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In order to meet the needs of deep, high temperature and low permeability gas reservoir fracturing in Shanbei Gas Field, this paper has studied and developed two key components of fracturing fluid used for fracturing and reconstruction of such gas reservoirs - high temperature delayed crosslinking agent and microcapsule slow-release gel breaker. In this paper, the research status and development trend of water-based fracturing fluid crosslinkers and microencapsulated slow-release gel breakers at home and abroad are comprehensively analyzed. Based on the actual situation of northern Shaanxi gas field, the boron and zirconium composite high-temperature delayed crosslinkers and microencapsulated slow-release gel breakers with urea-formaldehyde resin as the wall material are developed according to the characteristics of organic boron crosslinkers with good retarding cross-linking performance, small formation damage and high temperature resistance of organic zirconium crosslinkers.

Through the optimization of technical route, the synthesis process of high-temperature delayed crosslinking agent was determined, including reaction medium, material ratio, temperature, time and pH, etc. The structure of high-temperature delayed crosslinking agent was analyzed and characterized, and the crosslinking mechanism was discussed. Through the performance study of fracturing fluid, it is proved that the crosslinking agent has good retarding crosslinking performance and temperature resistance at low pH (usually around 10); The crosslinking time can be adjusted for 2-6 min, and the viscosity is 105 mPa · s at 140 ℃ and 170s-1 for 2 h. In particular, the use of adsorption inhibitors and special gel breakers has completely solved the problem of high damage of organic zirconium crosslinkers to supporting cracks. The damage of water-based fracturing fluid to the formation is largely related to the use of gel breaker. The use of high concentration conventional gel breaker can reduce this damage, but the viscosity of fracturing fluid gel will also decrease, so that the fracturing fluid can not achieve the functions of fracturing, extending fractures and carrying sand.

In this paper, a new microcapsule slow-release gel breaker was synthesized by in-situ polymerization using trichloromethane as the medium and polymeric monomer as the wall material. The gel breaker releases ammonium persulfate with the slow change of hydration, temperature and pressure through the wrapped polymer membrane, which has good slow release performance and high release rate; Add the gel breaker into the water-based fracturing fluid gel, shear at constant temperature and constant speed for 60 min, and the viscosity of the gel is still more than 60 mPa · s, and the viscosity of the gel breaker after 8 h is less than 2 mPa · s; Add 0.1% sample of HPG/organoboron fracturing fluid, at 80 ℃, the permeability is~0.018 μ The damage rate of core permeability of m2 is 14.8%. The optimum synthesis conditions were determined by experiment: the reaction medium was trichloromethane (pH2), the volume ratio of urea-formaldehyde resin and ammonium persulfate saturated solution was 1:2, and the reaction time was 1 h at 30 ℃, 40 ℃ and 50 ℃ respectively.