In recent times, the oil industry has shown increasing awareness towards the maintenance of optimum well productivity through better drilling/completion practices. Attempts are being made to control the invasion of high permeability sands by mad solids through the use of sized Loss Control Materials(LCM) which can be difficult to clean up. Recent developments involve the use of sized carbonate and sized salt systems with mixed results. Whilst these special systems have been known to provide efficient fluid loss control, the cleanup efficiency especially in horizontal wells has been known to be poor in certain instances. Likewise, the completion of long horizontal sections through highly permeable, unconsolidated reservoirs has witnessed increasing use of prepacked screens many of which are reported to be plugged/damaged by filter cake debris and completion fluid solids as well as formation fines. Gravelpacks are also known to have been damaged not only by formation fines but also by solids in completion fluids injected from the wellbore especially during cleanup operations.

The migration of sized particles in the drilling and completion fluids through high permeability reservoir sands, prepacks or gravelpacks is characterised by flow capacity decline which may be instantaneous or gradual depending on the migration process and the pore bridging phenomena. Therefore, accurate prediction of the prevailing pore blocking mechanism for a given pore throat-particle size relationship can provide a good basis for the estimation of the maximum allowable size of particles in various completion fluid systems and also provide a good guide to the design for optimum prepack and gravelpack performance through the use of properly sized gravels.

In this paper, attempts have been made to analyse the impact of a number of key parameters on productivity impairment. The analysis has been based on the results of in-depth research into particle-pore bridging phenomena. Based on rigorous experimental studies to define the phenomena, a number of deterministic models have been developed to mathematically define the unconsolidated pay sand/pack sand systems permeability decline profile as a function of invasion pattern, migration/pore blocking mechanism, production /injection rate, production time and fines concentration and fines textural properties. Application of the results to the optimisation of drilling/completion fluids design as well as prepack/gravelpack design and analysis are illustrated with specific case studies.

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