Application on low molecular weight polymer fracturing fluid system

In order to reduce the secondary damage of melon gum fracturing fluid for the low permeability alkali sensitivity reservoir, Optimized a kind of low molecular weight polymer fracturing fluid, and solve the technical problem of crosslinking time is not easy to control, fracturing fluid gel breaking difficult in low temperature. Through the study of the optimization of main additives, The fracturing fluid use low molecular highly active three yuan low molecular polymer as a thickener, which use of special synthetic process of polymerization with multiple active node. Its molecular weight is 200000 ~ 300000, water soluble speed is fast, site preparation easy, and thickening ability is strong. When the concentration is 0.2% ~ 0.6% in the fracturing fluid ,it can satisfy the requirement of fracturing operation. With organic zirconium and aldehyde compounds as crosslinking agent, The fracturing fluid have been achieved in good results in erlian uli of north China oilfield, ordos basin yishan slope of oil well and gas well fracturing.


Introduction
Wuliyasitai oilfield administrative is part of the Inner Mongolia Autonomous Region and there is 7km to the north of Xilin Gol League Dongwuzhumuqinqi.Its construction belongs to the erlian basin depression northeast side mani.North east to strip type distribution on the whole, Area of 3000 km2, Reservoir porosity is 8% ~ 10% on average, Permeability is 0.05 x 10-3 ~ 1.03 x 10-3 um2.Belongs to low porosity and low permeability reservoirs, At the same time have the feature such as water sensitivity and alkali sensitivity.Fracturing usually use gel fracturing fluid, But due to its basic crosslinking environment and the characteristics of large residue content.The damage of alkali sensitivity reservoir is bigger.Synthetic polymer has always been the research object of water-based fracturing fluid because of neutral, good solubility, no residue, etc.It is also the ideal substitutes of melon gel fracturing fluid, But due to the time of crosslinking polymer fracturing fluid is fast, not easy to control, not resistant to shear, Defects and poor heat resistance make the application very restricted.Experimental study of low molecular weight polymer fracturing fluid -hydrophobic low molecular weight polymer as the main association of the new residue-free fracturing fluid system, solve the above problems.Low molecular weight polymer fracturing fluid is the use of special synthetic polymerization process, it has plurality of nodes of low molecular weight active high active lowmolecular polymer three yuan.Its molecular weight of 200,000 to 300,000, Only about 1/10 of a molecular weight of guar gum.It crosslinked under acidic conditions, and it has a strong tendency to selfpolymerization association between molecules.In the high temperature polymer to form a similar spatial network structure by coupling side chain.While the rigidity characteristic side chain groups covalently effect multicore inside the bridging hydroxyl ion reduction, crosslinking increases steric hindrance, Gel formation with temperature, shear resistance and good features.In this paper, preferably a low, medium, high temperature and low molecular weight polymer fracturing fluid formulation series, And achieved good effect in Wuliyasitai Oilfield, Ordos Basin Yishan slope of oil and gas wells fracturing.

Experimental part
Chemical reagents and materials including the preparation of fracturing fluid thickener SD-3, organic titanium cross-linking agent, ammonium persulfate breaker, cleanup additive BSA-101, anti-swelling clay stabilizer BYH-8.Natural core were taken from 53 wells, 29 wells too, yan 620 wells.
Test instruments including MARS rheometer, K12 automatic table interfacial tension instrument, XB220A electronic balance, 30 to 70 high-speed mixing device, CL10 high-speed centrifuge, BINDER heated blast drying box, High temperature and high pressure core flow meter.
Fracturing fluid formulation: Adding a certain amount of water in the 30-70 high-speed mixing vessel, adding a certain amount of polymer SD-3 under stirring, Preparation of the required experimental fracturing fluid.
Fracturing fluid temperature shear resistance properties, gel breaking performance, the content of the residue was determined and the extent of fracturing fluid core damage evaluation: The method according to the oil and gas industry standards SY / T 5107-2005 "waterbased fracturing fluid performance evaluation methods" provisions.
Anti-expansion properties of fracturing fluid measurement: Based oil and gas industry standards SY/T5762-1995 method "Fracturing clay stabilizer performance measurement method" provisions.

Crosslinking performance
According to the SD -3 molecules contain -CONH2 and -COO-perssad, it can be crosslinked by aldehyde, dialdehyde, six methylene tetramine and polyvalent metal ion Al 3+ , Cr 3+ , Zr 4+ , Ti 4 + .But research shows that a kind of crosslinking agent to form gel performance is not ideal, optimizing the organic zirconium and aldehyde compounds as crosslinking agent.The pH value of base fluid has a big influence on the crosslinking fracturing fluid properties and heat resistance performance.When the pH of base fluid within the scope of 5.5 to 6.2, The fracturing fluid not only has a certain delay crosslinking, but also has good heat resistance performance.The experimental data are shown in table 3.

Temperature resistance to shear performance
Low molecular weight polymer fracturing fluid has good heat resistance shear resistance Continuous shear 90 min under 105 170 s-1 Viscosity retention value is still high greater than 100 mPa s It shows that this fracturing fluid shear resistance of heat-resistant performance is good it can meet fracturing created fracture and proppant requirements.The results shown in Figure 1.

Gel breaking performance
The residue generated after gel breaking of gel fracturing fluid, it is difficult to discharge from the fracture, which will greatly reduce the fracture conductivity, and cause permanent damage to the formation.In order to reduce the formation fracturing and fracture conductivity damage, fracturing fluid gel breaking should as thoroughly as possible.And residue content in the gel breaking liquid should less as far as possible.Low molecular weight polymer fracturing fluid using oxidant ammonium persulfate gel water, Measured the gel breaking liquid viscosity of 0.5% low molecular weight polymer fracturing fluid were 3.5, 20.4 mPa s. which use 0.03% ammonium persulfate solution respectively, 0.03% ammonium persulfate solution + reacted acid as gel breaker.Under the experimental conditions, 0.03% ammonium persulfate and residual acid pH 4 can make 0.5% low molecular weight polymer fracturing fluid gel breaking completely.Gel breaking liquid viscosity meet the provisions of the viscosity is not more than 5 mPa s requirement in SY/T 5674-2007 General technical conditions for fracturing with plant gum .

Performance Evaluation of core damage
Carried out a core damage experiment of tai 53 Wells and 29 well, yan 620 well with different fracturing fluid The results showed that: Low molecular weight polymer fracturing fluid for core damage rate is less than 20% Guar gum fracturing fluid for core damage greater than 40% As shown in Table 4.

Comprehensive performance
The comprehensive performance of low molecular weight polymer fracturing fluid shown in table 5.

The field test and the analysis of the results
In erlian basin wulisitai oilfield tai 27 wellblock use gum fracturing hydraulic fracture 4 Wells, The early daily oil production is 40.3 t, the average single well oil production is 10.08 t.Using low molecular weight polymer fracturing hydraulic fracture 4 Wells, The early daily oil production is 48.9 t, the average single well oil production is 12.23 t.The oil production increased by 21.3% than the melon gum fracturing fluid.In yishan slope of ordos basin using low molecular weight polymer fracturing hydraulic fracture 4 Wells, output has increased by 2 times above.Shown in table 6.Using low molecular weight polymer fracturing hydraulic crack Y413 and Y620 gas Wells.Testing for production, Natural gas production were 1.2 × 10 4 m 3 and 1.5 × 10 4 m 3 , The gas production increased by 25% than using hydroxypropyl melon gum fracturing hydraulic.Shown in table 7.

Conclusion
1 The SD-3 low molecular weight polymer thickener has good solubility in water and salt resistance, the thickening ability and the advantages of no residue, can form crosslinked gel under neutral acidic conditions.
When the room temperature is 25 , With water make different concentrations of low molecular weight polymer fracturing fluid, Under the condition of the speed of 300 r/min stir 15 min.Standing swelling 1h, Use six speed rotating measuring sol viscosity fluid viscosity.The results showed that: Low molecular weight polymer to the thickening ability of water, The concentration of 0.2% ~ 0.6% of the fracturing fluid can satisfy the requirement of fracturing operation.

Table 1 .
Put 0.5% low molecular thickener SD 3in 400 ml water, Stirring was continued at room temperature for 25 , 300r / min Condition, With six speed rotary viscosimeter test sol viscosity of the liquid in different time.The results showed that: Stir the 15 min, Viscosity can be up to 91.1% of the final viscosity.Low molecular weight polymer water soluble speed, liquor easily at the scene.The relation between low molecular weight polymer fracturing fluid viscosity of sol solution and stirring time.

Table 2 .
The relationship of Low molecular weight polymer fracturing fluid viscosity of sol solution and the concentration.

Table 3 .
The effect of PH value on the cross-linking properties

Table 5 .
Polymer fracturing fluid routine performance

Table 7 .
Gas well fracturing operation table

Table 4 .
Different fracturing fluid on the rate of core damage