Fulong Ning

Gas-hydrate formation,agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

Abstract One of the main challenges in deep-water drilling is gas-hydrate plugs, which make the drilling unsafe. Some oil-based drilling fluids (OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation, agglomeration and inhibition by an experimental system under the temperature of 4 °C and pressure of 20 MPa, which would be similar to the case of 2000 m water depth. The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF. The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles. The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later. Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.

ANALYSIS ON CHARACTERISTICS OF DRILLING FLUIDS INVADING INTO GAS HYDRATES-BEARING FORMATION

Formations containing gas hydrates are encountered both during ocean drilling for oil or gas, as well as gas hydrate exploration and exploitation. Because the formations are usually permeable porous media, inevitably there are energy and mass exchanges between the water-based drilling fluids and gas hydrates-bearing formation during drilling, which will affect the borehole’s stability and safety. The energy exchange is mainly heat transfer and gas hydrate dissociation as result of it. The gas hydrates around the borehole will be heated to decomposition when the drilling fluids’ temperature is higher than the gas hydrates-bearing formation in situ. while mass exchange is mainly displacement invasion. In conditions of close-balanced or over-balanced drilling, the interaction between drilling fluids and hydrate-bearing formation mainly embodies the invasion of drilling fluids induced by pressure difference and hydrate dissociation induced by heat conduction resulting from differential temperatures. Actually the invasion process is a coupling process of hydrate dissociation, heat conduction and fluid displacement. They interact with each other and influence the parameters of formation surrounding the borehole such as intrinsic mechanics, pore pressure, capillary pressure, water and gas saturation, wave velocity and resistivity. Therefore, the characteristics of the drilling fluids invading into the hydrate-bearing formation and its influence rule should be thoroughly understood when analyzing on wellbore stability, well logging response and formation damage evaluation of hydrate-bearing formation. It can be realized by establishing numerical model of invasion coupled with hydrate dissociation. On the assumption that hydrate is a portion of pore fluids and its dissociation is a continuous water and gas source with no uniform strength, a basic mathematical model is built and can be used to describe the dynamic process of drilling fluids invasion by coupling Kamath’s kinetic equation of heated hydrate dissociation into mass conservation equations.

Estimation of in-situ mechanical properties of gas hydrate-bearing sediments from well logging

Abstract Taking Well SH7 in South China Sea and Well Mount Elbert in Alaska North Slope permafrost as examples, mechanical properties of gas-hydrate-bearing sediments (GHBS) in ocean and permafrost were estimated based on the method used in conventional oil and gas reservoirs with log data, and the results were compared with other tests or calculations. The correlations between mechanical parameters and log velocities in conventional oil and gas industry can obtain reasonable strength parameters of oceanic GHBS, such as cohesion, internal frictional angle, tensile strength and shear strength. However, the estimations of shale content of oceanic GHBS, internal frictional angle of permafrost GHBS, elastic parameters such as Youngs modulus and bulk modulus of oceanic and permafrost GHBS have big errors. In the future, more efforts should be given to build suitable relations between mechanical parameters and velocities or more accurate correlations between mechanical parameters and hydrate saturation. Thus, with the aid of velocities and hydrate saturations from well logging, the mechanical properties of GHBS can be more accurately evaluated.

Experimental Research of Gas Hydrate Drilling Fluids Using Polyethylene Glycol as an Inhibitor

ABSTRACT Low temperature is the key for maintaining the wellbore stability of the gas hydrate well and for ensuring the safety in the well. In this article, the characteristics of drilling in the permafrost containing gas hydrates are analyzed initially. And later, the relative properties and the inhibitory mechanisms of the polyethylene glycol and the hydrate inhibitor of the drilling fluids ensuring drilling safety are discussed. On the basis of these, the effects of the molecular weight and the content of the polyethylene glycol, and its interaction with the salts on the performances of the four groups of drilling fluids under low temperature are mainly analyzed. The results of these analyses show that for the relative drilling fluid system, the 5% content of polyethylene glycol with a molecular weight of 10 000 combined with 15% NaCl+5% KCl or 20% NaCl is suitable.

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.

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.

High Pressure Control in Synthetic Experimental System of Gas Hydrates Simulation

The simulating experiment of gas hydrates often lasts a long time, has a high pressure (usually equal or more than 38bar) and has many steps. A high-pressure console was developed to lighten labor intensity and guarantee experiment safety in the Synthetic Experimental System of Gas Hydrates Simulation (SESGHS). Gas inlet/outlet, cell pressure test and overpressure protection are all managed on the console. The overpressure protection module included a gas-control valve NV1-20-6M-ATC, a solenoid valve VP542, a relay OMRON/ G2R-1, a multi-function card PCI-1711, a pressure sensor and the corresponding software. The acquired pressure data was judged whether to surpass the alarm value, if so, alarm would be send out. And if over the unloading value, number 1 will be send to the channel connecting PCI-1711 card and the relay, then relay is switched on to make electromagnetic valve open and let low pressure gas enter so that the gas-control valve can release high pressure automatically. The whole pressure-control module was operated successfully in the experiments of gas hydrates formation, drilling fluids tests and mini-drilling simulation. It worked steadily, operated conveniently and had high sensitivity for overpressure protection, which largely enhanced the safety of the experiments.

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.