Evaluation Method For Damage Mechanism Of Dual-medium Tight Sandstone Gas Reservoirs

Tight sandstone gas reservoir,qualified with compact structure,small pore throats,high content of clay minerals,always need fracture stimulation to obtain oil and gas flow of industrial value.The reservoir sometimes has dual-medium characteristic under special circumstance,to be the first,the growth of nature fractures and micro cracks act as main seepage channel which communicate with the base reservoir.Second,pressure fractures can connect the original channels,thus the reservoir has same Dual-medium feature after putting into production.Dual-medium tight sandstone gas reservoir have complicated and diversified damage mechanisms during drilling and developing processes because of their unique geological conditions and flow characteristics,restricting the efficiency of gas exploration and development.The laboratory reservoir damage evaluation is the basis of comprehensive study for damage mechanisms and their protection measures.However,the existing evaluation methods mainly refer to the routine ones,restricted by single evaluation medium,and dose not consider the effects of geological factors on evaluation results,lacking of pertinence and representativeness to dual-medium tight sandstone gas reservoir.This paper defines the potential damage mechanisms of the base block and the fracture in Dual-medium tight sandstone gas reservoir,basing on literature research and basic geological feature analysis.And then focus on the deficiency of present evaluation methods,carry out relative experiments to study the damage mechanisms and evaluation methods for dual-medium tight sandstone gas reservoir,the following results are achieved:(1)Using gas and liquid phase as the medium for displacement tests separately,while velocity sensitivity,water sensitivity and acid sensitivity damages are evaluated using base reservoir cores.The results show that,traditional liquid evaluation methods expand the fluid sensitivity in reservoir,nevertheness,traditional evaluation methods for stress sensitivity doesn't consider about the stress changing,water saturation and other geological factors,thus the evaluation results aren't comprehensive;(2)Water blocking damage is evaluated by the combination of two damage processes which contains of imbibition and residual water damage:Deionized water and simulated formation water are used as testing liquids for comparison.Results show that low salinity fluid can eliminate additional salting out damage.Moreover,high-pressure saturation-centrifugal weight evaluation-nuclear magnetic resonance(NMR)measurement is established for extra low permeability gas reservoir,compared to traditional fluid injection-gas drive method,the new method show more comprehensive and more applicable.(3)Improving the evaluation methods for solid phase invasion by eliminate stress sensitivity of artificial core fracture,and then evaluate the solid invasion damage to the cracks caused by drilling mud and fracturing fluid respectively.In conclusion,the evaluation experiments indicate that,conventional oil and gas reservoir damage evaluation methods and standards can not applied to dual-medium sandstone layer factually and objectively,sinse error value is existing.In this paper,conventional evaluation methods are improved and added basing on the consideration of stress changes,water saturation and other factors of dual-medium reservoir,the newly established evaluation methods for dual-medium tight sandstone gas reservoir have important guiding significance for the exploration and development of this kind of gas reservoir.