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Static HT/HP Fluid Loss, ML/30 Min X = 200 MD BEREA © = 600 MD BEREA

Figure 6-25. Correlation of dynamic fluid loss with static HT/HP fluid loss.

multifunctional circulating system shown in Figure 3-14. The results showed that the HTHP static loss correlated quite well with the short term (30 minute) dynamic loss, but had virtually no relationship with the long term (equilibrium) dynamic loss (see Figure 6-25).

Despite its shortcomings, the API or some similar static test is the only practical test for control of filtration at the wellsite. However, results should be interpreted in the light of correlations made in the laboratory between API filter loss and dynamic filtration rate (such as those made by Krueger), but one should use local muds and treating agents. Interpretation of the data would be greatly assisted if cake permeabilities were determined. Cake permeability can be calculated very simply from the API filter loss, cake thickness, and Equation 6-13. In measuring cake thickness, the soft surface layers should be excluded, since these are not present in the dynamic cake. It is, of course, impossible to know the thickness of the downhole cake, but comparative data would be obtained if some standard procedure were adopted. One method would be to use a cake thickness gauge,30 the piston of which would be weighted to penetrate layers of only a certain consistency. The required degree of penetration could be established in the laboratory by correlating sialic and dynamic cake thicknesses. Cake permeabilities thus determined would not be quantitative, but should be a better guide to downhole dynamic filler rates than is the API filter loss. Remember, though, that static cake thickness is the best guide when concerned with problems such as tight hole, differentia] sticking, etc. Most of these problems occur when the fluid is not circulating, in which case the gross thickness of the cake, including the soft surface layers, should be measured.

 A = Area of filter C = Constant defined by Equation 6-8 / = Coefficient of internal friction h = Thickness of filter cake k = Permeability md = Millidarcies P = Filtration pressure Q = Rate of filtration
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