Classification of Pore Structure and Identification of Multiple Types of Pore Fluids in Chang 7 Shale Reservoir of the Ordos Basin Based on Nuclear Magnetic Resonance

The presence of distinctive mineral components imparts a remarkably intricate microscopic pore structure, which results in a diverse array of pore fluid types. Utilizing the principles of low-field nuclear magnetic resonance, we undertook a classification of the pore structure of Chang 7 shale reservoirs using 15 shale samples. The T2 relaxation time boundaries for various types of pore fluids were identified and delineated through a combination of centrifugal testing and heat treatment. These efforts allowed for quantitative characterization of the complete pore size distribution characteristics of the target reservoir. The research findings ascertain that the pore structure of the target shale can be categorized into three types, I, II, and III, and three types of fluids coexist within shale pores. In Type I shale, free fluid predominantly occupies medium and large pores with a pore radius exceeding 24 nm with an average saturation of 30.5%. Clay-bound fluid, on the other hand, is confined to micropores with a pore radius less than 4.5 nm with an average saturation of 35.3%. Type II shale features free fluid residing primarily in large pores with a pore radius greater than 41.9 nm with an average saturation of 26.6%. Clay-bound fluid is found in micro- and small pores with a pore radius of less than 10.3 nm, exhibiting an average saturation of 40.7%. Type III shale exhibits free fluid concentrated in large pores with a pore radius exceeding 94.3 nm and an average saturation of 17.2%. Clay-bound fluid is present in medium and small pores with a pore radius less than 29.3 nm and an average saturation of 50.1%. Furthermore, r2C1 and r2C2, calculated by T2c1 and T2c2 through conversion coefficients, effectively distinguish the free fluid and clay-bound fluid. r2C2 can be employed as an indicator for assessing the boundary of the reservoir flow capacity.