Chemical Analysis of Flowback Water and Downhole Gas Shale Samples
- Ashkan Zolfaghari (University of Alberta) | Yingzhe Tang (University of Alberta) | Jordan Holyk (University of Alberta) | Mojtaba Binazadeh (University of Alberta) | Hassan Dehghanpour (University of Alberta) | Doug Bearinger (Nexen Energy ULC)
- Document ID
- Society of Petroleum Engineers
- SPE/CSUR Unconventional Resources Conference, 20-22 October, Calgary, Alberta, Canada
- Publication Date
- Document Type
- Conference Paper
- 2015. Society of Petroleum Engineers
- 6.5.4 Naturally Occurring Radioactive Materials, 2.5 Hydraulic Fracturing, 3 Production and Well Operations, 5.2 Reservoir Fluid Dynamics, 2 Well completion, 5.2 Reservoir Fluid Dynamics, 5 Reservoir Desciption & Dynamics, 5.8.2 Shale Gas, 5.8 Unconventional and Complex Reservoirs
- Flowback Chemical Analysis, Shale-Water Interactions
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Recently, flowback chemical analysis has been considered as a complementary approach for evaluating fracturing operations and characterizing reservoir properties. Understanding the source of flowback salts and the mechanisms controlling the water chemistry is essential but also challenging due to the complexity of water-shale interactions. In this study, samples of flowback water and downhole shales are analyzed to investigate the mechanisms controlling the chemistry of flowback water.
The water samples at different flowback times and the shale samples are collected from three wells completed in the Muskwa, Otter-Park, and Evie members of the Horn River Basin. The water samples consist of aqueous solution and precipitated salts. The water samples are digested in nitric acid to dissolve the precipitated salts, and are analyzed at both intact and acid-digested conditions using ICP-MS. The flowback salts are weighted and analyzed using XRD and SEM-EDXS. A sequential ion-extraction is performed on the shale samples; and the extracted ions are categorized into three tiers of loosely-, moderately-, and strongly-attached ions. Acid digestion is also performed on the shale samples to characterize their elemental composition.
The concentration of monovalent cations in both intact and acid-digested samples is higher than that of divalent cations. Also, the concentration of all cations is higher in the acid-digested samples compared with that in the intact samples. Chemical analysis of the flowback water samples suggests that the majority of the ions in the early flowback water are loosely-attached monovalent ions. These ions can be originated from the mixing with in-situ formation brine, dissolution of soluble precipitated salts, or leaching of exchangeable cations from the clay minerals. Similarly, the role of relatively slow water-shale interactions (such as leaching of divalent exchangeable cations, e.g. Ca2+) increases at the later flowback stages. XRD and SEM-EDXS analyzes of the flowback salts indicate that sodium chloride, potassium chloride, and calcium carbonate are the major salts. The sequential ion-extraction reveals that the majority of the monovalent cations are in the loosely-attached tier. However, majority of the divalent cations are moderately or strongly attached to the rock. The strongly-attached portion of the ions is determined by acid digestion of the rock sample at the final stage of sequential ion-extraction process. These strongly-attached ions cannot be easily released by hydraulic fracturing and therefore, has small effect on the flowback water chemistry. Flowback chemical analysis indicate that sodium is the major ion in the flowback water and its concentration is higher than the potassium concentration. While, according to the results of acid digestion of rocks, potassium is the major constituent of our shale samples and its concentration is about five times higher than the sodium concentration. Moreover, although barium is not a major constituent of our shale samples (~0.05%), the concentration of the barium ion is relatively high in the flowback water samples. These results may suggest that the water-shale interaction is not the major mechanism controlling the flowback water chemistry.
|File Size||3 MB||Number of Pages||17|
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