Drilling muds for deepwater: hydrates and rheology problems

When classical hydrate inhibitors fail

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The development of deepwater and ultra-deepwater operations brings new and more complex technical challenges due to the harsh conditions encountered at these depths. Adaptation and specific design of the drilling muds are needed.

The poor consolidation of the surface formation and the narrowness of the mud weight window represent some of the specific conditions which have to find adapted answers. Ranges of temperature and pressure (temperatures of 2°C and pressure up to 400 bars are not uncommon at the mud line) are extreme.

The drilling fluid, while flowing through the well and the riser length, will experience temperatures ranging from 0°C to 150°C and must keep its properties for this whole range. The mud rheological properties will strongly depend on the temperature and pressure variations, and these variations will be different with different mud formulations.

These pressure and temperature ranges are also favorable conditions for the formation of gas hydrates in drilling muds. Hydrates are solid structures formed from water and gas. Water contained in drilling muds will form, under certain temperature and pressure conditions, a solid structure with the gas molecules. Formation of these solid gas hydrates is likely to plug kill and choke lines as well as the annular spaces, and may cause interruption of the drilling operation and even destruction of the rig equipment.

Operators are aware of this problem as shown by the number of publications relative to this topic, and a certain number of operational solutions exist. These solutions are based on the use of thermodynamic inhibitor additives (mainly salt and glycol additives), which displace the equilibrium point of hydrate formation. According to Ouar et al, the following rules should be followed:

Water depth 1,000 ft: No hydrate occurrence

Water depth 1,500 ft: Without inhibition, risk of hydrate occurrence

Water depth 2,000 ft: Without inhibition, hyd rate occurrence

Water depth >2,000 ft: Limited experience - salts alone not sufficient.

In deep offshore conditions (over 2,000 ft), classical inhibitors may be ineffective and other solutions may have to be proposed. The first step is to define precisely the conditions of hydrate formation in the drilling mud, in order to identify the drilling phases where hydrate problems may occur.

Gas hydrates in mud

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A research program has been initiated at IFP through the Artep association in order to investigate and control these specific difficulties due to deep offshore conditions. In order to determine the dangerous (P, T) zones where gas hydrates may form into the mud, PVT cell measurements of the equilibrium conditions for gas hydrates have been carried out on a solid-free simplified mud formulation.

Concurrently, an innovative technique was elaborated, using differential scanning calorimetry under pressure (DSC) to determine the thermodynamic and kinetic conditions of hydrate formation in whole mud formulation. With this technique heat transfers between a fluid sample and a reference are measured as a function of time or temperature at different pressures and thus phase transitions can be detected.

Many advantages can be provided by this technique: simplicity of use, limited size of the sample, and possibility to work with fluids presenting various densities or viscosities. It is then possible to determine conditions of hydrate formation in a whole mud formulation including solids up to 100 bars or under a representative gas inflow.

Moreover, it is possible to work in isothermal conditions or with a specific thermal history and measure heat transfers as a function of time. When measurements are correctly interpreted, we have, by this way, access to the kinetic characteristics of the hydrate formation, which is usually not possible when doing classical PVT cell measurements. Measurements have been performed on an oil-based mud formulation, first on simplified formulations (emulsions without solids) in order to validate the technique, then on a complete mud including solids.

In an accompanying figure, points of dissociation temperature for methane hydrates in different simplified formulations measured with differential scanning calorimetry (DSC) technique are represented. On the same graph are shown temperature/pressure profiles (rendered by software) in an offshore drilling operation as a function of time, when mud circulation is stopped. It can be seen that hydrate formation is likely to occur in the brine of oil emulsions in these conditions.

The possibility of characterizing hydrate formation with this technique has been shown for the first time on an oil-based mud formulation. Investigation of the influence of several additives (salts, solids) will also be performed with this technique.

Rheological properties

Concurrently, rheological behavior of drillings muds at low temperature has been investigated with laboratory rheometers. Water-based mud and oil-based mud viscosity vary exponentially with decreasing temperature. Another accompanying figure shows an example of the viscosity variation of several base oils. Catastrophic rheological behavior at low temperature can be seen for some base oils.

Complementary experiments are underway to underline the occurrence and importance of mud gelation, especially for oil-based muds where the gel strength at low temperature may reach high values. This gelation process is thermodependent and varies with the thermal history of the mud sample.

Acknowledgement

The authors wish to thank the Artep association for permitting the publication of this work.

Reference

Ouar, H., Cha, S., Wildeman T., Sloan E., "The formation of natural gas hydrates in water-based drilling fluids," Trans I. Chem. E., 70A, 1992.

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