Research On Water Blocking Damage Evaluation Of Ultra-tight Sandstone Gas Reservoirs

Micro-sized and nano-sized pore are developed in hyper-tight sandstone with poor connectivity.The gas reservoir has the characteristics of high capillary pressure,high clay mineral content,strong heterogeneity and low water saturation.Only by the stimulations such as fracturing can we obtain oil and gas flow of industrial value.Because of its unique geological conditions and seepage characteristics,the water phase is easy to produce imbibition and difficult to flowback in the process of drilling,completion,development and production.Meanwhile,water blocking is the crucial damage form for this type of gas reservoir.The laboratory evaluation of water blocking damage is the basis of comprehensive study on the mechanism of water blocking damage and the prevention and control measures of this damage.At present,there are no unified standard for evaluation of hyper-tight sandstone gas reservoir water blocing damage at home and abroad.The existing evaluation criterions have many problems.Initial water saturation is not set up,formation water salt out,water sensitive damage and water blocking damage are superposed and coupled,permeability test has system error and the evaluation is not comprehensive and so on.The old methods are lacking of pertinence and representativeness of the evaluation of water blocking in hyper-tight sandstone gas reservoir.Based on the data collection,literature research and basic geological feature analysis,this thesis defined the potential damage mechanisms of the water blocking in hyper-tight sadstone gas reservoir,aiming at the shortage of present evaluation methods.Relative experiments to study the damage mechanisms and evaluation methods for hyper-tight sandstone gas reservoir was carried out,and the following results were achieved:(1)Water blocking damage was evaluated by the combination of two damage processes which contains of imbibition and residual water damage..The thesis utilized MRI technology to monitor the process of rock samples’ capillary spontaneous imbibitions in real time.(2)High-salinity brine,deionized water and fracturing fluid were used as testing liquids for comparison.This thesis also focus on the influence factors of quantitative water locking damage;(3)Meanwhile,the evaluation methodology of water blocking damage for this kind of gas pool was established by means of inactivation at high temperature,supercentrifugal process,displacement at constant pressure and NMR.The new method showed more comprehensive and more applicable.Furthermore,laboratory experiments using this methodology was conducted,and it can be used to guide the prevention and control of water blocking damage.In conclusion,the evaluation experiments indicate that,conventional oil and gas reservoir damage evaluation methods and standards can not applied to hyper-tight sandstone layer factually and objectively,since error value is existing.In this thesis,conventional evaluation methods were improved and added basing on the consideration of many controlling factors of water blocking in hyper-tight sandstone reservoir,the newly established evaluation methods have important guiding significance for the exploration and development of this kind of gas reservoir.