Mobilization of Waterflood Residual Oil by Miscible Fluids
- L.W. Holm (Union Oil Co. of California)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- July 1986
- Document Type
- Journal Paper
- 354 - 362
- 1986. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 5.4 Enhanced Recovery, 1.2.3 Rock properties, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 5.6.5 Tracers, 6.5.2 Water use, produced water discharge and disposal, 5.4.1 Waterflooding, 5.3.2 Multiphase Flow, 4.1.5 Processing Equipment, 5.3.1 Flow in Porous Media, 4.3.3 Aspaltenes
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A study was conducted to determine how the residual oil left by a waterflood is mobilized by miscible or low IFT fluids. Crude oils were used in the oil displacement experiments with cores and sandpacks. This was in contrast to the work by most previous investigators who used refined oils in their studies.
The residual crude oil was distinguished from injected oils by tagging one or the other with olefins, hydrocarbons which are not present in crude oils but which are easily and quantitatively present in crude oils but which are easily and quantitatively distinguishable by analytical chromatographic techniques.
The results of the study show that crude oil left as residual by a waterflood is more accessible to an injected miscible fluid than a residual refined oil, presumably through crude oil-wetted surfaces of the rock. The results of laboratory core flood displacements of residual or mobile refined oils (decane, soltrol, mineral oil, etc.) are not representative of reservoir formations containing indigenous crude oil. The displacement of residual crude oil by solvent is more efficient than displacement of refined oil by that solvent.
Also, the displacement of residual oil by an oil or solvent at favorable mobility ratio occurs as a bank of oil production at high flowing oil saturation, approaching one minus the immobile water saturation.
Enhanced oil recovery processes using miscible fluids and fluids which greatly reduce interfacial tension (IFT) between oil and water have as their target the oil which is left behind or would be left behind as residual to an immiscible flood such as a waterflood. It was the objective of this study to determine how waterflood residual oil is mobilized by miscible or low IFT fluids (i.e., micellar, microemulsions). For a miscible fluid to mobilize residual oil, it must contact that oil. To determine how the oil is contacted, it is necessary to know how the remaining oil is distributed. Generally, oil is left as a residual because either it is bypassed by the waterflood, or the capillary retention forces in pores of the rock are greater than the displacement forces applied. We can say further that the oil is left in three possible positions in the reservoir rock:
1. In contact with the surfaces of the pore network which we term oil wet rock.
2. As globules in the pores surrounded by water, termed water wet rock.
3. Some combination of 1 and 2. Some portions of the rock are water wet and some are oil wet.
When both water and oil are present in the rock, there is no direct way to discern or measure the wettability. Instead, wetting conditions have been inferred from fluid flow performance through the rock. Investigators also have been aware that, in the case of reservoir rock wettability, conditions may change during cutting and handling of formation samples.
Many papers have been published to describe how fluids flow when oils were used to miscibly displace resident oils from porous rock, particularly when the resident oil was immobile to water particularly when the resident oil was immobile to water injection . Most of these studies used Berea rock and refined oils (octane, decane, soltrol, mineral oil) and were conducted under water wet conditions. In a study in which residual crude oil was present in the core, gasoline and soltrol were the miscible displacement fluids, and both had unfavorable mobility ratios to the displaced crude oil .
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