Study of Tertiary Oil Recovery by Surfactant/ Polymer Flooding

Surfactants/Polymers (SP) has been in use since the 1950’s. Technological advancements during the 1980’s by major oil and services companies have resulted in a thoroughly tested and reliable technology. SP has been used worldwide and has been successful in recovering additional oil of about 15 to 30% of OOIP.

TOR program utilizing SPF
Depending on the characteristic of the formation being produced and the economics, the chemical components of the SP process are mixed with traditional water. Surfactants clean the oil off the rock and polymer spreads the flow through more of the rock.
SP flooding has been used extensively worldwide and has been successful in recovering and additional oil recovery from 15 to 25% of 00IP. Also field projects indicate that SPF can recover up to an additional 28% of reservoir oil. Field data proves SPF is an effective way to recover residual oil.

Oil Recovery Principles
There are three levels of oil recovery; primary recovery, where reservoir energy is used to produce oil and gas. The average recovery of this stage is about 15 to 20% of OOIP. Secondary recovery, where energy is given to reservoir by injection of water or gas recovers an additional 20 to 25% OOIP. Tertiary oil recovery, which historically follows secondary recovery, recovers an additional 15 to 20 % OOIP over secondary.
However, there are several factors that affect SPF. Such factors include reservoir depth, salinity, crude oil, reservoir temperature, ion-caution exchange, lithology and geology, and chemistry of formation water.
There are also factors that influence the actual SPF process. Such factors include mobility control design, surfactant concentration, residual permeability reduction, surfactant retention, dispersion of the surfactant slug, and rheological behavior of surfactant solution in porous medium.

Chemical oil recovery methods
To increase ultimate oil production beyond that achievable with primary and secondary methods, there are a few steps to undertake. First, an improvement of the sweep efficiency must ensue. This is then followed by a reduction of the amount of residual oil in the swept zone. Thirdly, there must be an increase in the displacement efficiency. And finally, there must be a reduction in the viscosity of thick oils.

Tertiary Oil Recovery (TOR)
TOR methods consist of three major categories:
1) Thermal Processes, which include steam flooding, steam stimulation and in-situ combustion
2) Chemical Processes, which include surfactant polymer injection, polymer flooding, and caustic flooding
3) Miscible Displacent Processes, which include miscible hydro ban displacent and carbon dioxide injection.

Why SPF?
The natural question that comes to mind is why exactly one should invest in SPF. The answer is quite simple. First, it is a low risk technology. Second, it focuses on mature fields with substantial existing infrastructure and finally, it increases production and cash flow. For the proper implementation of SPF, an appropriate project team must be composed. This team should consist of geologists, reservoir engineers, production engineers, marine specialists and economists.

Field Testing
There are two main types of pilot tests. The first tests the technical feasibility of the process, while the second is meant to assure optimization of operations and to prove their profitability. There are also laboratory experiments designed to determine microscopic sweep characteristics, residual oil saturation, adsorption, dispersion, degradation of injection products and reaction with formation water.

Modeling SP flooding
A compositional simulator is one that is based upon the mass conservation of each component. This leads to a set of partial differential equations. In addition to the material balance equations, there is also Darcy’ low and a large number of equations describing physical properties such as: phase behavior, interfacial tension, relative permeability, capillary pressure, and adsorption / dispassion.
The SPF project timeline is divided into several phases. The pilot program takes approximately 2-4 months; facilities bear another 5-8 months; initial response is 6-8 months; peak production is reached within 3 years; production plateau at 4 years and production declines at 10-30 years.
At times, however, the SPF process fails. There are several reasons for that, which include but not limited to low oil prices, inadequate understanding of process mechanisms, unavailability of chemicals in large quantities, and an insufficient description of reservoir (i.e., permeability heterogeneities, excessive clay content, high water saturation, bottom water or gas cap, and/or fractures).
There are also factors that affect the SP flooding process. Such factors include Mobility control design, Surfactant concentration, Residual permeability reduction, Surfactant retention, Dispersion of the surfactant slug, and Rheological behavior of Surfactant solution in porous medium.

The SP flooding process design consists of three aspects:
1) Mobility Control, where such control should be sufficiently superior in order for high sweep efficiency to occur in the reservoir
2) Reservoir, where characterization must take into account the reservoir characteristics
3) Core Floods, which should be conducted in reservoir cores in order to evaluate the oil recovery performance and surfactant retention

The SP flooding process sequence begins with a preflush, which is done in order to lower formation salinity. This is then followed by surfactant slug, which raises the slug volume from 5 to 20% pv. Then, the mobility buffer, which drives the slug and banked up fluids to the production wells. Finally, the chase water, which is essentially the fluid injected to reduce the cost of the continuous injection of polymer.

The typical range of surfactant slug compositions is as follows:

  1. Aqueous slug—0.5 to 10% surfactant, 0.05 to 0.2 % polymer, 0 to 10% co surfactant and brine is balance.
  2. Micro Emulsion—1to20% surfactant, 10 to 90% oil, 1 to 20% co surfactant, and brine is balance.

Fundamental requirements for a successful SPF Project
The essential requirements for a successful SPF project are that the surfactant must mobilize the residual oil and reduce its saturation essentially to zero by lowering the interfacial tension. Also, the retention of the surfactant must be sufficiently low so that it can propagate across the entire oil reservoir using only a small amount of injected surfactant.
A successful SP flood project must achieve three things for effective oil recovery. First, the SP slug should propagate at optimal conditions. Second, surfactant concentration should be big enough so that some of it is not retained by permeable surfaces. Third, and last, the active surfactant should sweep a large portion on the reservoir without being excessive due to dispersion.
In short, the SPF method increases the capillary number, where polymer increases viscosity of display fluid (water); heat reduces viscosity of displaced oil; and surfactant added to water decreases interfacial tension. The objectives of SPF are to increase the capillary number in order to mobilize residual oil while decreasing the mobility ratio for better sweep; in other words, the emulsification of oil to facilitate production.

A numerical calculation of SPF recovery from water to flushed zone can be seen as follows:

Increment ultimate oil recovery =
[OOIP x Boi]     (Sorw – Sore )  E/Bo
1 – Swi
OOIP  = Original oil in place (Stock tank barrels).
Boi      = Original oil formation volume factor reservoir barrels per stock tank barrel
Bo       = Current oil formation volume factor reservoir barrels per stock tank barrel
Swi      = Original water saturation
Sorw    = Residual oil saturation by water flood in swept zone
Sore     = Residual oil saturation by surfactant / polymer flow in swept zone
E          = Water flood and chemical flood volumetric sweep efficiency
The above equation provides an estimate of the incremental volume of oil that might by recovered by surfactant flooding over that which could ultimately by achieved by conventional water flooding.       

The Future for Egypt
At the moment, there is much interest in improved oil recovery in Egypt due to the fact that the country suffers from many abandoned oil wells and declining production from mature oil fields. In general, it is quite advantageous to recover oil with new techniques at low recovering cost.
In conclusion, in field practice, the most successful surfactant/polymer flood has been conducted in low temperature, low salinity sandstone reservoir, having a moderate to high permeability and containing relatively low viscosity crude oils.
Surfactant and polymer flooding is ideal for shallow fields with small wells. High temperature causes surfactant break down. The presence of magnesium and calcium ions in reservoir fluids can cause surfactant molecules to separate and settle out of solution.
In the next ten years, oil prices are expected to increase steadily. There is no doubt that the petroleum industry will take advantage of this favorable long-range economic situation to speed up the development of TOR processes.
Each of the TOR processes has a specific range of application depending on field characteristics, local consideration, market condition, and economic incentive.

By: Dr. Atef Abdelhady, Production General Manager
Egyptian Natural Gas Holding Company, Egas, Egypt


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