Abstract
The giant Iraqi Zubair oilfield is a hotspot for drilling activities. Many vertical and directional wells have been recently drilled in this oil field. Losing cement slurry into karstic facies of the Hartha formation is the main challenge on most wells. Achieving required top of cement behind the intermediate casing is a critical challenge because this casing is the main barrier against the corrosive sulfurous water of the underlying Um Er-Radhuma and Tayarat formations. In this field, in spite of the tremendous time and effort that have been spent to mitigate losses while drilling and before cementing, at least 55% of wells experience losses while cementing. Various conventional treatments have been implemented to bring cement to the surface in primary cementing operations, including one stage with lead and tail slurries and two stages with two cement slurries (lead and tail) for the second stage. Even when ample amount of extra excess of cement has been used for the second stage to compensate for the cement lost during first stage, there was still a need for top jobs subsequently after the cement job. Recently, the field applications of fiber cement have shown substantial success compared to the case of using only cement slurries as lost circulation material. This was confirmed by getting returns when displacing cement across the lost circulation zone (Hartha formation) in five out of six wells. The cement jobs of the six wells have been revised, and use of fiber cement has now become a standard practice for cementing the intermediate casing strings in this field. In addition to presenting six case studies in the use of fiber cement, this paper describes the previous efforts at solving the problem and the success criteria. This paper also discusses the mechanism and the properties of using fiber within cement slurries for curing lost circulation while cementing in fractured formations. Laboratory tests were conducted to evaluate the plugging efficiency and the magnitude of the developed compressive strength.
