Core Analysis and Core-Log Integration for Evaluation of Unconventional Formations
Hirasaki, George J
Doctor of Philosophy
Unconventional resources are of great importance in the global energy supply. This thesis develops new techniques and presents fundamental research serving unconventional formations evaluation for the petroleum industry. First, hydrocarbon composition is a critical input to formation evaluation. In this thesis, a new technique using laboratory Nuclear Magnetic Resonance (NMR) core-analysis integrated with downhole NMR logging is developed to estimate the hydrocarbon composition in an organic-rich chalk prospect. More specifically, the contrasts in T2 and T1/T2 distributions between fluids are used for fluid typing. Meanwhile, another technique based on the NMR laboratory-measured restricted diffusion of light hydrocarbons is proposed to estimate the mean pore size, heterogeneity length scale, and tortuosity of the hydrocarbon-filled porosity. Second, it has been commonly observed that in organic-rich shale, the saturating hydrocarbons have higher NMR T1/T2 ratio than the saturating water. However, the origin of the high T1/T2 ratio was not clearly understood until now. In this thesis, the organic matter (i.e., kerogen) in the organic-rich shale is isolated for investigation. It is confirmed that the saturating heptane in kerogen has higher T1/T2 ratio than water in kerogen and clays, which validates the fluid typing technique providing the wettability. This thesis also proves that the high T1/T2 ratio originates from dissolved heptane in kerogen and/or bitumen, where the dominant relaxation mechanism can be the 1H-1H dipole-dipole interaction, as a result of nanopore confinement. Last but not least, permeability is an indicator of the producibility of reservoirs, and thereby a critical petrophysical property during formation evaluation. The existing ultralow-permeability measurement approaches for unconventional formations, including both steady-state and unsteady-state approaches, confronting various challenges. In this thesis, a novel unsteady-state method is proposed to determine the permeability by history matching, which consists of 1D transient-pressure experiments and numerical simulation incorporating real-gas pseudo pressure and table lookup. This novel method helps to improve the experimental efficiency, simplify the set-ups, reduce the interpretation complexity, and alleviate the pressure-limit constraint. These new technologies and fundamental understandings could in principle be used to improve the evaluation of unconventional formations.
Formation Evaluation; NMR; Permeability; Simulation