Formation Damage and Fluid Loss in Crossflow Filtration of Drilling Muds




Di, Jiao

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A systematic study of the formation damage and filtrate loss associated with the dynamic filtration of both water and oil based muds is presented. Experiments were conducted in a specially designed core holder to measure the permeability impairment and changes in resistivity as drilling mud is circulated past the face of a core. Four sections of the core were monitored to measure the depth of damage. The fluid loss rate as well as the return permeabilities were also measured. Seven kinds of water based muds and four kinds of oil based muds were used in the experiments. The experimental results show that the depth of invaded mud particles is strongly dependent on the mud composition. A mud overtreated by a thinner will cause deep mud particle invasion (more than 8 inches). However, for most muds used, the depth of particle invasion is limited to 2.5 inches. The results of resistivity measurements indicate that a broad transition zone exists between the mud filtrate and the resident brine. The width of the transition zone depends on the mud filtration rate. In the range of the filtration rates considered, only the first one vi or two inches of the core closest to the invasion face were completely flushed by the mud filtrate. For all the oil-based muds tested, the dynamic filtration rate is much higher than the API static filtration rate. The filtrate loss rate more than doubles when a finite oil saturation is present in the core. This suggests that the API static filtration test is not appropriate for oil based muds, and dynamic test on oil saturated cores needs to be conducted. Oil-based mud cakes consist primarily of water droplets stabilized by colloidal particles and emulsifiers. These mud cakes are not as shear resistant as water-based mud cakes. The use of water-wet solids results in very poor quality external cakes and high fluid loss rates. Better external mud cakes are formed by adding the same amount of organophilic clay to the mud. There exists a critical overbalance drilling pressure below which no filter cake will be formed on the formation face. This is particularly important in low permeability formations (k < 1 md). By drilling at overbalance pressures just above this critical pressure (thus ensuring cake formation) the formation damage caused by the mud solids can be minimized. The rheological properties of the mud (flow index n and consistency constant c in a power law fluid) have a significant effect on cake formation. For example, flocculated muds (high c and n values) need a high overbalance drilling pressure and a low mud flow rate to form a cake even though the filtration rate for this mud is high. Equations are provided in the dissertation that allow both the minimum overbalance pressure and the critical permeability to be computed from laboratory rheological and dynamic filtration data. Mud cake properties, such as porosity, permeability, and compressibility, were measured in the laboratory for all water based muds used in the study. These properties were used as input parameters in a model for dynamic filtration. Good agreement is obtained between the model calculation and experimental results.


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