Research Of Microemulsion Cleanup Agent & Highly Efficient Flowback For CBM

The world is experiencing an energy evolution, the proportion of unconventional gas energy (include shale gas, coalbed methane and tight gas) is increasing year by year. Development of unconventional gas energy in China is also in full swing. Coalbed methane (CBM), one important part of unconventional gas energy, has obtained a lot of achievements, but also faces many development challenges.Coalbed methane reserves huge in China, but coalbed methane have a "three lows" characteristics, namely low gas saturation, low permeability and low formation pressure. Wells without stimulation yield little gas or no gas, so fracturing is one of the necessary parts of CBM production. However, a lot of fracturing fluid will be introduced into coal rock, thus fracturing fluid phase trapping caused by coal rock’s low porosity and low permeability is very serious, fracturing fluid retained in coal rock pore, cleats and cracks will hinder CBM desorption and transportation, reducing gas relative permeability, resulting stimulated wells had even cut production.It has been reported by foreign counties in recent years, the use of microemulsion as cleanup agent can be a good solution solving fracturing fluid phase trapping. Microemulsion make performance by reducing fracturing fluid surface tension and increasing contact angle between fracturing fluid and coal rock, in the meanwhile because of the smaller size of microemulsion, it can penetrate into smaller rock pore reducing water lock damage, increasing gas production. Field application shows that microemulsion cleanup agent can improve fracturing fluid recovery greatly and increase gas production.Microemulsion is thermodynamically stable system consisting of surfactant, co-surfactant, oil and water, the appearance of microemulsion is an optically isotropic phase. In this paper, surfactants, co-surfactant and oil and were screened based on coal rock characteristics. According to the drawing pseudo-ternary phase diagrams, microemulsion cleanup agent target area was identified, and further microemulsion phase behavior was determined by microemulsion conductivity curve. Microemulsion’s particle size ranges from 10 to 100 nanometer.The prepared microemulsion is yellowish transparent liquid, surface tension is about 30mN/m, microemulsion adsorb on coal rock surface as tile-like, contact angle between fracturing fluid and coal rock is increased. Two types of devices were developed according to the characteristics of CBM development, one was gravity drainage device, and the other is gas drainage device. The two devices were used to evaluate the flowback efficiency under fluid single-phase flow and gas-fluid two-phase flow, respectively. The gravity drainage results show that:microemulsion cleanup agent can improve 25% fluid recovery compared with formation water,10% higher than common surfactants; gas drainage results show that:micriemulsion’s gas drainage flowback efficiency under unit pressure is twice compared with common surfactant,15% higher than common cleanup agent. Microemulsion cleanup agent had good compatibility with guar gum system. Damage tests show that core damage of microemulsion was 8.83%, reducing about 10% compared with formation water, has less damage effect.Microemulsion cleanup agent was applied on one well in Qinshui basin, this well casing pressure recover quicker and earlier, production is 2.5 times compared with adjacent wells. Field application success.