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Friday Haggler


Which of the following are correct statements about sag (settling of drilling fluid weight material)?


(a) High mud weights and hole inclinations increase the risk of sag

Correct; if we assume that the high mud weight is produced by traditional solid weight material (barite or calcium carbonate), rather than using a solids free formate mud system (sodium, potassium or cesium) or micronized barite. Cesium formate can be used to produce a solids free mud system with a weight up to 2.20 SG. So, assuming we have a “traditional” system, the higher the MW (for a given mud system) the more weight material present and therefore more settling potential.

As the inclination increases, the distance the weight material has to fall to the low side of the hole decreases, decreasing the time it takes for the material to settle on the low side.

(b) A major challenge in reducing sag is modifying the ultra-low shear rate viscosity without effecting the high shear rate viscosities

Correct; increasing the ultra-low shear rate viscosity is beneficial for sag as a more viscous fluid at annular flow rates (represented by the low shear rheology) will decrease the settling velocity of the weight material, thus reducing sag. However, increasing the low-end rheology independently from the high-end rheology is often a challenge as most viscosifiers will do both. Increasing the high-end rheology is detrimental to standpipe pressure and can therefore limit flow rates for hole cleaning. This is less of a challenge with some modern mud systems and viscosifiers.

(c) Sag will be worse when lubricating (pumping with low flow) out of a high angle hole than when out the hole changing out the BHA

Correct; the low flow rate used for lubricating will ensure the gels are broken, producing dynamic barite sag. Also, fresh mud is being reintroduced into the well, carrying more weight material to add to the sag problem.

(d) Pipe rotation and high AV help to pick-up and reintroduce sagged weight material back into the drilling fluid in a high angle well

Correct; similar to hole cleaning, rotation and potentially some turbulence from high AVs will help mobilise weight material from the low side of the hole, if it has not formed a paste on the low side, which will be difficult to remove.

(e) The sag potential of a drilling fluid can be reduced by increasing the particle size (D50) of the weight material

Incorrect; Stokes law states that particle settling velocity (propensity for sag in our case) is directly proportional to the square of the particles radius. Therefore large particles will have an increased settling velocity, which is why some low sag drilling fluids use micronized barite as the weighting agent. See the expression below, derived by George Gabriel Stokes in 1851, referred to as Stokes law.

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