ABSTRACT:

A study is conducted to measure acoustic velocities of carbonates with varying CO2 saturation at reservoir conditions. Three samples from the proposed injection zone of K-field were tested in a modified triaxial rig with CO2 core-flooding capability for measurement of their acoustic velocities. To fully capture the fluid sensitivity of the core samples, acoustic measurements were conducted for 100% CO2 saturated and 100% brine saturated conditions. For varying CO2/brine saturations, the brine is displaced with CO2 at pre-determined volumes to achieve the target intermediate saturations. The results showed trends for both compressional velocity (Vp) and shear velocity (Vs) changes against the CO2/brine saturations. The 100% brine saturated sample translated to the highest acoustic velocities whereas the 100% CO2 saturated sample showed the lowest velocities. As the CO2 saturation increases, a steady decrease in acoustic velocities was observed. The fluid sensitivity effect (up to −10% and −12% for Vp and Vs, respectively) was observed up to 70% CO2/brine saturations which will be beneficial for seismic-based monitoring post-CO2 injection.

1. Introduction

In an effort to unlock high contaminant gas fields within offshore Malaysia, PETRONAS is currently embarking on Carbon Capture and Storage (CCS) technologies to ensure that the monetization of these gas fields will be undertaken in a sustainable manner. CCS is a technology concept of capturing carbon dioxide from large industrial sources and injecting them into deep underground reservoirs for permanent storage with the objective of reducing the impact of CO2 emission to the climate system. A carbonate gas field in offshore Sarawak has been identified (K-field) as a potential CO2 storage reservoir. One of the crucial aspects of any CO2 storage operations is the monitoring of the CO2 plume post-injection. This is typically done via 4D seismic monitoring and prior rock physics modelling (Lumley 2010; Rodrigues et al. 2012; Hao et al. 2016). However, for carbonate reservoirs, this can be quite complex due to the nature of the carbonate sediments which do not have a predictable porosity against velocity trend when compared to their siliciclastic counterparts (Eberli et al. 2003; Anselmetti and Eberli 1997).

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