To minimize formation damage due to drill-in and completion fluids solids must be sized to satisfy two important criteria:

  • they must be large enough to not invade the rock, and

  • they must be small enough to form filter cake that effectively filters drill solids and polymers from the filtrate entering the formation.

These criteria when used together with the model presented in this paper quantitatively define the particle size that should be used in drill-in fluids for a given formation permeability, overbalance pressure and mud formulation.

A model is presented that estimates the depth and degree of formation damage caused by solids of widely different sizes present in drilling or completion fluids. The depth of damage and permeability loss is calculated after the invasion of the mud and also after flowback. The effect of the particle size distribution in the fluid, particle concentration, overbalance pressure, and permeability of the formation are studied. It is demonstrated that particle invasion and flowback processes are largely dependent on the particle size in the mud and the permeability of the formation. The results of the model are shown to agree well with mud filtration experiments.

To better estimate the particle size distribution in drill-in and completion fluids, different methods for measuring particle sizes were investigated. These results show that the measured particle sizes can vary over two orders of magnitude depending on the technique used and on sample preparation. Based on a comparative analysis of several samples light scattering techniques are recommended for measuring the particle size distribution. Recommendations for sample preparation are also provided.


Fluids are used in the wellbore while drilling and completing a well. These fluids are maintained at a pressure higher than the formation pressure to prevent the reservoir fluid from flowing into the wellbore. Due to this "over-balance" pressure, the fluid invades the formation and can cause formation damage. The invading particles, which were initially suspended in the mud, tend to plug pores and hence reduce the rock permeability. The mud filtrate can interact with the formation minerals to cause mobilization and subsequent redeposition of in-situ fines, and induce wettability changes leading to a reduction of permeability. It is, therefore, important to minimize filtrate invasion as well as solids invasion in the formation. The damage caused by particles in the mud and the volume of filtrate loss is the primary focus of this paper.


The main factors that determine formation damage due to particle invasion are:

  1. Particle size distribution in the mud.

  2. Formation permeability / pore size distribution.

  3. Concentration of mud solids.

  4. Over-balance pressure.

  5. Mud circulation rate and rheology.

Glenn and Slusser (1957)1 studied the effect of injected mud particle size distribution on formation damage. They conducted filtration experiments on porous samples with muds containing different particle sizes. The experiments indicated that a certain particle size distribution in the mud is required for a given pore size distribution for minimum permeability impairment.

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