While modern oil and gas wells are constructed using layers of steel casing all the way to their total depth, this construction is a relatively recent development in the Appalachian Basin. Many wells were drilled and lined with a conductor pipe to support topsoil (approximately 25 feet) and followed by surface casing to seal off the fresh water table (approximately 200 to 500 feet). The remaining hole was left uncased in order to optimize primary and secondary oil recovery. These wells can present significant challenges for operators during P&A when shale formations below the surface casing begin to cave into the annulus. The collapsed formation will often create a bridge in the well which allows gas to seep through to surface while prohibiting passage to the original TD without significant well clean-out efforts.
P&G has deployed two cable tool rigs for a client that has held as many as one hundred wells in inventory of this construction. While outdated for use on modern wells, cable tools facilitate clean-out of caved formations and collapsed casing with much greater efficiency than their service rig counterparts. These P&G rigs run year-round for this client, and each will plug approximately 10 wells per year.
