Solids Content

Figure 10-17. Permeability impairment in a 3-in. Berea core caused by brines containing low concentrations of micron-sized particles, filtered brines, and a KCI based drilling fluid containing bridging solids. (From Krueger.30a Copyright 1986 by SPE-AIME.)

microns (100-mesh screen) should suffice for all formations except gravel beds and formations with open fractures or channels. Drilling fluids are sometimes deficient in bridging particles larger than 50 microns, and certainly will be if subjected to effective desanding and desilting. Particle size distribution should therefore, be watched closely when drilling in unconsolidated sands, and mechanical separators should be adjusted to maintain enough coarse particles for bridging. Coarse grinds of suitable material, such as calcium carbonate, should be added if necessary.

Apart from the productivity damage caused by deep mud particle invasion, lack of sufficient bridging particles causes slumping of unconsolidated sands and hole enlargement. As explained in Chapter 8, an unconsolidated sand has a coefficient of cohesion of zero, and will therefore slump into the hole unless a mud cake is formed. The pressure drop across a mud cake increases the cohesion and reduces the compressive stresses around the borehole. It is essential that a mud cake be formed quickly, because the turbulent flow around the bit creates highly erosive conditions, and hole enlargement is rapid. Failure to establish a mud cake quickly will result noi only in productivity impairment, but also will lead to sand production, casing buckling, and other production problems associated with enlarged hole.22 Correlation between particle sizes maintained in the mud and caliper logs will soon establish optimum bridging requirements for a particular reservoir

Reservoirs with Fracture Permeability. Some reservoirs, notably carbonates. have a very low matrix permeability, and production depends on flow through a network of microfractures The fractures are mostly less than 10 microns in width, but may be much wider. Because of the uncertainty of fracture size, and because of the geometry involved, bridging fractures is more difficult than bridging porous media. If the fractures are not bridged, fine m ud particles invade the fracture and filter internally against the sides of the fracture until it is filled with mud cake. Such internal mud cakes are not removed by backflow, and productivity is greatly impaired. Such reservoirs should therefore be drilled with a fluid whose solids are degradable.

When Gun Perforating. The damage caused by gun perforating with conventional muds in the hole has been recognized for some time.J' Even if a well is perforated with a completely nondamaging fluid, productivity is decreased by the formation of a zone of crushed rock around the perforation tunnel (see Figure 10-18). If the productivity of this zone is further reduced by mud solids or filtrate, the productivity of the well will be reduced severely. Based on experimental data, Klotz et al29 have calculated loss of well productivity resulting both from formation damage by mud solids and filtrate while drilling and from damage to the zone around the tunnel. Figure 10-19 shows that if a well is perforated with a completely non-damaging fluid and if the permeability

8 inch | perforation

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