Comprehensive evaluation of formation damage induced by working fluid loss in fractured tight gas reservoir

• We evaluate the formation damage degree and determine the damage range induced by working fluid loss.
• We establish the formation damage pattern induced by working fluid loss.
• Drill-in fluid loss induces serious formation damage including sensitive damage, particle plugging and water phase trapping.
• Subsequent kill fluid loss further aggravates the formation damage degree.
• Formation damage zone radius exceeds the acidizing treatment radius.

Western Sichuan tight gas reservoir is characteristic of developed natural fractures and ultra low matrix permeability. Developed fracture is beneficial for the economic and efficient development of tight gas reservoir. But it will lead to lost circulation of working fluid and induce formation damage. Lost circulation has frequently occurred during drill-in, completion and test process. Formation damage degree and damage range are the key indexes for the formation damage evaluation. To our best knowledge, few papers have been published on the comprehensive consideration of the above two indexes. In this paper, we conduct laboratory experiments and develop a mathematical model to evaluate the formation damage degree and determine the formation damage range. Based on the study results a formation damage pattern is established to analyze the mechanism and process of the formation damage induced by working fluid loss. The study results show that the average formation damage degree induced by drill-in fluid loss is 68.51% and increases to 78.70% when the kill fluid loss damage is taken into consideration. The radius of formation damage zone induced by working fluid loss is 15.8 m. The formation damage pattern is as follows: First the loss of drill-in fluid induces serious formation damage including sensitive damage, particle plugging and water phase trapping. Then the subsequent loss of kill fluid in the process of completion and test further aggravates the formation damage degree. Finally in the acidizing treatment the acidizing radius cannot exceed the damage zone radius so that the formation damage cannot be completely removed. The comprehensive evaluation and pattern of formation damage are necessary for designing reasonable reservoir protection and damage removal measures for the fractured tight gas reservoir.

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