Cementing outlet (spark plugs)

Outlet cementing is a process which is applied in the construction of oil-gas wells, to prevent the leakage of liquid into a well, either between formations or between a formation and the surface. As such, it must provide a hydraulic and mechanical seal.

Factors to consider during an outlet cementing

Any sealing operation presents a common problem where usually a cement powder is placed in a large volume of well fluid. Well liquids can contaminate cement and even after a reasonable wait at cement time, the result is poor, non-uniform or unclear. Closing situations often present unique issues that require sound engineering design and judgment. For these reasons, mechanical and chemical technologies are necessary for successful closure.

In addition to the issues usually considered for a primary cementing work, other factors must be carefully considered for an “outlet” work, such as:

Closing application

Closing operations are difficult because the working wave from a heavier cement base is supposed to be removed from its position on a lighter well fluid. Some of the different reasons for performing closure operations are discussed below.

Abandonment

To close selected intervals of a dry pit or a depleted well, operators can place a cement “plug” at the required depth. This is done to help prevent zonal communication and the migration of any fluid that may penetrate groundwater sources.

Directional drilling / bias

When bypassing a pit around an irreversible “fish”, e.g. a stuck bottom assembly or when changing the direction of drilling for geological reasons, it is often necessary to place a “cement plug” at the depth required to change the direction of the well. In this way, the support of a mechanical whip can also be helped, so that the part can be oriented in the desired direction.

Lost traffic control

When mud circulation is lost during drilling, lost returns can sometimes be restored by uncovering a cement stopper along the lost circulation area and then drilling back through the plug. When a closure work is planned efforts should be made to identify the source and reason for the lost returns. Factors that may contribute to turnover loss include:

Well control

Plugs, usually made of cement, are sometimes placed in a well when the well has reached a critical state in which no boundaries remain between pore pressures and fractions. For this reason, the drilling string is sometimes intentionally cemented. This is because it cannot be withdrawn without the risk of triggering an uncontrolled surface leak or a transverse leak from a high pressure zone to a weak or low pressure zone.

Zonal isolation / conformity

One of the most common reasons for closure is the isolation of a specific area. The goal may be:

In a well with two or more production intervals, abandoning an impoverished or unprofitable production area may be beneficial. A permanent cement “plug” is used to insulate the area, helping to prevent potential production losses or the migration of fluids from another interval. The integrity of such plugs is often mechanically improved by placing them above the bridge plugs or through and on the squeegees.

Formation testing

Plugs are occasionally placed in an area to be tested that is located at a considerable distance from the bottom, where other means of interval isolation are not possible or practical. Although cement is the most common plug material, the following materials can also serve as sealing agents:

Stability of the well

Sometimes during drilling, placing a sealing material through an unstable formation can be helpful. Polymer, resins, cement or combinations of these materials can be used to consolidate formations and change stresses near the well. A balanced cement plug is sometimes placed simply to “fill” a section of elliptical holes. In such cases, the plug is then drilled, leaving a cement wrap in place – to reduce or prevent further expansion of the well and to help return the well to its original diameter.