Waterflood Performance Under Bottomwater Conditions: Experimental Approach
- Ezeddin Shirif (U. of Regina) | Khaled ElKaddifi (U. of Regina) | Jonathan J. Hromek (U. of Regina)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2003
- Document Type
- Journal Paper
- 28 - 38
- 2003. Society of Petroleum Engineers
- 2.2.2 Perforating, 5.3.2 Multiphase Flow, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.4.1 Waterflooding, 1.6.9 Coring, Fishing, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.7.2 Recovery Factors, 2.4.3 Sand/Solids Control, 5.4.10 Microbial Methods
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In many light or moderately viscous oil reservoirs in Saskatchewan and Alberta, Canada, a high water saturation zone of varying thickness and extent ("bottomwater zone") occurs in communication with the oil zone above. As a result, the primary production period is short, and water breakthrough occurs very early in the life of the reservoir. Later, during the secondary recovery stage, such a zone can have an adverse effect on the waterflood efficiency. This paper addresses the problem of waterflooding such reservoirs.
This study was directed toward reducing water mobility in the bottomwater zone for more efficient oil displacement. Polymer in various concentrations was used as a blocking agent in the bottomwater zone and as a mobility-control agent in the oil zone. Different strategies were investigated to reduce the water mobility in the bottomwater zone and improve the vertical sweep efficiency. The variables examined were relative water-to-oil layer thickness, oil viscosity, polymer concentration, injection rate and injection point, and the effect of vertical and horizontal injection and production well combinations.
The results showed that oil recovery could be increased by minimizing crossflow between layers by blocking the bottomwater zone. It also was found that for an unfavorable mobility ratio, as the injection rate increases, the ultimate oil recovery increases. The injection of a polymer solution had a favorable impact on waterflood performance. Moreover, the worse the conventional waterflood performance was, the more effective the polymer was as a mobility and blocking control agent. The use of horizontal wells showed slightly better oil recovery over vertical wells in a conventional waterflood of reservoirs under bottomwater conditions. In addition, certain well combinations (horizontal production and vertical injection) gave better oil recovery because of the increase in the swept area.
The efficient and economic recovery of oil from reservoirs under bottomwater conditions is recognized as a formidable task. High water cuts and rapidly decreasing oil rates early in the production life of such reservoirs have, in many instances, prompted their suspension or abandonment at very low levels of oil recovery. Reservoir characteristics and rock and fluid properties combine to yield the single most important parameter (mobility ratio) in a waterflood. A number of chemicals such as polymers, emulsions, biopolymers, foam, and carbon dioxide-activated silica gel have been used to control the mobility ratio.
One of the oldest techniques for controlling the mobility of water in waterflooding is the use of polymers. This control agent was shown to be effective in the early 1960s by Pye.1 He performed numerous field and laboratory studies of polymer flooding with polyacrylamide solutions. It was observed experimentally that the viscosity of the water-soluble polymer solutions measured in the formation sample departed markedly from that obtained using a viscometer. He quantified the unusual departure of the measured values from the expected response as the resistance factor. It was assumed that the permeability was constant.
Pye pointed out that at constant flow, the injection pressure rose; this effect was not a core plugging problem because the system reached equilibrium after some time. It also was observed that the extent of departure from the measured viscosity value was most pronounced at low concentrations, but at higher concentrations, the effect was approximately proportional to the solution viscosity. It was suggested that this unusual behavior was a property of selected water-soluble polymers, among which were the extensive family of acrylamide polymers and copolymers. It was recommended that rapid laboratory flood rates should be avoided to keep the resistance factor constant.
The problem in recovering oil under bottomwater conditions was first recognized in the early 1960s, when Barnes2 suggested the use of a viscous water slug to improve waterflood efficiency in a reservoir partially invaded by bottomwater. The viscous water in Barnes' study referred to water thickened by a chemical additive such that the viscosity of water was greater than 1.0 cp. He argued that injecting a viscous water slug in bottomwater reservoirs would reduce the flood life, reduce lifting costs, and increase ultimate recovery. He also pointed out that the larger the size and the higher the viscosity of the viscous slug injected into such a system, the greater the crossflow of oil ahead of the displacing front, thus leading to a higher oil rate during displacement. His visual model studies showed that crossflow was most severe immediately ahead of the front and diminished to zero at the producing well. However, such a crossflow phenomenon was not described quantitatively in the study.
Barnes' study was directed toward increasing the viscosity of water. Other chemical slugs that could reduce relative permeability to water were not considered. Unlike the viscous water used by Barnes,2 polymers have the ability to lower the mobility by reducing the relative permeability to water and increasing its viscosity1 so that the mobility ratio is improved.
Islam and Farouq Ali3,4 carried out an intensive experimental study on the use of various chemical slugs in waterfloods conducted under bottomwater conditions. The chemical slugs included emulsion, air, foam, and carbon dioxide-activated silica gel. The study showed that emulsion performed better (i.e., high oil recovery and low water/oil ratio) than the other chemicals used.
Yeung and Farouq Ali5,6 introduced three different displacement techniques - the Emulsion Slug Process (ESP), the Alternating Water Emulsion Process (AWE), and the Dynamic Blocking Process (DBP) - to improve the vertical sweep efficiency while waterflooding bottomwater formations. For emulsion with low surfactant concentration, the DBP and AWE processes were found to give higher oil recoveries than the ESP process under bottomwater conditions. For emulsion with higher surfactant concentrations, the reverse was found to be true. According to Yeung and Farouq Ali, the crossflow was very prominent ahead of the flood front for high-viscosity fluids. It was concluded that a high surfactant concentration did not necessarily give a higher oil recovery for both homogeneous and bottomwater reservoirs.
In view of the foregoing, when a stratified reservoir is being studied for waterflooding, failure to account for crossflow can lead to large errors in oil-recovery prediction; however, under bottomwater conditions, this effect is aggravated because of the presence of the mobile water phase.
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