The estimation of bulk volume of irreducible water (BVI) is one of the earliest and the most widely used applications of NMR logging using either a fixed T2 cutoff value or Spectral BVI. NMR BVI assumes that bound fluid resides in small pores and producible free fluids (FFI) resides in large pores where the pore throat and pore body sizes are often related. As T2 distribution is related to a pore body size, a T2 cutoff can partition BVI & FFI.
In Carbonates there is no clear relationship of pore throat size to pore body size, thus measuring BVI T2 cutoff per rock type becames important although challenging. This paper covers the importance of following correct laboratory procedures, quality assurance of laboratory experimential data, and innovative methods to determine reliable T2 cutoff which otherwise are very low and not practical to apply in the NMR log domain.
NMR analysis was performed with a 2 MHz field instrument using a CPMG sequence and sufficient echo trains to acquire reliable T2 distribution. T2 cutoff was determined on core samples with a wide range of porosity, permeability, and rock types. Core samples were analyzed for T2 distribution at elevated temperature and pressure. First the cores were saturated with formation brine to 100% Sw, followed by desaturation using the porous plate pressure equilibrium method to Swirr. Initially, desaturation with gas provided very low T2 cutoffs; desaturation steps were repeated using a lab oil with reservoir property to investigate its positive impact on the T2 cutoffs and to use in the NMR log domain.
The T2 curve of the fully saturated plugs with brine shows a shift towards the shorter T2 time. Determination of T2 cutoff from the gas-brine system in the laboratory results in lower T2 cutoff values approximately 15 to 50 msec. This is due to the lower T2 response at the Swirr state from the combined affect of pores still partially saturated with brine and gas diffusivity affects. When desaturating with lab oil, the NMR response results in more reliable T2 cut-off of 55 to 100 msec.
NMR T2 cutoff for BVI using the oil-brine system has been determined using two methods, a conventional method where T2 cutoff is determined where the cumulative T2 value of the Sw at 100% brine equals the cumulative T2 value of the Swirr. The other method is where the T2 cutoff separates major peaks of bound and free fluid in the incremental T2 response. The conventional method posed a challenge in picking correct T2 cutoff due to various complexity as outlined in the paper. The latter method provided more reliable and better control on picking T2 cutoff to apply in the NMR log domain.