Response of Oceanic Hydrate-Bearing Sediments to Thermal Stresses

In this study we evaluate the response of oceanic subsurface systems to thermal stresses caused by the flow of warm fluids through non-insulated 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 build-up 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 non-uniform 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 G + NH H2O = G•NH H2O, 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 deposits1. 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 are1: (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). Objective and Problem Description. The main objective of this study is to evaluate the response of marine HydrateBearing Sediments (hereafter referred to as HBS) to thermal loading. Such thermal loading occurs when heat from hot reservoir fluids (produced from deeper reservoirs) flows into the HBS through uninsulated pipes. The resulting rise in temperature in the HBS can have serious consequences. Even before dissociation is attained, a rising temperature T is expected to affect the mechanical strength of hydrate-bearing sediments – possibly severely, given the narrow temperature range between hydrate stability and dissociation. When the temperature T reaches the hydrate equilibrium Te temperature (see Figure 1) at the prevailing pressure P (usually close to the hydrostatic pressure at the location), hydrate dissociation occurs by thermal stimulation. This leads to the rapid release of large amounts of gas, which can in turn result in the evolution of high pressures. This higher pressure can result in formation fracturing, with potentially serious consequences if the fracture plane crosses the confining (impermeable) top boundary of an underlying reservoir, thus allowing the escape of the reservoir fluids. It is also possible that the increased P, if sufficiently high, can have detrimental effects on the wellbore assembly, including cement fracturing and wellbore collapse. Another problem that can be potentially caused by thermal loading is the deterioration of the structural stability of the geologic formation in the vicinity of the wellbore. Hydrates are very effective cementing agents2, and their dissociation can lead to significant geomechanical changes in the thermally affected region (including substantial subsidence). Unless accounted for, these changes can pose a hazard to the structural stability of the wellbore assembly. The reason for the concern is demonstrated in the photograph of Figure 2, which shows a dissociating core of a marine HBS. While the more isolated inner portion of the core (where hydrate still remains) appears “solid” and structurally strong, the medium in the outer annular space (where hydrate dissociation is in progress or has already occurred) has a fluid and very weak consistency because of the loss of the cementing hydrate and shows evidence of escaping gases (bubbling). Because of its consistency, the remaining watery mud is characterized as “soupy sediment”. The impact of its evolution on the structural stability of marine HBS demands evaluation, especially in the case of compressible sediments such as muds and clays. Finally, during dissociation, the basal zone of the gas hydrate becomes underconsolidated and possibly over-pressurized because of the newly released gas3, leading to a zone of weakness akin to that indicated in Figure 2 (i.e., low shear strength, where failure could be triggered by gravitational loading or seismic disturbances), which can result in submarine landslides4,5. In this study, we employ numerical simulation to determine the effects of thermal loading on the behavior of marine hydrate deposits (assumed to be pure CH4 hydrates). More specifically, we determine the evolution over a 30-year production period of important conditions and parameters in the oceanic subsurface, including P, T, phase saturations, porosity (as affected by P and T), and salt concentration (as influenced by the release of fresh water from the dissociating hydrates). We also determine the magnitude and location of the maximum pressure evolving in the dissociating HBS, and we provide estimates of the rate of gas release and of the cumulative volume of released gas. These data can be used as inputs in standard geomechanical models to determine the geomechanical and structural response of the oceanic subsurface, and to evaluate the safety and adequacy of proposed well designs. The numerical simulation code. The numerical studies in this paper were conducted using the TOUGH-Fx/HYDRATE simulator6, which can model the non-isothermal hydration reaction, phase behavior and flow of fluids and heat under conditions typical of natural CH4-hydrate deposits in complex geologic media. It includes both an equilibrium and a kinetic model7,8 of hydrate formation and dissociation. The model accounts for heat and up to four mass components (i.e., water, CH4, hydrate, and water-soluble inhibitors such as salts or alcohols) that are partitioned among four possible phases: gas phase, liquid phase, ice phase, and hydrate phase. A total of 12 states (phase combinations) can be described by the code, which can handle any combination of hydrate dissociation mechanisms and can describe the phase changes and steep solution surfaces that are typical of hydrate problems. Contextual analysis of simulation predictions. Note that the TOUGH-Fx/HYDRATE simulator6 is a mass and heat flow and transport code, and does not include a full geomechanical component. Thus, it cannot internally estimate stresses and strains resulting from P and T changes in response to HBS heating and hydrate dissociation. As such, it cannot compute the effect of the resulting changes in stress and strain on the HBS hydraulic, thermal and geomechanical properties system, and on the overall system behavior (e.g., deformation and subsidence). However, TOUGH-Fx/HYDRATE can predict the evolution of all the other important hydraulic and hermal properties and conditions discussed earlier, which can then serve as inputs in subsequent geomechanical studies.