Fluid-solid coupling model for studying wellbore instability in drilling of gas hydrate bearing sediments

As the oil or gas exploration and development activities in deep and ultra- deep waters become more and more, encountering gas hydrate bearing sediments （HBS） is almost inevitable. The variation in temperature and pressure can destabilize gas hydrate in nearby formation around the borehole, which may reduce the strength of the formation and result in wellbore instability. A non-isothermal, transient, two-phase, and fluid-solid coupling mathematical model is proposed to simulate the complex stability performance of a wellbore drilled in HBS. In the model, the phase transition of hydrate dissociation, the heat exchange between drilling fluid and formation, the change of mechanical and petrophysical properties, the gas-water two-phase seepage, and its interaction with rock deformation are considered. A finite element simulator is developed, and the impact of drilling mud on wellbore instability in HBS is simulated. Results indicate that the re- duction in pressure and the increase in temperature of the drilling fluid can accelerate hydrate decomposition and lead to mechanical properties getting worse tremendously. The cohesion decreases by 25% when the hydrate totally dissociates in HBS. This easily causes the wellbore instability accordingly. In the first two hours after the formation is drilled, the regions of hydrate dissociation and wellbore instability extend quickly. Then, with the soaking time of drilling fluid increasing, the regions enlarge little. Choosing the low temperature drilling fluid and increasing the drilling mud pressure appropriately can benefit the wellbore stability of HBS. The established model turns out to be an efficient tool in numerical studies of the hydrate dissociation behavior and wellbore stability of HBS.

Experimental study on mechanical properties of methane-hydrate-bearing sediments

Mechanical properties of methane hydrate- bearing-sediments （MHBS） are basic parameters for safety analysis of hydrate exploration and exploitation. Young＇s modulus, cohesion, and internal friction angle of hydrate- bearing sediments synthesized in laboratory, are investigated using tri-axial tests. Stress-strain curves and strength parameters are obtained and discussed for different compositions and different hydrate saturation, followed by empirical expressions related to the cohesion, internal friction angle, and modulus of MHBS. Almost all tested MHBS samples exhibit plastic failure. With the increase of total saturation of ice and methane hydrate （MH）, the specimens＇ internal friction angle decreases while the cohesion increases.

Effect on the Performance of Drilling Fluids at Downhole Rock Surfaces at Low Temperatures

To maintain gas hydrate stability, low-temperature drilling fluids and high drilling speeds should be used while drilling in gas hydrate-bearing sediments. The effect of the drilling fluid on downhole rock surfaces at low temperatures is very important to increase the drilling rate. This paper analyzed the action mechanism of the drilling fluid on downhole rock surfaces and established a corresponding evaluation method. The softening effect of six simulated drilling fluids with 0.1 wt.% of four common surfactants and two common organic salts on the downhole rock surface strength was evaluated experimentally using the established method at low temperature. The experimental results showed that the surfactants and organic salts used in the drilling fluids aided in the reduction of the strength of the downhole rock surface, and the established evaluation method was able to quantitatively reveal the difference in the softening effect of the different drilling fluids through comparison with water. In particular, the most common surfactant that is used in drilling fluids, sodium dodecyl sulfate（SDS）, had a very good softening effect while drilling under low-temperature conditions, which can be widely applied during drilling in low-temperature formations, such as natural gas hydrate-bearing sediments, the deep seafloor and permafrost.