With the increase of low-permeability oil and gas reservoir development efforts, fracturing as main production and injection increasing of low-permeability oil and gas reservoir, has aroused more and more concern. The study is based on the Chinese people of the country and the country’s oil and gas industry standards SY/T5107-2005 “water-based fracturing fluid performance evaluation method”, to evaluate a new type of fracturing fluid thickening agent about its temperature and shearing resistance, salt tolerance and gel breaking performance, the experimental results show that the fracturing fluid thickener at 100℃, under 170 s^-1 during 90 min the constant temperature shear viscosity is greater than 50mPa.s, gel breaking liquid without residue, the surface tension of gel breaking liquid under 80℃ is 25.87mN/m, the construction site 8 times, and achieved good results to improve the average daily oil production, has good heat and shearing resistance, good sand carrying properties; gel breaking completely, less damage, for low permeability oil and gas reservoir fracturing mining has wide application reference value.

With the increase of market demand, people has stepped up efforts to develop low permeability blocks, also put forward higher challenge to the reservoir production. Fracturing is an important means of oil and gas reservoir to increase production and injection, fracturing fluid as an important part of fracturing technology is getting more and more attention. The commonly used fracturing fluid of domestic and foreign is water-based fracturing fluid, the traditional thickener of fracturing fluid is guar gum, fenugreek gum and other natural plant gum, but it takes long time to reach the maximum viscosity, high residue content, gel breaking incomplete. The residue remaining in the cracks, severely reducing the permeability of the reservoir: flowback difficulties, for low permeability reservoir damage is serious, make the effect of the reservoir deteriorated. The viscoelastic surfactant (VES) with a small flow friction, filter reduce little, less damage to reservoir, without crosslinking agent, simple composition, but the high cost is a certain extent limits to its application in fracturing T.

1. Experiment section

1.1 Drug and instrument

Drug: thickener(HDCF-1), crosslinking agent(HDCF-B); potassium chloride, sodium persulfate.

Main instruments: HAAKE RS 150 rheometer, HH-W600 humidity water bath, ZL2100 automatic interface tension instrument, TX500C full scale interfacial tension instrument, NP-03 shale expansion instrument.

1.2 Experimental methods

Referring to China’s oil and gas industry standards SY/T 5107-2005 “water-based fracturing fluid performance evaluation method”, SY/T 6376-2008 “general technical conditions of fracturing fluid”, determine heat and shearing resistance, fluid loss, gel breaking performance of fracturing fluid.

2. Results and discussion

The thickener is composed of polyacrylamide(PAM) formed by chemical modification from the type of hydrophobic association polymer, the appearance is white powder, the molecular weight of 200~400 million. Due to a small amount of hydrophobic groups into polymer chains, which in hydrogen bonding and van der Waals electrostatic forces between the molecules to produce “physical” association with a certain intensity, so that the polymer chains are formed three- dimensional network spatial structure, unlike guanidine gum system strong covalent bond or coordination bonds formed by the strong chemical crosslinking system.

2.1 Effect of thickening agent dosage

At 25℃, under the condition of 170s^-1, Figure 1 analyze the effect of thickening agent dosage on the viscosity of fracturing fluid. From Figure 1curve, with the increasing amount of thickener, the fracturing fluid viscosity increases. This is caused by the changes in micellar form of surfactant in solution. With the increase of thickening agent concentration, the micelle concentration in the solution increases, and gradually intertwined to form a three-dimensional reticular structure, so that the solution viscosity increases. Seen from Figure 1, the thickener can be adjusted its amount to adjust the base fluid viscosity, to accommodate different design requirements of fracturing fluid. In the process of adjust the field formula, considering the temperature factors, the amount of the thickener is between 0.3% ~ 0.4%.

2.2 Salt tolerance

The formation water of Xinjiang oilfield with high salinity, therefore, then need to test the salt tolerance of fracturing fluid. At 25℃, 170s^-1, the mass fraction of 0.35% thickener were separately added 2% KCl, 4% KCl, 6% KCl, after 8h placement, determination the apparent viscosity, the determination results are shown in Figure 2.

We can see from Figure 2: adding inorganic salt KCl will reduce the complex fracturing fluid viscosity. This is probably because the addition of inorganic salt, causing the improve of the polarity, resulting in weakening their interaction, coil volume decreases, affect the rheology of rolling base.

2.3 Crosslinking performance

The fracturing fluid crosslinking time is short, the viscosity after crosslinking is 115mPa.s, the storage modulus if 29.5Pa, the loss modulus is 29.7Pa, the pH value is 7.41. Its system is transparent viscous liquid(Figure 3), high viscosity, good fluidity.

Using HAAKE TS 150 rheometer, temperature control in 55℃, followed by the change of shear rate,evaluation the shear performance of clean fracturing fluid, shown in Figure 4.

Figure 4 shows that the fracturing fluid from the wellbore with high shear flow to formation at low flow speed, can quickly restore the viscosity, then to achieve good carrying capacity.

2.4 Temperature shear resistance

Figure 5 examines the temperature shear resistance of fracturing fluid. Seen from Figure 5 curve, the fracturing fluid viscosity decreases with the increase of temperature and gradually stabilized, after 15min, decreased slowly, and maintained at 50 mPa.s. This is due to the fracturing fluid after heating, more frequent collisions between molecules, the micelle was destroyed, and continue to reduce, and network structure in the fracturing fluid by molecular chain intertwined to form a shearing force under the strong damage, reduce the friction between the molecules, the viscosity decreased gradually. Experiments show that, the fracturing fluid has good temperature shear resistance.

2.5 Sand carrying ability

Thickener : 0.40%, crosslinking agent : 0.20%, the fracturing fluid formation viscosity 120.0 mPa.s( measurement environment : 30℃, 170s^-1. 3min), was measured using a static 20/40 ceramic sand properties of the fracturing fluid suspension system, observed the static 6h for sand phenomenon is not obvious, it shows the good carrying sand performance.

3. Conclusion

(1) The fracturing fluid viscosity increases with thickening agent dosage increased.

(2) Good salt tolerance.

(3) At 100℃, 170s^-1, possess good temperature shear resistance, and good sand carrying performance.

(4) The fracturing fluid gel breaking completely, low interfacial tension, low damage.

(5) The successful development of the fracturing fluid system, for large field continuously mixing fracturing technology provides technical support and protection. 

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