Turning a North Sea oil giant into a gas field - Depressurization of the Statfjord Field

Abstract

Abstract The Statfjord Field has produced about 635 million Sm[3] (4 billion bbl) of oil and exported 68 billion Sm[3] of gas. This equals to an oil recovery factor of 65% and a gas recovery factor of 48% of volumes initially in place. Currently, the field is producing at an oil rate of approximately 20 000 Sm[3]/d, which is about 17% of the plateau production rate. The predominant drainage strategy has been pressure maintenance by water and gas injection. However plans toextend production life for the Statfjord Field even longer will require changing drainage strategy from pressure maintenance to depressurization. Estimates show that implementation of the new drainage strategy will lead to an increased ultimate gas recovery from 53% to 74% and an oil recovery factor of 68%. Lifetime for the Statfjord Field will be extended by approximately 10 years. A change in focus from oil production to gas production has consequences both subsurface and topside. Key elements for implementing the depressurization of the Statfjord Field reservoirs are drilling and recompletion of approximately 80 wells which will be equipped with artificial lift and sand control, as well as topside modifications on the three existing platforms. This results in a high offshore activity level for several years on a field in production. A new gas export pipeline to the UK FLAGS system (Far north Liquid And Gas System) is necessary to provide sufficient offtake of the produced gas. Implementation of a new drainage strategy on the Statfjord Field has also regional effects. Prolonged life time of the Statfjord Field installations leads to increased recovery for the Statfjord Satellites and opens for business opportunities in a longer term. Introduction The Statfjord Field is the largest producing oil field in Europe in terms of recoverable reserves. It is located in the Tampen Area of the North Sea, 200 km northwest of Bergen, Norway, straddling the border between the Norwegian and the UK sector, see Figure 1. The field is approximately 27 km long and 4 km wide with a STOIIP of approximately 1 billion Sm3 and an estimated ultimate recovery factor for oil of 68%. It is developed with three concrete platforms and each platform is a combined drilling and production unit. In addition, the Statfjord satellite fields (Statfjord Øst, Statfjord Nord and Sygna) and the Snorre Field are connected to the Statfjord Field facilities. The Statfjord Field is located on a late Jurassic rotated fault block. The Statfjord Field's two main reservoir sand stone units, members in the Brent Group and Statfjord Formation, are divided by the Dunlin Group which mainly consists of shale. The Brent Group is divided in Upper and Lower Brent. The Main Field which contains 85% of the STOIIP consists of a rotated fault block with the Brent Group and Statfjord Formation reservoirs. It has a dip of approximately 6–7 degrees towards west-northwest. The East Flank consists of slump fault blocks generated by gravitational failure at the crest of the field. The East Flank is structurally and stratigraphically complex. It is heavily faulted with internal faults and small scale structures making reservoir mapping challenging. The communication from the Main Field to the East Flank is generally good, with some restrictions as one moves to the east of the field.