Barite Segregation in Incline boleholes_jcpt97-76

Barite Segregation in Incline boleholes_jcpt97-76 - Special...

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Special Edition 1999, Volume 38, No. 13 Journal of Canadian Petroleum Technology PAPER: 97-76 Introduction In weighted drilling mud barite tends to segregate slowly. In directional drilling operations the settling process is accelerated. Barite settles in the lower side of the borehole and starts sliding when the borehole has an inclination above 30˚. This phenomenon is known as barite sagging. Sagging can lead to drilling and com- pletion problems; a density variation or non-linear hydrostatic pressure gradients which can lead to pressure control problems, while thick and tight barite beds can lead to high torque and drag, stuck pipes and plugged boreholes, and even lost circulation. The sag problem is related to the so called Boycott effect (1) , first described in 1920. Hanson et al. (2) have investigated the phe- nomenon and found that most of the sagging occurs while the mud is circulating. The same conclusion was reached by Bern et al., (3) the sagging tendency is highest at low annular velocities. Zamora and Jefferson (4) presented a method for tracking drilling fluid density variations, which helps to detect, but not to predict drilling mud instability. Jamison and Clements (5) developed a test method to characterise settling and sag tendencies in static drilling fluids, however their equipment was not able to distinguish between settling and sliding. They also found that their data, based upon standard API rheological parameters like PV and YP, were unsuitable for prediction of sagging behaviour. It is apparent from previous works that the most difficult part of the problem is the prediction of drilling mud instabilities for both static and flow- ing drilling fluids. To date there are no API test procedures for sag testing. A new simple laboratory tool was therefore designed for the purpose to study and develop a method to predict sagging. Drilling Mud Separation Settling of Particles In vertical wells the settling of weighting material is generally not a problem due to the long settling distance. In horizontal wells the distance to the lower side of the wall is only about 0.2 m, which leads to rapid generation of solids beds. The settling veloci- ty of a single spherical particle, v s,l , in a fluid is expressed by Stoke’s law (6) : ............................................................... (1) A barite particle ( ρ p = 4,200 kg/m 3 ) with a diameter (d p ) of 20 μ m in a fluid of density ( ρ fluid ) 1,500 kg/m 3 and a viscosity ( μ29 of 40 cP will settle at a rate of 53 mm/h. With an increase in particle concentration or volume fraction, c, the settling velocity at low concentration (c < 0.01) will decrease only due to the reduced cross-sectional flow area (7) . At higher concentrations hydrody- namic interference will arise. Based on geometric considerations for dispersed particles in laminar flow, the slip velocity at higher concentration, v s,c , can be expressed as (6) : .................................................................... (2) A concentration of 10 vol% barite (c = 0.1) in water results in a settling velocity v s,c = 0.59 v s,l , i.e. the slip velocity is reduced by 41%. Agglomeration/clustering, collision, flow regime at parti-
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