Summary Typically, water-based fracturing treatments consume a large volume of fresh water. Providing consistent freshwater sources is difficult and sometimes not feasible, especially in remote areas and offshore operations. Therefore, several seawater-based fracturing fluids have been developed in an effort to preserve freshwater resources. However, none of these fluids minimizes fracture-face skin and proppant-conductivity impairment, which can be critical for unconventional well treatments. Several experiments and design iterations were conducted to tailor raw-seawater-based fracturing fluids. These fluids were designed to have rheological properties that can transport proppant under dynamic and static conditions. The optimized seawater-based fracturing-fluid formulas were developed such that no scale forms when additives are mixed in or when the fracturing-fluid filtrate is mixed with different formation brines. The tests were conducted using a high-pressure/high-temperature (HP/HT) rheometer, coreflood, and by aging cells at 250 to 300°F. The developed seawater-based fracturing fluids were optimized with an apparent viscosity greater than 100 cp at a shear rate of 100 seconds–1 and a temperature of 300°F for more than 1 hour. The use of polymeric- and phosphonate-based scale inhibitors (SIs) prevented the formation of severe calcium sulfate (CaSO4) scale in mixtures of seawater and formation brines at 300°F. Controlling the pH of fracturing fluids prevented magnesium and calcium hydroxide precipitation that occurs at a pH value of greater than 9.5. Most importantly, SIs had a negative effect on the viscosity of seawater fracturing fluid during testing because of their negative interaction with metallic crosslinkers. The developed seawater-based fracturing fluids were applied for the first time in an unconventional and a conventional carbonate well and showed very promising results; details of field treatments are discussed in this paper.
Summary Previous studies of citric acid/calcite have been limited to coreflood tests and bench-scale experiments. However, the kinetics of a citric acid reaction with calcite has not been measured. This paper gives, for the first time, the kinetics of citric-acid/calcite reaction, which will provide a better way to model the performance of citric acid as a standalone stimulation fluid. In this paper, the rotating disk apparatus was used to study citric-acid/calcite reaction at a pressure of 1,000 psi, temperatures from 25 to 50°C, citric acid concentrations from 1 to 7.5 wt%, and disk rotational speeds from 100 to 1,000 rev/min. The reaction rate of citric acid with calcite was found to be dependent on the initial citric acid concentration, disk rotational speed, and temperature. For example, at 50°C, the reaction was reaction-rate-limited at rotational speeds greater than 500 rev/min, using high initial citric acid concentrations (3, 5, and 7.5 wt%), while at low acid concentrations (1 and 2 wt%), the reaction was mass-transfer-limited even at high rotational speeds (1,000 rev/min). During the reaction of citric acid with calcite, calcium citrate precipitation occurred at different acid concentrations and rotational speeds. The amount of this precipitation was found to be a function of both the initial citric acid concentration and disk rotational speed. More calcium citrate precipitated at high initial citric acid concentrations, especially at high rotational speeds. Calcium citrate precipitation occurred at the calcite surface, even at low initial citric acid concentrations. Because of this precipitation, the equilibrium of the citric-acid/calcite reaction was disturbed and the reaction shifted toward the forward reaction (toward the products). Therefore, the overall reaction rate was governed mainly by the rate of the forward reaction, and, hence, it was modeled by a new simplified reaction-rate equation; rate = kf{Ka1·CB}n/2 The average value of the reaction order (n) was found to be 0.833. In addition, the value of the reaction-rate constant (kf)was determined at various temperatures. The effect of temperature on the reaction-rate constant was found to follow the Arrhenius law, where the activation energy was found to be 63.1 kJ/mol.
Borate crosslinkers are the most commonly used crosslinker in fracturing fluids. However, they exhibit lower performance at high temperature, high pressure, high water salinity, and low pH applications. Consequently, zirconium crosslinkers are utilized to address these limitations. Zirconium crosslinking chemistry is complex and depends on many factors such as pH, metal to ligand ratio, ligand order, ionic strength, and type of polymer used, which in turn influence the delay time, thermal stability and shear resistance performance. This work evaluates the rheological performance of four different zirconium crosslinkers with a biopolymer and a synthetic polymer. The tested crosslinkers are manufactured in different chemical structures. The two polymers tested are 40 lb/1,000 gal CMHPG and 40 lb/1,000 gal synthetic polymer. The rheological performance was measured through HPHT rheometer (100 s−1 shear rate) at 200-400°F for 2 hours. The shear tolerance performance was also evaluated under a custom shear rate schedule (100-1000 s−1). The results show significant variation in crosslinking performance due to the changes in crosslinker chemical structure and type of polymer used. Zirconium lactate and propylene glycol crosslinker shows the highest enhancement in shear and thermal stability with CMHPG based fracturing fluids. Surprisingly, the same crosslinker performed the least with synthetic polymer-based fracturing fluids. However, Zirconium triethanol amine and lactate showed significant enhancements in shear and thermal stability with synthetic polymer-based systems. The results also show and discuss the influence of systematically changing crosslinker ligand order in CMHPG and synthetic polymer-based fracturing fluids. The work studies the influence of the zirconium crosslinker chemical structure on the rheological properties of both biopolymer and synthetic polymer-based fracturing fluids. The performance evaluation shows that delay time, shear and thermal stability can be enhanced by manufacturing the appropriate crosslinker chemical structure, thus reducing additional additives required used and saving cost.
Hydraulic fracturing activities in tight gas wells in Saudi Arabia have been exponentially increasing to meet domestic demand for natural gas. During each fracturing stage, up to 125,000 gallons of groundwater is currently being used. The need to reduce groundwater usage during fracturing treatments has been set as a priority, and alternative water sources for fracturing applications that can significantly reduce groundwater usage have been intensively explored. One such alternative water source is seawater as a base fluid for hydraulic fracturing. The primary challenge for this application is the tendency for scale precipitation due to the high sulfate content in seawater and its potential incompatibility with formation water. Without proper prevention and mitigation measures, this scale precipitation can induce formation damage and reduce the fracture conductivity. To minimize scaling tendencies, an in-house multidisciplinary team has performed extensive collaborative research to identify a scale inhibitor appropriate for Arabian Gulf seawater and formation water. Scale precipitation can be further mitigated by filtering the seawater with a nanofiltration system to dramatically reduce the sulfate ion as well as lower the calcium and magnesium ions. The successful application of seawater-based fracturing fluid in Saudi Arabia opens up the door to minimizing consumption of groundwater in hydraulic fracturing operations. Millions of gallons of groundwater could be saved and development of sustainable water resources could be achieved. This paper will describe the optimization of a scale inhibitor and fracturing fluid system, the selection of the nanofiltration system, and the first field applications of the seawater based fracturing fluid system in high-temperature gas wells in Saudi Arabia.
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