Research On The Injection System Of Nitrogen Gas Foam For Offshore Oilfield

The increasing need for petroleum resource of China promotes the development of offshore petroleum industry. As the development of offshore oil-filed, similar problems as in land oil-field were also appeared, such as high absorption wells, water-sensitive formation, pressure loss of oil reservoir and so on. Foam fluid is suitable for the drilling, well completion and well workover for such oil-field because of its low density, low static fluid column pressure, high viscosity, strong carrying capacity and weak reservoir damage. In order to popularize the foam fluid to the offshore oilfield, it is necessary to develop a set of injection system of nitrogen gas foam for offshore oil-field combining with its need and characteristic.According to the environment of offshore oil-field and safty demand, sea water and nitrogen gas were selected as the liquid phase and gas phase of foam fluid respectively. The Ross-Miles method and the Warning-Blender method were adopted in laboratory to evaluate the performace of some usual foam agents. The influence of salinity, crude oil and temperature on performance of foam agent was investigated. Foam agent product and the optimum concentration were determined by experiment. Stabilizer system fitting with the foam agent was selected by stability experiment.The effect of temperature, pressure and mass ratio of gas and liquid on foam density was investigated by experiments. Calculation correlations for different mass ratio of gas and liquid were obtained by matching the experimental data. The comparison among the calculation results, experimental data and the results from routine method was conducted. Foam viscosity was measured with six-speed rotary viscosimeter. The results showed that the rheological model of foam fluid was consistent with the power-law model. The expressions of rheological parameters of foam fluid were obtained by matching the experimental data.Injection system of nitrogen gas foam for offshore platform and land oil-field was developed. Foam producer designed autonomously had the merits of simple structure, strong corrosion and pressure resistant and continuous working. It can be used to complex operation conditions. The autocontrol system based on fuzzy control theory can be used to realize real time monitoring and controlling, which can assure the reliability and safty.The mathematical models of the foam density, pressure and temperature for the foam flowing in well bore were set up based on the principle of mass conservation, momentum conservation and energy conservation. Four-order Runge-Kutta method was adopted to solve the model of foam flowing in tubing. The results showed that the foam density, pressure and temperature were influenced each other. The determination of foam density should be based on pressure and temperature. The influence of temperature on foam performance must be considered for deep wells and super-deep wells at high foam quality.In order to apply the injection system of nitrogen gas foam to offshore oilfield, the carrying capability of foam fluid was analyzed by software of FLUENT. A mathematical model for well cleanout of sand washing was obtained. The model was solved by iterative method of constant annular back-pressure and corresponding software was programmed. Process flow was designed according to the special requirement of offshore oil-field and field applications were carried out. Application experience showed that the sand washing by foam fluid, not only the technical goal can be achieved efficiently, but it has the merits of lowering leakage, reducing pollution and volume. The safty, reliability and adaptability of the injection system of nitrogen gas foam were excellent. The system and the corresponding technology had good economic benefit and deployment value.