Abstract

Fluid loss is encountered in almost all drilling operations. Depending on the severity of lost circulation and the size and type of the thief zone, different lost-circulation materials (LCMs) are used for curing. Cement is one of the most common LCMs. Various types of cement have been used as LCMs in the past.

Recent developments in cement technology and the understanding of lost circulation have produced customdesigned applications utilizing effective cement types and compositions. Applications also vary depending on the drilling fluid type and its properties. Custom-designed applications include thixotropic and ultrathixotropic cement slurries; slurries containing cello flakes, mica, and CaCO3 for mechanical bridging; unique spacers and surfactant packages; and foamed cement for controlling loss.

Selection of proper cement type and injection procedure calls for specific information such as formation properties, wellbore conditions, and thief zone characteristics. Laboratory experiments are recommended in this process. Field observations are also critical in making the final decision for selecting the optimum treatment fluid train and application strategy.

This paper discusses the application of various cement types (and process designs) as LCMs. Solutions to problematic field cases are provided. The criterion for selecting the best cement compositions is outlined. Optimal strategies are also presented.

Introduction

Lost circulation is the partial or complete loss of drilling fluid or cement during drilling, circulation, running casing, or cementing operations. According to a 1991 API survey, lost circulation occurs during drilling on approximately 20 to 25% of wells drilled worldwide.1

Loss of drilling fluid can result in increased cost, loss of time, plugging of potentially productive zones, blowouts from decreased hydrostatic pressure in formations other than the thief zone, excessive inflow of water, and excessive caving of the formation. Therefore, application of an immediate solution to lost circulation is an essential part of drilling engineering.

Lost circulation occurs through existing high-permeability zones such as highly fractured, vuggy, or cavernous structures or induced fractures when the hydrostatic pressure of drilling fluid exceeds the breaking strength of the formation. Carbonates are good examples of the former case, while the latter types typically occur in sandstones. For lost circulation to occur, the size of the pore openings of the included fractures must be larger than the size of the solid particles in the drilling fluid.

Lost circulation in naturally fractured, cavernous/vugular and unconsolidated formations cannot be avoided completely. The following preventive measures can be used however in other cases:1

  • set casing to protect weak formations,

  • maintain a minimum safe drilling fluid density,

  • avoid excessive downhole pressures caused by improper drilling fluid rheology, hydraulics, high flow rate, thick filter cakes, surge pressures during tripping in the well, bridges occurring in the annulus, and high shut-in surface pressures.

Problems that emerge due to lost circulation in the early and later stages of the drilling operation are different. For example, in surface holes, lost circulation has been known to cause massive washouts that in extreme cases, lead to the loss of the drilling rig.

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