Compound drilling fluid for natural gas hydrate (NGH) reservoir in muddy siltstone stratum

Abstract

The invention discloses a compound drilling fluid for natural gas hydrate (NGH) reservoir in a muddy siltstone stratum, and relates to the field of NGH exploitation. The drilling fluid comprises following components: sodium silicate, sodium chloride, glycol, potassium chloride, low viscosity carboxymethyl cellulose (LV-CMC), potassium polyacrylate, xanthan gum, polyvinyl caprolactam, ethylene glycol butyl ether, tetra-n-hexyl trimethyl ammonium bromide, and make-up water. The drilling fluid is suitable for NGH drill wells in muddy siltstone stratum reservoir, is resistant to low temperature, and can inhibit and reduce the solid phase. The drilling fluid is compounded with a kinetic inhibitor and a synergist to inhibit the generation of hydrates. Moreover, the components of the drilling fluid are basically harmless to the environment, and the drilling fluid has the advantages that the efficiency is high, the environment is protected, and the raw materials are cheap and easily available.

Description

technical field [0001] The invention relates to a muddy siltstone natural gas hydrate formation compound drilling fluid, in particular to the field of natural gas hydrate exploitation. Background technique [0002] Natural gas hydrates are mainly distributed in the deep-water areas of the continental margins and the permafrost areas on land, and the former accounts for the vast majority of the discovered quantities. The proportion of sand and mud in natural gas hydrate storage has a great influence on the properties of the formation itself and the saturation of hydrate in it. The storage is divided into silty mudstone, argillaceous siltstone, siltstone, sandstone and glutenite. Its hydrate content can reach up to 95%; the argillaceous siltstone structure is granular and has good water permeability. The main component is silt, containing a small amount of clay minerals and cement. It is 70%-50%; the argillaceous siltstone reservoir has unique structural characteristics, main...

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Problems solved by technology

[0004] (1) When drilling in natural gas hydrate formations, due to the release of formation stress near the well wall and bottom of the reservoir, the formation pressure will decrease; and the drill bit will cut rock, and the friction between the bottom drilling tool and the well wall and core will generate a large amount of Thermal energy destroys the temperature and pressure conditions for the stable existence of hydrates; thus triggering the decomposition of hydrates, resulting in instability of the well wall and adverse effects on drilling operations

[0005] (2) The invasion of drilling fluid and the decomposition of hydrates will lead to the increase of formation pore pressure in different degrees, and the increase of pore pressure will increase the density of drilling fluid required to maintain the stability of the wellbore wall, which not only seriously affects the penetration rate, but also increases the pressure of the wellbore wall. Seepage increases the pore pressure around the well, which further reduces the stability of the well wall, forming a vicious circle

Moreover, the decomposition of hydrates will also increase the permeability of the formation, and the drilling fluid will seep into the well wall to generate osmotic pressure, which will increase the collapse pressure of the formation and reduce the fracture pressure, making the well wall more likely to collapse and fracture, reducing the stability of the well wall

[0007] (4) For argillaceous siltstone natural gas hydrate storage, due to the composition of argillaceous siltstone and high clay content, a large amount of clay enters the drilling fluid during drilling, which increases the viscosity of the drilling fluid and makes it easier to form in the drilling fluid. Gas hydrates, clogging conduits and blowout preventers, causing serious safety incidents

However, none of the above drilling fluids addresses the problem of excessive clay content during drilling in muddy siltstone reservoirs.

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