The slick-water polymer adsorption damage and control measures in shale were examined using a shale pack model of the Ordovician Wufeng Formation–Silurian Longmaxi Formation in the Changning block of the Sichuan Basin. The adsorption law of slick water under different displacement time, concentrations, pH values and temperatures of polymer were tested by traditional displacement experiment and UV-Vis spectrophotometer. The adsorption equilibrium time was 150 min, the amount of adsorption was proportional to the concentration of the polymer, and the maximum adsorption concentration was 1 800 mg/L.
GUO Jianchun, LI Yang, WANG Shibin
State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation in Southwest Petroleum University, Chengdu 610500, China
Received date: 14 Oct. 2017; Revised date: 10 Jan. 2018.
With the increase of pH value, the adsorption capacity decreased gradually, the adsorption capacity increased first and then decreased with the increase of temperature, and the adsorption capacity was the largest at 45°C. The adsorption patterns of polymers on shale were described by scanning electron microscopy and magnetic resonance imaging. It is proved that the adsorption of polymer on shale led to the destruction of the network structure of anionic polyacrylamide molecules, and the shale adsorption conformation was characterized qualitatively.
Finally, according to the adsorption law and adsorption mechanism, it is proposed to reduce the adsorption quantity of polymer on shale surface by using hydrogen bond destruction agent. The effects of hydrogen bond destruction on four kinds of strong electronegative small molecules were compared, the hydrogen bond destroyer c was the best, which lowered the adsorption capacity by 5.49 mg/g and recovered permeability to 73.2%. The research results provide a reference for the optimization of construction parameters and the improvement of slickwater liquid system.
Shale reservoirs are typical low porosity and low permeability reservoirs[1-3], and hydraulic fracturing is the key tech nology for developing this kind of resource[4-5]. The usage rate of slick-water fracturing fluid system exceeds 80% in shale fracturing[6-7]. However, slick-water has low viscosity and poor proppant carrying capacity. In order to enhance the proppant delivering capacity, the pumping rate of slick water fracturing is much higher than that of conventional fracturing, which results in sharp increase of operation friction[8-11]. Therefore, the resistance reducing agent in slick-water (usually anionic polyacrylamide) is one of the important guarantees for shale fracturing[12-14]. However, the hydrophilic groups in the anionic polyacrylamide form a chemical bond with the oxygen-containing groups in the shale, the adsorption and retention occur in the wall of the rock and the pore of the matrix. And the oil and gas flow channels are reduced or even blocked by fracturing. It restricts the effect of fracturing seriously[15-17].
At present, the main reservoir protection techniques in conventional reservoir fracturing are to reduce the concentration of fracturing fluid, shorten the operation time and reduce the relative molecular mass of the polymer, etc. However, the pore throat radius of the shale reservoir and the polymer molecules are both nano-scale, which would cause larger damage to the fracture and matrix permeability[23-24].
Taking the shales of the Ordovician Wufeng – Silurian Longmaxi formations as examples in the Changning block of the Sichuan Basin, this study examines the adsorption law and influencing factors of polymer in slick-water. Combined with the microscopic analysis of adsorption morphology, the measures for reducing the adsorption damage to shale reservoir are put forward.
Experiments of shale adsorption damage
The main component of resistance reducing agent in the slick-water system is usually anionic polyacrylamide. To verify whether shale would adsorb anionic polyacrylamide, shale samples were pulverized into 0.21 to 0.30 mm (70-50 mesh) particles and put into a sand pack tube first. The shale had a quartz content of 60.2%, calcite content of 14.7%, and clay mineral content of 13.9% (mostly illite and chlorite). The displacement with anionic polyacrylamide solution of 1000 mg/L last 5 h, and then nitrogen gas was used for reverse displacement for 1 h (to prevent mechanical retention). The experimental device is shown in Fig. 1.
Fig. 1 Experimental integrating shale displacement unit and ultraviolet spectrophotometer.
The infrared spectra of the shale sample before and after displacement were tested, combined with the absorption peak characteristics of anionic polyacrylamide infrared spectrum and the absorption peak characteristics of shale infrared spectrum[25-26], it can be seen that the shale adsorbed anionic polyacrylamide during slick-water flooding process.
Rules of shale adsorbing polymer
In this study, the concentrations of anionic polyacrylamide before and after adsorption were measured by chromogenic method of starch and chromium iodide, and the adsorption capacity was calculated by the concentration change.
Based on the test results of adsorption capacity, the adsorption rules of polymer in shale at different time, concentrations, relative molecular mass, pH values and temperatures were figured out.
Adsorption rules at different time
The slick water samples with anionic polyacrylamide concentrations of 500, 1 000, 2 000 mg/L were prepared (with relative molecular mass of 1 800-104). Adsorption capacity test was carried out according to the above experimental process, the concentration of the sample was tested every 10 min, through a total test time of 240 min, and the adsorption equilibrium curves are shown in Fig. 2.
Fig. 2. The adsorption equilibrium curves of slick-water samples in shale (at 25 ºC and pH of 7.0).
As shown in Fig. 2, adsorption capacity increases with time, and the adsorption equilibrium time is independent of the concentration. The three kinds of slick water samples all have adsorption equilibrium time of 150 min, which is the time reaching the saturated adsorption. The adsorption of slick water on shale surface accords with the Langmuir adsorption law. The adsorption sites of the rock surface are completely occupied by anionic polyacrylamide, and the adsorption reaches the state of dynamic equilibrium.
Adsorption rules at different concentrations
Three kinds of slick water samples with different relative molecular mass of anionic polyacrylamide were prepared, respectively, diluted to 200-3 000 mg/L (every 200 mg/L for a concentration point). After displacement for 150 min, the absorbance was measured and the adsorption capacity was calculated. The results are shown in Fig. 3.
Fig. 3. The limiting concentration curves of slick-water samples in shale (at 25 ºC and pH of 7.0).
It can be seen from Fig. 3 that the amount of adsorption increases with the relative molecular mass of anionic polyacrylamide. When the molecular mass is 1 800-104, the adsorption capacity is up to 8.6 mg/g. At the same time, with the increase of polymer concentration, the adsorption capacity increases rapidly. Increase of adsorption capacity slows down when the concentration exceeds 800 mg/L, and the limiting concentration is reached at 1 800 mg/L.
The main reason for the increase of adsorption capacity is that the interaction between anionic polyacrylamide chains enhances with the increase of concentration, leading to the decrease of permeability of shale pack model[29-30]. When the polymer concentration in slick water is more than 1 800 mg/L, the adsorption sites on the surface of the shale are completely occupied by the polymers, and the adsorption capacity will not increase with the rise of polymer concentration anymore.
Adsorption rules at different pH values
The pH value of the polymer solution would change the distribution of the molecular chains of the polymer, but the molecular structure of the polymer will be destroyed if the acidity or alkalinity of the solution is too strong (slick-water system in field hydraulic fracturing is generally weakly alkaline). Considering the systematicity of the study on adsorption mechanism, the scope of the acidity and alkalinity range was expanded in this research. The adsorption laws of polymer solution with pH values of 3-11 were tested. The experimental results are shown in Fig. 4.
Fig. 4. Adsorption laws of slick water samples with different pH values in shale (at the temperature of 25 ºC and polymer concentration of 1 800 mg/L).
With the increase of pH value, the amount of adsorption decreases on the whole, and the process can be divided into 5 stages. A (pH < 4.0), C (pH = 6.0-7.5) and E (pH > 10.0) are intervals where the adsorption quantity doesn’t correlate with the pH value, because under the strong acidity and alkalinity, the molecular structure has been destroyed, and the molecular structure of the polymer will not change much under the neutral condition, so the adsorption quantity is unaffected.
B (pH= 4.0-6.0) and D (pH = 7.5-10.0) are intervals where the adsorption capacity is negatively correlated with the pH value. Because the anionic polyacrylamide is in linear state under the acidic condition, and the molecular chains curl with the increase of pH value, which is not conducive to the combination of polymer and shale adsorption sites, and leads to rapid reduction of the adsorption capacity[31-32].
Adsorption rules at different temperatures
Reservoir temperature is a key factor affecting fracturing, and also an important variable affecting the law of adsorption. The variation curve of adsorption capacity and equilibrium time of shale with reservoir temperature of 20-100 ºC is shown in Fig. 5.
Fig. 5. Relationship between temperature, adsorption capacity and adsorption equilibrium time (at pH of 7.0 and polymer concentration of 1 800 mg/L).
It can be seen from Fig. 5 that with the increase of temperature, the adsorption amount increases first and then decreases. X section (T < 45ºC) is the section where the adsorption capacity is in positive correlation with temperature, as the temperature rises, the diffusion speed of polymer molecules accelerates, and more polymer molecules get away from the binding of water molecules, the collisions between polymer molecules and shale surfaces increase, thus leading to the rapid increase of adsorption capacity.
Y section (T = 45-70ºC) is the section where the adsorption is in negative correlation with temperature, as the temperature continues to rise, the desorption rate of polymer molecules will accelerate at the same time, which will cause the polymer molecules to separate from the shale surface and melt into the slick water again, so the adsorption capacity decreases rapidly. Z section (T >70º) is a section where adsorption is not correlated with temperature.
When the temperature is above 70 ºC, the amount of adsorption no longer changes with the temperature. The adsorption capacity is stabilized at about 2.7 mg/g and the adsorption and desorption of the polymer on shale surface reach dynamic equilibrium at this time.
With the increase of temperature, the adsorption equilibrium time decreases continuously. The adsorption equilibrium time decreases sharply at 20-45ºC, and the adsorption equilibrium time is in exponential negative correlation with the temperature. At 45-70 ºC, the adsorption equilibrium time decreases continuously, and is in linear negative correlation with the temperature. The equilibrium time will not change at 70-100 ºC.