells are testament to the poor correlation between compressive strengths and cement integrity. Additional mechanical properties such as tensile and flexural strengths, elastic modulus, and Poisson's ratio are being taken into account with increasing frequency for maximizing the cement sheath performance during the life of the well. Unfortunately, all such measurements are performed on samples that have been cured, either under wellbore conditions (for example, pressure and temperature), or laboratory conditions (for example, atmospheric pressure) but tested at atmospheric pressure and temperature. Such properties may at best be useful for comparing different formulations in the selection process but do not provide information about the cement properties under downhole conditions.Using ultrasonic shear wave and compression wave combination measurements, dynamic mechanical properties, such as elastic modulus, bulk modulus, and Poisson's ratio and compressive strength, are measured under pressure and temperature. These measurements are compared with mechanical properties obtained from load vs. displacement under static conditions and acoustic compression and shear wave measurements at atmospheric pressure and temperature. Correlations are made for several slurries. The results are presented. The results also will present cases where the measurements made using this method demonstrated unique advantages over the conventional load vs. displacement techniques.
This paper discusses the tests conducted to determine the mechanical properties and the integrity of the cement sheath when subjected to cyclic loads. In addition, lower-density cement systems are discussed in this paper, along with how the density was lowered by incorporating conventional additives such as:Pozzolanic beads (cenosphere)Hollow glass beadsGas bubblesWater-binding additivesSilica fumeFly ash Primary cementing compositions for oil-well applications are becoming increasingly complex and challenging because of the extremes encountered in well-operating conditions. The stresses exerted on the cement sheath during well operations could be severe enough to damage the cement sheath and negatively affect the safety and the economics of the well. The results discussed in this paper should help operators design a cement sheath that can withstand the stresses from well operations and thus help improve the safety and economics of the wells. Introduction The oil and gas industry is exploring and producing oil and gas from extreme environments, such as high-pressure and high-temperature (HPHT), deepwater, shallow gas, and accelerated production rates. The stresses exerted on the cement sheath from these extreme operating conditions could be severe and could damage the cement sheath. Examples of well operations that could exert stress on the cement sheath are:Cement hydrationChangeover from displacement fluid to completion fluidHydraulic stimulationHydrocarbon productionFluid injectionGas lift These types of operations could change the pressure and temperature of the cement sheath after the slurry is placed in the annulus. The cement sheath could be damaged if the magnitude of pressure or temperature change is large and the stresses in the cement sheath exceed key values of the cement sheath. The key values are measured values and vary depending on cement slurry formulation. Some major consequences of damage to the cement sheath, such as sustained pressure on the annulus side or damage to the casing, could force well shutdown or result in high remedial costs. Other consequences of damage to the cement sheath, such as loss of hydrocarbon production, production of unwanted fluids (e.g. water), and growth of wellhead, could negatively affect the safety and economics of oil and gas assets because the remedial jobs are expensive to impossible in some cases. Hence, the integrity of the cement sheath should be considered during the early stages of well construction and designed for uninterrupted, safe, and economic production of hydrocarbons. A detailed engineering analysis should be conducted to evaluate how the different well operations affect the integrity of the cement sheath. In other industries, for example, bridge construction, applying engineering analysis is a common way to optimize material properties. The oilfield is slowly adopting these techniques. This adoption has occurred because of a combination of increased risk to cement-sheath integrity in expensive wells operating in extreme operating environments, and increased safety standards.1–7 A three-step approach, outlined in Fig. 1, should help operators construct a well that can produce hydrocarbons safely and economically. Step 1 is the engineering analysis. The outcome of the engineering analysis is to help provide the optimum cement sheath properties needed to withstand the well operations. Step 1 is discussed and presented by Bosma et al., Ravi et al., and other authors.8–12
Combining meawiernent, simtilafion,..m. dbnaging technologies into an integ@ted program .can help operators achieve the best hydraulic f~cture treatment possible. Hydrocarbon production can be signi@iMy increased when fractures tire extended to the planned length, and fracturing fluid is.rektined within t& zone of interest. Fractures that briali out of zbne incfesse the risk of excess water "production with the hydrocarbon. Consequefitly, the ability to select suitable operational parameters for hydraulic fracturing is critical to job SUCCS.3S.An evacuation of formitkiii piopeiti& and potenfib amiem to hydraulic fracttiijng can bc made with three-"" "dimensional (3D) simulatiori to integrate data tzken from wirelirie logs, wzveforni sonic-logs, and.microfrai measurements. In-situ stress "orietitationis determined by use of a downhole extensonieter, oriented cores, mrelastic stmin rccovefy (ASR)"measnrements, and -" borehole imaging logs.-Sidewall cores. can be taken perpendlculzr to wellbore walls without distorting-the. borehole or the core tafcen; orientation of the cores can be determined with imagingReferences at the erid of the paper. 575 logs run after coring. Nat@ tka@rres can be viewed with a downfrole video csmera lowered into the well on fiberoptic cable."Effectiveness of fracture &eatmerrts may be evalnated with vsrious gamma ray logging techniques aod production Iogiiorgparirig expectsd production to scturd zonal contribution. Refried procedures that re:nlt from 'after-fiat-analysis can be,used to-pkm field development for optimal reservoir drainage.
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