Xiang Wu

Comparison and application of different empirical correlations for estimating the hydrate safety margin of oil-based drilling fluids containing ethylene glycol

Abstract As the oil and gas industries continue to increase their activity in deep water, gas hydrate hazards will become more serious and challenging, both at present and in the future. Accurate predictions of the hydrate-free zone and the suitable addition of salts and/or alcohols in preparing drilling fluids are particularly important both in preventing hydrate problems and decreasing the cost of drilling operations. In this paper, we compared several empirical correlations commonly used to estimate the hydrate inhibition effect of aqueous organic and electrolyte solutions using experiments with ethylene glycol (EG) as a hydrate inhibitor. The results show that the Najibi et al. correlation (for single and mixed thermodynamic inhibitors) and the Ostergaard et al. empirical correlation (for single thermodynamic inhibitors) are suitable for estimating the hydrate safety margin of oil-based drilling fluids (OBDFs) in the presence of thermodynamic hydrate inhibitors. According to the two correlations, the OBDF, composed of 1.6 L vaporizing oil, 2% emulsifying agent, 1% organobentonite, 0.5% SP-1, 1% LP-1, 10% water and 40% EG, can be safely used at a water depth of up to 1900 m. However, for more accurate predictions for drilling fluids, the effects of the solid phase, especially bentonite, on hydrate inhibition need to be considered and included in the application of these two empirical correlations.

EFFECT OF SDS AND THF ON FORMATION OF METHANE- CONTAINING HYDRATES IN PURE WATER

ABSTRACT Gas hydrate formation generally involves gas dissolution, formation of nuclei and growth of new nucleus. On condition of synthesizing experiments without agitation, the formation of hydrate nuclei is comparatively difficult and needs an induction period which is considerably uncertain and random. Some additives such as surfactant sodium dodecyl sulfate (SDS) can increase the formation rate and reduce the induction time. A hydrate formation and mini drilling experimental system was used to carry on methane hydrate formation experiments with small quantity of SDS and SDS- tetrahydrofuran(THF) in deionized water. The reactor is a high pressure cell (40Mpa) made of titanium alloy with 4 transparent windows and an inner volume of about 2.8 liters. The effect of SDS and THF hydrate on the formation rate and amount of methane hydrate was studied by comparative testing and analyzing the collected data of temperature and pressure. According to the results of the tests, the formation rate of methane hydrate in the SDS-THF solution was faster than that in the SDS solution. As a water-soluble hydrate former, THF hydrate nucleation may be benefit of methane hydrate nucleation. A small amount of SDS and THF could dramatically promote the formation of methane hydrate in the pure water, and rapidly increase the amount of methane hydrate too. Therefore, a great deal of time for experiment was saved, which established a good basis for the coming mini drilling and drilling fluid experiments.

Numerical Simulations of Drilling Mud Invasion into the Marine Gas Hydrate-Bearing Sediments

When drilling through the oceanic gas hydrate-bearing sediments, the water-based mud under overbalanced drilling condition will invade into the borehole sediments. The invasion behavior can influence the hydrate stability, wellbore stability and well logging evaluation. In this work, we performed the numerical simulations to study the effects of density (i.e., corresponding pressure), temperature and salinity of mud on the mud invasion and hydrate stability around borehole. The results show that the mud invasion will promote greatly the hydrate dissociation near wellbore sediments if the temperature of mud is higher than that of hydrate stability. Under certain conditions, the higher mud density, temperature and salinity, the greater degree of mud invasion and heat transfer, and the more hydrate dissociation. The gas produced from hydrate dissociation can reform hydrates again in the sediments, and even the hydrate saturation is higher than that in situ sediments due to the displacing effect of the mud invasion, which forms a high-saturation hydrate girdle band around the borehole.

The Experimental System of Gas Hydrates Integrative Simulation and its control module

The Experimental System of Gas Hydrates Integrative Simulation (ESGHIS) was built to study the thermodynamics and kinetics of gas hydrates formation and dissociation, distribution characteristics of temperature and pressure in gas hydrate well drilled for exploration and exploitation, and gas hydrate drilling technologies. A control module was developed to lighten labor intensity and guarantee experiment safety in the ESGHIS. A multi-function card PCI-1711 was selected as the core of the control module, which has high-speed data acquisition and digital/analog output. A relay OMRON/G2R-1 was used to automatically manage the startup/stop of units. A solenoid valve VP542 and a gas-control valve NV1-20-6M-ATC were used to operate overpressure protection. PLC TEMI550 was used to control programmable high-low temperature test chamber HLT705P through connecting it to a computer using a RS232/RS485 converter I-7520. A proportional valve and a stepper motor were used to control lifting and rotary speed of the drilling pipe in mini-drilling module. The corresponding software was coded by using multithreading and Visual C++6.0 language. It had good quality on interface, maintainability and expansibility. The whole module was operated successfully in the experiments of gas hydrates formation and mini-drilling simulation. It worked steadily, executed quickly and had high sensitivity for overpressure protection, which largely lightens the experimental intensity of the labor and enhances the safety of the experiments.