Gas hydrates are crystalline compounds that occur under appropriate pressure-temperature conditions when water forms a cage-like structure around a smaller guest molecule. The guest molecule can be methane, ethane, nitrogen, carbon dioxide etc. Gas hydrates are found in natural environments such as sea bottoms, under permafrost areas etc. Gas hydrate prospects on earth are the biggest unlocked recourses of energy in the world. The estimated amount of organic carbon in the form of gas hydrate in earth is 10000 giga tones which is equal to 100,000 to about 300,000,000 trillion cubic feet of gas reserves [4, 5]. Over the years several countries have started research programs aimed at discovery and production of gas from offshore gas hydrate deposits. Deep drilling, operation of oil and gas wells, and operation of oil and gas pipelines in the deep sea are vastly complicated by the presence of natural gas hydrates. The necessary requirements for formation of gas hydrates are; presence of water or ice, suitable sized gas/liquid molecule and suitable pressure-temperature conditions. Temperature and pressure condition is a function of gas/liquid and water compositions. Gas hydrate formation in the production tubing can cause severe safety problems. Pressure and thermal stimulation, inhibitor injection and a combination of these methods can be used in gas recovery from gas hydrates.
Scope of this paper is to analyze effects on tubular integrity due to pressure and thermal simulation in recovering gas from the gas hydrate bearing formation. The vertical well is producing through the production tubing and production liner is the final casing in the wellbore. Increasing the system temperature (temperature stimulation - steam injection from an offset well to gas hydrate bearing reservoir ) and reduction of system pressure (depressurization) helps in preventing gas hydrate formation in production tubing. This is attributed to shifting of phase equilibrium away from formation of gas hydrates. System pressure and temperature induces tri-axial stresses (hoop stress, radial stress and buckling stress) in production tubing. The von Mises Criterion, also known as the maximum distortion energy criterion, is used to estimate the yield of production tubing. von Mises stress is further compared with the yield strength of the tubing off set by the factor of safety. The factor of safety gives the safe margin for operation avoiding the tubular failure. This paper is aimed to analytically study the effects of system temperature and pressure on mechanical strength of the production tubing inside production liner. This paper also proposes performance envelope of system temperature and pressure which enables gas production avoiding gas hydrate formation ensuring tubular integrity. In this paper maximum system temperature and pressure is also established beyond which production tubing will yield.