Abstract

Total loss of circulation is a common problem during drilling through the fractured limestone in western offshore fields and coal seams in the CBM fields of eastern India. Most of the the times the wells are completed in total loss conditions either by drilling blind or by use of sub hydrostatic drilling fluid as conventional loss control measures were not effective. Moreover requirement of long cement column to cover multiple coal seams with excellent zonal isolation for fracturing in CBM wells during the production stage makes the cementation a big challenge in these wellls. To overcome these challenges, an inert dispersible fiber based cement system along with an improved placement techniques have been tested in the field with great success the details of which has been presented in this paper.

An innovative inert fiber based cement system has been formulated and evaluated in the laboratory for plugging fractured formations under simulated conditions prior to its application in total loss condition. Computer simulation studies were performed with field data and permissible slurry density and displacement rate were determined to minimize losses and formation damage during cementing. Limiting friction pressure by adjusting the rheological properties & displacement rate of the slurry during slurry placement also minimizes the lost circulation problems. Using this combination of improved cement system and placement techniques, circulation of the slurry was regained while cementing and the designed cement top could be achieved thereby avoiding anticipated repair jobs. Case histories of wells drilled with total loss conditions where successful implementation of this cementation technique prevented cement invasion into the production zone and lead to improved zonal isolation are presented in this paper.

This paper established the significance of application of these innovative cement system and cementing techniques in terms of better well completion and improved productivity in cementing across formations having total loss condition during drilling.

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. It is one of the most troublesome and costly problems encountered in drilling a well. Numerous lost-circulation treatments are available in the oil industry, but due to the mixed results of such treatments, the loss circulation problem is still a major issue of concern. Lost circulation has also been blamed for reduced production because it resulted in failure to secure production tests and samples, while the plugging of production zones have led to decreased productivity. 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 such as natural or induced fractured formations, vugular or cavernous formations, highly permeable formations, and unconsolidated formations. Lost circulation in naturally fractured, cavernous/vugular and unconsolidated formations cannot be avoided completely but can be minimized with proper treatment/techniques.

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