Movable Fluid Distribution and the Permeability Estimation in Tight Sandstones Using NMR
An accurate understanding of movable fluid distribution and permeability are crucial for the successful exploitation of tight oil reservoirs. However, tight oil reservoirs typically show a wide pore size distribution with pore sizes ranging from several nanometers to several hundred microns. Now existing permeability estimation parameters are no longer suitable for tight sandstones. Therefore, this paper explores the applicability of nuclear magnetic resonance (NMR) on movable fluid distribution and the permeability estimation in tight sandstones. Six sets of NMR experiments are carried out on samples from Chang 6 formation in Ordos Basin. The first one is performed on fully water saturated plugs to obtain the fluid distribution. And the remaining five ones are conducted on the same plugs after centrifuged at different centrifugal force to determine movable fluid distribution. In order to further confirm the pore-throat morphology occupied by movable fluid, we carry out scanning electron microscopy on core slice. Based on the statistics of movable fluid distribution, a modified Coates model is developed for permeability prediction in tight sandstones. In terms of the study, the following conclusions are arrived at. The optimum centrifugal pressure for the Chang 6 formation is 208 psi and pores with radii less than 0.1μm show no obvious difference with throats. Movable fluid is mostly controlled by throats with radii less than 1μm, especially throats radii ranging from 0.3μm to 1μm. Movable fluid is stored in pores distributed around the right peak of the bimodal distribution with radii ranging from 10μm to100μm.These pores are residual interparticle pores and dissolution pores. Four types of pore-throat combination are identified using scanning electron microscopy, among which shrinking throats and flaky throats are common. A formula describing permeability based on the Coates model for a tight sandstone oil reservoir derived from the results matches the formation’s permeability with an excellent correlation factor of 0.9612 when the value in the Coates model equals 18. Petrophysical characterization by NMR technique provides an effective approach to better understand pore throat structures and storage capacity of tight oil reservoirs.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018