Summary
The Foam Assisted Water Alternating Gas (FAWAG) project has been a full-scale field demonstration of foam for gas mobility control. It was carried out in the Snorre field on the Norwegian Continental Shelf from 1997 to 2000, with support from the European Commission's Thermie Program.
A production well treatment to reduce the producing gas/oil ratio (GOR) was performed in 1996. The FAWAG was initiated in the Central Fault Block (CFB) of the Snorre field in August 1998. A commercial surfactant system, AOS (alpha-olefin-sulphonate), with a carbon chain length mix of C14/C16, was chosen as the foaming agent. Approximately 2000 tons of commercial grade AOS surfactant have been injected.
Foam for mobility control in the CFB operation had to be aborted because of operative problems in the target injector P-25A. The main operational conclusion from the CFB operations was that surfactant alternating gas (SAG) injection is preferable to coinjection. Operationally, SAG injection is almost identical to water alternating gas injection (WAG), which is a well-known production method.
The concluding demonstration was performed on the Western Fault Block (WFB) in well pair P32-P39. The target injector and producer wells are approximately 1500 m apart. A total of 380 tons of commercial grade surfactant was used. The surfactant was divided into two slugs, each followed by gas injection that lasted until original gas injectivity was restored.
The production from WFB has shown that large volumes of gas have been stored, either temporarily or permanently, in the reservoir. It has been estimated that the FAWAG treatment has contributed approximately 250 000 Sm3 of oil. The cost of the treatment in WFB was approximately U.S. $1 million.
Introduction
Foam is a method to improve sweep efficiency during gas injection, and several field applications of foam have been reported.1–6 In the North Sea, foam application before FAWAG has mainly involved production well treatments.4–6 In 1996, a foam treatment was performed on production well P-18, located in the CFB of the Snorre field.6 Foam was used to reduce the producing GOR.
The FAWAG project commenced in 1997 on the CFB of the Snorre field. Snorre is one of the major oil fields on the Norwegian Continental Shelf in the North Sea, located about 150 km off the coast. The reservoir is a massive fluvial deposit within rotated fault blocks. The field was originally developed with water injection as the main drive mechanism and came on stream in 1992. One of the first measures taken to increase production was implementation of a downdip WAG pilot in the CFB. This was later expanded to cover the three main fault blocks in the field.
The demonstration of FAWAG was carried out in the CFB and WFB, as described in Fig. 1. The main target for the FAWAG was the Upper Statfjord reservoir zones S1 and S2. Upper Statfjord is a sandstone reservoir with upward coarsening sequences. The permeability is in the range of 400 to 3,500 md, and the blocks are dipping 5 to 9° toward the southwest. The injection is downdip. In the CFB, there is vertical communication between S1 and S2; this seems not to be the case in the WFB target area. In WFB, the injection is below the original water/oil contact.
The Snorre oil is originally undersaturated by 260 bar. The injection gas used is identical to the export gas and is rich in intermediate components. Laboratory studies conclude that the gas is miscible with reservoir oil at pressures above 282 bar. The gas and water are injected downdip in all but one fault block in order to use existing water injectors and producers. In areas with direct communication from injector to producer, breakthrough times of gas on the order of 1 month were observed for well distances in excess of 1 km.
In the Upper Statfjord sands, the gas will rapidly segregate and move updip. The gas will mix with the oil when the phases are in contact, but the amount of oil contacted is limited in later cycles. Local attics in the reservoir will be well swept by gas, and structural attics behind producers will form secondary gas caps.
The principles behind FAWAG are illustrated in Fig. 2. The high mobility of the gas may result in early breakthrough of gas in the producers. On the Snorre field, it is believed that gas either moves on top of the reservoir zone or through other high-permeable zones. By generating foam in the reservoir, it is anticipated both that the gas sweep efficiency is improved and that the oil production is increased.
To create the foam, a suitable foaming agent (surfactant) must be used. The surfactant can be applied in different ways. Both injection in a SAG mode and coinjection of aqueous surfactant solution and gas have been investigated in the FAWAG project.
As part of the qualification plan for foam, it was decided to carry out two foam pilots: one producer treatment for gas shutoff, and one in-depth treatment to control gas mobility. The mobility-control operation had to be aborted because of operative problems in the target injector. Consequently, the operation was moved to the WFB of Snorre in 1999 for the concluding demonstration.
This paper will give a summary of all foam applications on Snorre. Experiences from the foam tests involve logistics, evaluation of the gas-blocking effect, injection design, and gas storage.
P-18 Gas Shutoff Treatment
The producer treatment was carried out in well P-18 in July 1996,6 where a total of 32 tons of commercial-grade surfactant was used. P-18 was suffering from high GOR caused by premature gas breakthrough from WAG injection. The objectives of the test were to reduce the GOR in P-18 and to bring P-18 in production. The foam was placed in the target reservoir zone, which was isolated in the well during injection by a packer. It became apparent from downhole pressure measurements that a strong foam had been generated in the formation.
The treatment resulted in a GOR reduction of more than 50% over a period of 2 months, resulting in a significantly increased oil production from lower reservoir zones. It is expected that a more gentle opening of the well after treatment could have increased the effective treatment period. Other North Sea foam treatments to reduce the producing GOR are discussed in Refs. 4 and 5.
