Cathodic Protection (CP) Well Abandonment
In the oil and gas industry, plugging and sealing of production wells that have reached the end of their natural life has been a common practice for decades. The oil and gas industry uses the term “plug and abandon” (P&A) for this operation. For oil and gas wells that often extend several thousands of feet into the earth, this can be an expensive undertaking. In the cathodic protection (CP) world, deep anode groundbed systems are installed at much more shallow depths (generally no deeper than a few hundred feet) and have typically been excluded from the same filling and sealing requirements. Cathodic protection well abandonment was merely a matter of removing the surface equipment (rectifier, junction box, meter loop and pole) and cutting the wires connecting to the anode(s) down hole. (For background reading, see Corrosion Prevention in Buried Pipelines.)
Decommissioning a deep anode well, or ground bed simply by removing the surface hardware and walking away is often no longer acceptable. This is due to concerns with environmental impacts and safety hazards associated with the deep hole into which the anode assembly was installed. A growing number of states and local jurisdictions both within the United States and abroad have passed regulations requiring owners of deep anode groundbed cathodic protection systems to follow similar plug and abandonment requirements as those in place for production wells. Basically, the owner is required to seal the hole completely prior to abandoning the cathodic protection well.
Concerns over Cathodic Protection Boreholes
The primary reason for abandoning a cathodic protection borehole is that it could become a conduit for contaminating drinking aquifers. While many cathodic protection boreholes are not deep enough to reach into or cross aquifers or other sub-surface water zones, the very existence of a borehole can be perceived as a problem. As noted in the California Well Standards:
“Cathodic Protection wells, along with other types of wells, can allow groundwater degradation to occur. Improperly constructed, maintained or abandoned cathodic protection boreholes can constitute a preferential pathway for the movement of poor quality water, pollutants and contaminants.”
An additional concern with improperly abandoned cathodic protection wells is the potential safety hazard that they may present. (Learn more about pipeline safety in The Impact of the Pipeline Safety Act of 2011 on the Industry.) Fortunately, cathodic protection boreholes have small diameters, are almost always backfilled and plugged at the top, and therefore are not a hazard to children or animals.
Cathodic Protection Well Abandonment Steps
In general, plugging and abandoning a cathodic protection well consists of three basic requirements:
- Remove the deep anode groundbed surface materials, including the surface casing. The first step in the process is to safely de-energize and remove any electrical equipment such as AC power cabling, transformer/rectifiers, junction boxes and cabling. The surface casing and hole plug should then be removed.
- Drill into the old groundbed borehole. After safely de-energizing the surface components of the cathodic protection system, a drill rig is needed to re-drill the borehole. Care must be taken handling all of the boring spoils to collect these safely and dispose of them properly. During cathodic protection operation, the area around the anode can become very acidic with a pH approaching 1 in some cases.
- Fill the abandoned borehole with an approved material (typically cement or bentonite or a combination of the two). Bentonite (or bentonite clay) is a clay mixture frequently used in drilling mud because of its colloidal properties and because it expands when wet and is capable of absorbing several times its dry mass in water. The swelling property of bentonite makes it a useful sealant creating a self-sealing, low permeability barrier. It is also less expensive than cement backfills. Cement backfill generally falls into two categories, either neat cement, which is a mixture of Portland cement and water, or cement grout, which is a mixture of Portland cement, water and sand. In both cases, the recommended installation method is to use a tremie pipe installed in the hole that allows the cement to be pumped to the bottom of the hole. As the cement fills the hole the tremie pipe is slowly removed, assuring the hole is completely filled.
Prior to undertaking cathodic protection well abandonment, the deep anode groundbed owner should familiarize themselves with any specific state or local regulations or requirements to assure compliance.