Formation damage mechanisms and protection technology for Nanpu nearshore tight gas reservoir

Nanpu nearshore tight gas field, with rich natural gas resources, is a relatively small continental fault depression located in northern Bohai Bay Basin, China. Because of the complex geological conditions, formation damage mechanisms were not well understood, which further resulted in the difficulties of formation protection. For two adjacent wells producing from the same zone, one is high production well, the other is low producing well, two wells have significantly different gas production rates. Based on the measurement and comparison of recovery of gas- and oil-phase permeabilities before and after fluid flow of different fluids, the results analysis show that water blocking damage in gas-bearing formation is 30-50% higher than that of oil-bearing formation. The synergy of reverse osmotic effect and anti-water blocking was utilized to develop 180 °C high temperature polymeric micelle drilling fluid system, water loss of drilling fluid at HPHT (high pressure/high temperature) conditions was only 8 ml, membrane efficiency was improved from 0.11 to 0.5, and the recovery of core's gas-phase permeability reached 91%. This drilling fluid system was applied in NPAX-C Well, and it showed an excellent clay shale inhibiting effect and excellent gas formation protection, which meet the urgent needs of exploration and development of Nanpu nearshore tight gas reservoir. The technological ideas and innovation of this drilling fluid can be a valuable reference and demonstration effect for other tight gas reservoirs' exploration and development in China and around the world.