Study On High-density Brine-based Fracturing Fluids

With the development of deep well,ultra deep well and tight oil reservoirs,the treating pressure of the wellhead is increasing more than 100 MPa.Because of the ground equipment limitation of operation under high pressure and the capacity of the pipeline,the use of conventional density fracturing fluids will affect the fracturing effect and construction safety.In view of this problem,technology of weighting fracturing fluid has been studied.That is,utilizing high-density brine-based fracturing fluids to increase the static liquid column pressure of fracturing fluids at wellbore,and further decrease the wellhead treating pressure.In this paper,the crosslinking system which is suitable for brine-based fracturing fluids is determined:HPG-organic boron system and CMHPG-organic boron system.The appropriate thickening agent and crosslinking agent are selected.According to the influence of the weighting agents on the viscosity and density of the fracturing fluids,two kinds of weighting agents were optimized:sodium bromide and sodium nitrate.The viscoelasticity,stability,corrosion,gel breaking property,temperature resistance,shearing resistance,filtration and compatibility of the fracturing fluids were studied by means of rheometer and discourteous tester and other equipment.The results showed that the gel breaking property of CMHPG-organic boron system is better than that of HPG-organic boron system,while the temperature and shear resistance of HPG-organic boron system is better.By contrast,the fracturing fluids density of sodium bromide salt solution can increase up to 1.5g/cm³,besides the corrosion rate of the steel is small and the syestem is stable at room temperature as well as low temperature.Despite the gel breaking property is poor,it can withstand temperature of138?.The solution density of sodium nitrate fracturing fluids is increased up to 1.36g/cm³.Results showed temperature and shear resistance is good,and this system is easy to break.But the corrosion rate of steel sheet is high and the stability is poor at low temperature.It can withstand temperature of 160?.Based on the study of single brine-based fracturing fluids,two kinds of weighting agents were compounded to form the NaNO₃-NaCl compound brine-based fracturing fluids and NaBr-NaCl compound brine-based fracturing fluids.Performance of the compound brine-based fracturing fluids were investigated.The results showed that the optimum compound ratio of NaNO₃-NaCl fracturing fluids is 3:1,which maximum density can reach up to 1.349g/cm³ and withstand temperature above 142?.Comparing NaNO₃ fracturing fluids,this compound fracturing fluids decrease cost under the same density.The optimum compound ratio of NaNO₃-NaBr fracturing fluids is 1:3,which maximum density can reach up to 1.555g/cm³ and withstand temperature about 138?.Comparing NaBr fracturing fluids,this compound fracturing fluids increase density under the same cost.