Measuring and testing – Gas analysis – By thermal property
Reexamination Certificate
2002-02-06
2003-06-03
Kwok, Helen (Department: 2856)
Measuring and testing
Gas analysis
By thermal property
C048S127500
Reexamination Certificate
active
06571604
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of drilling operations, in particular deep offshore and very deep offshore drilling. These operations generate increasingly complex technical problems considering the extreme conditions encountered at such water depths. It is for example possible to observe temperatures close to 0° C. and pressures close to 400 bars at the water bottom (mud line). As a consequence, the drilling fluid circulating in the well, subjected to these conditions, must keep its properties within a very wide temperature range, for example between 0° C. and 200° C.
The above-mentioned bottomhole temperature and pressure conditions are particularly favourable to the formation of gas hydrates. Gas hydrates are solid structures containing water and gas. The water contained in the drilling fluids forms, under certain temperature and pressure conditions that essentially depend on the composition of the aqueous phase, a solid cage which traps the gas molecules. Formation of these solid gas hydrates can have particularly serious consequences as a result of the agglomeration and deposition of hydrate crystals that may eventually clog the wellhead, the auxiliary control lines and the annulus.
The loss of the rheological properties of the mud (due to the breaking of the water-in-oil emulsion by the hydrate crystals in the case of inverted oil-emulsion muds, and to the growth of the crystals in the case of water-base muds) can lead to an interruption of the drilling operations or even to the loss of the well, not to mention the safety problems linked with the dissociation of the hydrates formed (high-velocity propulsion of solid hydrate slugs). Furthermore, during mud backflow to the surface, large amounts of gas can be released at the surface.
BACKGROUND OF THE INVENTION
The operational solutions conventionally used by operators consist in using water-base or oil-base muds comprising thermodynamic hydrate formation inhibitors. The most commonly used inhibitors are salts and glycols, used in high proportions (conventionally 20 to 30% salt concentrations), which entails considerable corrosion and toxicity or logistic problems.
Determination of the pressure/temperature zones where gas hydrates are likely to form in the drilling mud (thermodynamic conditions of use) is currently based on tests carried out in reactors on aqueous solutions (simplified or model formulations) or on thermodynamic models validated from PVT cell experiments on simple or model fluids. The action of inhibitor additives is generally tested on model hydrates (THF or freon) allowing to work safely at the atmospheric pressure.
At the present time, there is no simple, fast and reliable method for determining the conditions of gas hydrate formation in drilling fluids that could be directly applicable in the field, at temperatures close to 0° C. and under natural gas pressure. The importance of working on real muds, i.e. mud samples taken at the surface, is particularly linked with the influence of the constituents, notably the solids, whose action on the formation of hydrates cannot be quantified a priori.
The existing techniques for determining the hydrate dissociation points in drilling muds use measurements in PVT cells or in reactors, and they follow the gas consumption and the pressure variation (at constant volume). The drawbacks of these techniques are linked with the implementation weightiness (long experiment time) and with the difficulty in working with complex fluids, particularly those containing solids.
Practically any physico-chemical phenomenon characterized by an enthalpy change (chemical reaction, transition, fusion . . . ) can be characterized by DSC (Differential Scanning Calorimetry). However, application of this technique to the characterization of hydrates has been limited to model hydrates that can form at atmospheric pressure.
Handa's published work (Handa, Y. P., (1986a), Compositions, entbalpies of dissociation and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter, J. Chem. Thermodynamics, 18, 915-921. Handa, Y. P., (1986b), Calorimetric studies of laboratory synthesized and naturally occuring gas hydrates, in Proc. AIChE Annual Meeting, Miami Beach, Nov. 2-7, Handa, Y. P., (1988), A calorimetric study of naturally occuring gas hydrates, Ind. Eng. Chem. Res., 27, 872-874) is well-known. He has developed a calorimetric technique for determining the compositions, enthalpies of dissociation and specific heats of xenon, krypton, methane, propane, ethane and isobutane hydrates, as well as natural gas hydrate samples. He has used, for this study, a SETARAM BT Calvet type calorimeter allowing to work on samples of several grams, which of course reduces the usable temperature scanning speed range (because of thermal transfer problems in the sample), but allows very precise enthalpy and thermal property measurements.
Koh et al. (1998) of King's College in London (Koh, C. A., Westacott, R. E., Hirachand, K., Zugic, M., Zhang, W., Savidge, J. L., (1998), Low dosage natural gas hydrate inhibitor evaluation, in Proc. 1998 Intern. Gas Research Conference, San Diego, USA, November 8-11, Vol. I, 194-200) have recently used the DSC technique to test hydrate inhibitors. Since their device does not work under pressure, they have studied model THF hydrates that form at atmospheric pressure. They used cooling and temperature scanning to determine the supercooling degrees according to the inhibitor type and also carried out studies under isothermal conditions after fast quenching of the sample to observe the crystallization of the THF hydrates as a function of time. They have thus been able to draw curves referred to as THF (time-temperature-transformation) curves which allow to compare the kinetic effect of the inhibitors on the formation of hydrates.
Fouconnier et al. (1999), of the University of Compiègne (Fouconnier, B., Legrand, V., Komunjer, L., Clausse, D., Bergflodt, L., Sjöblom, J., (1999), Formation of trichlorofluoromethane hydrate in w/o emulsions studied by DSC, Progr. Colloid Polym. Sci., 112, 105-108) have used the DSC technique at atmospheric pressure to study the formation of model trichlorofluoromethane hydrates in water-in-oil emulsions stabilized by Berol 26. The formation of hydrates has been observed by means of the DSC technique with temperature scanning.
SUMMARY OF THE INVENTION
The object of the present invention is to have, on a drilling site (in mud logging and monitoring cabs), a device for determining risks of hydrate formation on a real well fluid, by measuring the hydrate dissociation temperature at a given gas pressure, according to the DSC (Differential Scanning Calorimetry) technique. These measurements allow the operator to predict dangerous zones with hydrate formation Pressure/Temperature conditions, and therefore to select the mud that is best suited to the current or future drilling conditions, or even to carry out in-situ tests on hydrate inhibitor additives under conditions that are very close to the real conditions. In the case of oil-base muds, which are inverted water-in-oil emulsions, it is also possible to determine whether hydrate formation is likely to break the emulsion, in which case the fluid loses its rheological properties. The combined use of a software allowing to determine the thermal profile in the mud during drilling allows the risks of hydrate formation during the operation to be precisely determined.
The present invention thus relates to a method for determining the gas hydrate formation conditions in a well fluid, said method comprising the following stages:
taking a fluid sample,
placing this sample in a calorimetry cell,
performing on this sample a reference thermogram in a temperature range between T1 and T2,
performing on the same sample a second thermogram in the same range and under a pressure Ph of a hydrocarbon gas, T1 being a temperature low enough to obtain the format
Audibert Annie
Dalmazzone Christine
Herzhaft Benjamin
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Kwok Helen
Politzer Jay L.
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