Abstract

During the turn of the year to 2003 the sidetrack A-19 A was drilled from the motherwell A-19 as a Through Tubing Rotary Drilling (TTRD) well, on the Veslefrikk Field in the Norwegian sector of the North Sea.

A whipstock with holes to produce the motherwell through was set in the 5 in. liner. 4 1/8 in. polycrystalline diamond compact (PDC) bit and 2 7/8 in. high torque motor with adjustable kick off (AKO) setting of 3.8° were run on 2 7/8 in. drill pipe through 5 1/2 in. tubing.

The sidetrack started with 25° inclination and the wellpath was horizontal after 24 m true vertical depth (TVD) drilled. An average dogleg severity (DLS) of 42° /30 m was achieved in the build up section. An 80 m horizontal section was drilled and geo-steered using gamma ray (GR) and multi propagation resistivity (MPR) measurements. The same bottom hole assembly (BHA) was used for the horizontal section as for the build up section of the sidetrack.

A pre-perforated 2 7/8 in. liner was installed in the 4 1/8 in. open hole. The operations were performed using a Rig Assist Snubbing (RAS) unit in cooperation with the rig system. The pre- and past-drilling operations were performed in underbalance while the drilling operation was in overbalance through the snubbing BOPs.

The motherbore and the sidetrack were set in production immediately after the liner was run.

In the future Statoil are looking at performing a similar drilling operation in underbalance. It should then be possible to make a sidetrack in approximately 10 days.

Introduction
General Veslefrikk Field Information

The Veslefrikk field in the North Sea outside the west coast of Norway is an oil field well into the decline phase of production, going into the tale production phase. It is high focus on drilling low cost wells to develop the remaining pockets of oil.

Although the field is divided into segments and contains several faults, the full field reservoir simulation model gives relative good control of remaining oil pockets. In addition, new 3D seismic has been gathered in order to improve mapping of remaining reserves. The drilling- and well- operations are executed from a fixed platform, the Veslefrikk A platform. A floater, the Veslefrikk B platform, with the processing plant is located next to and connected to the fixed platform by flexible hoses. All the twenty-four wellslots on the Veslefrikk A platform are utilized. The wells, consisting of 14 oil producers, 5 Water Alternate Gas (WAG)-injectors, 4 water injectors, and 1 gas injector diverge across most of the field. Due to low reservoir pressure, all oil producers are equipped with gas lift mandrels and Annular Safety Valves (ASV).

As there are no available slots and the possibility to reach most of the field by creating (short) sidetracks from existing wells, it is important to qualify TTRD as a drilling technique for Veslefrikk.

Geological Information

The A-19 A sidetrack was drilled to develop bypassed oil in the Etive 3 layer close to the A-19 original well.

The Etive formation is divided into three layers based on reservoir properties (see Fig. 1). The middle layer (Etive 2) is a Low angle Cross-stratified Conglomerate (LCC) with very high permeability. Below (Etive 1) and above (Etive 3) the LCC there is sandstone with not as high, but still good permeability. The injection water, which supports the pressure in the Etive formation, has swept most of the oil in the middle and lower part of the Etive formation leaving remaining oil in the Massive Sandstone (MS) in Etive 3.

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