Phase behaviour of hydrate formations in oil and gas production in subsea conditions

Flow assurance of hydrocarbon streams in subsea systems, such as pipelines, is a very important subject in the oil and gas industry. Some of the flow assurance risks in subsea pipelines include gas hydrates, which form when light hydrocarbon molecules, such as methane, are trapped in water molecules. This solid crystalline compound can agglomerate and plug pipelines under suitable thermo-equilibrium conditions. Plugging of subsea pipelines can lead to costly operational problems or fatal accidents. Predicting the conditions when hydrates can form is very critical to successful oil and gas production. In this paper, PVTSim - a computer simulation program - is used to predict the hydrate formation conditions and generate equilibrium pressures and temperatures for two gas samples in experimental case studies. The PVTSim results are found to be in good agreement with the results in the case studies. As a result, PVTSim is used to analyze the phase behavior hydrate formation phase, generated for the two gas samples. The hydrate phase equilibrium (also known as hydrate formation or dissociation) curve separates the hydrate-formation and hydrate-free zones. The phase behavior of the hydrate formation is simulated by modifying the gas compositions with the addition of H₂S (hydrate former) and methanol (hydrate inhibitor). The results show that the hydrate-forming zone expands with addition of H₂S, increasing hydrate formation potential while the addition of methanol shrinks the hydrate-forming zone, lowering the potential for hydrate to form.