【本周專題：油田化學】新型耐鹽減阻劑體系實現產出水100%回用

To continue to treat multiple clusters in longer laterals effectively, even for stages near the toe, a robust friction-reducer (FR) system typically is required to overcome pipe friction. Additionally, using a single FR system throughout the entire treatment that can tolerate various water sources of varying salinity up to 300,000 ppm is imperative. This paper discusses the field trials of a new salt-tolerant FR system in the Marcellus Shale.

Introduction

FRs are used to enable high-rate pumping of water while maintaining lower treating pressures in both slickwater fracturing and coiled-tubing stimulation applications. Typically, the performance of conventional acrylamide/acrylic acid copolymer or partially hydrolyzed polyacrylamide-based FRs diminishes as the dissolved-ion content of the source water increases and can be dependent on both the type and the concentration of ions present in the source water. Because of this limitation, flowback and produced water used for slickwater applications are often diluted with a freshwater source to maintain adequate friction-reducing performance. To enable the use of 100% produced water, new friction reducers have been developed that are capable of providing near-freshwater performance in flowback and produced water exceeding 300,000 ppm total dissolved solids (TDS).

The new salt-tolerant FR consists of FR systems that are designed to work in a specific range of high salinity to enable the use of 100% flowback and produced water in slickwater hydraulic-fracturing applications without requiring dilution with fresh water. The salt-tolerant FR service can handle TDS levels greater than 300,000 ppm; is effective in water containing a variety of dissolved salt ions, including chlorides, sulfates, sodium, calcium, and magnesium; and can tolerate a variety of unknown contaminants that might render conventional FRs ineffective. The new FR system is effective at low concentrations (0.25 to 1 gal/1,000 gal) in clay-control brines and flowback and produced-water sources.

This new salt-tolerant FR system consists of a water-in-oil cationic polymer and an inverter. Unlike other FRs, the distinctive advantage of the new FR is that the ratio of polymer to inverter can be readily adjusted on the fly to achieve maximum friction reduction. During the pumping operations, it was demonstrated that the inverter was sufficiently quick to invert and release the polymer from oil to water and the cationic polymer was extremely efficient at reducing additional pipe friction, even with severely impaired water. Additionally, the use of a single FR reduces inventory stock and simplifies on-location quality assurance of material usage.

Flow-Loop Tests

Laboratory friction loop testing is typically conducted to determine the performance of various FRs. This testing provides an indication of the friction reduction of the FR in the treatment or source water, which ultimately will affect treating pressures and the hydraulic-horsepower requirements during a stimulation treatment. These data can then be compared with those from conventional FRs in fresh water, and a conclusion can be drawn. Fig. 1 above displays the performance of the new salt-tolerant FR. The new salt-tolerant FR displayed superior performance in field-supplied water sources compared with the conventional FR. These results show that salt-tolerant FRs enable the use of undiluted flowback and produced water in fracturing operations, consequently reducing the need for excess expenses and logistics for water that otherwise would be considered waste stream for disposal. In addition, the use of this fluid chemistry enables recycling of flowback and produced water, which can be economically and environmentally sustainable solutions.

During actual pumping on location, the pipe friction reduction is affected by several parameters, such as FR loading, FR effectiveness, and how well the FR is dispersed in the fracturing fluid. In particular, the chemistry of the FR is crucial. Most commonly used anionic FRs, such as partially hydrolyzed polyacrylamides, tend to agglomerate and do not stretch fully in high-TDS water (e.g., water with greater than 60,000 ppm TDS), which results in poor friction reduction. Cationic FRs, however, can overcome this challenge because they are less affected by the presence of cations, which directly correlates to the TDS level in the fracturing fluids. In other words, they can still stretch and extend to some extent.

Field Trials

A slickwater system is typically pumped at higher rates to cover the rock volume as specified in the fracture design. When the FR does not provide sufficient reduction in the average treating pressure (ATP) at the average treating rate (ATR), the FR concentration is usually increased. This salt-tolerant cationic FR system does not require increasing the concentration yet allows lower treating pressures and pumping rates compared with conventional FRs. A three-well trial was executed in the Marcellus Shale.

For Well 1, the conventional FR was pumped for the first 10 stages, after which the FR was switched to the salt–tolerant FR. Both the ATP and the ATR immediately responded, and an ATR greater than 90 bbl/min was largely maintained. During the 20th stage, the ATR dipped below 90 bbl/min, which is attributed to the operator adjusting the ratios between the salt-tolerant FR and the inverter on the fly.

For Well 2, the field-standard anionic FR was pumped for the first 14 stages. The ATP remained high and fluctuated between 8,700 and 9,100 psi. As soon as the conventional FR was switched to the salt-tolerant FR at the 15th stage, the ATP immediately dropped to 8,100 psi and remained between 8,000 and 8,700 psi, with the fluctuations resulting from the ratios between the FR and the inverter being adjusted on the fly. In addition, the ATR increased to 100 bbl/min -immediately after the switch and was maintained at that rate for the remaining stages.

When looking at the ATP for both the conventional FR and the salt-tolerant FR system as a function of the weight percentage of impaired water, the data illustrate that the salt-tolerant FR was able to maintain relatively low treating pressure (less than 9,000 psi) at 100% impaired water. In terms of the ATR, the data strongly suggest that the salt-tolerant FR system was able to raise the treating rate to 100 bbl/min compared with the field-standard FR, despite the use of 100% impaired water.

Comparing the FR concentrations at each stage for the conventional and salt-tolerant FR systems, the salt-tolerant FR generally maintained a higher pumping rate, even at a concentration lower than 1 gal/1,000 gal.

Conclusions

A salt-tolerant FR system was formulated and pumped in slickwater fracturing treatments with up to 100% produced water. The ATP was maintained lower and the ATR was maintained higher compared with those observed with the conventional FR. Furthermore, the salt-tolerant-FR concentrations, for most of the pumped stages, were maintained below 1 gal/1,000 gal. The salt-tolerant FR system is customizable through independent inverter dosage, which results in improved friction-reduction performance for use in 100%-produced-water conditions.