Fluid-Rock Characterization and Interactions in NMR Well Logging

Abstract

Objective: To characterize the fluid properties and fluid-rock interactions that are needed for formation evaluation by NMR well logging.This project is a partnership between Professor George J. Hirasaki at Rice University and Professor Kishore Mohanty at University of Houston. In addition to the DOE, a consortium of oil companies and service companies supports this project.The advances made in the understanding of NMR fluid properties are summarized in a chapter written for an AAPG book on NMR well logging. This includes live oils, viscous oils, natural gas mixtures, and the relation between relaxation time and diffusivity.Oil based drilling fluids can have an adverse effect on NMR well logging if it alters the wettability of the formation. The effect of various surfactants on wettability and surface relaxivity are evaluated for silica sand.The relation between the relaxation time and diffusivity distinguishes the response of brine, oil, and gas in a NMR well log. A new NMR pulse sequence in the presence of a field gradient and a new inversion technique enables the T 2 and diffusivity distributions to be displayed as a two-dimensional map.The objectives of pore morphology and rock characterization are to identify vug connectivity by using X-ray CT scan, and to improve NMR permeability correlation.Improved estimation of permeability from NMR response is possible by using estimated tortuosity as a parameter to interpolate between two existing permeability models. 3 EXECUTIVE SUMMARYThe fluid properties characterization is summarized as a chapter written for an AAPG book on NMR well logging. It describes the departure of live oils from correlations based on dead oils. Mixing rules are developed for gaseous mixtures of methane, ethane, propane, carbon dioxide, and nitrogen. The coupling between the relaxation time distribution and the diffusivity distribution is demonstrated with a binary mixture of hexane and squalene. Measurements of crude oils show that the T 1 , LM and T 2 , LM are equal at low viscosity but the T 1 , LM approach a plateau that is a function of viscosity and Larmor frequency. The hydrogen index of live oils can be estimated from measurements on the stock tank oil and live oil PVT data.Oil based drilling fluids can have an adverse effect on NMR well log interpretation if it alters the wettability from the natural condition of the reservoir.To gain an understanding of the effect of additives on NMR response, several surfactants were added to a base oil and its effect was evaluated on silica sand.A fatty acid, stearic acid, has no effect. However, a cationic surfactant and a commercial emulsifier alter wettability and increase the surface relaxivity of oil.New down-hole nuclear magnetic resonance (NMR) measurement and interpretation techniques have substantially improved fluid and reservoir characterization. These techniques take advantage of the magnetic field gradient of the logging tools to make diffusion sensitive NMR measurements.New NMR pulse sequences called "diffusion-editing" (DE)...