Effect of initial water saturation on the performance of fracturing fluids with and without polyallylamine under simulated EGS conditions

Journal Article · Sat Apr 15 00:00:00 EDT 2023 · Geothermics

DOI:https://doi.org/10.1016/j.geothermics.2023.102715· OSTI ID:1987312

Jian, Guoqing ^[1]; Sarathi, Ramesh S. ^[1]; Burghardt, Jeffrey A. ^[1]; Bonneville, Alain ^[1]; Fernandez, Carlos A. ^[1]

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1. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Objectives/Scope: StimuFrac (US Patents 9,873,828 B2 and 9,447,315 B2), a CO₂- reactive polymer aqueous solution [polyallylamine (PAA) 1wt% in water] combined with CO₂, can be used as a potentially less water-intensive fracturing fluid for enhanced geothermal systems (EGS). Our previous results show that in hot dry rock (HDR), PAA/CO₂ fracturing fluids outperformed other fluids such as water, CO₂, and CO₂/water in generating large fractures with less fluid consumed. The objective of this work is to investigate the effect of initial water saturation on the performance of StimuFrac by conducting hydraulic fracturing tests with ½ foot cubic rock samples held under representative EGS stress/temperature conditions and by using cyclic injection strategies (under constant injection rate). The resulting fracture hydraulic conductivities, breakdown pressures, and volumes of fluids required are compared. Methods/Procedures/Process: To simulate geothermal reservoir conditions, in all tests, the rock sample was held under triaxial confinement and at 200 °C, and different volumes of water were initially injected into the rock sample before any fracturing processes were initiated. For the single-cycle PAA (or water) alternating CO₂ (PAG or WAG) injection fracturing experiments, one complete cycle consisted of two steps: (1) injecting a PAA slug (or water slug) followed by (2) injecting CO₂ to initiate and propagate the fracture. For experiments involving multiple injection cycles, the CO₂ injection pressure is increased until it peaks and begins to decline (indicating fracture initiation at this moment), and then continued being injected for another 30 seconds to propagate the fracture. Then, these two-step cycles [injection of PAA (or water) followed by CO₂ injection (up to 2-4 mL/min)] are repeated. Applications/Significance/Novelty: The results of this study suggest that water saturation significantly affects the fracturing fluid transmission into the rock pore space, thus affecting the fracture initiation and propagation. In this study, fracturing tests via a single injection cycle or multiple injection cycles were performed. Splitting the rock samples in half after testing reveals that fracture propagation is significantly limited under high water saturation conditions (three-day initial water injection) compared to stimulation experiments performed in hot dry rock. The fractures propagate less than 1/3 of the distance from the wellbore to the outer rock surface, and in some cases, no fracture is generated. This may be caused by leak-off dominating the fracturing process and the fluid injection rate is insufficient to overcome leak-off, even under high injection rate conditions. Additionally, CO₂ could be leaking off into the wellbore annulus and this may be making it more difficult to generate sufficiently high-pressure gradients away from the near-wellbore region. Under low water saturation conditions (dry rock or after 1-day initial water injection), PAA/CO₂ consistently generated significantly larger fractures compared with the other fluids. CO₂ generated large fractures only in the hot dry rock and only when using high injection rates, though data variability is high.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE; USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Geothermal Technologies Office
• Grant/Contract Number: AC05-76RL01830; AC06-76RL01830
• OSTI ID: 1987312
• Alternate ID(s): OSTI ID: 1999871
• Report Number(s): PNNL-SA-177490
• Journal Information: Geothermics, Journal Name: Geothermics Vol. 111; ISSN 0375-6505
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CO2-reactive polymer
Fracturing
Initial water saturation
Polyallylamine (PAA)
StimuFrac