Study On Evaporation And Salt Formation Of High Salinity Formation Water In Gas Field Near Well

For high temperature and high pressure gas reservior with high salinity formation water,the function of water-miscible hydrocarbon increases.With the decrease of pressure,a lot of formation water evaporates,increasing water salinity,resulting in salt deposititon,causing the reservoir clogging,and reducing well productivity. By integrating experiment study with theory simulation,this paper studied on the law of formation water evaporation and salt out.This will results in great significance on understanding the complex phase behavior,liquid distribution and production dynamic of hydrocarbon-water in HT/HP gas reservoir.The following works have been done:(1) The paper has carried out a series of physis experiments,including formation water normal pressure and high pressure evaporation experiment, formation water high pressure and high pressure flash evaporation experiment, reservoir rock-fluid interaction experiments and porosity-permeability damage salting experiments.(2) The thermodynamic and kinetic mechanism of salt mineral has been analysised. The four-phase gas-liquid liquid-solid thermodynamic model estabilished.(3) The paper build the hydrocarbon-water-solid model simulation with the water evaporation and salt out.Simulation of the effects of formation water evaporation and salting out in the process of temperature,pressure,salinity.Based on this, the impacts of the vaporous water in nature gas on the gas properties and P-T phase diagrams were further discussed.(4) On the grounds of two phase flow theory of gas ans water,the water saturation equation model in the process of formation water evaporation was derived.The new material balance model of gas reserve has been established on the impact of the water evaporation. Joint the formation water evaporation model and salting out model,the releationship between the salinity and formation water saturation has been established, and analysised the salting out production.(5) The single well radial numerical simulation was built. Through combining with the phase equilibrium model,The simulation of the production process has been built on the factor of water vapor content of gas, salinity and salting out yield. Analysis of the reservoir properties (permeability, irreducible water saturation, temperature and water salinity) and dynamic production (gas production and minimum bottomhole pressure) on the evaporation of water and salting out. Simulated the effect of salting out on the well protential production,porosity and permeability.Based on the above works, some achievements and conclusions are as follows:(1) When the reservoir pressure is reduced to 20MPa, the vaporation evaporation significantly strengthened, mineral salt is mainly sodium chloride. When the formation salted, average porosity reduced about 7%, average permeability reduced about 17%.(2) Salt particles settlement may occur in the pore through deposition, bridge and caputure. The gas-liquid-liquid-solid material has been built, selecting the condensate as a first phase and the formation water as the second phase. Choosing the water as reference phase, the four phases thermodynamic has been built which using the Pitzer model to calculate the formation water.(3) The water evaporation in gas increases with temperature increases, pressure decreases and salinity decreases. The CO2 and H2S has a great improvement for water evaporation. The evaporation of formation water has influnce the gas volume factor, deviation factor and viscosity. The amount of salting increases with temperature increases, pressure decreases and salinity increases. Salting has much influence on formation water than gas. The P-T phase diagram diagram of gas HT/HP gas reservoir considering formation water evaporation can reflect the real phase behavior characteristics well.(4) Considering the water evaporation, the water saturation model has been built, which modified the impact of salinity on formation water evaporation. The influence of production time,production,salinity and temperature on the water distribution has been analysised. The new material balance model of gas reserve has been established on the impact of the water evaporation. Joint the formation water evaporation model and salting out model,the releationship between the salinity and formation water saturation has been established, and analysised the salting out production.(5) The formation water salt deposition occurs mainly in the well around 10m. With the formation pressure drops, evaporation enhances, salinity increases, and the amount of salting increases. The lower permeability, the higher irreducible water saturation or higher temperature, make the water content in gas higher, the amount of salting higher. The higher salinity, the stronger the inhibition of formation water evaporation, but the higher the yield of salt. The higher gas production or the lower the minimum bottomhole pressure, the evaporation of water and the amount of salting are more obvious higher. Formation water evaporation and salting cause well productivity decreases, the maximum decrease of about 60%; resulting in the formation of Salt reservoir porosity and permeability were lowered, the maximum porosity decreases by about 1%, permeability damage becomes 2.5%-8.5%.