A novel NMR surface relaxivity measurements on rock cuttings for conventional and unconventional reservoirs

Abstract

Surface relaxivity (ρ) is an intrinsic rock property that makes low-field NMR a valuable tool for determining pore size distribution of geological formations. Its value can vary considerably in different rocks depending on the type and distribution of the rock minerals, rock surface wettability and petrophysical properties. Therefore, laboratory measurement of ρ is required on a case by case basis. However, existing NMR methods are expensive, complex, and most often require supplementary and expensive non-NMR measurements. Rock samples are also required to be well consolidated and cylindrical so that experiments can be run at high pressures – a barrier for measurements of unconsolidated and thinly laminated unconventional rocks. This study proposes a new NMR method termed ‘NMR grain sizing method’, that uses different grain sizes of a given rock to generate pores of corresponding sizes. Using a geometric constant, grain specific surfaces are transformed to pore surfaces, and the governing NMR equation is rearranged so that we have a method that measures ρ from only NMR measurements on rock cuttings. This approach is simple, inexpensive, and does not require supplementary measurements. It offers an added advantage for characterizing thinly laminated formations. A proof of concept is demonstrated for three unconventional rocks (Qusaiba shale, Jafurah carbonate mudrock and Eagle Ford) and two conventional rocks (Berea sandstone and Indiana limestone). The ranges of surface relaxivity are 96.9–134.5 μm/s (Qusaiba shale); 62.07–106 μm/s (Jafurah carbonate mudrock); 55.9–68.72 μm/s (Eagle Ford); 35.51–43.2 μm/s (Berea sandstone); and 33.2–40.54 μm/s (Indiana limestone). All ρ measurements are reproducible with 95% confidence interval. The measured values for Indiana limestone and Berea sandstones were validated with micro CT based pore network models and also compare well with literature data. Measurement of Qusaiba shale and Jafurah carbonate source rocks are apparently reported for the first time here.