Fluid Saturation

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Drilling Mud Effects on the Fluid Saturation

Water Base Muds

Water base drilling muds (Figure 1) have a range of additives, clays and chemicals incorporated into the water to create a homogeneous blend. The clay is usually a combination of native clays that are suspended in the fluid while drilling, or specific types of clay such as bentonite. Barite is commonly used to increase the mud weight (Figure 2). Many different chemicals can be added to a water base mud system to achieve drilling operational efficiencies (Figure 3). Water is the continuous phase.

Water base drilling mud, fluid saturation, drilling, Drilling Mud Effects on the Fluid Saturation — **Figure 1**: Water base drilling mud

Barite additive for water base muds, Barite additive, water base mud, fluid saturation, Drilling Mud Effects on the Fluid Saturation — **Figure 2**: Barite additive for water base muds

Components of water base drilling fluids, fluid saturation, water base mud, Drilling Mud Effects on the Fluid Saturation — **Figure 3**: Components of water base drilling fluids

In general, the use of water base drilling muds results in a moderate diameter of invasion, and thus a measurable invaded zone. In cases where the water loss is excessively high or the mud column highly overbalanced, invasion diameters can be quite large. In such instances, the measurement of R_xo is easy, but that of R_t is more difficult, and the corresponding value for S_w is less reliable.

Most importantly, the mud filtrate invasion is always water. This water will replace moveable hydrocarbons within the invaded zone, with a consequential increase in the water saturation of the invaded formation.

Oil Base Muds

Oil base drilling muds, in general, have oil as their continuous phase and water as the dispersed phase in conjunction with emulsifiers, wetting agents and gellants. The oil base can be diesel, kerosene, fuel oil, selected crude oil or mineral oil. The water phase of oil base mud can be fresh water, or a solution of sodium or calcium chloride. The external phase is oil and does not allow the water to contact the formation. Components of oil base drilling muds are shown in Figure 4.

Components of oil base drilling fluids, oil base drilling, mud oil base drilling, Drilling Mud Effects on the Fluid Saturation — **Figure 4**: Components of oil base drilling fluids

Oil base drilling mud systems generally produce an oil filtrate. In water-bearing formations, the relative permeability to oil is often low, and invasion is slight. In oil-bearing formations, reservoir oil may be replaced by invading oil filtrate, and normally not any of the formation water is displaced. Consequently, S_xo is likely to be equal to S_w when oil base muds are used. Most importantly, oil is the mobile phase.