Laboratory assessment on factors controlling the acoustic properties of carbonates: A case study from Bombay offshore

Abstract

Carbonates are known to have a heterogeneous, complicated pore structure, characterized by textural variations. This complex nature of carbonates makes it challenging to establish correlations to interpret petrophysical properties from acoustic properties. The present study focuses on investigating various parameters that influence carbonates by integrating parameters estimated from laboratory based measurements along with digital image analysis (DIA) technique, to capture the heterogeneity in pore architecture. A generalized rock physics model for carbonates of Bombay offshore basin has been developed based on compressional wave velocity (Vp) measured at laboratory conditions using ultrasonic measurement techniques. Our study maximizes the use of information obtained from the laboratory analysis that is usually constrained by accurate geological details. For this purpose, we measured Vp of carbonate core plugs using transducers of 54 kHz frequency by utilizing the through transmission technique. The parameters investigated at the laboratory include bulk density, porosity, permeability, mineral content, pore size and pore aspect ratio. Statistical analysis was performed using single as well as multiple parameter function. Mineral content was correlated with Vp and porosity. Bulk Density and porosity showed greater correlation with Vp as single parameter function while pore size showed poor correlation. A combination of bulk density, porosity and mode pore size was able to explain more than 83% of the deviation of Vp. It was observed that the predictability of Vp increases when it is modeled as a multi-parameter function of bulk density, porosity and pore size. The effectiveness of the velocity model improved when Vp was modeled as a function of these investigated parameters collectively rather than in isolation. Thus, an integrated study such as multi-regression analysis would help to present a more realistic picture that affect the relations between petrophysical, mineralogical and acoustic properties. Furthermore, while considering different saturation conditions, our study suggest that porosity and mineral composition have an effect on Vp, while effect of porosity is more prominent. We observed approximate 50% decrease in Vp values when porosity varied from 2 to 40% for different saturation conditions. Hence, the results potentially allow the interpretation of different petrophysical properties from acoustic data and thereby help in connecting geology and seismology.