Mechanism of Transit Time Increase and Its Interpretation After Water Injection into Reservoir M in the Lao Jun Miao Oil Field

Abstract

Summary Reservoir M in the Lao Jun Miao oil field is a very massive sandstone oil reservoir about 60 m [200 ft] thick. Within the reservoir, there are no evident shale barriers and no interlayer standard water zones. This reservoir is classified as a fractured porous reservoir. The reservoir rocks have a high shale content; montmorillonite is the main clay mineralwith a maximum percentage content of up to 60%. After water injection, the sonic transit time was increased substantially in the flooded rather than in the nonflooded intervals. This paper gives an idea of the mechanism for the increase in sonic transit time in flooded intervals of the reservoir on the basis of the measurement and analysis of the noninvaded cores. Study results show that the increase in sonic transit time after flooding is linked to the rock texture in relation to the high montmorillonite content. It is well known that montmorillonite expands when it absorbs water and can plug the formation. However, it is possible to have better communication in the pores locally. These concepts are important and useful for better oilfield development and in the evaluation of the sweep efficiency of waterflooding. This paper also gives an in-house measurement technique for samples with high montmorillonite content. On the basis of core analysis and well testing data, we propose a comprehensive interpretation technique for determining flooding or nonflooding intervals that uses the transit time from sonic logging as the main data. Good results have been obtained.