Grain size distribution of formation sands is one of the crucial parameters in a sand control program. Such information is commonly derived from sieve or laser particle size analysis (LPSA) techniques on a limited number of core samples from the target well or from a neighboring well in the same field. These laboratory-based grain size analysis methods require proper disaggregation of grains, a process which often causes breakup of some grains resulting in distortion in the grain size distribution. Moreover, variations of grain sizes in the formation may not be captured by the limited number of core samples.
This paper presents a new method of grain size determination from nuclear magnetic resonance (NMR) logs using micro-structural rock modeling (MSRM). NMR probes the pore space as the relaxation time distribution of the wetting-phase fluid correlates to pore size distribution while the non-wetting phase fluid carries no pore size information. To derive grain size distribution from NMR relaxation time measurements, MSRM is applied to reconstruct the pore/grain distribution and to simulate NMR responses by considering mineralogy, partial water saturation, surface relaxivity, and the surface roughness factor of the grains. Extensive core studies were performed to validate this method. A ternary diagram is developed to correlate surface relaxivity and the surface roughness factor with the amount of quartz, feldspar, and clay as a triangle in the ternary diagram.
Field application case examples are presented. Continuous grain size distributions are calculated on foot-by-foot basis that capture the variations of grain size in the formation sands along the depth. The log-derived grain size distribution shows good agreement with the core data at depths where core samples are available. D50 and D90 characteristics are derived from the continuous grain size distributions to assist the completion decisions, e.g., the selection of screen mesh size in the openhole standalone screen sand-control approach, or the selection of gravel size in the gravel-packing sand-control approach.