Pore Network Characterization Utilizing LWD Acoustic Pore Aspect Ratio and NMR Rock Typing for Enhanced Well Placement

Pore aspect ratio (PAR) is defined as the ratio of the short axis to the long axis of an oblate ellipsoid that represents the average pore shape. In carbonate reservoirs, a combination of Nuclear magnetic resonance (NMR) rock typing, through classification using T2 (or T1) and porosity, and acoustic PAR provides an indicator of the effect of dolomitization on rock quality. Integration of these analyses indicates that dolomitization alters rock quality through changes to pore structure, including PAR, pore size, and porosity. The main objective of this study is to further test the above hypothesis in situ and explore its applications for enhanced well placement. In this study, the average acoustic PAR was computed from logging-while-drilling (LWD) acoustic logs, using an established methodology (Ma et al., 2023). Formation rocks types are classified based on another established method (Hursan and Van Steene, 2023), which classifies rocks into four types represented as four quadrants in the NMR T2 logarithmic mean) (T2LM) and porosity domain. Rock type 1 (RT1) and rock type 2 (RT2) have the highest porosity with T2LM varying from high to low, while rock type 3 (RT3) and rock type 4 (RT4) have lower porosity than RT1 and RT2, also with T2LM from high to low. Since both T2LM and porosity are statistically proportional to permeability, RT1, the highest quality rock, would have the highest permeability, while RT4 the lowest permeability. RT3 may have higher permeability than RT2, even though RT2 has higher porosity, if the influence of T2LM is more dominant than porosity on permeability. With dolomite volume obtained from log analysis, data cross plotted between dolomite volume and acoustic PAR can be grouped by NMR rock types, illustrating the effects of dolomitization and its diagenesis on rock quality in terms of changes in porosity and permeability. The above method of analyzing and integrating data of acoustic, NMR, and other logs is applied to LWD data in a vertical well that crossed multiple reservoirs and in a dual-lateral horizontal well placed in carbonate formations, where, the highly dolomitized rocks have a higher average PAR, indicating that the pores are more spherical than in the more calcitic rocks. The dolomitized zones mostly belong to RT1 and RT3 with longer T2LM, associated with larger pore size, indicating that dolomitization has increased the rock quality in terms of pore size and also PAR, though not necessarily porosity. This effect of dolomitization occurred in both high and lower porosity rocks. The better reservoir quality in the dolomitized zone was confirmed by high mobility from formation testing data. In this study we present a novel approach analyzing and integrating acoustic, NMR, and other logs to provide quantitative information about the diagenetic process of dolomitization and its effect on rock quality, supporting rock typing and rock quality definition in situ in real time for enhanced well placement.