25. Acidizing

Sometimes, petroleum exists in a formation but is unable flow readily into the well because the formation has very low permeability. If the formation is composed of rocks that dissolve upon being contacted by acid, such as lime stone or dolomite, then a technique known as acidizing may be required.

Acidizing is usually performed by an acidizing service company and may be done before the rig is moved off the well or after the rig is moved away. In any case, the acidizing operation basically consists of pumping anywhere a quantity of acid down the well. The acid travels down the tubing, enters the perforations and contacts the formation. Continued pumping forces the acid into the formation where it etches channels; these provide a way for the formation's oil or gas to enter the well through the perforations.

26. Fracturing

When sandstone rocks contain oil or gas in commercial quantities but the permeability is too low to permit good recovery, a process called fracturing may be used to increase permeability to a practical level.

To fracture a formation, a company providing this service pumps a specially blended fluid down the well and into the formation under great pressure. Pumping continues until the formation literally cracks open.

Sand, walnut hulls or aluminium pellets are mixed into the fracturing fluid. These materials are called proppants. The proppant enter the fractures in the formation. When pumping is stopped and the pressure is allowed to dissipate, the proppant remains in the fractures. Since the fractures try to close back together after the pressure on the well is released, the proppant is needed to hold or prop the fractures open. These propped-open fractures provide passages for oil or gas to flow into the well.

27. Installing the Christmas Tree

Even though the oil or gas can flow into the casing after it is perforated, the well is not usually produced through the casing Instead, a small diameter pipe called “tubing” is placed in the well to serve as a way for the oil or gas to flow to the surface. The tubing is run into the well with a packer. The packer goes on the outside of the tubing and is placed at a depth just above the producing zone. Then the packer is expanded, it grips the walls of the production casing and forms a seal in the annular space between the outside of the tubing and the inside of the casing. As the produced fluids flow out of the formation through the perforations, they are forced to enter the tubing to get to the surface.

When casing is set, cemented and perforated, and when the tubing string is run, then a collection of valves called a “Christmas Tree” is installed on the surface at the top of the casing. Like so many terms in the oil industry, no one knows why this device on top of the well is called a Christmas tree. Perhaps all the valves and piping reminded someone of the traditional Christmas tree. The tubing in the well is connected to the Christmas tree, so as the well's production flows up the tubing, it enters the Christmas tree. As a result, the production from the well can be controlled by operating or closing valves on the Christmas tree.

Usually, once the Christmas tree is installed the well is complete. The drilling contractor has done his job according to the drilling contract and he can move the rig to another location to start another well drilling process all over again.