Research On Shear Strength And Wellbore Stability In Gas Hydrate Formation

With the growing demand for oil and gas energy and developing of natural gas hydrates exploration, it is planned to exploit gas hydrates commercially. However, as a result of gas hydrates dissociation, the mechanical properties of hydrate formation would change, and this will result to the possibility of borehole damage and reservoir unstable during hydrates drilling process, so it's necessary to study the mechanical properties and borehole stable problems. Confronted with the problem of maintaining reservoir stability in the process of drilling, this paper measures the shearing strength of reservoir bearing hydrates by theoretical and experimental method, and studied the stability of hydrate reservoir during drilling in order to provide the theoretical guide and technique supports for security of the natural gas hydrates exploration.This paper makes research on the physical properties of hydrates reservoir, and summarizes the porosity, saturation, permeability and mechanical properties, etc of hydrates'reservoir systematically. On the base of investigation of shearing test around the world, the author develops a shear strength measuring apparatus of hydrates reservoir, which could provide the low-temperature and high-pressure conditions of hydrates existing stably, measures the strength of sand models with different saturation, and gets that with the increase of the saturation, the friction cohesion and friction angle will increase too. By regression of experiment curve, this paper gets the relationship of cohesion and friction angle with saturation, and provides parameters for calculating the collapse pressure of reservoir bearing hydrates.The author found that the key to this problem is how to determine and control the drilling fluid to be low-temperature and high-density by studying the method of keeping borehole stable during drilling process. Taking into account of the gas invasion due to rock debris and decomposition of gas hydrates, based on mass, momentum, energy conservation principle and the theory of heat transfer, we established temperature and pressure distribution model of annular air-liquid two-phase flow by using the micro-element analysis and finite difference method, and did sensitivity analysis to each affecting elements. Through the comparison of annular space temperature & pressure and the hydrate phase curve & reasonable drilling fluid density window, we proposed two key parameters of hydrate decomposition pressure and critical inlet temperature of drilling fluids which can keep borehole stable during the hydrate reservoir mining, and gave the calculation methods of these two key parameters.