Well Completion Requirements for ARID System

After a receiving aquifer has been identified and the ARID System is installed properly, there is rarely an issue in pumping water out of the coal zone and into the receiving aquifer. It is critical that the CBM Well is drilled and completed properly in order for ARID to be most effective.

There are various CBM well requirements that will impact the use of the ARID System as follows:

Size of Well Casing

The ARID System can be fitted to work in almost any well casing of any size. While standard size well casings may range for 5.5 inches, 7 inches, and 14 inches, the ARID mandrel can be made for most any size well bore casing.  Additionally, the gas riser pipe size can also be adjusted to accommodate the requirements of the well.

Vertical and Horizontal CBM Wells

The ARID System will work with all vertical CBM wells and with CBM wells that have been drilled horizontally at the base of the well. In each of these situations the ARID Mandrel is located in the vertical portion of the well.

Pump Type

The Arid System can be utilized with a submersible pump, beam pump, rod pump, pc pump, etc. With any of these pump types, the water from the coal seam is pumped to the top of the ARID mandrel where it can be diffused through the perforated well casing into the selected recharge zone. If the water is pumped to a recharge zone that is located below the coal seam, then an inverted submersible pump is required that will be provided by Big Cat.
See below.

Location of Recharge Zone

The majority of the time, a recharge zone will be located in an available aquifer that is above or shallower than the coal seam. The recharge zone must have water characteristics that are similar to the water produced from the coal seam. Often the water produced by the coal seam is better than the water in the recharge zone, which is also acceptable. When necessary, the ARID System can also be positioned to pump water to a recharge zone that is located below the coal seam. In that situation the ARID Mandrel is inverted and positioned below the coal seam. A specially designed inverted pump provided by Big Cat is located at the coal seam that pumps the water from the coal seam down through the ARID Mandrel and into a recharge zone located below the coal seam. In this situation the well must be over drilled to accommodate a perforation in the well casing at a receiving zone that is deeper than the coal seam.

Properly Setting the Well Casing

In situations of either under reaming or drill through of the coal seam, properly setting the well casing at the correct depth is important to insure that fines are minimized from entering the well bore. With the ARID System in place, fines that are pumped out of the coal seam and moved above the mandrel can cause the receiving zone to become filled with this material. While the receiving zone above the mandrel can accept some amount of fines, if they are excessive, the fines may fill the receiving zone and limit the rate at which the receiving aquifer can accept water. Additionally, fines in the water ultimately reduce the life of the submersible pump adding unnecessary replacement cost to the well.

Properly Cementing the Casing

A complete and proper cementing of the well casing is imperative after the casing is lowered into the well bore. In all situations it is critical that the exterior of the well casing is fully cemented from the bottom of the well to the ground surface. If this is not completed properly there is a risk of migration of shallower coal seam water and other aquifers/zones that were penetrated initially when the well was drilled. With the ARID system in place, separate perforations are made in the well bore adjacent to the receiving aquifer. If cementing of the exterior of the casing is incomplete, water may migrate back to the coal zone, thus negating some or all of the water pumping process and reducing gas production.

Maintaining Well Head Pressure

It is important that well head pressure above the ARID mandrel is monitored and maintained at the appropriate pressure in order to properly migrate the produced water into the receiving zone. While in most cases a receiving aquifer will accept all the water that is produced by the CBM well, if a receiving zone cannot handle the produced water at the rate required to produce gas, then the water pressure above the ARID mandrel should be set to allow the receiving zone to properly accept water while the water overburden is vented with a pressure release valve through the top of the well head for discharge or disposal. While not a requirement, it is beneficial to have pressure sensors installed below and above the ARID mandrel, or to periodically test the pressure manually. This insures proper well head pressures and reduces electrical use and pump wear.

Pipe Integrity

In any production well it is critical to have pipe casing integrity to insure that the well maintains pressure and water can be effectively and efficiently removed from the coal seam, and that water from the CBM well does not improperly migrate to an inappropriate aquifer. In accordance with regulatory requirements, new wells are pressure tested for integrity. With the ARID system there is a separate gas riser pipe that allows gas to flow up out of the well head. Since this riser pipe runs up through the pressurized casing, it also must have integrity so that water that has been pumped out of the coal seam to the top of the ARID mandrel does not migrate back into the adjacent riser pipe and back down to the coal seam, rather than moving into the specified receiving zone.