History Matching the Clean-up of a Well at the Oselvar Field Using a Transient Multiphase Flow Simulator
Authors: Solvang B. , Krogstad M.K. , Lysne D. , Ashrafian A. , Quale E.A. 
Published: May 2016

Abstract
This paper presents the history matching of a clean-up operation of a long horizontal oil well at the Oselvar field in the North Sea using a commercial multiphase flow simulator. Data recorded during the actual clean-up is presented in the paper, and is compared with simulation results.
Well clean-up is the process of flowing the drilling and completion fluids out of a new well, removing formation damage and filling the well with formation fluids. The role of upfront simulation of well clean-up is to optimize the operation and provide the basis for the operational procedure to be used onsite. Also, after the well clean-up has been carried out, the simulation tool may be used for history matching in order to gain understanding of what happened during the operation and assess the quality of the upfront simulation model.
The data from the Oselvar clean-up operation revealed that the liquid flow rates at the initial choke size were significantly different from the rates predicted by the upfront simulations. Also, the time it took until no more drilling mud arrived topside was longer than expected. The data shows that the heel of the well was producing for several hours before there was any production from the toe of the well. This is attributed to the high initial productivity of the heel of the well, and to unexpected rheological behavior of the drilling mud. Constructing a transient downhole boundary condition based on the recorded data and on interpretation of petrophysical well data, a history matched simulation model was built that gave good agreement with the data.
The work in this paper contributes to improved understanding of mud retention in wells during clean-up operations. The data and simulation results demonstrate why the modeling approach widely used in the industry may lead to a conservative estimate of the pressure margin before a well is killed, and an optimistic estimate of the time that is required to clean the well.

Introduction
Well clean-up is the process of flowing chemicals, drilling and completion fluids out of new wells in preparation for regular production. As the well is kicked off and the clean-up progresses, these fluids are displaced by hydrocarbons produced from the reservoir. At the end of a successful clean-up operation, the drilling and completion fluids are fully removed from the well such that all production zones and all laterals are able to contribute positively to the overall productivity of the well according to design.
The clean-up strategy used for a particular well depends largely on the well design, the infrastructure tying the well back to the production unit, and the reservoir properties of the pay zones. In wells where there is a risk of formation damage, the drawdown may need to be controlled during the clean-up to limit local flow velocities at sandface. The same is true for openhole completions with sand control equipment where excessive flow velocities could result in screen plugging or erosion while drilling fluids containing solids are flowing through the lower completion. Wells drilled from existing production platforms or subsea templates can usually be cleaned up shortly after the well has been commissioned and handed over from the drilling unit to the production unit. However, subsea wells drilled in new field developments may have to wait for months or even years before clean-up to the production unit is possible. In such cases, clean-up to the drilling rig may be preferable if formation damage is expected from leaving the drilling fluids in the well for a prolonged period of time. Also, clean-up to the drilling rig may be the best option if it is feared that producing mud and debris into the pipeline will cause operational problems for the production facility, or that the additional pressure head from a long tieback pipeline may kill the well.