Primary Completion Procedures
Once a well has been drilled, the decision must be made: Can this well become a productive? Some wells are thoroughly tested to "try" and determine profitability before completion, but these methods are no where near precise. Modern shale well are not even tested before completion. Since a stimulation technique is needed in order for them to be productive test results at this point are not accurate. If the well is determined unproductive it is plugged and abandoned. If its is deemed productive the completion process is started. Well completion incorporates the steps taken to transform a drilled well into a producing one. These steps include casing, perforating, stimulation and installing production equipment.
The first step in completing a well is to case the hole. Casing ensures the well will come deteriorate as well as isolate the producing formation. Casing is a series of 30ft joints of steel pipe which are screwed together to make what is referred to as a continuous ?casing string?. There are different levels of casing, these include production casing, intermediate casing, surface casing and conductor casing. The casing string (or sometimes various strings) runs the entire length of the well. It is lowered into the hole and cemented in place.
In order to achieve production, the casing and cement are perforated to allow the hydrocarbons to enter the well bore. This process involves running a perforation gun into the wellbore, Once the reservoir level has been reached, the gun then shoots holes in the sides of the casing to allow the hydrocarbons to enter the well bore.
Stimulation is performed on a well to increase production. Sometimes, a well initially exhibits low permeability, and stimulation is employed to commence production from the reservoir. Other times, stimulation is used to further encourage permeability and flow from an already existing well that has become under-productive.The Oil and gas industry has been utilizing well stimulation (including hydraulic fracturing techniques) since long before ?fracking? became a household term. The most common techniques are Acidizing and Hydraulic Fracturing (Fracing).
- Acidizing is performed in an effort to increase the permeability of the reservoir rock. Well acidizing is achieved by pumping acid into the well to dissolve limestone, dolomite and calcite cement between the sediment grains of the reservoir rocks. This process enlarges the existing voids to improve permeability.
- Fracing s a technique in which rock is fractured by a pressurized liquid. The process involves the high-pressure injection of 'fracking fluid' (primarily water, containing sand or other proppants) into a well to create cracks in the specific rock formations through which oil and natural gas will flow more freely. When the hydraulic pressure is removed, the frac fluid flows out leaving small grains of proppants to hold the fractures open. It makes existing small fractures larger and creates new fractures which allow the hydrocarbons to flow into the well bore.
The only thing visible at the well head after the drill rig leaves the site is a series of valves and gauges connected vertically to each other and attached to the top of the well. This allows the amount of hydrocarbons to flow from the well and it prevents leakage at the surface. This structure is referred to as the Christmas Tree (see image).
All of the techniques discussed thus far for removing the hydrocarbons from the reservoir and bringing them to the surface are referred to as Primary Recovery Techniques. Primary techniques rely entirely on natural forces within the reservoir trap. And primary recovery accounts for a large portion of the total volume of hydrocarbons in the trap, but not all of it. Less than 40% of hydrocarbons present are recoverable by means of Primary Recovery.
In most reservoir traps, initial pressure is sufficient to push the oil to the surface of the production well with only minimal help from a down hole pump. But, with declining well pressures, it becomes more difficult to get the hydrocarbon to the surface. Sometimes, artificial OIL lift (pumping unit) is needed.