Completing and Perforating the Well

The final decision on whether a formation should be put into production is based on economics. If considered to be of commercial value, the zones classified as productive are "pay". Zones that are likely to produce water or amounts of hydrocarbon deemed to be non-commercial are "non-pay". Pay discrimination focuses on porosity, permeability, and water-cut. Permeability is a measure of the formation's ability to transmit fluids and is related to the geometry of the pore space, in particular the connectedness of the pores. Water-cut refers to the amount of water that is produced along with any hydrocarbons.

Higher porosities mean greater volumes of pore space that can contain hydrocarbons and usually higher permeability that results in high fluid flow rates. The water saturation is also used to judge whether the water-cut of the hydrocarbons is too high, because produced water must be disposed in conformance with environmental regulations. The distinction of pay from non-pay is usually made by reference to porosity and water saturation cut-offs. The numbers vary by formation and local experience, but specify minimum porosity and maximum water saturation values for pay.

Although various techniques can be used to estimate the porosity, permeability, and water saturation from well logs, in the current version of the Small County exercise, you will determine potential pay intervals primarily based on oil staining in the drill-cuttings log and positive deflections on the resistivity log indicative of increased hydrocarbon saturation.

If no productive zones are located, then the well is determined to be a "dry hole" and the well is plugged and abandoned. A cementing company is called to the drill site and cement plugs are required within the well bore at intervals where porosity has been detected to isolate these porosity zones and prevent the movement of formation fluids from one formation to another. The cement is pumped into the well bore through the drill string. The plugging procedure is done to conform with environmental regulations and to ensure that shallow formations whose waters are used for irrigation or drinking water supplies are not contaminated.

However, if a decision is made to complete the well as a producer, steel casing is delivered to the site. The casing is threaded together and inserted into the well bore much in the same manner as the drill string. Cement is mixed at the surface and then pumped down the casing , filling the space between the casing and the well bore. A well-perforating company is then called to perforate holes in the casing at the depths of productive zones to allow the oil and gas to enter the casing. When the perforating tool has been lowered to the desired depth, gas perforate the casing, the cement, and the surrounding rock. A smaller-diameter pipe, called tubing, is then threaded together and inserted into the casing to allow oil or gas to flow to the surface At the surface, a well head is installed which is equipped with valves to control the flow of oil or gas from the well. The well head is known as a "Christmas tree".