Abstract

With the increasing number of deep offshore drilling operations, operators and service companies are now faced with new problems related to the possible formation of gas hydrates in drilling muds. Actually, the appearance of gas hydrate crystals in drilling fluids can lead to dreadful effects and safety problems, like modification of mud rheological properties, interruption of the drilling operations due to plugging and even destruction of rig equipment when gas hydrates dissociate. Propulsion of gas hydrate plugs at very high velocity is also a great risk. To prevent these problems, formulations of drilling muds (WBM or OBM) have to be optimized with thermodynamic inhibitors of hydrate formation (salts and glycols), which cause important problems of density adjustment, corrosion and toxicity.

In previous papers (SPE 62962, 71379), an innovative calorimetric technique (DSC) was presented to characterize hydrate formation in drilling mud up to 100 bar. This rapid, easy and reliable technique was applied to fluids of increasing complexity, from solutions and emulsions to complete oil-base and water-base muds. Results were validated from classical PVT measurements.

In this study, this work was continued on thermodynamic properties of hydrate formation in complex solutions. The main objective was to establish phase diagrams of ternary and quaternary mixtures (water-CH4-salt and water-CH4-salt-glycol) and also to understand the mechanisms that govern hydrate formation in such mixtures.

In parallel, a new microcalorimeter was developed, that allows measurements on gas hydrates up to 400 bar, whatever the mud composition (WBM or OBM). This apparatus can analyze complete muds, in presence of solids, and is designed to be implemented on drilling platforms. Hence, it will be possible to follow the risk for hydrates formation as a function of drilling conditions on site. Furthermore, this apparatus will allow the establishment of a database on different muds. A coupling of this database with a software that calculates the thermal profile along the riser should give birth to a predictive method for gas hydrate formation risks.

Introduction

Gas hydrates are ice-like structures of a water lattice with cavities, which contain guest gases. These crystalline compounds belong to a group of solids called clathrate and are formed from mixtures of water and low molecular weight gases at high pressures and low temperatures. Gases likely to form hydrates include among others light alkanes (methane to iso-butane), carbon dioxide, hydrogen sulfide, nitrogen, oxygen or argon.

A noteworthy property of hydrates is the amount of gas trapped in a given volume: 1 m3 of gas hydrates can contain as much as 170 m3 of gas. Therefore, when hydrates decompose because of reduced pressure and / or increased temperature, they can produce large volumes of gas and cause serious safety problems.

The problem of the formation of gas hydrates is well known by the oil industry and has been studied for a long time in the field of oil and gas transportation1. The great amount of studies on natural gas hydrates over the past 60 years is largely connected to the problems encountered in the natural gas transmission industry: blocked transmission lines and resulting pressure fluctuations.

For the drilling industry, formation of gas hydrates can also lead to serious operational and safety problems. As water depths increase, the potential for gas hydrate formation during drilling operations also increases. The higher hydrostatic pressures at the seafloor as well as the lower temperatures encountered in deep and very deep water drilling strongly increase the probability of hydrate formation in kill and choke lines, drilling risers and of course blowout preventers (BOP's).

The occurrence of gas hydrate formation in deep offshore drilling operations has been previously described2. It may lead to the interruption of the drilling operation due to plugging and even destruction of the rig equipment.

By now, the way to prevent all the problems related to gas hydrates is to add thermodynamic inhibitors to drilling fluids formulations (WBM or OBM). Classical inhibitors are salts and glycols3, which may cause important problems of density adjustment, corrosion and toxicity. Consequently, optimization of drilling fluids formulations becomes more and more critical.

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