Effect Of Hydrate Dissociation Research Articles

Deformation Coupled Effective Permeability Change in Hydrate-Bearing Sediment during Depressurization

Methane hydrates found in the sediments of deep sea and permafrost regions draw global interest. The rate of gas production from a depressurized well is governed by the effective permeability of the hydrate-bearing sediments around the wellbore. During depressurization, a decrease in pore pressure leading to soil compaction and hydrate dissociation results in a dynamic change in the effective permeability. To describe the change in the effective permeability in detail, in this study, a simple coupled compressibility–permeability analysis method is proposed to identify the conditions under which the effective permeability increases or decreases after depressurization. An analytical solution is derived for the effective permeability change with pore pressure and temperature, considering hydrate dissociation and soil compaction. We found that when there is a sufficient heat supply, hydrate dissociation dominates the effective permeability during hydrate dissociation, but after hydrate dissociation, soil compaction is the governing factor for permeability change. When there is an insufficient heat supply, however, compaction mainly determines the permeability, and the effect of hydrate dissociation is limited. This work will be helpful for rapid reservoir assessment.

3D numerical simulation on drilling fluid invasion into natural gas hydrate reservoirs

The drilling fluid invasion into hydrate-bearing sediments (HBS) would trigger geological risks. However, invasion mechanisms and formation responses during drilling the gas hydrate reservoirs, especially the fluid-loss characteristics and control mechanisms of hydrate dissociation, remain poorly understood. Thus, we develop a three-dimensional (3-D) coupled thermal-hydro-chemical model to investigate the drilling fluid invasion process and dynamic responses of gas hydrate reservoirs. This model deals with the fluid-loss properties and flow field characteristics as well as well-formation interactions considering the effect of hydrate dissociation. The results indicate that the invasion characteristics mainly depend on drilling fluid pressure and permeability, while the temperature affects the hydrate dissociation. Besides, the fluid-loss velocity increases slowly after a sharp decrease at initial stage of invasion due to the increase of permeability induced by hydrate dissociation. Afterward, characteristics and mechanisms of drilling fluid invasion into hydrate reservoirs are determined by the invasion process coupled with hydrate dissociation. Given the unique characteristics of HBS, the invaded formation is divided into flushed zone, transition zone, and undisturbed zone, presenting a better description of the dynamic filtration process. Moreover, optimization strategies and drilling technology are proposed to prevent hydrate dissociation and control geological risks during drilling hydrate.

Investigation of CO2 hydrate slurry flow characteristics with particle dissociation for carbon storage and transportation

CO2 capture and storage technology is an effective method to mitigate the greenhouse effect and hydrate slurry is a high-efficiency way of CO2 transportation. In this study, in order to further reduce the cost and improve the transportability, a new CO2 hydrate slurry transport mode which allows a small part of hydrate particles to dissociate during flow process is proposed. To investigate the CO2 hydrate slurry flow characteristics and particle micro behaviors, a multiphase flow model coupled with hydrate dissociation kinetic model and population balance model is established. Experimental results are used to verify this integrated model. Furthermore, the effects of hydrate dissociation on CO2 transportability are also analyzed. The results indicate that the dissociation rate of hydrate during flow process can be divided into rapid dissociation stage and weakened dissociation rate stage. The agglomeration of hydrate particle can reduce the total surface area, further resisting the dissociation rate. The hydrate particle size experiences a process of growing to dynamic balance. The hydrate dissociation can decrease the mean viscosity of CO2 hydrate slurry flow, which can reduce the pressure drop and energy loss compared with the flow without hydrate dissociation. The generated bubbles can accelerate the CO2 hydrate slurry flow, increasing the transportability and reducing the risk of hydrate blockage in pipe induced by agglomeration.

Effect of Hydrate Dissociation and Axial Strain on the Permeability of Hydrate-Bearing Sand during the Creep Process

Summary The relative permeability (kr) and capillary pressure (Pc) are essential components to predict the gas and water flow in hydrate-bearing reservoirs. In this study, we analyze the dynamic gas and water relative permeability (krg and krw) during the continuous creep process of hydrate-bearing sand (HBS) under conditions of thermal-stimulated hydrate dissociation using computed tomography (CT) and the pore network model (PNM). The dynamic sample region extraction method for CT images is adopted by considering the deformation of HBS in the vertical direction. The pore structure of the HBS was visualized and reconstructed by CT scanning. The 3D pore network is built after image processing; then, gas and water flow processes are analyzed by the PNM. The results show that krw is highly consistent with two main pore structure factors: the pore space size and connectivity variation. krg is greatly affected by an increasing number of narrow flow channels in the HBS during the creep process. In addition, the irreducible water saturation (Swir) during the creep process is mainly affected by the joint effect of the pore size, throat size, and pore space connectivity. The preferential flow directions of the gas and water change from vertical to horizontal along with the progression of creep.

Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Vøring Plateau, offshore Norway

This paper presents a new data set, including single-channel airgun seismic lines, OKEAN long-range side-scan sonar data and gravity cores, acquired on the edge of the Voring Plateau and in the region of the Storrega Slide. The data acquisition was part of the 8th Training Through Research (TTR-8) expedition with R.V. Professor Logachev. The acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom-simulating reflectors (BSRs) at about 350 ms TWT subbottom depth. The lower BSR zone is located on the slope of the Voring Plateau in the immediate vicinity of the northern headwall of the Storegga Slide and, in some places, below the slide deposits. This zone runs parallel to the general trend of the continental slope. The spatial distribution of the upper BSR zone, located upslope in an area where diapir-like structures are found, does not demonstrate any topographic control. Interpretation of high-backscatter patches on the OKEAN sonographs associates the observed structures with fluid escape features on the seabed. Most of them are pockmarks, but in a few places, diapirs are cropping out and form dome-like elevations. After analysing the behaviour of the BSR (sometimes crosscutting) and its acoustic characteristics (reversed polarity), and after applying seismic inversion processing to estimate the acoustic velocity change across the BSR, this reflector is interpreted to represent the base of the local gas hydrate stability field (GHSF). This information was used to derive the regional geothermal gradient. Spatial variations of the inferred geothermal field appeared to be negligible. The observation of two separated BSR zones suggests different environmental controls for the growth of the hydrates. Enhanced reflectors observed in the intermediate zone can be explained by the presence of strata-bound free gas accumulations and migration combined with overlying permeability barriers. Therefore, a model for gas hydrate formation in relation to directional fluid migration processes, fluid escape features and the presence of diapiric structures is proposed in this paper. Our model is supported by sedimentological analyses of seafloor samples, which demonstrate the presence of stiff clays with evidence of gas migration, and by paleontological studies of the cores retrieved from the pockmarks. The presence of a BSR in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event. This observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm/year. Finally, the effect of hydrate dissociation on slope instability is considered. Assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re-distributed through the whole sedimentary section above the paleo-BSR (with consuming heat and increasing pore pressure, dissociation tends to shift the PT-conditions back to equilibrium values), it appears that the dissociation of hydrates due to sliding would cause only 0.2% increase of the pore pressure, which would hardly contribute to further slope instability.