Response of Oceanic Hydrate-Bearing Sediments to Thermal Stresses
Authors: Moridis George J. , Kowalsky Michael Brendon 
Published: December 2007

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):depressurization, in which the pressure P is lowered to a level lower than the hydration pressure Pe at the prevailing temperature T,thermal stimulation, in which T is raised above the hydration temperature Te at the prevailing P, andthe 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).