Summary
In this study, we evaluate the response of oceanic subsurface systems to
thermal stresses caused by the flow of warm fluids through noninsulated well
systems crossing hydrate-bearing sediments. Heat transport from warm fluids,
originating from deeper reservoirs under production, into the geologic media
can cause dissociation of the gas hydrates. The objective of this study is to
determine whether gas evolution from hydrate dissociation can lead to excessive
pressure buildup, and possibly to fracturing of hydrate-bearing formations and
their confining layers, with potentially adverse consequences on the stability
of the suboceanic subsurface. This study also aims to determine whether the
loss of the hydrate—known to have a strong cementing effect on the porous
media—in the vicinity of the well, coupled with the significant pressure
increases, can undermine the structural stability of the well assembly.
Scoping 1D simulations indicated that the formation intrinsic permeability,
the pore compressibility, the temperature of the produced fluids and the
initial hydrate saturation are the most important factors affecting the system
response, while the thermal conductivity and porosity (above a certain level)
appear to have a secondary effect. Large-scale simulations of realistic systems
were also conducted, involving complex well designs and multilayered geologic
media with nonuniform distribution of properties and initial hydrate
saturations that are typical of those expected in natural oceanic
systems. The results of the 2D study indicate that although the
dissociation radius remains rather limited even after long-term production, low
intrinsic permeability and/or high hydrate saturation can lead to the evolution
of high pressures that can threaten the formation and its boundaries with
fracturing. Although lower maximum pressures are observed in the absence of
bottom confining layers and in deeper (and thus warmer and more pressurized)
systems, the reduction is limited. Wellbore designs with gravel packs that
allow gas venting and pressure relief result in substantially lower
pressures.
Introduction
Background. Gas hydrates are solid crystalline compounds in which gas
molecules (referred to as guests) are lodged within the lattices of ice
crystals (called hosts). Under suitable conditions of low temperature and high
pressure, a gas G will react with water to form hydrates according
to
(Eq.)
where NH is the hydration number. Of particular interest
are hydrates formed by hydrocarbon gases when G is an
alkane. Natural hydrates in geological systems also include
CO2, H2S, and N2 as guests. Vast amounts of
hydrocarbons are trapped in hydrate deposits (Sloan 1998). Such deposits occur
in two distinctly different geologic settings where the necessary low
temperatures and high pressures exist for their formation and stability: in the
permafrost and in deep ocean sediments.
The three main methods of hydrate dissociation are (Sloan 1998): (1)
depressurization, in which the pressure P is lowered to a level lower
than the hydration pressure Pe at the prevailing temperature
T, (2) thermal stimulation, in which T is raised above the
hydration temperature Te at the prevailing P, and (3)
the use of inhibitors (such as salts and alcohols), which causes a shift in the
Pe –Te equilibrium through competition with
the hydrate for guest and host molecules. Dissociation results in the
production of gas and water, with a commensurate reduction in the saturation of
the solid hydrate phase.
Gas hydrates exist in many configurations below the sea floor, including
massive (thick solid zones), continuous layers, nodular, and disseminated, each
of which may affect the seafloor stability differently. The hydrates in
all of these configurations may be part of the solid skeleton that supports
overlying sediments, which ultimately support platforms and pipelines needed
for production from conventional oil and gas resources, and from hydrate
accumulations (when it becomes economically and technically viable).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
9 May 2006
- Revised manuscript received:
29 January 2007
- Manuscript approved:
10 February 2007
- Version of record:
20 June 2007
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