By setting sand control screens for open hole completions in the same fluid that was used for drilling into the reservoir, considerable cost savings can be realized through the elimination of the need for displacing to a specialized completion fluid. The control of the particle composition of fluids is crucial in preventing the plugging of the sand control screens. Conditioning the fluid with fine grade shaker screens for removing the larger fluid particles is sufficient to decrease the amount of screen plugging when back producing the drilling fluid in the annulus between the screen and formation. Fluids with a lower solids loading are also beneficial for reducing screen plugging. Particle size distribution measurements have been found to be insufficient in comparison to direct laboratory evaluation the screens at predicting sand screen plugging.
Large cost savings are possible if long, horizontal wells can be completed with a sand screen in an open hole instead of gravel packing. The completion costs can be reduced even further if the well can be drilled and completed in the same mud, without using any clean-up fluids or dissolvers. Also, the use of clean-up fluids to remove the drilling mud and filter cake has been shown to increase formation damage in many cases.
Drilling fluids are designed to plug the pores in the reservoir rock and form a thin, impermeable filter cake. It is reasonable to assume that in a sandstone the largest pores will have approximately the same size as the smallest sand grains. Because sand control screens are design to retain the formation sand, they have slots that are narrower than the largest sand grains. Thus, the slots of the sand control screens are not much larger than the largest pores in the reservoir rock.
This means that an operation where the drilling mud must be able to plug the largest pores in the formation sand without plugging slots of the sand control screens, requires very careful planning and control, both of the drilling mud, and of the slot width of the sand control screens. The risk of plugging the sand control screens will be high. For the safe execution of such operations a good understanding of the plugging mechanism is required. One must be able to predict the screen slot width that is required to avoid plugging by a mud with a certain particle concentration and particle size distribution. It would also be useful if one could predict the volume of drilling mud that could pass through a certain screen without plugging it.
When the sand control screen and drilling mud have been chosen, the particle size and concentration of the drill cuttings in the mud must be carefully controlled when drilling through the reservoir. The size of the mud solids can be controlled most easily during drilling by changing the screen in the shale shakers. For this reason it is very important to know what size of openings in the shale shaker screens are required to avoid plugging of a certain sand control screen.