Formation damage of gas wells can occur during drilling and completion operations causing a first sensible impairment of the potential productivity.

When drilling gas wells in low permeability reservoir with strongly inter-layered formation, as in the Adriatic Sea, oil based muds (OBM) are often used in order to minimize differential sticking and clay destabilization problems. The OBM's filtrate generates through the mud filter cake, when exposed to completion brine may form emulsion droplets that are not easily and fully removed during production, causing a permanent damage to rock permeability.

This paper presents the study carried out to understand the damage mechanism which gas wells undergo when drilled with OBMs and the results obtained in engineering a system able to enhance production recovery for new wells.

Bulk experiments and coreflood tests have been performed both to investigate the interaction effect of OBM's filtrate with high density water-based pill and to assess its impact on the well deliverability. In bulk conditions, viscosity and microscopy characterization of tight w/o emulsions (up to 99:1 W/O) highlighted that low shear rate and very low emulsifier concentration in the mud filtrate promote very viscous and thermally stable emulsion. Besides linear coreflood tests proved dramatic permeability impairment when flushing CaCl2 brine through a mud filtrate saturated core, confirming the formation of a blocking phase in the porous media.

A prevention system has been engineered as a peculiar surfactant to be added to completion brine, able to prevent emulsions from forming instead of simply breaking it up.

Surfactant selection guidelines have been provided as result of various additives screening being focused on the main characteristics affecting the surfactant efficiency, like HLB and solubility in brine.

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