Foam Cementing in the Volga-Ural Region: Case Study

Abstract

Most wells drilled in 2015–2017 in the Volga-Ural Region of Russia experienced serious downhole problems related to mud losses, which account for a substantial share in total non-productive time (NPT). With conventional methods such as cement plugs, etc. used to solve the lost circulation problem, it takes on average 4 to 6 days per well, or 6 to 10% of the total rig time. In some cases, however, the losses are so heavy that there is no mud return to surface at all, and it may take up to 30 days (about 50% of the rig time) to stop these disastrous losses. Besides, there may be several thief zones at different depths with different loss initiation points. Conventional methods of lost circulation control generally fail. In many cases, conventional cement plugs used to stop mud losses are inadequate to meet such challenges. This paper describes the application of special cryogenic equipment and chemicals to offer oil and gas well operators an alternative solution based on foaming base cement slurries, spacers, and drill muds with inert gas (usually nitrogen) over a wide range of densities. When cementing is used for controlling lost circulation, computer-based simulation is essential to determine hydraulic model parameters. A new proprietary cement service performs high-precision foam cement calculations based on actual well data. The key is to select the correct concentration of nitrogen in foam water spacer and foamed cement to reduce hydrostatic pressure below the point at which losses are initiated in a weak horizon (formation). This special foam cementing equipment is capable of controlling nitrogen concentration automatically and injecting nitrogen into base cement with the foamer to maintain the design density of foamed cement. Cement and service water are foamed under high pressure on the surface in a high-pressure pipeline loop system. Automatic foaming means that three main units are simultaneously in operation: a cementing unit, an N2 unit, and an automatic chemical injection skid. Foamed cement is pumped under pressure to drilling tools. Similarly, service water is foamed to produce foam water spacer with a density of 0.3 g/cm3 to 0.9 g/cm3, which is injected into the well before foamed cement. Foam water spacer is injected first in the thief zone followed by foamed cement. The high-viscosity foam water spacer prevents foamed cement from being washed away by formation fluid and reduces the flow of formation water in the thief zone. As a result, the linear velocity of foamed cement in the lost circulation horizon is reduced, which makes it possible for the cement to achieve the required consistency and isolate the weak formation in the near-wellbore area. Foam water spacer injected into the well lowers hydrostatic pressure and raises the static level of fluid in the well to the wellhead to help ensure returns to surface. The new technology has proved to be the most efficient among other solutions used to mitigate or eliminate mud losses during well drilling in the Volga-Ural region. Foam cementing has reduced the time required to address loss-related problems to two days. This paper discusses the case study of foam cementing used to resolve the lost circulation problem by plugging the thief zone with foamed cement.