Feasibility Study on Offshore Polymer
Flooding, Forecasting Production Through Integrated Asset Modelling, A
Technical and Economic Approach
Arash Farhadi, London
South Bank University and Primera Reservoir Ltd, Alejandro Primera, Jesus
Aponte, Primera Reservoir Ltd, Maria Astrid Centeno, London South Bank
University
Introduction
Substantial
percentage of current world oil production derives from mature fields and the
rate of replacement of the produced reserves by new discoveries has been
declining in the previous years. In order to sustain such upsurge in the demand
for economical energy throughout the world, the recoverable oil resources in
known reservoirs can be produced economically by applying EOR techniques.
The following work is a
comprehensive review of offshore polymer flooding through Integrated Asset
Modelling (IAM). Polymer flooding has been one of the emerging EOR techniques
in offshore environement in the recent decades. The pilot implementation of
polymer flooding has proved to be challenging due to the difficulties
associated with the operational facilities and the high Capital Expenditure
(CAPEX) required to initiate the project. Coupling the IAM technique to such
project would provide valuable insight to the current and future field
production levels and expected operating conditions. IAM can add essential
values in areas of field and well optimization, production forecasting,
operational decision making and effect of extending field life on surface and
subsurface facilities.
Objectives
The
objective of this work was to determine the performance and feasibility of
polymer flooding in an offshore environment. Areas under scope were those such
as ideal polymers for such environments, typical range of polymer concentration
and slug size, injection rates, rock integrity and fracture pressure, polymer
adsorption, along with the perspective need for required surface facilities
such as polymer mixing unit, storage tanks, dispersion units, desalinization
unit etc. The surface facilities required for such projects, which require
incremental investment on platform, have proved to be an obstacle faced by the
field operators in the process of polymer flooding implementation.
The
execution of such projects would normally require significant initial
investments which would go towards refurbishing or building the surface
facilities required. Once the facilities are in place and the flooding is
implemented, it would then take an average of 10 to 15 years before any
significant incremental oil is recovered. To this end, it was essential to
determine whether this project would be identified as rewarding considering the
current oil price and the waiting time before any major lucrative incremental
oil recovery.
Finally,
the role that UK government has played in the recent times through the sanctioning
of legislations such as Brown Field Allowance (BFA) and its corresponding
effect on company’s desire to tackle such projects was analysed.
Polymer Flooding
Polymer
flooding is one of the chemical enhanced oil recovery mechanisms which has been
used in the oil and gas industry since the late 1960s. It involves addition of
water soluble polymers to injection water in order to increase injected fluid
viscosity and enhance the oil displacement in the reservoir. The resulting
increase in injected fluid along with the decrease in aqueous phase
permeability would cause a reduction in mobility ratio, therefore increasing
the volumetric sweep efficiency and lowering the swept zone oil saturation (Russell
T, et al. 2014).
A
typical polymer flooding consists of mixing and injecting polymer solution over
an extended period of time until certain amount of reservoir pore volume has
been injected. Subsequently, through long term waterflooding, the polymer slug
and the oil bank will be driven towards the production wells.
Polymer Flooding
Screening
In
order to have an effective polymer flooding, the field under investigation
needs to have the desired characteristics such as moderate reservoir
temperature, permeability etc. Figure below is an illustration of screening
criteria for polymer flooding.
Figure 1: Illustration
of the polymer flooding criteria
Polymer types and selectivity
criteria
There
are numerous types of polymers available in the market, however, choosing the
right type of polymer out of those available is of paramount importance. In
this work, Hydrolyzed polyacrylamide (HPAM) and Xanthan polymers were chosen as
ideal polymers. The reason behind this decision was based on numerous factors
such as their performance as a viscosifier, thermal stability, prices etc.
HPAM
More
than 90% of polymer floodings around the world have been performed by HPAM.
HPAM which is a synthetic type of polymer is relatively cheap compared to other
types of polymers. It has high molecular weight ranging between 2 – 20 million
Dalton. HPAM solution can reach high viscosity levels at low concentration. It
is however known to be sensitive to temperature, salinity and hardness (Russell
T, et al. 2014).
Xanthan
Xanthan
is from the category of biopolymers which has an even higher molecular weight
ranging between 2 – 50 Million Dalton. Compared to HPAM it comes at a higher
price, however, due to its molecular structure, it has higher tolerance to
temperature, hardness and salinity conditions. It is however known to be sensitive
to bacterial degradation (Russell T, et al. 2014).
Table 1: Polymer screening
Surface facilities
on FPSO/Platform
The
minimum required surface facilities in a typical polymer flood comprises of
water treatment and mixing facilities, piping, valves, injection pumps and
metering equipment. In most cases, the mixing process is operated at low
pressure since there is no need of high pressure to be effective. Some of the
aforementioned facilities could be the actual source of mechanical degradation
of polymer. In some cases, special separation equipment is required at the surface
due to the emulsion created by the produced polymer (Russell T, et al. 2014).
Figure
2 is an illustration of the surface facilities such as mixing unit, storage
tanks, desalinization unit etc, on FPSO/Platform.
Figure 2: Surface
facilities required on FPSO/Platform
Challenges
associated with polymer flooding implementation and post-production surface
facilities
The
design, construction and operation of surface facilities for offshore polymer
flooding has always been a challenging task for operators. In addition,
considering the potential emulsion occurrence at production level as a result
of produced polymer demands emulsion treatment facilities, oil/water separation
and water disposal facilities (Binayak K, et al., 2011)
Considering
the large quantity of polymer being injected on daily basis, the logistical
challenges in terms of transportation, handling and storage of polymer
throughout the entire supply chain, from manufactures site to floating
facilities, cannot be ignored.
In
some cases where the existing facilities have been in operation for years the
integrity of the assets become a factor. Some facilities will be approaching
their end of design life. The facilities would need to be upgraded considering
the potential rejuvenation and the respective extension life of the field facilities
(Binayak K, et al., 2011).
Figure 3:
Challenges associated with implementation and post-production facilities
required (Alvarado,V et al., 2013)
Sensitivity
Analysis
Through
experimental design, sensitivity cases were generated in order to envisage the factors
driving the value of the project. The sensitivity parameters were categorized
as technical and economic.
Table 2: Sensitivity
parameters
Results
Graph 1: Summary
of recovery factors obtained for polymer cases
Economic Analysis
Subsequently
polymer and Waterflooding NPV cases were generated and compared
Graph 2: Summary
of NPVs obtained for polymer flooding and waterflooding cases
- From the sensitivity analysis it was determined that the most sensitive parameter was the incremental CAPEX associated with the surface facilities required for polymer flooding. Small swings in CAPEX could make or destroy the value of project.
- IAM is a robust approach to incorporating the physics of facilities and fluids in the wellbore.
- During the simulation runs, it was noted that the polymer concentration, adsorption and slug size have significant effects in the effectiveness and the productivity of the flooding. The lower the polymer adsorption, the more effective sweep of residual oil in the swept zones were noted.
- As the polymer concentration and therefore the slug size were increased, the adsorption levels also increased. However, yet higher incremental oil was recovered with the increasing slug size. This did not indicate a linear relationship between concentration level and the incremental recovery, along with higher the polymer concentration levels, high injection rates are required. In this case the rock integrity and the corresponding fraction pressure needs to be taken into account. A reasonable polymer concentration should be used along with a sufficient rate of injection.
Conclusion
- Considering the current decline in the oil production rates in offshore fields in the North Sea, in order to preserve the present production levels, the polymer EOR infrastructure, facilities and technologies are considered as good areas of investment.
- The move of Polymer EOR to offshore environment to meet world’s growing requirements for crude oil is becoming more appealing to operators, however considering the current volatility oil prices, more in-depth study needs to be carried out around the area.
- Such projects are expensive and require significant incremental CAPEX. Considering the late recovery in incremental oil and the subsequent impact on company bottom line, some operators may be more cautious to invest in such projects at this stage. To this end, companies should consider the ultimate recovery over immediate recovery when evaluating such projects.
- Governments can play significant roles when it comes to such investments on brown fields. Tax incentives for brown fields are appealing methods of promotion for the implementation of offshore polymer flooding. To this end, in September 2012 the UK government introduced a tax relief known as Brown Field Allowance (BFA) for producing fields in the North Sea with the intention of encouraging investments in mature assets therefore delaying the decommissioning. However, the sanctioning of the BFA in the recent budget proposal in the UK parliament could prove to be a step in the wrong direction in terms of encouraging companies to invest in offshore CEOR projects.
- Challenges associated with implementation of such projects in offshore environments are:
- Platform space and weight limitations.
- In case of remote locations it may be difficult to
transport the polymers through pipeline from shore. The high shear degradation
of polymers are the limiting factors.
- Sea water can influence the performance of the polymer
due to its high salinity levels. To avoid this, there would be a need for water
treatment facilities on the platform (Alvarado,V et al., 2013).
- High injection rates may result in an acceleration in production, however, other limiting factors such as the rock integrity in terms of formation pressure need to be taken into account. Also, along with higher injection rates there will be higher degradation rate of polymers.
References
- Alvarado, V., E, Manrique, Engineering Design Challenges and Opportunities beyond Waterflooding in Offshore Reservoirs (OTC-24105). Offshore Technology Conference, Houston, Texas, USA, May 2013
- Larry W. Lake, Russel T. Johns, William R.Rossen, and Gary A.Pope (2014). Fundamentals of Enhanced Oil Recovery. 2nd ed. Texas: Society of Petroleum Engineers. 279-306.
- Kui-Fu Du, Binayak. Agarwal, Sho-Wei Lo, Simon Ritom, Wichaya Mekarapiruk, Gary Adamson and Chon Fui Chai, Sarawak SHELL Berhad; Azmi Bin Din and Norlia Azizan PETRONAS Carigali Sdn. Bhd and Nur Atiqah, Zakaria, Maisarah. Jamaludin, PETRONAS, Evaluating Chemical EOR Potential of St.Joseph Field, Offshore Malaysia, (SPE-144655). Kuala Lampure, Malaysia, July 2011
Strong focus on decision making and uncertainty analysis combined with advanced, state of the art and complex workflows.
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