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Evaluation of Enhanced Oil Recovery Application in Tight-Oil Formations
- Author / Creator
- Tran, Son Thai
The rapid decline rates and low oil recovery factor (typically less than 10% of the original oil in place) of primary production are well-known challenges in the development of tight-oil formations. Several enhanced-oil-recovery studies and field trials have been conducted with promising results in these formations, however, key oil-recovery mechanisms are poorly understood. This research evaluates mechanisms controlling oil recovery during a natural-gas huff ‘n’ puff (HnP) and fracturing-fluid (FF) leakoff/flowback processes in tight-oil formations.
The main focuses are to investigate gas-transport and oil-recovery mechanisms during a gas HnP process under zero irreducible water saturation (Swirr); and changes in oil effective permeability (𝑘𝑜eff) caused by FF-leakoff/flowback under Swirr > 0. First, systematic phase-behavior and natural-gas (C1 and a mixture of C1/C2:70/30 mol%) HnP studies using Montney fluid/rock samples are conducted to understand the controlling mechanism of gas transport into the plug during injection/soaking and oil recovery during the whole process. Furthermore, Péclet number (NPe) analysis is performed to quantify the contribution of gas-transport mechanisms during the soaking period. Second, coreflooding tests are performed on Midale carbonate core plugs with a range of porosity and permeability to simulate leakoff/flowback processes. Measured 𝑘𝑜eff before (baseline) and after the leakoff process are compared to evaluate the effects of leakoff fluid properties (FF with nonionic surfactants and fresh water), shut-in duration (3 and 14 days), and rock properties on regained permeability values.
Key conclusions drawn from the phase-behavior and natural-gas HnP studies show that molecular diffusion is the dominant gas-transport mechanism during the soaking period of the HnP tests (NPe = 0.26 to 0.62). The advective-gas flow caused by differential pressure during gas injection leads to improved gas transport into the plug (NPe = 1.58 to 3.03). Total system compressibility, oil swelling, and vaporization of oil components into the gas phase are the recovery mechanisms observed during gas injection and soaking periods, while gas expansion is the main recovery mechanism during depressurization phase. Overall, gas expansion is the dominant recovery mechanism, accounting for approximately 90% of the oil recovery. During the “puff” period, the expansion and flow of diffused gas drag the oil along its flowpaths, resulting in a significant flow of oil and gas observed on the surface of the plug. The enrichment of injected gas by 30 mol% C2 enhances the transport of gas into the plug and increases oil recovery compared to pure C1 cases. According to the results of constant-composition-expansion tests and minimum-miscibility-pressure measurements, increasing C2 mole fraction in the injection gas significantly reduces minimum miscibility pressure of the oil/gas system and increases the oil-swelling factor.
The results of leakoff/flowback experiments show that adding appropriate surfactants in FF not only significantly reduces 𝑘𝑜eff impairment caused by leakoff, but also improves 𝑘𝑜eff compared with the baseline for a tight plug. FF (with surfactants) significantly reduces effects of aqueous phase trapping and improves 𝑘𝑜eff during flowback compared with fresh water. In terms of shut-in duration, extending the shut-in time does not significantly affect the 𝑘𝑜eff in high-permeability plugs. However, an increase in 𝑘𝑜eff is observed in the tight plug, and this improvement significantly increases by increasing shut-in time. Overall, the improvement in regained 𝑘𝑜eff is primarily because of the reduction of interfacial tension by the surfactants. In the tight plug, the interfacial tension reduction is attributed to the effective mixing of the invaded FF with initial oil/brine, supported by pore-throat size distribution and measured water saturation data.
- Subjects / Keywords
- Graduation date
- Spring 2021
- Type of Item
- Doctor of Philosophy
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