An Efficient Treatment Technique for Remediation of Phase Trapping Damage in Tight Carbonate Gas Reservoirs
Authors: Nazari Moghaddam Rasoul , Van Doorn Marcel , Dos Santos Auribel 
Published: November 2020

Abstract
Aqueous or hydrocarbon phase trapping is among few formation damage mechanisms which is capable of significant reduction in effective permeability even near to 100%. Phase trapping can occur by physical fluid displacement (during drilling, completion, stimulation or workover operations), crossflow from wet zones, co-current and counter-current imbibition or condensate banking around the wellbore. The remediation techniques including bypassing damage (e.g. hydraulic fracturing) or removal techniques (e.g. solvent or surfactant injection) are highly expensive or only effective temporarily. In this study, a new chemical treatment is proposed for efficient remediation of water or hydrocarbon phase trapping damage in low permeability porous media.
Phase trapping or capillary trapping occurs inside pore structure when viscous forces are not strong enough to overcome the capillary forces. Increasing viscous force is not always attainable, especially in low permeability formations; hence decreasing the capillary forces through IFT reduction or wettability alteration is being used for many years. While in this paper, it is focused on pore structure alteration to reduce the capillary forces. An efficient chemical solution is introduced as a slow-reacting treatment agent to change the pore structure of damaged zone around the wellbore or around the hydraulic fractures where the effective permeability of porous media is significantly impaired due to the phase trapping. The impacts of the treatment on rock pore structure and the resulting capillary forces were evaluated at both pore and core scale. The effective permeabilities of tight carbonate rocks were measured before and after phase trapping damage and after the treatment at HPHT conditions. In addition, the feasibility of field application of the new treatment technique was investigated by compatibility and corrosion testing.
It is found that the treatment solution is reacting with rock minerals at very slow rate such that it can penetrate deeper into the formation before it is spent. It is shown that the active ingredient can be dispersed into the aqueous wetting layer, enlarging the pore body and pore throats by rock dissolution. The results from the coreflood testing showed that the capillary reduction is effective enough to reduce the trap phase saturations and enhances the effective gas permeability significantly (up to 400%). It is concluded that this capillarity reduction (due to the pore enlargement) occurs with greater extent at the region near to the wellbore where the highest phase trapping is reported. When the high concentrated solution was injected, zero capillary zone was also achievable. It is also verified that the treatment solution is fully compatible with typical formation brines and the corrosion rate is far below the accepted corrosion level of 0.05 lb/ft2. Finally, from the cost-analysis results, it is shown that this new treatment is comparable to the solvent/surfactant injections while it benefits from its longevity and lower operational risks.
The new treatment method proposed here is cost effective and experimentally proven to be efficient and long-lasting. Such chemical treatment is recommended to alleviate gas flow in tight gas with aqueous trapping damaged zone or in gas condensate reservoirs with condensate banking problem.