STUDY OF PERMEABILITY STRESS-SENSITIVITY CHARACTERISTICS AND ITS EVALUATION METHOD OF ULTRA-LOW PERMEABILITY RESERVOIR USING DIFFERENT TYPES OF FLUID MEDIA

Understanding the permeability stress-sensitivity characteristics is vital for effective development of oil and gas reservoirs and nuclear waste underground disposal.Different types of fluid media(nitrogen,brine and kerosene) permeability stress-sensitivity have been comparatively studied in ultra-low permeability reservoir and outcrop sandstone rock samples obtained from Daqing oilfield and Changqing outcrop reservoir,respectively.Those factors including rock permeability,pore throat deformation fluid compressibility and fluid-solid coupling were also analyzed on the effect of permeability stress-sensitivity.The experimental results indicate that the magnitude of permeability stress-sensitivity of different types of fluid media is significantly different.Gas permeability and oil effective permeability at irreducible water saturation dramatically declined during the early increase of effective stress.The comparative results also suggest that water permeability stress-sensitivity is stronger than that of gas permeability when the rock gas permeability is larger than 1×10-3 μm2.But the gas permeability stress-sensitivity is stronger than that of water permeability when the rock gas permeability is less than 1×10-3 μm2.That the larger pore throats were firstly compressively deformed is the main factor causing the permeability dramatically decreased during the early increase of effective stress.Fluid compressibility and fluid-solid coupling differences among different types of fluid media cause different magnitudes of gas,water and oil permeability reduction.Oil phase should be used as the flow media in oil reservoir permeability stress-sensitivity evaluation.A clear distinction between rock core and petroleum reservoir permeability stress-sensitivity experiment and evaluation method were also proposed.The permeability stress-sensitivity of ultra-low permeability reservoir is weaker when the reservoir effective stress range is considered.