Analysis of system and methods for improved cementing of oil- and gas wells
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Cement is a crucial part of well construction. If a good cement bond between the casing and well bore wall cannot be achieved in the primary cementing, expensive remedial cementing jobs or possible loss of the well bore can occur. When cementing casings and liners in deep waters or pressure depleted reservoirs there is often a small pressure margin between pore pressure and fracture pressure, this makes cement operations more complex. New cementing techniques and cements are being developed to solve the problems in these areas. Light weight cements have for a long time been used as a means to avoid fracturing the cemented formations. The first light weight cements used water and water absorbing solids to lower the cement slurry density. More modern techniques involve using nitrogen gas, hollow glass spheres or a combination of both to make the cement slurry light enough. These methods have proven useful and clearly have their advantages in certain areas. However, the light weight methods also have their limitations so a new method for getting heavy conventional cement down in narrow pressure margin areas has been evaluated. The method is based on reducing the hydrostatic head of fluid above the cement column by displacing the mud in the riser by a lighter fluid. A choke valve placed in bypass over a closed BOP will make it possible to control the pressure so that a constant bottom hole pressure is achieved. The Macondo well in Gulf of Mexico has been used as a case to evaluate the new method. Hydraulic simulations have been done for cementing a full casing string and for cementing a liner as a production casing in this well. Results from the simulations together with a standard liner cementing procedure have been used to make complete cementing procedures for the new method. Results from the simulations show that the new method could be used with success on the Macondo well. Further analyses of the results show large differences in case of what well geometry the method is used in, and limitations on where the method can be used in terms of water depth. The new method is not applicable in shallow waters as the riser volume in these conditions will be too small. As the riser volume is a limiting factor for the method, completing a well with a full casing string will also reduce the chances it can be used. Even though this thesis shows promising results for the new method for cementing in narrow pressure windows, there is still much work to be done in examining the different aspects of the method further, before an eventual field test.