Evaluation of IOR by CO2 Injection in Gullfaks

Hafsteinn Agustsson ( haag@statoil.com ), Reservoir Engineering Specialist with Statoil in Bergen and George Grinestaff, Senior Reservoir Engineering Advisor and Vice President with PetroTel Inc, Anchorage ( ghg@gci.net ) have undertaken a detailed evaluation of the potential to redevelop Gullfaks using CO2 injection in miscible WAG mode. An earlier presentation on aspects of this work is available at (http://www.mi.uib.no/CO2/Presentations/agustsson.pdf). They are also presenting the detailed results at the SPE/DOE 14th Symposium on IOR at Tulsa during April 2004 (SPE 89338). Below is an overview of the findings from the study.

Geology

Gullfaks is located 180 km NE of Bergen. Production is from sandstones in the Brent Group and the underlying Statfjord, Cook and Lunde formations. The Brent Group represents the major part of the producing formations in Gullfaks. It consists of Jurassic sands, with shallow marine to fluvial deposits at around 1800 m subsea.

Figure 1: Gullfaks Reservoir Structure (Click for larger view)

Reservoir quality is generally high to very high, with porosity around 30-35%, and HCPV weighted average horizontal permeability of the order of 800 mD. The reservoir pressure and temperature are around 310 bar and 74 °C respectively, at datum (1850 m TVD SS). The oil gravity is between 32-36°API and the Brent GOR is around 100 Sm3/Sm3. The reservoir is extensively layered and severely faulted (Figure 1), with numerous intercommunicating compartments. Apart from the lateral segmentation, with the major N-S faults mostly sealing, the Brent may be roughly divided into three main hydraulic units vertically, referred to as Upper, Middle and Lower Brent, nevertheless with a degree of vertical communication.

These units also communicate when juxtaposed across the many faults in the field. These aspects pose a challenge to both modelling and managing the reservoir.

Development

The field began production in 1986, and has (as of July 2003) produced roughly 300 million Sm3 of the currently estimated 342 million Sm3 recoverable reserves. The current production strategy is through pressure maintenance at above saturation pressure (230 bar in the Brent) by water injection, augmented by gas injection and a modest natural water influx. The plans envisage continued production to 2020, with an overall recovery factor of 58%.

Production is to three large gravity-base concrete platforms, each with processing and water injection capabilities, and two with gas injection facilities. Gas export is by pipeline to shore and the stabilised oil is stored in tanks and exported by tanker. Third party processing of fluids from the nearby Tordis and Vigdis fields, as well as the recent development of the subsea Gullfaks Satellites, boosts production, but still leaves spare process capacity. Primary drilling on Gullfaks is now complete. Only seven out of a total of 136, drilling slots are currently vacant, and 120 platform wells are in operation.

IOR and Gullfaks

Gullfaks has been the focus of IOR activity from the outset. Even with a projected recovery factor of 58%, a significant amount of oil remains as a prize. Associated gas has been reinjected from the early 1990s and this has recovered a significant amount of attic oil. Surfactants and polymers were also studied but deemed uneconomic. Well-by-well conformance control, both mechanical and chemical, sand control, hydraulic fracturing, and smart wells have achieved additional success. Infill drilling, targeting remaining oil mapped through 4D seismic has made a large contribution. Inexpensive through-tubing drilling is expected to account for a significant portion of future IOR.

In order to lift the recovery even higher miscible gas injection on a field scale has been investigated. Enriched hydrocarbon gas would be too expensive so the focus turned to CO2 injection. Experiments showed the minimum miscibility pressure of CO2 in the Brent to be below the bubble point pressure of 230 bar. The current average reservoir pressure is close to the initial pressure of 300 bar so miscible conditions prevail.

Well Arrangement

Gullfaks has been developed with water injected into key "super injectors" in the aquifer below the original OWC. The current well network is therefore primarily designed for long distance volume replacement with water through the aquifer. As time has passed some injectors have been placed closer to producers, in the flushed zone. However, this strategy is unsuitable for miscible WAG. The long distance between the peripheral injectors and oil zone producers would result in late solvent (CO2) breakthrough. A shorter distance is desirable in order to achieve a better vertical sweep and more rapid reuse of the expensive solvent. Control over the slug size applied to each flood pattern becomes impossible with the long throughput time and uncertain flow paths in the absence of solvent breakthrough. Also, significant volumes of solvent would be lost to the aquifer. Rapid circulation of solvent through the IOR target accelerates the recovery. The well arrangement needs to be such that a pattern can receive concentrated flood treatment before injection is moved on to the next. The number of patterns can be expanded as more solvent becomes available from back production.

To achieve timely and efficient sweep of the IOR target, many watered-out producers are reassigned to miscible WAG injection. In all, 33 wells are assigned to miscible WAG injection from 2008. Most of the wells will require a degree of workover such as plugging, zone isolation, re- and additional perforating and tubing replacements. Of the 33 wells, 14 are currently injectors (including one gas injector) and the rest are producers, including 5 future well targets that will be turned to miscible WAG injection, either immediately or in due course. The operations required to prepare each well for injection are routine on Gullfaks, and the associated time, equipment and costs well known. It is assumed that all the well work can be completed in 2007 and 2008.

Segregated Processing and Direct Reinjection

A redevelopment strategy which is relatively simple in terms of infrastructure upgrades and modifications is termed "Segregated Processing and Direct Injection". Gullfaks is a processing centre for third party production from nearby fields. Many of these rely on gas sales as a main source of revenue. If contamination of the sales gas by CO2 is to be avoided, this can be achieved by dedicating one of the two processing trains to the production stream from the miscible WAG areas on the Gullfaks A and C platforms, whilst the other trains are reserved for non-CO2 contaminated production. Additionally, to simplify the facilities, the entire gas production stream of CO2 and hydrocarbon gas from the WAG trains is mixed with the imported CO2 and reinjected. Calculations show that the inclusion of the hydrocarbons in the injectant has only minor negative consequences on the displacement process.

CO2 Miscible WAG Injection and Production Profiles

The field-wide performance of a CO2 miscible WAG project has been assessed using a synthesis of 1D, 2D and 3D full-field and sector simulation models. These involve both conventional black oil and compositional finite-difference models ( Eclipse ) and more novel streamline-tracer simulations ( Frontsim ).

The evaluations assume a CO2 supply of approximately 5 million tonnes per year over a 10-year period, an amount that can be accommodated within the platform capacities, with manageable upgrades where necessary.

Figure 2 shows the oil production profiles for three separate cases:

1. Conventional water and gas injection (current strategy) in blue.
2. Water flood only using the 33 well injection pattern from 2008 in black, and
3. CO2 miscible WAG in red.

Figure 2: Oil Production Profiles (Click for larger view)

The conventional profile is run to 2020 only, as the economic limit for the field life with the current strategy is around 2018. The other two profiles are run to 2030 in the expectation that the economic life of the field under these scenarios can be extended well beyond 2020. In fact, with miscible WAG, production is maintained above the limiting rate until around 2029.

Figure 3: Solvent Injection and Production Profiles (Click for larger view)

Figure 3 shows the solvent injection (red instantaneous, green cumulative) and back production (black instantaneous, blue cumulative) profiles for the CO2 miscible WAG case. Tabular results for this case are presented below:

Around half of the IOR benefit is achieved through water injection alone, using the well pattern that has been developed for miscible CO2 flooding. This may represent a significant opportunity for Gullfaks in its own right, but detracts from the potential for a miscible CO2 flood.

Figure 4: Oil Saturation in 2008 (Click for larger view)

Figure 4 shows the oil saturation at top reservoir in 2008 and Figure 5 the solvent (CO2 plus recycled gas) saturation in 2029, indicating the high degree of reservoir coverage.

Figure 5: Solvent Saturation in 2029 (Click for larger view)

CO2 Supply

Commercial injection using external CO2 sources is not currently taking place anywhere in the North Sea . There are no significant natural sources of CO2 in the vicinity, nor any infrastructure for removal or transport of CO2. An entire infrastructure from capture, to transport and injection has therefore to be built from scratch to realise a commercial IOR project.

One scenario considered for Gullfaks is to import CO2 from Danish coal fired power plants. The CO2 could be supplied by a direct pipeline, or by ship to a CO2 hub located in western Norway and then by pipeline to the field. The latter option has greater flexibility with regard to adding other sources, but is more complex logistically.

Upgrades of Field Facilities

A CO2 miscible WAG scheme in Gullfaks would require the facilities to be upgraded contributing significantly to the total project Capex:

• New wells and well conversions
• De-bottlenecking of production systems
• CO2 injection systems and manifolds on all three platforms
• Additional intra-field pipelines, and
• Material upgrades

Gullfaks installations were extensively upgraded during the 1990s to 13% Cr specifications in anticipation of increasing H2S production. Also, all the producers are completed with tubing of the same material. This makes Gullfaks, amongst other fields of similar age in the North Sea , uniquely competent to handle corrosive well and process streams. Nevertheless, some material upgrades would be necessary. Although not trivial, they are considered manageable.

Economics

Gullfaks has technical potential for CO2 IOR but current economic conditions make the project non-commercial.

The procurement of CO2 represents the greatest single cost element. Even if reductions in the other main elements, Capex and Opex, could be achieved, these are likely to be modest. Therefore, a radical reduction in the cost of CO2 is required for commercial viability. There is considerable uncertainty in the potential income associated with CO2 quota trading and its value to possible future sequestration in Gullfaks once commercial oil production has ceased. CO2 tax credits and delayed field abandonment costs are only modest income elements.

To make the project economically attractive, efforts could be made to form alliances to share the cost of the necessary infrastructure to supply a consortium of several fields (e.g. CENS ), also national fiscal systems need to adapt to the late life production situation to provide incentives to introduce IOR on this scale.

Although a miscible CO2 flood is not at present economic, a significant opportunity to enhance the recovery by water flooding through a radical redesign of the injection pattern has possibly been identified.

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