Fill Removal Modeling

Transcription

1 CTES, L.C 9870 ozos Lane Conroe, Texas phone: (936) fax: (936) Fill moval Moeling Subject Matter Authority: Bharath Rao May 10, 1999 Summary Operations involving fill removal or wellbore cleanouts are the most common of coile tubing applications. The wash flui is pumpe through the CT at a sufficient rate for a certain perio of time in an effort to clean out the wellbore. The flui returning in the annulus carries the fill particles (usually san, rille cuttings, or other wellbore ebris) to the surface, where the particles settle out in surface tanks. Common wash fluis inclue foams, water, brines, light oils, an polymer gels. This ocument presents a simple moel for the fill removal operation base on a steay-state analysis. Tech Note CTES, L.C. 1

2 In recent years, fill removal (also referre to as wellbore cleanout) ranks as one of the primary applications of coile tubing [Sas-Jaworksky II (1993)]. The operation involves circulation of a flui through the coile tubing own to the fill (san/rille cuttings) in the wellbore. The flui mixes with the san particles an carries the particles up to the surface through the annulus forme between the coile tubing an the prouction tubing/casing. The cleanout operation shoul be esigne so that the flow rate in the annulus is sufficiently high to transport the fill (usually san) up to the surface while maintaining pressure in the coile tubing below the maximum working pressure. The pressure uring the operation shoul also have no averse effect on the reservoir, formation, an wellbore. For horizontal or eviate wells, a san be may form at the lower sie of the annulus, which may result in incomplete cleanouts, an may sometimes even cause the coile tubing to become stuck. Therefore, a successful esign of a fill removal operation involves optimum selection of various parameters such as pump rate an flui type in orer to ensure a complete cleanout with the least possible time uner safe operating conitions. In the past ecae, a few articles [ursell an Moore (1992); Sas-Jaworsky II (1993); Appah an Ichara (1994); Gu et al. (1994); Walton (1995)] on fill removal in coile tubing applications have been reporte in literature. Many important aspects of the fill removal operation can be capture by a simple analysis base on steay state flow. The fill removal moel in Cerberus is base on such a steay-state analysis an is presente below. The amount of fill that is picke up for any given flow rate in a cleanout operation is expresse in terms of the lifting ablility of the flui. The lifting ability of the flui (L A ) is efine as L A Qλ, EQ 1 where Q is the flow rate an λ is the maximum loaing (maximum concentration of paticles in the flui meium) for that particular flow rate. Variables are in consistent units here an throughout this ocument, except where note. The penetration rate (R ) can be foun from the lifting ability as R La ( 1 ϕ)ρ A F EQ 2 where ρ is the ensity of the fill particle, ϕ is the porosity (represents the voi volume fraction), an A F is the fill cross-sectional area. The term (1 - ϕ)ρ in Eq 2 is sometimes referre to as the fill packing ensity (ρ D ) an is provie as an input along with ρ, ϕ, an λ. Tech Note CTES, L.C. 2

3 Once the fill particles are suspene in the flui meium, it is important that they remain suspene an be carrie all the way up the annulus. In orer to ensure this, the magnitue of the annular flui velocity must be sufficiently greater than the terminal settling velocity (v ) of fill particles. A popular rule of thumb is that, in vertical wells, the flui velocity in the annulus must be atleast twice the terminal settling velocity of fill particles. For horizontal wells, this factor is close to 10 to prevent be formation an maintain particle motion in the flow irection [Sas-Jaworsky II (1993)]. Again, this is by no means a scientifically soun analysis an, therefore shoul be treate accoringly. Moore (1974) has propose a correlation for etermining v from a particle ynols number ( ) efine for non-newtonian fluis as ρv µ a D e EQ 3 where µ a Kγ n-1. Here, n is the behavior inex, K is the consistency inex, an γ is the shear rate. The expression for v in Moore s (1974) work is obtaine from Stokes law as v 4 3 g C ρ ρ ρ 1 2 EQ 4 where is the iameter of the fill particle (assume to be spherical) an C is the rag coefficient. The value of C is epenent on the magnitue of p an is etermine as follows. C C ( p > 300; turbulent flow) EQ 5 ( p 3; laminar flow) EQ 6 C 22 (intermeiate region) EQ 7 Tech Note CTES, L.C. 3

4 Settling velocities may also be estimate by use of the correlation [Chien (1994)] v 5.030α µ e B ρ [ 1] EQ 8 where B α ρ ρ e p 1 ρ µ EQ 9 In Eq 8 an Eq 9, units of µ an p are centipoise an inches respectively, an ρ an ρ p are in pouns per gallon. In aition, α is calle the sphericity an refers to the rounness of the fill particle. It is efine as the ratio of the surface area of a shape having the same volume as the particle to the surface area of the particle. Both Moore s (1974) an Chien s (1994) correlation are incorporate in Cerberus to etermine the particle terminal settling velocity. The magnitue of particle settling velocity obtaine from these correlations is compare with the require annular velocity in a particular well. As mentione earlier, epening upon whether the well is vertical, horizontal, or eviate, a suitable velocity factor (range between 2 an 10) is use to etermine the minimum annular velocity for a complete cleanout. The minimum flow rate an system pressures that satisfy the annular velocity requirements are subsequently etermine. Nomenclature C rag coefficient iameter of fill particle (ft) K consistency inex (lbf-s n /ft 2 ) L A n R v lifting ability of flui (lbm/s) flow behavior inex rate of penetration (ft/s) terminal settling velocity of particles (ft/s) Tech Note CTES, L.C. 4

5 Greek Symbols α sphericity γ shear rate (1/s) µ viscosity of flui (lbf-s/ft 2 ) ϕ porosity ρ ensity of fill particle (lbm/ft 3 ) ρ D packing ensity (lbm/ft 3 ) Subscripts F fill p particle terminal velocity ferences 1. Appah, D. an Ichara, M.: Empirical Moel Determines Energy to Clean San from Wellbore, Oil an Gas J., (Feb. 28, 1994) Chien, S. F.: Settling Velocity of Irregularly Shape articles, SE presente at the 69th Annual Technical Conference an Exhibition, New Orleans, LA (Sept , 1994). 3. Gu, H., Walton, I.C., an Bray, B.H.G.: A Computer Wellbore Simulator for Coile Tubing Cleanout Operations, Computer methos an Avances in Geomechanics, Balkema, Rotteram, Netherlans (1994) 3, Moore,. L.: Drilling ractices Manual, etroleum ublishing Co., Tulsa (1974). 5. ursell, J. C. an Moore, B. K.: How to San Wash Large Tubulars with Coile Tubing, etroleum Engineer Intenational, (Aug. 1992) Sas-Jaworsky II, A.: Coile Tubing Operations an Services, art 4, Worl Oil s Coile Tubing Hanbook, Gulf ublishing Co. (1993), Walton, I. C.: Computer Simulator of Coile Tubing Wellbore Cleanouts in Deviate Wells commens Optimum ump Rate an Flui Viscosity, paper SE presente at 1995 rouction Operations Symposium, Oklahoma City, OK, (Apr. 2-4, 1995). CTES Tech Note CTES, L.C. 5