Hydraulic fracturing involves breaking subterranean rock with hydraulic pressure in order to create higher permeability for hydrocarbon fluids and gas through the wellbore.
To keep the fractures open, ceramic proppant (or sand) is pumped as slurry into the fracture. Recent advances in fracturing technology have allowed for the economic production of unconventional gas deposits such as shale gas and coal bed methane.
Chemical additives are essential to the success of hydraulic fracturing, with dozens of additives that can potentially be used in any given fracture treatment. However, the use of chemicals in fracture applications has come under increased scrutiny worldwide; particularly in North America where fracturing has become a common practice. As a result, much focus has been made on the development of environmentally-acceptable chemistries for both bespoke applications and for the replacement of potentially environmentally-hazardous chemistries currently in use.[Page Break]
One such chemical is 2-butoxyethanol (2-BE, EGMBE, BGE, butyl cellosolve), which forms part of the pre-flush treatment and acts as a mutual solvent. Use of this chemistry can provide significant benefit by removing water blocks, preventing emulsion formation and allowing better flowback of the fracture treatment and therefore faster well clean up. Yet like many chemicals, the use of 2-BE has come under scrutiny primarily due to toxicity concerns.
Experimental studies are available and report that high exposure has allegedly caused birth defects and that ingestion can cause breathing problems, low blood pressure, lowered levels of hemoglobin, blood in the urine and metabolic acidosis (Harris et al., 1998). The key environmental test parameters for 2-BE are summarised in Table 1 - it can be generally summarised that while it is a readily biodegradable compound, it is able to bioaccumulate and is toxic.[Page Break]
Clariant Oil Services initiated an intense R&D programme aimed to identify a replacement for 2-BE, with superior environmental performance while maintaining the desirable features this chemistry offers during fracturing and flowback. The final product was chosen for its mutual solvency and wettability modification properties.
The VES-based fracture additive package, when untreated, formed an emulsion with condensate, commonly produced alongside gas in shale formations. Using 2-BE it was shown that quick and clean resolution of the emulsion occurred. The new environmental replacement chemistry for 2-BE exhibited the same ability to resolve the emulsion and also contributed to preventing the emulsion from forming in the first place.[Page Break]
The advantage shown by the new chemistry was faster resolution of the emulsion. From this testing it was concluded that a 10 per cent solution of the new replacement chemistry for 2-BE in water could be used to replace the current 2-BE based spearhead. Core flood work proved that when using a mutual solvent pre-flush of the new, environmentally-acceptable chemistry, superior permeability regain was demonstrated compared to a VES system.[Page Break]
Field data was gathered from dozens of fracture treatments on wells that used a 2-BE pre-flush, along with wells that did not use any mutual solvent in the pre-flush. The gas rates from these two sets of wells were averaged and this data is plotted in Fig.1. A clear product benefit can be seen when 2-BE was used as a pre-flush additive due to a faster clean-up from the fracture treatment and quicker return to full production. The cumulative production benefit when using 2-BE after 1000 days of the treatments was over double. Wells without mutual solvent produced 4414 e3m3 (156 MMscf) while wells that did use mutual solvent produced 9048 e3m3 (320MMscf) over the same time period.
Fig. 2 shows field data comparing wells fractured using 2-BE as a pre-flush additive compared to wells fractured with the alternate, environmentally-acceptable chemistry. An even further production benefit was realized when using the environmentally-acceptable chemistry. The total production for 145 days of wells fractured with 2-BE was 11,287 e3m3 (399 MMscf) and for wells that used the environmentally-acceptable alternative was 16,217 e3m3 (573 MMscf), an increase of approximately 40 per cent.[Page Break]
Conclusions
An environmentally-acceptable replacement chemistry for 2-BE has been identified, tested and field proven. The conclusions are:
- Using toxicity, biodegradation and bioaccumulation as the key environmental performance indicators, the alternative chemistry has a superior environmental profile when compared to 2-BE.
- Bottle tests performed in the laboratory show the alternate chemistry to have greater mutual solvency efficacy when compared to 2-BE.
- Core flood experiments showed a greater regain to permeability when using mutual solvent when compared to no mutual solvent.
- Field data summarised from multiple applications showed that production flowback performance of wells where no mutual solvent was applied was half that of wells pre-flushed with 2-BE.
- Field data shows that when the new environmentally-acceptable alternate mutual solvent was applied, the production benefit over that of wells pre-flushed with 2-BE was an incremental 40 per cent.
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References:
Harris O, Wilbur S, George J, and Eisenmann C (1998) Toxicological Profile For 2-Butoxyethanol And 2-Butoxyethanol Acetate. US Department of Health and Human Services Public Health Service Agency for Toxic Substances amd Disease Registry.
Jonathan J Wylde is UK Business Manager, Clariant Oil Services, Dyce, Aberdeen, UK. www.oil.clariant.com
