Study On Drilling And Completion Fluids Of Low Permeability Gas Reservoir With Miro-fracture In Bozi Block

Bozi block is a typical low permeability gas reservoir with miro-fracture in Tarim basin in the Xinjiang Uygur Autonomous Region,China.In this block,the skin factors of each well were high according to well test interpretation and the reservoir damage was serious.Therefore,This paper aims to study the drilling and completion fluid technology for protecting low permeability gas reservoirs with miro-fracture,and researches comprehensive performance evaluation of used drilling and completion fluids based on clear reservoir geological characteristics and damage mechanism.By optimizing the treatment agents with excellent performance and combining with the reservoir protection performance,developes the drilling and completion fluids system to protect low permeability gas reservoirs with miro-fracture of Bozi block.Considering the influence of temperature on the sensitivity,the high temperature and pressure sensitivities have been evaluated by core flow experiment in which the gas was used as the experimental fluid,the result showed this block has the characteristics of low velocity sensitivity,medium to strong water sensitivity,medium to weak salt sensitivity,medium to weak alkali sensitivity,weak acid sensitivity.The high temperature stress sensitivity damage was both strong damage by adopting two kinds of loading methods.And the existence of fractures could increase the stress sensitivity,the wider the fractures,the stronger the stress sensitivity.It was predicted that there was serious damage to the water phase trapping in the Bozi block based on a variety of water phase trapping prediction model.By nuclear magnetic resonance T₂ test,core self-absorption experiments and core displacement experiment,clearing that Bozi block has significant water blocking damage.There exists the damage of solid to miro-fractures in this block because the drilling fluids could not effectively seal the micro-fractures based on the theory of shielded temporary plugging,ideal filling theory and D₉₀ rule.Four kinds of water and oil based drilling fluids and two kinds of organic salt completion fluids used in site were evaluated.The result showed that the rheological property of the drilling fluid fluids could meet the drilling requirements in general.However,the stability at high temperature,resistance to contamination,lubrication performance,settlement stability and sealing performance of two kinds of water-based fluid fluids should to be improved,especially in reservoir protection performance,the permeability recovery value of four kinds of drilling fluid was less than 80%wherein the maximum value was 74.21%,which would cause high permeability damage to the reservoir.The anti-water-blocking performance of the completion fluids was poor,whose permeability recovery value was low,and they would also cause some damage to the reservoir.In view of the main damage mechanisms of liquid lock damage,water/salt sensitivity and the damage of solid and liquid to miro-fractures,a set of water-based drilling fluid have been built and the oil-based drilling fluids have been optimized by taking plugging performance,inhibition performance and anti-water-blocking performance as the main indicators.And the treatment agents have been preferred by using water blocking suppression technology and film sealing technology.Meanwhile,the excellent compound-plugging agent was optimized by using the ideal filling technology.The basic performance of the optimized drilling fluids could meet the requirements of the field,which also have better reservoir protection ability,and the permeability recovery value was more than 85%.The completion fluid was optimized by analyzing oil-based filter cake removal and oil-based filtrate demulsification.The optimized completion fluid could effectively eliminate the damage caused by oil-based drilling fluids and improve the core permeability.The experimental permeability recovery value of cores which early polluted by oil-based drilling fluids then treated by the optimized completion fluid reached 100%.