Cement is one of the most common lost circulation materials (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 custom-designed 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 process designs and application of various cement types as LCMs. Solutions to four problematic field cases are provided. The conditions that require cement as an LCM and the criteria for selecting the best cement compositions are outlined along with optimal strategies. Introduction Lost circulation is the partial or complete loss of drilling fluid or cement slurries into formation voids during drilling, circulation, running casing, or cementing operations. It was reported that lost circulation occurs during drilling on approximately 20 to 25% of wells drilled worldwide.1 It is one of the most troublesome and costly problems encountered in drilling a well. Millions of dollars are being spent every year due to lost circulation and the detrimental effects it causes, such as lost rig time, stuck pipe, blow-outs and, frequently, the abandonment of expensive wells. Lost circulation has also been blamed for reduced production in that losses resulted in failure to secure production tests and samples, while the plugging of production zones have led to decreased productivity2. Lost circulation problems may be encountered at any depth when the total pressure exerted against the formation exceeds the formation breakdown pressure. Generally, four types of formation are responsible for lost circulation:Natural or induced fractured formations;Vugular or cavernous formations;Highly permeable formations, andUnconsolidated formations.3 Lost circulation in naturally fractured, cavernous/vugular and unconsolidated formations cannot be avoided completely. However, the following preventive measures can be used in other cases:1 a) set casing to protect weak formations, b) maintain a minimum safe drilling fluid density, and c) 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. High penetration rates in large boreholes produce large quantities of drill cuttings, which, if not immediately removed, cause excessive increase in mud density, and, in turn, an increase in circulating pressure. In intermediate holes, most lost-circulation problems are caused when encountering a depleted zone whose formation pore pressure is markedly lower than that of the upper formations. Sudden increases in the hydrostatic pressure of mud due to surging effects can fracture weak formations and lead to lost circulation. Lost circulation during cementing operations is also a common problem. Using proper cement composition and cementing technique (or design) is critical in successful cementing jobs with minimal loss. This paper presents solutions prepared for four challenging field cases where severe cement and/or drilling fluid loss were encountered. In two cases, cement loss was minimized by selecting the proper cement type and applying optimum design during casing cementing. In two other cases, cement was used to cure drilling fluid loss. The keys to successful applications for all these cases are outlined.

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 CaCO 3 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.

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