GB/T 41029-2021Specification for offshore well abandonment operation of oil&gas drilling (English PDF)

3.1 Submarine wellhead remnants

After the well abandonment operation, the objects left on the bottom of the seabed (the bottom of the water body in the case of internal water) and above the mud surface near the wellhead.

3.2 Special well

Oil and gas wells that meet one or more of the following conditions, depending on their final state before they are abandoned.

a) Gas wells and oil wells with a gas-oil ratio exceeding 350 (m3/m3) under standard conditions;

b) Wells that contain corrosive fluids such as H2S and CO2 in the formation and are prone to corrosion after abandonment;

c) Wells with maximum formation pore pressure equivalent density higher than 1.30g/cm³.

4.1 General Requirements

4.1.1 Before the well abandonment operation, the integrity of the wellbore should be assessed, and remedial measures should be taken if the integrity requirements are not met.

4.1.2 After the well abandonment operation, the oil and gas layers and other permeable formations should be effectively isolated, so that the formation fluids between different pressure layers cannot interact with each other.

For channeling, there should be at least two barriers to ensure that the formation fluid does not leak into the channel of the seabed mud surface.

4.1.3 Casings and annuluses that do not achieve effective cement isolation shall not be used as isolation barriers for permanent well abandonment.

4.1.4 The first cement plug in the perforation section, in the casing above the open hole or above the liner hanger shall be probed to the top of the cement plug and the pressure test shall be qualified.

4.1.5 The length of the last sealing cement plug for permanent well abandonment shall not be less than 50m, and the top surface of the cement plug is located 4m~30m below the seabed mud surface.

4.1.6 When the well is permanently abandoned, when the flow channel between the oil, gas and water permeable layers is blocked with cement plugs, the length of a single cement plug shall not be less than 50m;

The vertical height of the cement plug above the top of the oil, gas and water permeable layer in inclined wells and horizontal wells shall not be less than 30m.

4.1.7 When the well is permanently abandoned in the waters within the territorial sea, the cutting position of all casings, piles, etc. shall be 4m deeper than the seabed mud surface.