This work presents a laboratory investigation of miscible ethane foam for gas EOR conformance in low permeability, heterogeneous, harsh environments (<15md, 136,000ppm total dissolved solids with divalent ions, 165°F). The use of ethane as an alternative to CO2 presents several operational and availability strengths which may expand gas EOR applications to depleted or shallower wells. Coupling gas conformance also helps improve displacement efficiencies and maximize overall recovery. Minimum miscibility pressure displacement tests were performed for dead crude oil from the Wolfcamp Spraberry trend area using ethane and carbon dioxide. Aqueous stability, salinity scan, and static foam tests were performed to identify a formulation. Subsequent foam quality and coreflood displacement tests in heterogeneous carbonate outcrop cores were conducted to compare the recovery efficiencies of three processes: a) gravity–unstable, miscible ethane foam; b) gravity–stable, miscible ethane, and; c) gravity– unstable, miscible ethane processes. Slimtube tests comparing ethane to CO2 resulted in a lower MMP value for ethane. We identified a stable surfactant blend capable of Type I microemulsion and persistent foams in the presence of oil. Core floods conducted with gravity-unstable miscible ethane foam, gravity stable miscible ethane, and gravity-unstable miscible ethane recovered 98.4%, 61.9%, and 42.6% OOIP respectively. Our work shows that miscible ethane injection processes result in significant recoveries even under gravity-unstable conditions. The addition of foam further enhances overall recovery at laboratory scale, showing promise for field applications. Unconventional plays present a challenging set of operational conditions which include high temperature, high salinity, low permeability, and fracture networks. Aggressive development of plays and low primary recovery values reveal a potential for enhanced oil recovery methods. Our work demonstrates that miscible ethane foam has the advantage of better conformance control availability that can satisfy these requirements.
Enhanced oil recovery methods have been instrumental in recovering additional oil from reservoirs after primary recovery cycles. Gas injection EOR, in particular, has contributed to the profitable recovery of oil from deep fields with low permeabilities and light to medium oils (Taber et al., 1997). Gas injection processes employ the use of nitrogen, hydrocarbon, or carbon dioxide gases to increase incremental oil recovery; they can be classified as miscible where the important mechanisms of oil displacement are miscibility and interfacial tension (IFT) reduction or immiscible where viscosity reduction and oil swelling play notable roles (Lake et al., 2014). A recent worldwide biennial survey of EOR projects shows carbon dioxide (CO2) and steam EOR as dominant production processes (Moritis, 2010). Miscible CO2 processes in the United States recently eclipsed steam EOR processes at 308,564 b/d compared to steam EOR's 300,762 b/d (Oil & Gas Journal, 2012). Apart from general gas injection issues such as viscous fingering and stability, CO2 flooding has several specific operational drawbacks. Poor selection of metals in production tubing for wells producing from CO2 flooded fields can result in corrosion, delays, and increased capital expenditures due to the presence of carbonic acid in upstream and midstream operations (Kermani and Morshed, 2003). Additionally, the formation of carbonic acid near injectors can cause dissolution and subsequent precipitation of rock minerals and asphaltene precipitation (Marques and Pimentel, 2016). Commercially profitable CO2 EOR projects also require sufficient transport infrastructure as well as vast quantities of naturally available injectant gas (Martin and Taber, 1992).
Asphaltene deposition and plugging of pipelines during oil production and transportation is considered a challenging flow assurance issue. Instead of adding dispersants, the concept proposes to remove asphaltenes from the flow stream by means of electro–deposition prior to transportation to prevent later deposition. This study mainly examined the effect of molecular composition on the efficiency of electro-deposition. Two sources of asphaltene, namely asphaltenes from coal tar ("AS-C") and asphaltenes from bitumen ("AS-B") with different molecular composition were collected in this study. Elemental analysis revealed that both AS-B and AS-C possessed transition metals (V and Ni) and heteroatoms (O, N and S). The effect of oil components on the stability of two asphaltenes was studied. After conducting the electro–deposition of both asphaltenes with various oil components and electric field strength, the deposition charge and recover rate was recorded and compared. During stability test, the amount of precipitated AS-B decreased with increasing aromaticity of solvent, while that of AS-C was constant. For electro–deposition, the electro–kinetic behavior of AS-C reveals strong sensitivity to the oil components. Interestingly, both asphaltenes exhibited a change in the net charge, which occurred under 6000 V/cm and 12000 V/cm for AS-B and AS-C respectively, as evidenced by a change in the electrode upon which deposition ocurred. Based on the results, the efficiency of electro–deposition is confirmed to depend upon the metal and heteroatoms of asphaltenes; in addition, and by interaction with these elements, the oil composition and electric field affected the stability, net charge, and electro–kinetic behavior of apshaltene. However, our study is the first to show that the current density plays a role in the net charge of the asphaltene molecule and offers an explanation to the controversy over the polarity or the charge sign of asphaltenes, which gives a clue to understanding the microstructure of asphaltenes. In addition, this is the first study to include the effect of oil components and electric field strength on the performance of deposition, which makes further optimization of the proposed process possible.
Kostarelos, Konstantinos (University of Houston) | Martin, Clint (University of Houston) | Tran, Kyo (University of Houston) | Moreno, Jose (University of Houston) | Hubik, Aaron (University of Houston) | Ayatolli, Shahab (Sharif University of Technology)
Asphaltenes represent the heaviest fraction of crude oil, which are known to precipitate when the crude is added to aliphatic solvents such as n-pentane or n-heptane and yet remain soluble in light aromatic solvents such as benzene or toluene (Gawrys et al. 2006; Borton et al. 2010). They are characterized by highly complex structures that contain multiple aromatic rings and have a large hetero-atom content (e.g., nitrogen, oxygen, and sulfur) and metal content (e.g., vanadium and nickel) (Yarranton 2000; Hashmi and Firoozabadi 2012).
Asphaltenes tend to self-associate on a molecular level, depending on the composition, temperature, and pressure of the system. Precipitation of the particles out of solution results in flocculation, where they begin to deposit on hydrophobic surfaces such as metal pipes and surface equipment used for the production and transportation of crude oils (Khvostichenko and Andersen 2009). These tendencies result in reduced flow or complete blockage of producing wells and surface equipment, including pumps, pipelines, and separators.
Currently, the only methods of treatment are through the use of chemical dispersants and inhibitors, which increase the stability of asphaltenes to prevent deposition. Once asphaltene deposition has occurred, running a “pig” through the pipeline is often the method used to scrape the solids that accumulated on the walls of the pipe. It is known that asphaltene molecules can be polarized, gaining an electric charge by introducing an electrostatic field (Hashmi and Firoozabadi 2012; Khvostichenko and Andersen 2009; Khvostichenko and Andersen 2010; Hosseini et al. 2016). The polarity of the asphaltene particles is directly related to its hetero-atom content, with a higher hetero-atom content giving increased levels of polarity and a higher rate of aggregation (Hosseini et al. 2016).
Experimental DeviceOur ultimate goal is to build a device (Fig. 1a) that would remove asphaltenes from crude oil near the point of production, using electrokinetics. Thus, a scaled-down device (Fig. 1b) was fabricated and tested using a model oil to prove the concept and study some of the parameters that would influence the design of a larger-scale device.