Achieving good cementation of shallow structural casing is very important. It provides structural support for the well construction, ensures isolation of shallow aquifers and safeguards against any shallow gas zone. The drilling of shallow section however encounters soft to unconsolidated formation, which could result in excessive hole enlargement and lost circulation. These conditions make it difficult to ensure good cementation. Presence of any shallow zone that can flow further complicates the cementation.

A conventional method is to use low weight cement slurry and pump 300 – 400% excess to take care of possible excessive hole enlargement and losses. If cement returns are not seen at surface, carry out top jobs by pumping cement from annulus until cement surfaces. Slurry weight as low as 16 ppg can successfully be managed. I can also provide acceptable compressive strength and setting time if designed property. Use of light weight slurry reduces the hydrostatic head and hence chances of losing in weak zones. This allows a full column of cement that provides required support to the structural casing and annular isolation to be able to safely proceed with the planned well construction.

Alternatively, there are many light weight and ultra-light weight slurry options that are available with most leading cementing companies. Light slurry 9 ppg – 13 ppg weight can be prepared with these speciality cements. Light weight slurries usually exhibit lower compressive strength and slower compressive strength development. However, depending on conditions, these could be in acceptable range. Most lightweight slurries are designed through either nitrogen gas injection or addition of bentonite or replacing portions of cement with low-specific gravity materials, such as microspheres, fly ash, silica fume or blast furnace slug.

A cost Vs benefit and risk Vs reward analysis should be carried out as the special light weight slurries cost more than the conventional cement slurries. Operational complexities on the rig for foam equipment set up or managing two different cement blends on the rig need to be evaluated and addressed at the planning stage. If the distance of the site from the blending station is extensive, possibility of segregation of microspheres from rest of the cement should also be considered.

For offshore wells, using an ROV to monitor cement returns at seabed is helpful. Adding mica in spacer would further help detecting returns at seabed through ROV. Adding fiber-based sealing agents on the fly while carrying out cementation helps in minimizing losses, improving success possibilities.

Cement plugs are an essential part of oil & gas well drilling. Drilling a well is fraught with downhole problems, which cost companies dearly. Many of them pose serious risks to assets and human lives. A cement plug becomes part of the solution in many instances.

Common applications of cement plugs in oil & gas wells are as below:

Plug & abandon a well: Once an exploratory or appraisal well is drilled to its planned depth, target reservoirs are tested to evaluate their potential. Depending on the results, the decision may be made to either temporarily or permanently abandon the well. In both scenarios, it is paramount to ensure that all the exposed zones are safely isolated to avoid them flowing any reservoir fluid to the surface in future. Multiple cement plugs along with mechanical plugs are set in the well to achieve these objectives. The number and setting depth of plugs vary depending on company policy, well design, and reservoir fluid. The plugs are tagged and tested to ensure that they hold pressures and provide effective isolation.

Isolate a depleted zone: Many wells have multiple producing zones. With time, some zones could deplete. Having them open with a higher pressure zone would result in a cross-flow. Hence the depleted zone needs to be isolated from other zones in the well. Cement plugs are used to provide an isolation barrier between the zones.

Sidetrack well: There could be many situations that result in the drill string getting stuck in the well. If the string is not freed in spite of all efforts, the last resort is to disconnect or sever the string and recover the part that is not stuck, leaving a fish in the hole. The well is then sidetracked to redrill the hole bypassing the fish. A cement plug is set above the fish based on the sidetrack well design. The hard cement plug ensures that the kick-off assembly drills the softer formation rather than trying to go back into the abandoned part of the well.

Lost circulation mitigation: Losing circulation is a very common problem. The reasons vary a lot. It could be either, seepage losses, formation cavities, weak formation, or depleted zone. Cement plugs are commonly placed across the loss zone to seal the crevices or pores and avoid the loss of drilling fluid. Special types of cement and additives are used for cement to be more effective in sealing a lost circulation zone.