A Case Study ofNMR Interpretation in Complex Reservoirs Using Variable T2 Cutoff

A complete formation evaluation requires determination of irreducible fluids, movable fluids, and formation permeability. Analysis of NMR measurements to determine these parameters is strongly dependent on formation lithology and mineralogy, especially in complex and heterogeneous reservoirs. Therefore, NMR measurements must be combined with measurements from conventional logs for comprehensive formation evaluation. In this paper, an interpretation method that combines these data is discussed and is applied to complex formations An important advantage of the method presented in this paper is its capability to generate a variable T2 cutoff for NMR interpretation that depends on the volumetrics of the calculated formation lithology. In NMR analysis, the T2 cutoff is required to divide effective porosity into irreducible and movable porosity. In traditional T2 interpretation, the T2 cutoff is usually a constant value throughout a formation; however, the methodology proposed in this paper uses a variable T2 cutoff calculated on a foot-by-foot basis from log-derived lithology. Comparisons between the constant and variable T2 cutoffs and their effect on formation evaluation are included, and recommendations are discussed. An error minimization technique is used to solve a system of equations derived from the log data. In this system of equations, the NMR effective porosity is used as the formation effective porosity. To evaluate the goodness of the fit between the original logs and the reconstructed logs, the chi-square of the difference between logs is calculated. In this methodology, weighting factors can be applied to the log data, and geological and petrophysical constraints can be imposed.