The Influences of Microscopic Pore Structure on Spontaneous Imbibition of Tight Gas Reservoir

Abstract

In the process of tight gas reservoir development, there is a high possibility of spontaneous imbibition, which will cause water lock and seriously affect the effect of gas reservoir development. Investigating the change of gas and water distribution during the process of tight gas reservoir spontaneous imbibition by using nuclear magnetic resonance (NMR) technology, we can draw a conclusion that after excluding the influence of wettability and the viscosity of the formation water, the gas-water distribution in sandstone is related to the connectivity of the pore throat, and the microscopic pore structure plays a significant role in the process of spontaneous imbibition. Then, this research use constant-speed mercury injection, high pressure mercury injection and nitrogen adsorption methods to obtain the characteristic parameters, such as average pore throat ratio, specific surface area and pore size, which characters the microscopic pore structure. On the basis of all the results obtained, the effects of microscopic pore structure on spontaneous imbibition within tight gas reservoir was studied. The experimental results show that the proportion of small pores involved in imbibition is reach up to 98.23%. Pore-permeability ratio and permeability show a positive correlation with recovery efficiency, spontaneous imbibition is easy to happen in gas reservoir with good reservoir permeability. There is a critical value of the average pore-throat ratio, when the wetting phase fluid can pass through the throat smoothly, the decrease of the average pore-throat ratio has no obvious influence on the imbibition recovery. Specific surface area of tight sandstone shows a negative correlation with the recovery efficiency. The proportion of small pores plays a decisive role in the recovery efficiency of spontaneous imbibition. Pore diameters of large and medium pores are much larger, as a result, the capillary force is greatly reduced, so the increase of proportion of large and medium pores has a negative effect on the process of spontaneous imbibition.