Impact of H2S in Predicting the Storage Efficiency of CO2 Injection in a High Pressure High Temperature (HPHT) Carbonate Aquifer - A Case Study in a Sarawak Offshore High CO2 Gas Field, Malaysia

Abstract

Carbon capture and storage (CCS) has been identified as transformative technologies to achieve maximum reductions in CO2 emissions specified under the Kyoto Protocol. Historically, CO2 was first stored in reservoir indirectly back in 1970s during Enhanced Oil Recovery (EOR) process. Subsequently in 1996, the Sleipner CO2 storage project was officially implemented in Norway providing reference to future CO2 storage projects. As one of the main players in the oil and gas industries, PETRONAS has been proactive in finding an end-to-end technology development for CCS. The S field, a non-associated gas field, located 250km Offshore Sarawak Malaysia has recently been identified as a potential pilot for hydrocarbon production and CO2 storage as well as for surface and subsurface technologies maturation. The S field, a carbonate builds up platform is considered heterogeneous type of reservoir with porosity ranging from 15% to 40% and permeability ranging from 10md to 1300md. The S field contains 70mol% CO2 gas contents/30mol% hydrocarbons with some amount of impurities, N2, H2S and Hg. As a baseline development strategy to support this technology project, pure CO2 injection was considered, taking into account the separation technology efficiency to separate CO2 from other contaminants back into the same reservoir aquifer. However, due to some limitation on the surface technologies and economy limits, traces of impurities might be present in the system. There is a possibility of CO2 being co-injected with impurities, for instance H2S. The effect of the contaminant in the injected CO2 stream needs to be accessed to ensure the success of the CCS operation. Changes in base CO2 solubility may ultimately affect storage integrity and capacity. Since abundant solubility data of CO2 in water or brine are available in the literature and less data for this ternary system indicates that the understanding in quantifying the impact of this H2S impurity on CO2 solubility is essential. In this communication, extensive laboratory experiments on vapour-liquid equilibrium on the binary CO2-H2O (for experimental method validation) and ternary systems, CO2-H2S-H2O and CO2-H2S-Brine (0.35M) have been conducted at 423.15K, under pressure range of 1-35 MPa. The experimental method used in this work is the static-analytic type, taking advantage of two magnetic capillary samplers (Rolsi®, Armine’s Patent) developed in the CTP laboratory. This method allows the sampling of the mixture in both vapour and liquid phases and their analysis by Gas Chromatograph (GC). Thermodynamic modelling using the simplified Cubic Plus Association (sCPA) equation of state was able to satisfactorily describe the phase behaviour of the ternary systems. The impact of this H2S on CO2 injection for CCS has been further evaluated using numerical reservoir simulation. Existing reservoir dynamic modelling has been improved by incorporating the new experimental data. Simulations results were analysed based on the volume of CO2 dissolved in water and the total volume of CO2 gas injected. It shows that the introduction of H2S in injected CO2 promote a reduction of CO2 dissolved in water by 15%, however it eventually offers a similar amount of CO2 storage over long period of time.