A quantitative analysis method of formation conditions for a low resistivity annulus zone

Abstract

The low resistivity annulus zone is one phenomenon that during oil exploration can be used for interpretation and evaluation. For oil–water flow reservoirs, both the formation characteristics and water saturation determine the relative permeability. Furthermore, when the formation water resistivity is lower than the mud filtrate resistivity, the transitional zone between the invaded zone and the undisturbed zone can form a low resistivity annulus by mud filtrate invasion. The low resistivity annulus zone resistivity value is mainly affected by the difference between the mud salinity and formation water salinity and a relatively high water saturation. In addition, the mud circulation time determines the low resistivity annulus zone location. Many experts have modelled the mud invasion process using numerical models. However, they cannot give the low resistivity annulus zone formation conditions. This paper analyses in detail the basic condition of the low resistivity annulus using theory deduction, concludes that the water saturation and mud filtrate resistivity value ranges are ascertained and builds a correlated chart. In this paper, the low resistivity annulus zone resistivity is determined, from the qualitative method to the quantified method, to favourably find the oil layer.