Simulation and Analysis of Low-field Nuclear Magnetic Resonance Transverse Relaxation Response of Fractured Shale Reservoir
Abstract
Microfractures in shale reservoirs are extensively developed, and are typical of a dual pore system of fractures and matrix. Porous rocks are composed of pores of different sizes and hydrocarbons are mixed by complex hydrocarbons. Therefore, the log-Gaussian function distribution is used to characterize the pore size distribution as well as the bulk relaxation and diffusive relaxation of the rock. A fracture-containing shale core model was developed based on digital cores of shale, and the pores and fractures were simplified into spherical and ellipsoidal models, respectively, to simulate the NMR response characteristics in shale biphasic media. The effects of fracture parameters (shape, size, density, etc.) on the NMR logging response were simulated and analyzed based on shale rock physical parameters, respectively. The results show that the NMR response characteristics of the fractures are obvious and are related to multiple factors such as fracture morphology, fracture length, fracture density, and the fluid filling in the fractures. The NMR response characteristics of different types of fractures are different, and overall, the NMR T2 spectrum shifts to the right as the fracture length increases. With the increase of fracture length, the T2 spectrum of needle-shaped fracture changes from "double-peak" to "single-peak"; the T2 spectrum of ellipsoidal fracture shows a trend of "double-peak-single-peak-bouble-peak"; the T2 spectrum of pie-shaped fracture shows a trend of "double-peak-single-peak". The T2 spectrum of pie-shaped fractures is "triple-peaked". As the fracture density increases, the saturation of the fluid in the fracture increases, and the peak representing the fracture component gradually increases and shifts to the right; the peak representing the pore component gradually decreases and shifts to the left. When the fracture is saturated with oil, the short relaxation component represents the water in the pore space and the long relaxation component represents the oil in the fracture. As the oil saturation in the fracture increases, the long relaxation component increases and the short relaxation component decreases.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMMR53A..07G