Mud Flow

Figure 6-11. Invasion of a permeable formation by mud solids. Schematic. (From the Sept., 1975 issue of Petroleum Engineer Internationa!. Publisher retains copyrights.)

When adequate bridging particles are lacking, the API filter test may give grossly misleading results. A mud might give a neglible loss on filter paper, but give a large one on a permeable formation downhole. The point was well illustrated by experimental data obtained by Beeson and Wright,21 extracts from which are shown in Table 6-3. Note that the discrepancy between the gross loss on paper and that on the porous media was greater with unconsolidated sand than with consolidated rocks, even when the permeability of the latter was higher. Note also that the discrepancies between the net filter loss on paper and on porous media increased with increase in spurt loss. Evidently the mud spurt plugs the cores to such an extent that the pressure drop within the core becomes significant, thereby reducing the drop across the cake, and reducing cake compaction. Similarly, Peden et al2la got higher dynamic filtration rates on coarse synthetic filter media than on natural sandstones.

With regard to the critical size required for bridging, it was shown by Coberiy-2 that because of jamming, particles down to one-third the size of a circular screen opening would bridge that opening. Abrams19 showed that

Table 6-3

Effect of Filtration Medium on Mud Spurt*

Filtration medium

API Test Unconsolidated Whatman 50 sand, 219 md Consolidated Consolidated filter paper to 299 md rock, 520 md rock, 90 md

Bentonite, 1 04 SG:

Gross filter loss

0 0

Post a comment