International Journal of Petroleum Science and Technology ISSN 0973-638 Volume 6, Number (01),. 143-15 Research India Publications htt://www.riublication.com/ijst.htm Effects of Mud Proerties, Hole size, Drill String Triing Seed and Configurations on Swab and Surge Pressure Magnitude during Drilling Oerations *Uduak Mme and Pål Skalle Deartment of Petroleum Engineering and Alied Geohysics, NTNU, Trondheim, Norway *Corresonding Author E-mail: uduak.mme@ntnu.no Abstract Factors which affect swab and surge ressures must be recisely selected in order to control kicks or blowouts and to revent loss of circulation. In this aer a sensitivity analysis is carried out to investigate the effect of drilling fluid density, lastic viscosity, yield oint, bore hole diameter and drillcollarsto-drillie length ratio on swab and surge ressures. The analysis is based on: i. The theory of hydrostatic drilling fluid mechanics, taking the effect of mud clinging constant into consideration. ii. iii. Develoing a Matlab code to comute and simlify the flow equations Transforming the outut of this code into grahical form from which effects of mud roerties, drillstring triing seed, and hole and drill string configuration; on swab and surge ressures could be redicted. The develoed code requires fairly simle inut data which can be obtained from the rig site mud engineer. Imortant conclusions and industrial imlications of the findings are 1. Mud weight, rheology and drill collars length and size are redetermined factors which have been chosen carefully and maintained at the desired level during drilling and triing oerations in order to avoid blowout or loss of circulation roblems. These can be varied only to a limited degree.. Safe triing seeds are greatly influenced by hole size. 3. The develoed comuter rogramme can be used to redict safe drill string triing.
144 Uduak Mme and Pål Skalle 4. Triing seed is the only maniulative arameter with resect to controlling surge and swab ressure. This has been built into a grahical view for certain critical circumstances. 5. Whenever a critical situation demands that triing-out should be suorted by simultaneous uming, a grah has been develoed for this urose. Keywords: Mud roerties, Kick, Loss circulation, Mud clinging constant, Swab and Surge ressure Introduction When triing, the drillstring acts as a large iston moving through the borehole. This movement creates ressures due to friction losses between the moving string and mud. Swab ressures are created when the string moves uward and surge ressures when the string moves downward. If swab ressures are large enough, they can reduce the mud hydrostatic ressure below the formation ressure and cause a kick. Large surge ressures can fracture the formation and result in loss of circulation. Also, during the drilling oeration, it is often necessary to remove the drill string in order to change the drill bit. It may also be necessary to remove the drill string for other reasons, such as to erform desired logging oerations. After the bit is changed or after such logging oerations are comleted, the drill string must return to the borehole. The rocess of ulling out and running in the drill string is known as triing oerations. It has long been observed in the field that circulation is frequent lost after making a tri. Similarly, blowouts often occurred during the ie removal art of a tri due to formation gas (tri gas) entering the hole and lightening the mud column. Exeriment (Goins etal, 1951) and analytical (Cardwell, 1953) work showed these roblems to be largely due to down hole ressure variations caused by the iston-cylinder action of the ie and borehole. Swab and surge ressures have been studied by many researchers (Marken, 199, Mutomino, 1995; Zhong, 1995). Hussain and Sharif (009) indicated the reduction of surge ressure with the increase in eccentricity. For a artially blocked eccentric annulus with cuttings bed, the surge ressure decreases with the increase in the bed thickness. Ahmed et al. (010) resented hydraulic model to redict ressure losses while drilling and circulating drilling fluid. Hydraulic analysis of annular flow with axial motion of the inner ie have been carried out (Haige and Xisheng 1996, Fili and David 003) for different ie/borehole configurations and fluid models. Unfortunately, most of these studies have develoed models which lack in simlicity and require comlex inut arameters. In our work, effects of mud roerties, drillstring triing seed, and hole and drill string configuration; on swab and surge ressures were investigated by develoing a comuter code and transforming this code into a grahical technique. The develoed code require only simle data which could be gotten from the rig site mud engineer.
Effects of Mud Proerties, Hole size, Drill String Triing Seed 145 Mud Flow Physics The flow behaviour of drilling fluids is comlicated by the variation of aarent viscosity with rate of shear or flow. Consequently, the Newtonian fluid equations are altered for alication to tyical drilling mud systems. Swab and surge ressures can be comuted using a calculation stes based on basic mud flow Equations 1 to 10. The calculation rocedure is based on the theory of hydrodynamic viscous drag resented by Maidla and Wojtanowicz (1987) for Bingham-lastic fluids in wellbores. Hydrodynamic viscous drag is defined as the friction force between the ie string and the drilling fluid, which resist against ie movement. It deends on drilling fluid roerties, triing velocity, flow regime, ie outer diameter and wellbore diameter. The more viscous drilling fluid results the more viscous drag force.it also shows effect of clearance between string and the wellbore. The calculation rocedures includes: Ste 1 Calculation of the average effective annular velocity, V ae around the drill collars and around the drillie Average effective annular velocity is the mud velocity which roduces the viscous drag comonent of surge or swab ressure and its frame of references is the wellbore wall. The value of effective mud velocity due to moving ie wall is related to ie velocity, V by mud clinging constant C c. The value of C c deends uon the ratio of ie and hole diameters. δ + C 1 δ Vae = V c 1 Where δ reresents the ratio of ie diameter to borehole diameter δ δ lnδ 1 1 δ C c = = +, for la min ar flow (1 δ )lnδ lnδ 1 δ And 4 δ + δ δ C 1 c = + δ, for turbulent flow 3 1 δ Ste Is the mud flow laminar or turbulent? The flow critical velocity, V c around the drillcollars and around the drillie is calculated as follows: V c 1.08μ + 1.08 = μ + 9.3ρ( d ρ( d h d) h d) Y 4 Ste 3 If the flow is laminar, the ressure dro around the drillcollars and around the
146 Uduak Mme and Pål Skalle drillie is calculated as follows: L μ Vae ΔP = Y + 300( dh d) 5( dh d) 5 L is the length of drillie or drillcollars as the case may be. Ste 4 If the flow is turbulent, the ressure dro around the drillcollars and around the drillie is calculated as follows: flρvae ΔP = 6 5.8( d d) h Ste 5 The friction factor, f can be calculated using the following equations: Re 96.4 ρ V ( d d) ae h = 7 μ f = e c c1 + Re + c 3 ln(re) Where, c = 3.5378591164 c c 1 3 = 300.66099 = 0.16153971 Ste 6 Surge and swab ressure can be calculated from Equation 9 and Equation 10 resectively. Psurge = ΔPa + ΔPac + 0.05ρh 9 P swab = 0.05ρh ΔP ΔP 10 a ac Where ΔPa is ressure dro around drill ie and ΔPac is ressure dro around drillcollars. Formation fracturing ressure can be estimated using the following equation (Hubbert, 1957): ν Pf = σ ov + σ t 11 1 ν Normally, the tensile strength of reservoir rocks is neglected as a worst case could be encountered (equal zero). The overburden stress for normally stressed formations 8
Effects of Mud Proerties, Hole size, Drill String Triing Seed 147 can be evaluated as follows: Psi σ ov = 1 h 1 ft Formation fracture gradient for dee wells can be estimated using the following equation: ν G f = 13 1 ν Assuming that average Poisson s ratio, ν for most reservoir rocks equal to 0.5, the formation fracturing ressure gradient (comuted using Equation 13) equals to 0.67 si/ft. Formation ore ressure gradient normally constant and assumed to be 0.45 si/ft in this study. Figure 1: Flow chart for the develoed Matlab code
148 Uduak Mme and Pål Skalle Figure 1 indicates the flow chart for the Matlab code that has been develoed to do all the above mentioned calculations. Based on the above rocedure a sensitivity analysis was erformed to investigate the effect of fluctuation of the model arameters on surge and swab ressures including: i. Mud roerties (mud weight, lastic viscosity and yield oint) ii. Hole diameter iii. Drillcollars-to-drillie length ratio iv. Formation ore fluid ressure v. Formation fracturing ressure Table1: Matlab Code Inut Data Parameter Value or Range Unit Mud weight, ρ 8.5-1.5 g Mud lastic viscosity, μ 15-55 c Mud yield oint, Y 5-00 Ib/100sq.ft Well deth, h 6000-15000 ft Hole size, d h 7.875-9.875 inches Drillcollars-to-drillie ratio 0.0345-0.1538 fraction Drill ie size (ID,OD) 3.86, 4.5 inches Drillcollars size (ID,OD).813, 6.75 inches Drill string triing seed 0-18 ft/sec Formation ore fluid ressure gradient 0.45 Psi/ft Formation fracturing ressure gradient 0.67 Psi/ft Poisson s ratio 0.5 fraction By using the inut data resented in Table 1 above and the develoed comuter code, a sensitivity analysis was conducted. Results and Discussions Based on the results of the sensitivity analysis, Figures to 6 were lotted. Critical ie running seeds for both surge and swab cases were clarified in the Figures. Drilling fluid roerties including density, yield oint, and viscosity are imortant factors which greatly affect the magnitude of surge and swab ressures generated during tri in and tri out oeration resectively. Figure 3 shows the effect of mud weight on critical ie triing seeds range.
Effects of Mud Proerties, Hole size, Drill String Triing Seed 149 Figure : The effect of mud weight on surge and swab ressure magnitude The critical ie triing seed is the seed beyond which loss of circulation or blowout could haen. When the mud density increases the range of safe ie tri in seed decreases This effect is attributed to the large increase in mud ressure (in addition to the iston-cylinder action caused by the drill string) oosing the formation being drilled and the results will be serious fractures leading to loss of circulation roblem. On the other hand, the increase in mud density can easily control formation ore fluid ressure, therefore, wide range of safe ie tri-out seeds can be alied as shown in Figure. Safe ie triing seeds are affected by mud rheological roerties such as mud lastic viscosity and yield oint. Figure 3: The effect of mud lastic viscosity on surge and swab ressure magnitude
150 Uduak Mme and Pål Skalle This effect could be due to the fact that viscous mud magnify the iston-cylinder action roduced by drillstring u and down movements as shown in Figures 3 and 4. Figure 4: The effect of mud yield oint on surge and swab ressure magnitude Drillcollars has a diameter bigger than the normal drillies. Therefore it adds and extra ressure on the formation during triing in and drain the formation ore fluid into the wellbore during triing out due to the iston-cylinder action as shown in Figures 5. Figure 5: The effect of drillcollars to drillie length ratio on surge and swab ressure magnitude
Effects of Mud Proerties, Hole size, Drill String Triing Seed 151 Hole size have a large effect on safe drillstring triing seeds. This is because during triing out, large hole diameter allows the drilling fluid to raidly fill in the lace which was occuied by the drillstring, therefore the formation ore fluid ressure is easily controlled and avoid blowouts. During ie tri-in, large hole diameter rovides bigger assage area for the mud, therefore, the iston-cylinder action acting oosite to the formation is minimized and the formation fracturing can be avoided as shown in Figure 6 (Hole size). Figure 6: The effect of hole size on surge and swab ressure magnitude Thus, by lotting data obtained from the develoed comuter rogramme, effects of mud roerties, hole size, drill string triing seed and configurations on swab and surge ressure magnitude can be redicted during drilling oerations Conclusions Based on the erformed analysis, the following conclusions can be drawn: Mud weight, rheology and drill collars length and size are redetermined factors which have been chosen carefully and maintained at the desired level during drilling and triing oerations in order to avoid blowout or loss of circulation roblems. These can be varied only to a limited degree. Safe triing seeds are greatly influenced by hole size. The develoed comuter rogramme can be used to redict safe drill string triing. Triing seed is the only maniulative arameter with resect to controlling surge and swab ressure. This has been built into a grahical view for certain critical circumstances. Whenever a critical situation demands that triing out should be suorted by simultaneous uming, a grah has been develoed for this urose.
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