Development and Field Application of a New Ultralow Guar Gum Concentration Weighted Fracturing Fluid in HPHT Reservoirs

A weighted fracturing ﬂuid with superlow guar gum concentration was developed by synthesizing a polyboric acid cross-linker. The density of this ﬂuid is 1.365g/cm 3 and can withstand very high temperature up to 175 ° C. In this study, a weighting agent was selected, and crosslinking ligands and boric acid were optimized. The crosslinking performance, base ﬂuid viscosity, rheology, and gel-breaking performance of this fracturing ﬂuid were studied. Compared with the conventional weighted fracturing ﬂuid, the concentration of guar gum in the new weighted fracturing ﬂuid can be reduced by 30% at the same temperature condition; moreover, crosslinking can be delayed by 2 minutes. The concentration of gel breaker used in this ﬂuid can be signiﬁcantly reduced to 0.005% ∼ 0.01%. Two ﬁeld tests were conducted in Jidong oil ﬁeld, and both of them achieved great success.


Introduction
Weighted fracturing fluid (also called high-density fracturing fluid) has become more and more important for fracturing of high-temperature and high-pressure reservoirs (especially over 160°C). Adding weighted materials into the fracturing fluid is an effective way to increase fluid density and thus reduce wellhead pressure. For example, for a 5000 m well, every 0.1 g/cm 3 increase in fracturing fluid density will contribute to a 5 MPa wellhead pressure decrease [1][2][3][4][5].
However, a major challenge is that the fluid performance might change a lot with the addition of weighting agents. Adding a large amount of weighting agents into the solution will result in inhomogeneity and long thickening time, which will lead to poor temperature and shear resistance. Viscoelastic surfactant is a good choice for the high-density fracturing fluid due to its good salt resistance, but its poor temperature resistance (below 120°C) and expensive cost of the raw material have prevented its use in the oilfield. Guar gum and hydroxypropyl guar gum cost much less than the viscoelastic surfactant and can mitigate the solubility problem of thickeners in saline solution, together with the organic baron crosslinking agent, which has excellent shear recovery, and can delay crosslinking. erefore, the combination of hydroxypropyl guar gum and organic boron cross-linker is believed to be an ideal weighted fracturing fluid [6]. e temperature resistance of the fracturing fluid is highly demanded for high-temperature reservoirs (above 160°C). A large amount of guar gum and temperature stabilizers are needed to meet the temperature and shear resistance of the high-temperature fracturing fluid [7]. Besides, as temperature varies with seasons, plus the dissolution and endothermic of sodium nitrate and other salts may absorb heat, the temperature of the weighted fracturing fluid often decreases (especially in winter), which will lead to an increase in apparent viscosity of the base fluid. Furthermore, high-apparent viscosity base fluid will affect fluid supply and increase pipe friction during fracturing process. e addition of the temperature stabilizer will also increase the concentration of the gel breaker in the fluid system. In addition, collected field data revealed that the gel-breaking property of the existing high-density fracturing fluid is poor, and the concentration of the gel breaker is relatively high (0.05-0.1%), which affects gel flowback rate after fracturing treatment [8][9][10].
In order to overcome these difficulties, we optimized the boric acid source and crosslinking ligand grade, added ester bonds to the molecular structure, and increased the usage of boric acid to improve the rigidity and flexibility of the coordination bond. As a result, a polyboric acid crosslinking agent was synthesized, and a weighted fracturing fluid with superlow guar gum concentration was formed in this study. Two field tests were conducted in Jidong oil field, and both of them achieved great success.

Experimental Instruments. WT-VSA-2000B Warring
Mixer, offered by the Beijing Institute of Mineral Exploration Engineering, was used to configure the base fluid of the fracturing fluid. RS6000 Rheometer, provided by ermo Fisher Scientific, was used to test the temperature and shear resistance of the fracturing fluid. Fourier-transform infrared spectrometer (Nicolet 6700), provided by ermo Fisher Scientific, was used to analyze the functional groups of the sample. EDS (EDAX Genesis), provided by EDAX (USA), was used to analyze the elements of the sample and the mass fraction of each element.

Preparation of Polyboric Acid Cross-Linker.
e solution was prepared by dissolving 13 g glycerin in 39 g distilled water under magnetic stirring. en, 25 g boric acid 1 (Russia) was slowly added into the solution under constant stirring at room temperature. After that, 16 g sodium gluconate (food grade) and 7 g ester organic ligand 2 were added, and the mixture was slowly stirred for 3 hours at 60-70°C. en, sodium hydroxide was added to adjust the solution pH value to 7. After that, the final polyboric acid cross-linker (JDY-1) was obtained.

Characterization of JDY-1.
1∼2 mg dried JDY-1 were grinded into powder in an agate mortar and then mixed evenly with 100 mg dried potassium bromide (KBr, AR class) powder. e FTIR spectra of the sample were measured using a Fourier-transform infrared spectrometer (Nicolet 6700) provided by ermo Fisher Scientific. Elementary analysis of cross-linkers was investigated by EDS.

Performance Evaluation of the Weighted Fracturing
Fluid.
e apparent viscosity of the base fracturing fluid was tested using six-speed viscometer at 25°C with a shearing rate of 170 S −1 . Temperature and shearing resistance of gel were measured via RS6000 Rheometer provided by ermo Fisher Scientific (Germany) at different temperatures and sheared at 170 S −1 . Gel-breaking performance was tested at different concentrations of ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) at different temperatures.

Optimization of Crosslinking Ligands.
e first procedure of synthesis of polyboric acid cross-linker is to select a suitable crosslinking ligand. In this study, industrial-grade sodium gluconate 1 and food-grade sodium gluconate 2 were evaluated as crosslinking ligands of the fracturing fluid. As shown in Figure 1, the apparent viscosity of the fracturing fluid using sodium gluconate 2 (food grade) is higher than that of sodium gluconate 1 (industrial grade). e result confirmed food-grade sodium gluconate 2 had better temperature and shear resistance than industrial-grade sodium gluconate 1. is is due to the extremely high purity and low concentration of heavy metal ions of food-grade sodium gluconate. erefore, the fracturing fluid using food-grade sodium gluconate 2 as a crosslinking ligand has better temperature and shear resistance. e characteristics of sodium gluconate 2 and crosslinking ligand 2 were also evaluated. e test was conducted at 150°C and sheared at 170s −1 . e apparent viscosity of gels formed by sodium gluconate was higher than that of crosslinking ligand 2, indicating that the temperature resistance of sodium gluconate 2 was better than that of crosslinking ligand 2 at 150°C.
As a crosslinking ligand, sodium gluconate has strong temperature resistance and can improve rigidity of crosslinking bonds. e temperature resistance of gel can be improved by increasing the concentration of boric acid in a certain range; once it exceeds the range, gel dehydration phenomenon occurs. e gel would become brittle and lose part of its shear resistance. However, when using the combination of ester crosslinking ligand 2 and sodium gluconate, dehydration phenomenon did not occur with the increase of boric acid concentration (Table 1). is is due to the reversible esterification reaction which provided more crosslinking points, which improved the flexibility of crosslinking bonds, and thus increased the probability of recrosslinking under dynamic conditions.

2
Journal of Chemistry As a result, the combination of food-grade sodium gluconate 2 (16 wt%) and ester crosslinking ligand 2 (7 wt%) was determined to be the optimal result of the new crosslinking ligands. e apparent viscosity of gel reached 381 mPa·s after 1 hour at 150°C.
As shown in Table 2, experimental results indicated that, after 1 hour of shearing, the apparent viscosity of gel 2 (189∼381 mPa·s) was lower than that of gel 1 (215∼408 mPa·s), which means the temperature resistance of gel 1 was better. is is mainly due to the difference in processing procedures and technology, as well as the quality of raw materials of boric acids. erefore, the formula of 16% sodium gluconate 2, 7% crosslinking ligand 2, and 25% boric acid 1 was determined to be the optimum formula.

Characterization of the Crosslinking Agent.
e structure of the cross-linker was measured using a Fouriertransform infrared spectrometer (Nicolet 6700). e structure of the cross-linker was confirmed by FTIR as illustrated in Figure  Elementary analysis of cross-linkers was investigated by EDS as illustrated in Table 3   crosslinking ligand and sodium gluconate, and the ester crosslinking ligand has more carbon atoms than sodium gluconate (C 6 H 11 O 7 Na). As expected, the results confirmed the element composition of JDY-1. is might be explained that industrial-grade sodium nitrate generally contains Fe 3+ (the concentration of Fe 3+ less than 0.005 wt%), which will crosslink with guar gum. Sodium nitrate B may be of higher standard and did not contain Fe 3+ , so crosslinking was not observed in sample B. As a result, weighting agent B was selected.

Viscosity Test of the Base Fluid.
e objective of this test is to measure apparent viscosity at different conditions when weighting agent B was added to the solution. Table 4 shows the apparent viscosity of the weighted fracturing fluid is always greater than that of the conventional density fracturing fluid (density 1.0 g/cm 3 ). Moreover, the apparent viscosity of the conventional density base fluid did not change with ambient temperature, while the apparent viscosity of the weighted fracturing base fluid increased significantly with ambient temperature decreased. For example, at 25°C, when guar gum concentration was 0.5 wt%, the corresponding apparent viscosity of the conventional density base fluid (density 1.0 g/cm 3 ) was 72 mPa·s; however, at the same condition (25°C, concentration 0.5 wt%), the corresponding apparent viscosity of the weighted base fluid (density 1.32 g/cm 3 ) increased to 96 mPa·s. When the temperature falls to 15°C, the corresponding apparent viscosity of the weighted base fluid (density 1.32 g/cm 3 ) would increase to 114 mPa·s (guar gum concentration 0.5 wt%) ( Table 4).
e reasons can be explained as follows: the salt proportion in the weighted fracturing fluid was high, so the actual amount of water contributing to the fluid was relatively low, which would increase the apparent viscosity of the weighted fracturing fluid. In addition, due to the influence of melting and heat absorption of sodium nitrate and other salts, the temperature of the fracturing fluid often decreased,   which leaded to an increasing viscosity of the base fluid. Furthermore, since the temperatures of reservoirs can be as high as 160°C or even above, the amount of thickener added to the fracturing fluid was relatively high in order to ensure the temperature resistance of the gel. From field experience in Jidong oil field, the apparent viscosity of the base fluid exceeding 90 mPa·s will increase pipe friction and limit the flow rate, which will affect pumping process and cause serious troubles during operation. erefore, 0.45 wt% was determined to be the upper limit of guar gum concentration to make sure the apparent viscosity of the base fluid remained under 90 mPa·s.

Optimization of the Cross-Linker.
Two kinds of borate cross-linkers, JDY-1 (polyboric acid cross-linker) and ZCY-10 (conventional organic boron cross-linker), were evaluated in this study. Besides, high temperature shear tests were carried out at 160°C and sheared at 170 s −1 . Figure 3 indicates JDY-1 and ZCY-1 have a big difference in temperature resistance when sheared at 170 s −1 under the same guar gum concentration and density. At 160°C, JDY-1, with 0.4% guar gum concentration, can withstand 120 minutes of shearing and ended up with a 60 mPa·s apparent viscosity, while ZCY-10, with 0.5% guar gum concentration, can only withstand 40 minutes of shearing and ended up with a 40 mPa·s apparent viscosity. As a result, JDY-1 performed better and was determined to be the final crosslinker of the fluid system.

Crosslinking Performance.
e crosslinking tests of fracturing fluids with different densities were carried out. For conventional fracturing fluids (density 1.0 g/cm 3 ), crosslinking is not a problem. However, the crosslinking performance of the weighted fracturing fluid may be affected by the concentration of the cross-linker and regulator. e experiment results confirmed that increasing cross-linker concentration would improve the crosslinking performance. Table 5 indicates for conventional fracturing fluids (density 1.0 g/cm 3 ), crosslinking gels were good and stable, and crosslinking time was acceptable. As fluid density increased, the crosslinking of the fracturing fluid costs more time, and the crosslinking performance was weakened, indicating the addition of sodium nitrate affected the crosslinking performance. However, the crosslinked gel can be improved significantly, and the crosslinking time can also be shortened by increasing the dosage of the cross-linker.
JDY-1 was used as a cross-linker in formulas 1 and 2, while ZCY-10 in formula 3. e tests were conducted at different temperature conditions. e rheology characteristics of these samples were studied. Detailed descriptions and figures were given. Figures 4 and 5 compare the temperature and shear resistance of the weighted fracturing fluid and conventional (1.0 g/cm 3 ) fracturing fluid. Figure 4 shows that the weighted fracturing fluid could withstand very high temperature (up to 160°C), while the conventional fracturing fluid can only withstand 130°C. at is to say, compared with the conventional density fracturing fluid, the temperature and shear resistance of the weighted fracturing fluid improved significantly. Figures 6 and 5 demonstrate the temperature and shear resistance of both high-density and conventional density fracturing fluids using formula 2. Figure 6 indicates the highdensity fluid system can withstand very high temperature up to 175°C. e apparent viscosity was 68 mPa·s after 90 minutes at 175°C. Figure 7 compares the apparent viscosity of the conventional fluid system at 140°C and 150°C. It is obvious that the conventional fracturing fluid using formula 2 can withstand 140°C, but cannot withstand 150°C. At 140°C, the apparent viscosity of the conventional density fluid using formula 2 was 104 mPa·s after 120 minutes of shearing, while at 150°C, the apparent viscosity was only 44 mPa·s after 120 minutes of shearing. Figure 8 shows the temperature and shear resistance of the formula 3 fluid system. Formula 3 used 0.3% temperature stabilizer, 0.45% HPG concentration, and ZCY-10 as the cross-linker. e fluid system can only resist up to 150°C. Since 0.45% was determined as the upper limit of guar concentration in Section 3.2.2, formula 2 was determined as the optimal formula at 175°C. e results also confirmed     JDY-1 had excellent temperature resistance. Furthermore, it can effectively reduce guar gum concentration and overcome the difficulties that the high-apparent viscosity base fluid may cause, such as fluid supply and pipe friction issues.

Gel Breaking.
e gel-breaking properties of the weighted fracturing fluid using JDY-1 and ZCY-10 were studied. Tables 6 and 7 demonstrate the gel-breaker profiles at 120°C and 140°C. At 120°C, JDY-1 gel has good performance.     Journal of Chemistry e gel can be completely broken to less than 5 mPa·s with very low gel-breaker concentration (0.005%). However, at the same condition, ZCY-10 gel cannot be broken thoroughly, even with very high gel-breaker concentration (0.05%). At 140°C, JDY-1 gel also showed very good gel-breaking property; moreover, at the same condition, ZCY-10 gel with very high gel-breaker concentration (0.1%) can also be thoroughly broken for first few hours, but after 12 hours, the gel appeared again.
e reasons for this result can be explained. First of all, oxidized gel breakers, such as ammonium persulfate (NH 4 ) 2 S 2 O 8 ) and potassium persulfate (K 2 S 2 O 8 ), were used to oxidize crosslinking bonds and polymer chains in order to break the cross-linked gels. As oxidized gel breakers can produce [O], the acetal bonds of vegetable gum and its derivatives were oxidized and degraded. en, the oxidative degradation reaction of guar gum occurred under alkaline conditions. e active mechanism of the temperature stabilizer (such as sodium thiosulfate (Na 2 S 2 O 3 ), triethanolamine (C 6 H 15 NO 3 ), and sulfite) was to improve the thermal stability of thickener by removing free oxygen in the liquid and reducing its catalytic effect on thermal degradation of polymer, in order to stabilize the performance of the fracturing fluid in high-temperature environment.
erefore, the addition of the temperature stabilizer will inevitably consume oxidized gel breaker, which will result in the incomplete gel breaking of the fracturing fluid. Moreover, the addition of a large amount of salt prevented the oxidation reaction between ammonium persulfate and acetal bond, which would also lead to incomplete gel breaking.

Field Application
e new weighted fracturing fluid was used in the fracturing project of an old well in Nanpu oilfield, Jidong, China. e reservoir has a vertical depth of 4823∼4863 m, thickness of 10.5 m, and temperature of 168°C. e NPXX well has used 3 1/2 tubing size with RTTS packers. According to the data of adjacent wells, the closure stress gradient of this well was estimated to be 0.021 MPa/m, and breakdown pressure was predicted to be 102 MPa using the conventional density (1.0 g/cm 3 ) fracturing fluid system. Figure 9 demonstrates the fracturing construction curve of NPXX. e treatment was successfully constructed with the new weighted fracturing fluid system with a density of 1.36 g/cm 3 . During the process, the construction pressure ranged from 75 to 83 MPa, the maximum injection rate was 5.2 m 3 /min, and the total fluid and proppant volume were 830 m 3 and 54 m 3 , respectively. e maximum sand concentration was 525 kg/m 3 . After fracturing, a 5 mm nozzle was used for flowback, and the flowback rate was 85%. Furthermore, daily oil production reached 10 t/d, which was higher than adjacent wells. e success of this treatment implies the new ultralow guar gum concentration weighted fracturing fluid can completely meet the requirements under reservoir conditions. Moreover, it provides strong technical support for reservoir reconstruction in high-temperature deep wells in Jidong oilfield.

Conclusion
e following conclusions can be drawn: (1) A polyboric acid cross-linker (JDY-1) was synthesized by optimizing the source of boric acid and the grade of sodium gluconate, compounding the crosslinking ligand (16 wt% sodium gluconate 2 and 7 wt% ester ligand) and increasing the amount of boric acid to 25 wt%. (2) e composition of the new weighted fracturing fluid was confirmed: 0.45% HPG + 0.5% regulator + 0.15% Na 2 CO 3 + 0.3% cleanup additive + 0.5% antiswelling agent + 1% KCl + 0.6% JDY-1 + 0.1% temperature stabilizer + 40% sodium nitrate B. is fluid system can withstand very high temperature up to 175°C with a density of 1.365 g/cm 3 . After optimization, the guar gum concentration was effectively reduced in the new weighted fracturing fluid, and the difficulties during the pumping process in the high-temperature reservoir (≥150°C) have been solved. (3) e new weighted fracturing fluid has excellent gelbreaking performance. e gel can be broken thoroughly to less than 5 mPa·s by using gel-breaker with very low concentration (0.005% ammonium persulfate) at 120°C within 2 hours. is fluid system has solved many problems that the conventional weighted fracturing fluid would cause, such as high gel-breaker usage and difficulties in gel flowback.
Data Availability e figures and field data used to support the findings of this study are included within the article and are available from the corresponding author upon request.

Conflicts of Interest
e authors declare no conflicts of interest.

Authors' Contributions
Yi Liu (liu0yi@163.com) contributed to the overall thinking of the manuscript, synthesis of JDY-1, optimization evaluation of the new weighted fracturing fluid, and preparation of the corresponding parts of the manuscript. Jing Liu (liujing188@petrochina.com.cn) contributed to the optimization of crosslinking ligands, synthesis and characterization of JDY-1, optimization of the weighting agent, and preparation of the corresponding parts of the manuscript. Yunzi Li (liyunzi2018@petrochina.com.cn) contributed to the literature research, current progress in weighted fracturing fluid research and its applications, performance evaluation of the weighted fracturing fluid, and the translation of the manuscript. Hui Yang (zcy_liuyi@ petrochina.com.cn) contributed to the performance evaluation of the weighted fracturing fluid and preparation of the corresponding parts of the manuscript. Fei Yan (zcy_yan-fei@petrochina.com.cn) contributed to the optimization of boric acid and performance evaluation of the weighted fracturing fluid. Jiapeng Zheng (38076671@qq.com) contributed to the performance evaluation of the weighted fracturing fluid. Jun Wu (940568031@qq.com) contributed to the selection of the test wells and the field application of the weighted fracturing fluid. Jianyi Huang (huangjy188@ petrochina.com.cn) contributed to fluid preparation and fluid operation. Zuohao Wu (gsp_wzh@petrochina.com.cn) contributed to the rheology performance evaluation of the weighted fracturing fluid. Cheng Luo (jd_luocheng@petrochina.com.cn) and Junkai Lu (lujunkai@petrochina.com.cn) contributed to the fracturing engineering design of the NPXX well (using the new weighted fracturing fluid system).