Estimation of Variable Fluid-Mixture Density With 4D-NMR Logging

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 109051, "Estimation of Variable Fluid-Mixture Density With 4D-NMR Logging," by Chanh Cao Minh, SPE, Schlumberger; Emmanuel Caroli, Total; and Padmanabhan Sundararaman, SPE, Chevron, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11-14 November. The paper has not been peer reviewed. No technique is available to estimate the apparent fluid density measured by the density log. In wells drilled with oil-based mud (OBM), the near-wellbore-fluid composition and distribution can be very complex. Current practice for computing density porosity is to use an ad hoc constant fluid density. This method could lead to erroneous estimation of porosity. Radial fluid profiling with advanced magnetic-resonance tools provides radial information that can be inverted jointly with conventional 3D nuclear-magnetic-resonance (NMR) data to determine fluid mixture and radial distribution. Introduction Computing total porosity corrected for hydrocarbon effect from density/neutron logs gives an estimate of the hydrocarbon density when the matrix density is known. It assumes that the fluids are distributed uniformly within the investigated zones. Noting that near-wellbore fluids can vary radially and accounting for the different depths of investigation (DOIs) of the density and neutron logs enable computing total porosity in gas-bearing formations. Density and NMR log data can be combined to estimate total porosity in gas-bearing formations in the so-called DMR technique. However, the various sensors assess the same fluids. In spite of their shallow DOI, each sensor has a distinct DOI and fluid-averaging scheme. For example, NMR has a shallow DOI on the order of 1 in. to a few inches and a narrow sensitivity zone in the formation. Classical nuclear tools have deeper DOIs, ranging from a few inches for the density tool to 1 ft for the neutron tool, along with much broader investigation zones than NMR tools. Even though NMR and density logs might agree in many circumstances, they may not assess the same fluids. 3D NMR is a standalone technique to evaluate near-wellbore fluids at a fixed DOI. In some cases, complex fluid mixtures including water, oil, OBM filtrate, and gas are observed. Slow logging speed and unfavorable vertical resolution prevent routine use of the technique to estimate total porosity. 4D-NMR logging adds a radial dimension and can deliver continuous fluid mapping at multiple DOIs called shells, typically at 1.5, 2.7, and 4 in., as shown in Fig. 1. The simultaneous processing of data from all shells is intended to optimize the fluid results. Therefore, the variable fluid-mixture density can be estimated with 3D NMR at multiple DOIs and then used in the density-log-porosity computation, assuming that the matrix density is known.