Operational Field Rules - Bentonite Plugging
November 15, 2004
Following are guidelines for sodium bentonite well plugging operations. These guidelines are applicable as field rules in the Bakersfield and Coalinga districts and are subject to change.
- Well plugging programs requiring a squeeze job may not use bentonite as the squeeze material.
- Bentonite may not be used in wellbores below 4,000 feet (ED) or where the zone pressure differential is 500 psi greater than an upper zone without prior evaluation and consideration by the Division.
- Bentonite may not be used in wells with greater than 20 degrees deviation without prior approval by the Division.
- Any bentonite plug that has bridged above the intended interval must be drilled out.
- The bentonite must consist of sodium bentonite exhibiting a minimum specific gravity of 2.0 (dry)/1.5 (hydrated), a pH between 6 and 12, and a chemical makeup approximately 63 percent SiO2, 21 percent Al2O3, and 16 percent other. No additives may be used with the bentonite.
- Bentonite must be in compressed form. Flake or powdered bentonite may not be used without prior approval by the Division.
- Bentonite size/shape (using samples) must be preapproved by the appropriate District Deputy.
- Proper weight or volume indicators must be utilized to determine bentonite dry weight prior to placement in the wellbore.
- The bulk density conversion factors will be furnished to the Division prior to commencing operations.
- Wellbores must be cleaned out at least 25 feet into the uppermost set of effective perforations.
- Spacer material shall comply with Section 1745.7, Chapter 4, Division 2, Title 14 of the California Code of Regulations. “Any interval of the hole not plugged with cement shall be filled with mud fluid of sufficient density to exert hydrostatic pressure exceeding the greatest formation pressure encountered while drilling such interval.” The word bentonite is inferred as a replacement for “cement” in the section.
- Previously approved spacer or base materials may be allowed across the perforations to within 25 feet of the uppermost effective perforation. In wells with multiple-zone completions, each zone must be separated by a bentonite plug.
- Wellbores must be flushed with at least one hole-volume of water prior to the placement of bentonite. The purpose of flushing is to remove contamination, primarily oil, from the wellbore. Circulation is required with tubing on bottom. “Bradenhead flushing” is never acceptable.
- Rate and method for delivery of bentonite into the wellbore must be in a manner that the potential for bridging is kept to a minimum, and that proper volumetric calculations can be made prior to placement.
The location of all bentonite plugs shall be witnessed by Division staff. Generally, a bentonite plug may be tagged by a coiled tubing rig or conventional rig with sufficient weight to verify the plug is present. Tagging a plug by any other means such as bailer or sinker bar must be approved by the Division. Other methods may be approved by the Supervisor.
- Water will be added to the wellbore until all bentonite plugs are properly hydrated. Generally, a fluid level must be maintained over a bentonite plug for a minimum of 24 hours to ensure full hydration.
- Wells must be equipped with adequate BOPE as specified in the permit.
- If plugging is required in a wellbore with uncemented casing, a 10-foot section (minimum) cavity shot shall be placed opposite a competent shale or formation and a bentonite plug shall be placed across the entire cavity shot section.
- A minimum 20 lineal foot bentonite plug shall be placed directly below the base of any proposed cavity shot prior to detonation of the shot.
- Bentonite is temperature sensitive and may prematurely hydrate in wellbore fluids exceeding 120 degrees F.
- Bentonite should not be used in casing/tubing less than 2-7/8 inch OD.
- If an operator wishes to change an abandonment program from bentonite/mud to bentonite from bottom to top, the operator must file a Supplementary Notice. This notice may be filed after the work is done provided the work was in progress when the decision to change the program was made.
- If the well is a single-zone, shallow-reservoir well (i.e. Kern River zone well in Kern River oil field, or a Tulare zone well in Midway-Sunset oil field, etc.), the placing of a bentonite plug from cleanout to surface is acceptable. No bottom hole tag is necessary. The placement of the plug should be witnessed by Division staff.
- If the well has overlying hydrocarbon zones (i.e. not a single zone as in Note 4 above), a bottom hole plug tag is required. The plug must be fully hydrated (i.e. a 24-hour hydration period is necessary).
Special considerations for Coalinga and Jacalitos oil fields:
- Plugging the A point in a non-steam area: When one uncemented casing string is encountered and a cavity shot is required, the use of bentonite is approved as a plugging material. If two or more uncemented casings are encountered, bentonite is not approved and cement must be used.
- Plugging the Base of Fresh Water interval: Bentonite is approved only when there is cemented casing across the interval.