Effective Well Cleanouts With Seawater Using Downhole Measurements in Subhydrostatic Environment in Norway

Abstract

Critical coiled tubing treatments, such as fill cleanouts of sub-hydrostatic wells and perforations of small reservoir intervals have historically posed a high uncertainty for Norwegian operators due to nature of the complex completions i.e. long and large monobore completions. The complexity is exacerbated with the strict offshore environmental regulations that limit the fluids that can be used for intervention operations. Fill cleanouts on these wells with CT face the difficult challenge of achieving the desired rate for pumping seawater which leads to the need for better understanding the downhole conditions while performing the operation. Without understanding these conditions it can be costly and create operational complications. A way to provide insight to the changing conditions during the cleanout operation is to use CT enabled with real time downhole measurements. The use of real time downhole measurement allows recognition of the wellbore response due to changes in hydrostatic pressure as the fill/debris are removed from the wellbore and adjustment of the operational parameters needed for effective treatment. Following the cleanout, this ability to adjust the parameters allows efficient well kick off by optimizing the use of nitrogen. This paper will present several case histories incorporating real time downhole measurements for effective and efficient clean outs as well as optimized well kick offs in the Norwegian sector of the North Sea. Introduction The Valhall field is an Upper Cretaceous, asymmetric, chalk anticline that forms an overpressured, undersaturated, oil reservoir located in the Norwegian sector of the North Sea. It is characterized by high porosity (25 to 48%) and high oil saturation (92 to 97%).1,2 Fill cleanouts are often necessary during the life of these wells as the unconsolidated nature of the reservoir and the compaction mechanism contribute to production of particles and fines which plug reservoir perforations and obstruct wellbore access. Cleanouts are also challenging in these wells due to the unpredictable nature of the fill to be found and the possible wellbore damage that can exist especially in old wells.2 Improved method for cleanouts It is generally accepted that many, if not all, coiled tubing downhole applications can be optimized with the availability of real-time downhole information. Many of the existing treatment simulators/monitors for these operations use calculated values for downhole parameters based on extrapolation from surface measurements - giving at best an approximate result. Realtime downhole measurements allow interpretation and job optimization with services delivered through coiled tubing. It provides the information needed to adjust job parameters immediately, to improve effectiveness, reduce risks, and optimize performance with the operation still in progress.