In September 1999, Statoil completed Well A-13 in the Åre formation of the Heidrun field with a 130-m openhole gravel pack. After only a few days of production, a high gas-to-oil ratio (GOR) required the operator to shut the well in. Free gas was believed to be flowing from a gas-filled sand located immediately above the 9 5 / 8 -in. casing shoe. Production analysis indicated a high-perme-ability flow channel between this layer and the upper zone of the gravel pack. The operator and a major service company jointly evaluated different chemical gas shut-off systems and several placement methods. A new generation of organically crosslinked polymer gel technology was selected. In addition, the operator desired that the gel be injected with open-ended CT and without mechanical packers to reduce operational risk. Therefore, a chemi-cal diversion system was required for protecting the lower zone.
In February 2000, a temporary blocking agent consisting of a crosslinked hydroxypropyl guar polymer was placed in the lower part of the gravel pack. Then, the permanent sealant was injected into the gas-producing zone. After 72 hours, CT was used to wash the well. Finally, the well was lifted with production gas. The entire operation was completed in a single run with CT.
Currently, the well is producing oil at a rate of 350 Sm 3/d and no significant free gas. Production paid for the cost of the sealant treatment within 1 month. Additionally, this treatment prevented the operator from having to recomplete and sidetrack the well. This paper describes the well's gas-flow mechanism, the polymer gel systems used for sealing the gas-flow zone, the placement technique, and the operational aspects of the selective gas shut-off treatment.
After completion, estimated oil production from this well was 2900 Sm3/d with a GOR of 70 Sm3/Sm3. During lifting, the well produced 2000 Sm 3/day with a GOR of 700 Sm3/Sm3. The operator had to shut the well in to prevent gas-treatment capacity limitations from lowering total production from the field. Additionally, if the GOR could not be reduced, the well would have to be sidetracked.
The operator needed to develop a better understanding of the source and mechanism of the unexpected gas production and develop a method for selectively shutting off the high GOR zone without eliminating the option of sidetracking the well. Operators are often faced with the dilemma between successfully shutting off unwanted water or gas and the operational risk related with such a treatment. A mechanical barrier is often used in such situations, but mechanical barriers increase operational risks and can be bypassed in the reservoir matrix.
Several solutions and placement methods were considered for Well A-13. Hopefully, the engineering process described in this paper will serve as an example for operators working on other North Sea wells with unwanted water or gas production. The treatment performed on Well A-13 demonstrates that unwanted gas can be successfully shut off, and it provides an example of a low-risk, reliable conformance solution. Other fields in the Norwegian sector have been treated with silicate systems. 1–2 The system used here has high temperature resistance and gel strength.
An increasing number of wells on the Norwegian shelf are producing greater volumes of water.3–4 Therefore, the successful application of the conformance technology described here is an important milestone in the ongoing quest for cost-effective and environmentally sound oil production.
Conoco discovered the Heidrun Field in 1985 and completed the field's exploration and development phases. In 1995, after the field's first oil production, Statoil assumed production responsibility.