To evaluate formation petrophysical properties, a common methodology is to determine the reservoir porosity, permeability and water saturation through conventional measurements including gamma ray, resistivity, density, and neutron porosity. At least two or three logging tools need to be connected in the bottom hole assembly (BHA) together with a radioactive source. Several interpretation parameters, such as a (tortuosity factor), m (cementation exponent), n (saturation exponent), and formation water resistivity need to be determined through local knowledge or core measurements. In short, we look for alternatives to estimate water saturation (Sw) is without the above constraints.
A newly published method allows water saturation to be derived from nuclear magnetic resonance (NMR) transverse relaxation time (T2) distribution. Because it is independent of the resistivity measurement, it is viewed as a way to address the challenges mentioned above. The workflow builds on recent advancements to extract maximum information from minimal NMR data acquisition, such as factor analysis, a statistical method to determine various T2 cutoffs, and fluids substitution, a method to replace all hydrocarbons with water in T2 distribution. It is summarized below:
Identify fluids T2 cutoffs from factor analysis of the T2 distribution.
Perform T2 fluid substitution to get the 100% water T2 distribution.
Establish T2 log-mean, T2LM of 100% water (T2LM100%water) and T2LM of the hydrocarbon (T2LMhc), then, calculate Sw by interpolating the measured T2LM between the 1water and hydrocarbon T2LM endpoints.
Using the proposed workflow, we conducted a case study on two wells in Bohai Bay, China. The target reservoirs are heterogeneous shaly sand formations that produce light oil. Both wells have logging-while-drilling (LWD) triple combo, NMR and core data in the main reservoir intervals.
Overall, Sw derived from T2 shows good agreement with Sw calculated from Archie equation in both wells. The feasibility of the proposed NMR method is well demonstrated in the study field.
By replacing neutron-density tools with NMR, the operator is able to eliminate radiation hazard. With Sw computed from T2 distribution, the interpreter can corroborate resistivity-based interpretation or provide an independent reference when conventional interpretation is not conclusive.