This document describes the minimum standards and general practices for abandoning wells.
1. Abandonment or Temporary Suspension shall ensure that: Abandonment of the wells shall be designed and conducted in such a way that the environment is preserved and protected permanently.
2. Flow from the formation fluid to surface is prevented.
3. Permeable zones are isolated from one to the other by an approved tested barrier.
4. Reservoir management shall not be compromised. I.e independent reservoir layers must be isolated from one to the other to prevent cross-flow.
5. Compliance with barrier requirements which is a minimum of 2x independent tested barriers. In the case of abandonment or suspension, this shall exclude any hydrostatic column.
6. Compliance with local Government regulation. It is noted that regulations on abandonment vary in clarity and definition between regulatory authorities. Considerable attention should be paid to both regulations and definition to ensure all criteria are met.
7. Isolation of fresh water bearing formations.
8. Only wells that are planned to be re-entered within two years shall be defined as “suspended”.
Special Cases
9. A cement plug of a minimum 100m length shall be placed across potential weak points such as DV collars during abandonment.
10. In case of poorly cemented casing where a leak path or hydrocarbon migration is deemed possible, a window of 30m will be cut in the casing. The open hole exposed will be opened to the drilling diameter of the section plus minimum half an inch, or more in case of caving, and a 100m cement plug set across the window.
11. The formation fracture pressure at the window depth shall be greater than the contained pressure below the barrier.
1. PREPARATION ABANDONMENT
1. Prior to preparation of the abandonment or suspension programme the following shall be established between the appropriate departments:
a. Identify and agree permeable zones and identify those that contain hydrocarbons.
b. Ascertain the potential maximum formation pressure.
c. Identify zones that are capable of cross flowing.
d. Identify zones of good and poor cement quality in the casing annuli and any potential for leakage.
e. Identify and quantify any annular pressures.
f. Agree whether the well is to be abandoned or suspended.
g. If suspended establish any limitations on the nature of the suspension (Time frame).
2. An abandonment/suspension programme shall be prepared and approved by the Well Operations Manager and, as required, by the appropriate representatives from both production and Reservoir departments.
3. The abandonment programme shall include:
a. A statement of the objectives.
b. A schematic of the well showing the different casing setting depths, characteristics, pressure tests, LOT values, top of cement (theoretical and CBL).
c. Details of the formations, permeability’s, reservoir fluids and pressures.
d. An abandonment or suspension diagram.
e. A procedure for the operations.
f. Details of the abandonment suspension fluid.
g. Details of cement to be used.
4. Where primary cement in the casing annuli is inadequate, absent, or when annulus pressures are present, the abandonment cannot be completed until this is rectified. The abandonment programme shall specify a remedial programme that may involve:
a. Cutting and retrieval of casing and setting cement plugs.
b. Cutting a window in the casing.
c. Perforation of casing and remedial squeeze operations.
d. Surface squeeze remedial work.
5. Generally a notice shall be sent to the supervising authority advising the intention to abandon or suspend giving an outline of the proposed methodology. This shall include:
a. Reason for abandonment/suspension.
b. Supportive logs and test data.
c. A description and schematic of the proposed abandonment/suspension.
2 EXPLORATION WELL ABANDONMENT
2.1 OPEN HOLE SECTIONS
1. Open Hole Sections shall be abandoned as follows.
2. All identified permeable zones that may flow to surface or allow flow of fluid from one formation to another shall be isolated.
3. This shall be carried out by setting a balanced cement plug(s) across the formation(s) so as to extend to a height of 50m above the permeable zone.
4. The normal maximum setting height of a cement plug is 200m. Cement plugs shall be weight tested to confirm their integrity. However, it is permissible to set a series of cement plugs one on top of another. In this instance it is required to weight test at least the upper plug to confirm its presence.
5. Key to design is that permanent well barriers shall extend across the full cross section of the well, include all annuli and seal both vertically and horizontally. As such, the complete borehole shall be isolated. Diagramatically this may be shown as in Figure 1.
6. Verification shall be a weight test to 5 tons.
7. Pressure testing on plugs inside casing shall be 70 bar above measured formation fracture pressure.
FIGURE 1
2.2 PERFORATED CASED HOLE SECTIONS
1. Perforated Cased Hole Sections shall be abandoned as follows.
2. All perforated zones shall be cemented such that leakage or flow is prevented to other zones and squeezed.
3. This shall be carried out by setting balanced cement plug(s) across the perforated zones and squeezing. The completed plug(s) shall extend a minimum of 50m above the perforations.
4. In deviated sections or where the setting of balanced cement plugs may be difficult, set cement retainers and squeeze below, and set a minimum of 50m of cement above the retainer.
5. Cement plugs shall be weight tested to confirm their integrity. However, it is possible to set a series of cement plugs one on top of another. In this instance it is required to weight test only the upper plug to confirm integrity, unless stipulated otherwise by local regulation.
6. The last casing shoe above the open hole section or the first casing shoe above the perforations, shall have a 100m balanced cement plug set across open-hole and casing.
7. If the hole deviation at this point does not allow the setting of such cement plug or if it is deemed too risky to set a balanced plug, it is allowable to set a bridge plug close to the shoe and a cement plug on top of this of a minimum length of 50m. This plug shall be weight tested.
8. In the case of a liner, a 100m cement plug shall be set across the liner top, if the placement of a balanced plug is deemed too risky, a cement retainer can be used.
9. If during the primary cement job of casing set to isolate a permeable zone a problem has occurred, an assessment of the cement quality must be done (CBL/VDL) and a proper barrier re-established by milling a window of minimum 15m and setting a cement plug across or perforating in all directions and squeezing cement. Note that the best way would be to cut the casing to allow access to the full annulus.
FIGURE 2
2.3 RETRIEVING THE WELLHEAD
1. The wellhead shall be removed and casing shall be cut and retrieved:
a. At cellar level in the case of land wells.
b. At mudline level in the case of swamp barges.
c. At approximately 3 to 4m below mudline in the case of offshore wells.
d. A surface cement plug shall be set of a length not less than 50m with the top of the cement approximately at the mudline or in the case of a land rig, at bottom cellar level.
3 PERMANENT ABANDONMENT OF PRODUCTION WELLS
1. The general principals are the same as in the case of the abandonment of an exploration well except that bottom hole equipment may under certain circumstances be totally or partially left in place. Each case shall be considered on its own merits following detailed engineering study and risk assessment.
2. For tubing to be left in place above a permanent packer the following criteria must be met:
a. The reservoir(s) shall be oil or water and shall not be capable of flow to surface now or in the future i.e. non eruptive well.
b. The size and geometry of the tubing should allow a cement plug(s) to be placed through the tubing. To be considered as a barrier, cement should be on both sides of the tubing and adjacent casing.
3. For production wells that do not meet the above criteria, abandonment may be completed as follows:
a. The tubing is pulled above the packer.
b. A cement plug of a minimum length of 100m is placed on top of the packer. Note: if there is a liner lap below the packer this is still acceptable.
c. The abandonment is carried out as per an exploration well.
4. In the case of multiple packers abandonment may be achieved by:
a. Perforating between packers and filling the space between by cement squeezing.
b. Setting a cement plug of a minimum length of 100m is placed on top of the upper packer as long as the adjacent casing is properly cemented.
c. The abandonment is carried out as per an exploration well.
7 ABANDONMENT FLUID
Abandonment fluid shall not be considered as a barrier.
8 ABANDONMENT QUALITY CONTROL
1. All primary barriers require testing. There shall be a minimum of two primary barriers for each potential leak path.
2. Cased-hole plugs shall be placed adjacent to areas of good cement in the adjacent casing annulus in order to achieve full lateral coverage and guard against corrosion, otherwise the cutting of a window shall be evaluated.
3. Cased hole cement plugs should generally be set on top of bridge plugs as a bridge plug cannot be classed as a permanent barrier due to potential for corrosion.
4. Where it is practicable to carry out pressure tests, these shall be to 7 MPa (1000 psi) above injection pressure below that point or to 60% of the rating of the casing whichever is the less.
5. Where weight testing is carried out on any plug the minimum value shall be 5t (10,000 lb).
9 FILING
1. Records of temporarily abandoned wells shall be carefully kept in the affiliate Well Operations Office and also in the E&P Docs archives.
2. Records should contain at least the following detailed information:
a. Definitive well co-ordinates.
b. Definitive well trajectory and surveys record.
c. Status diagrams including the relevant depths of casing shoes, pressure tests values, LOT values, casings characteristics, formation tops and completion details.
d. Top of cement in each annulus and the means by which it was verified, i.e. CBL, calculation, pressure.
e. Position of each barrier and the means by which the barrier was tested i.e. pressure/inflow test, tagging, etc.
f. Formation and residual pressures of all permeable and especially overpressure or hydrocarbon bearing zones.
g. Details of the wellhead arrangement and any corrosion caps fitted.
h. Information related to the removal of wellhead and casing strings at/below the seabed/surface, highlighting the possibility of pressure migration and trap below the caps and the way to check that.
i. Seabed clearance survey/certificate.
j. Final schemes of any permanent abandonment of a well will be part of the final drilling report of the well.
REFERENCES
API RP 57
Oil & Gas UK Guidelines for the Suspension and Abandonment of Wells
NORSOK (Well Integrity in Well Operations)