Experimental investigation on hydrate dissociation in near-wellbore region caused by invasion of drilling fluid: ultrasonic measurement and analysis

Abstract

As we all know, development and utilization of clean energy is the only way for society to achieve sustainable development. Although natural gas hydrates is a new type of clean energy, uncontrollable hydrate dissociation and accompanying methane leakage in drilling operation threaten drilling safety and marine environment. However, dissociation range of natural gas hydrates around wellbore can't be reasonably and clearly determined in previous investigations, which may lead to the inaccurate estimation of borehole collapse and methane leakage. Then, the marine environment will be greatly damaged or affected. The purpose of the present work is to experimentally explore the dissociation characteristics of natural gas hydrates around wellbore in drilling operation, and analyze the in uence law and mechanism of various factors on hydrate dissociation. It is expected to provide reference for exploring effective engineering measures to avoid the uncontrolled hydrate dissociation, borehole collapse and accompanying methane leakage. The experimental results demonstrate that acoustic velocity of hydrate-bearing sediment can be accurately expressed as quadratic polynomial of hydrate saturation, which is the theoretical basis for determination of hydrate saturation in subsequent experiments. Owing to the fact that hydrate dissociation is an endothermic reaction, hydrate dissociation gradually slows down in experiment. Throughout the experiment, the maximum dissociation rate at the beginning of the experiment is 8.69 times that at the end of the experiment. In addition, sensitivity analysis found that the increase of stabilizer concentration in drilling uid can inhibit hydrate dissociation more than the increase in hydrate saturation. Hydrate dissociation was completely inhibited when the concentration of soybean lecithin exceeds 0.60wt%, but hydrate dissociation de nitely occurs in the near-wellbore region no matter what hydrate saturation is. In this way, based on the requirements of drilling safety and environment protection, hydrate dissociation and accompanying methane leakage can be controlled by designing and adjusting the stabilizer concentration in drilling uid.

Undrained Shear Properties of Shallow Clayey-Silty Sediments in the Shenhu Area of South China Sea

“…The conventional stability analysis that follows the safety factor aimed to avoid the ultimate stress states of the deposit under the load of the offshore infrastructure [46]. Both the total stress method and the effective stress method can be used to calculate the safety factor [47,48]. For a total stress analysis, the results in this study could provide a reference for quantifying changes in S u varying with effective confining pressures and OCRs regardless of pore pressure responses.…”

Undrained Mechanical Properties of Overlying Soils of Gas Hydrate Reservoirs in the South China Sea

“…In this equation, γ is the bulk density, which is the weight of dry soil divided by the volume of the sample; G s − the specific gravity of the soil, which is 2.71 g/cm 3 .…”

“…However, the extraction of hydrates will alter the equilibrium state of NGHs reservoirs. 3 Hydrates will decompose into natural gas and water, causing an increase in the pore pressure, a decrease in the effective stress, and a reduction in the reservoir strength. This may lead to deformation and subsidence of the hydrate layer and its overlying soil layers (OSLs), the decrease of gas production rate, 4 as well as engineering safety issues such as the collapse of brittle zones and underwater landslides.…”

“…If large-scale commercial utilization of the NGHs can be achieved, it could become the future star of energy, ensuring the security of the world’s fossil fuel supply. However, the extraction of hydrates will alter the equilibrium state of NGHs reservoirs . Hydrates will decompose into natural gas and water, causing an increase in the pore pressure, a decrease in the effective stress, and a reduction in the reservoir strength.…”

Experimental Study On the Effect of Hydrate Filling On the Mechanical Properties of Marine Sediments in the Shenhu Area, South China Sea

“…For different formations, the basic physical properties of the strata are different, which leads to uncertainty in the effect of hydrate filling on the mechanical properties of the formation. , However, for the same formation, hydrate can greatly increase the strength and stiffness of the formation by constraining the relative slip between particles by improving the cementation between skeletal particles, − making hydrate filling a key factor affecting the mechanical response of the formation. − With increasing hydrate saturation, the cementation and confinement effects become stronger, which seriously affects the cohesion, internal friction angle, stiffness, stress–strain pattern and volumetric strain characteristics of the samples. − Numerous experiments have shown that the cohesion and stiffness of the samples gradually increase with increasing hydrate saturation. , Masui et al pointed out that the strength of hydrate crystals is much smaller than that of the stratigraphic skeleton, and the friction between hydrate crystals and skeleton particles causes crystal breakage, which in turn affects the internal friction angle of the formation…”