Application of Borehole Image-Based Porosity Analysis in Carbonate Reservoirs to Assist in Permeability Calculation and Its Integration With Resistivity Inversion to Distinguish Productive Vuggy Zones and Tight Zones
Authors: Priyanka E. 
Published: March 2023

The “C” carbonate in Bukit Tua Main, KP Field, Northeast Java Basin is characterized by highly heterogeneous porosity distribution. By performing carbonate heterogeneity analysis using the input of borehole image log in well X-2, different porosity types were distinguishable, from matrix-porosity-supported zones to secondary-porosity-supported zones. The latter can further be distinguished according to the secondary porosity types. In this well, the predominant secondary porosity types are connected vugs, with subordinate solution-enhanced bed boundary and isolated vugs. Solution-enhanced bed boundary and connected vugs are typically the porosity types that are expected to contribute better to permeability. When compared with effective porosity calculated from conventional openhole logs (density-neutron), zones with an abundance of solution-enhanced bed boundary and/or connected vugs showed relatively higher effective porosity values (i.e., up to and exceeding 20%). Therefore, the study interval can be divided into vug-dominated zones and matrix-porosity-dominated zones to ensure that appropriate permeability equation is applied accordingly, depending on the textural (pore) characteristics. Permeability measurement during pressure buildup analysis from well testing was performed. Separately, permeability calculation throughout the interval was also performed using empirical equations for vug-dominated or karstified limestone that was derived from core porosity and core permeability relationship in Bukit Tua. By excluding the zones that are not supported by either solution-enhanced bed-boundary or connected vugs and, at the same time, have relatively lower porosity (i.e., below the average porosity of connected-vug- and solution-enhanced bed-boundary-dominated zones that possess relatively high porosity), a similar mean permeability value was obtained between the two methods. This confirms that because a borehole image-based heterogeneity analysis result could distinguish vug-dominated zones from matrix-porosity-dominated zones, appropriate permeability equations can be applied to those intervals with different textural characteristics, which results in a reliable permeability log as evidenced from the similarity of mean value with the direct permeability measurement. Both porosity heterogeneity analysis and calculated permeability logs indicate that the “C” carbonate has several zones of interest mainly supported by connected vugs and solution-enhanced bed boundary with permeability values ranging from 50 to 200 mD. In addition, when borehole image log is combined with another log such as the resistivity inversion, the two are useful to indicate which vuggy zones may have vugs connected further into the formation vs. those that are tight despite the presence of vugs. Zones with resistivity inversion showing green to cyan veins spectrum; i.e., moderately high conductivity, are typically consistent with zones that have an abundance of solution-enhanced bed boundary and connected vugs, as well as higher porosity and calculated permeability values. Conversely, zones with dark red veins; i.e., high resistivity, albeit having an abundance of vugs, indicate that these vugs possibly do not penetrate deep into the formation.