Abstract

Gas reserves of approximately 210 trillion cu-ft containing approximately 70% CO₂ have been found in the Natuna "D" Alpha Field. One of the alternatives for the field development proposal is to maximize the utilization of CO₂ by transforming this gas into several petrochemical products, such as artificial hydrocarbons, methanol, and urea. If this option is not feasible, however, the CO₂ will be stored in the carbonate aquifers below the Natuna "D" Alpha Field. The pore volume of the aquifer is approximately 40 times the pore volume of the gas field itself.

The storage efficiency of the aquifer depends strongly on a number of factors such as i) the solubility of CO₂ in water at reservoir pressure and temperature; ii) the pore volume availability; iii) the compressibility of water and the rock, iv) dispersion and diffusion and v) viscous fingering and segregation. In order to investigate the storage efficiency of the aquifer of the Natuna "D" Alpha Field, several laboratory experiments have been conducted.

The experiments were performed at two different reservoir temperatures, i.e. 200°F and 250°F. The CO₂ solubility in water at constant volume was determined through the use of a PVT cell. The solubility increases dramatically as the injection pressure is increased up to 4000 psig. In contrast, increasing temperature tends to reduce the solubility of CO₂ in water.

A core-flooding rig was used to simulate the storage efficiency of the aquifers. Core plugs from carbonate rock samples with different permeabilities were collected to represent the variation of aquifer conditions. Firstly, these plugs were saturated with water at reservoir pressure. Next, CO₂ was injected until the injection pressure reached a level near to the fracture pressure. Applying an initial injection pressure of 1600 - 2000 psig and final injection pressure of 5000 psig the storage efficiency of the saturated plugs was measured in the range of 13.6 to 17.7% of pore volume.

During the experiments, a reaction between the CO₂, water, and carbonate occurred. This was indicated by a rise in the level of Ca-ions and a reduction in the level of Mg-ions in the solution. After the experiment porosity of the plugs increased while the permeability tended to decrease. Analysis of results of SEM and thin-section work indicated several changes, such as carbonate-mud dissolution, changing by micrite mineral formation, and suture stylolite fractures.

Ideally, having completed the laboratory experiments, these should then be followed by field testing based on a computer simulation scenario.