Gas Hydrate Formation in Drilling Mud Characterized With DSC Technique

Abstract

Abstract Gas hydrates have been studied by petroleum industry for more than 60 years, but up to now, the major part of the research was linked to the problem of oil transportation, prevention or remediation of pipe lines plugging. The development of deep offshore drilling has extended gas hydrate problem to drilling muds area. The consequences of hydrate formation in drilling muds can be dramatic: loss of mud rheological properties, accumulation of hydrate crystals leading to plugging of the lines, the BOP or the annular, interruption of the drilling operations and even destruction of rig equipment. To prevent these problems, operators must use drilling muds (WBM or OBM) with thermodynamic inhibitors of hydrate formation as salts and glycols, which cause important problems of density adjustment, corrosion and toxicity. The usual way to determine the thermodynamic conditions of hydrate formation in drilling mud formulations is to use batch reactors or thermodynamic models validated from PVT experiments on simple or model fluids. An innovative methodology was elaborated using Differential Scanning Calorimetry (DSC) to determine the thermodynamic equilibrium properties and kinetics of hydrate formation in mud formulations, particularly in the presence of large amounts of mineral. This technique allows the measurement of heat transfers as a function of time, temperature and pressure and thus detects phase transitions. We have shown recently that this rapid, versatile, easy to use technique permits to determine the dangerous zones for hydrate appearance. We present here new results of hydrate characterization. Using DSC, hydrate appearance in several drilling muds formulations (WBM or OBM) was shown and quantified. The influence of gas composition i.e. methane or natural gas on hydrate formation has been studied for different mud continuous phases, the effectiveness of several inhibitors was measured. A methodology using isothermal experiments to quantify kinetics of the formation of hydrate was proposed, the bubbling of gas into the measuring cell allowing to simulate agitation and progressive gas incorporation. Effect of time and temperature are by this way quantified. Through this work, DSC has proven to be an efficient technique for characterizing hydrate formation in drilling muds. Introduction Gas hydrates are solid structures composed of water and gas: depending on thermodynamic conditions, water molecules will form a solid cage entrapping gas molecules. The problem of the formation of these insertion compounds is well known by the petroleum industry and has been studied for a long time in the field of oil and gas transportation1. In this area, the issue is to cure or prevent pipe plugging by solid hydrates. More recently, with the development of deep and ultra-deep offshore operations, the problem of hydrate formation into the drilling mud has become a major concern. With the pressure and temperature conditions encountered during these operations (400 bars and 4°C are not uncommon), water contained in the mud may form gas hydrate with gas coming from the formation. It is all the more true during circulation stop where the mud temperature may decrease a lot2. The consequences of this hydrate formation in drilling muds can be dramatic: accumulation and aggregation of solid hydrates may plug the lines (kill and choke lines), the BOP or the annular blocking the drill string. Even accumulation around the drill bit can lead to an interruption of the drilling operation. Destruction of the rig equipment due to a rapid liberation of gas when hydrate crystals dissociate is also to be feared: 1 m3 of hydrates can very rapidly liberate 170 m3 of gas. Propulsion of gas hydrate plugs at very high velocity is also a risk. Deep offshore operators are aware of this gas hydrates formation problem as shown by the number of publications relative to this topic, but it is difficult to find in official literature descriptions of real field cases. However hazards due to gas hydrates exist and must be evaluated. According to Barker & Gomez3, every drilling operation should evaluate and analyze gas hydrate formation risk, considering every shut-in period. The first issue, before addressing specific formulations for hydrates inhibition is to determine with precision the thermodynamic conditions of hydrate appearance, i.e. P and T windows as a function of the formulation composition.