Optimization of hydraulic program for liner drilling
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Drilling with casing and liner are unconventional drilling methods developed to reduce both the non-productive time spent on drilling operations and drilling time in general. A casing or liner string is used as the drillstring, and the number of operations is therefore reduced from two to one as drilling and casing of the wellbore are performed in one operation. Even though there are many benefits with this drilling method, there are also some drawbacks and operational considerations. Statoil and Baker Hughes have developed a rotary steerable liner drilling system, called the Steerable Drilling Liner (SDL). Two offshore test wells have been drilled with this system, and Statoil is now planning to utilize it on more wells in the North Sea. The Oseberg field is a candidate field for liner drilling due to stability problems in the Draupne and Ness formations. Two wells on the Oseberg field have therefore been chosen as the candidate wells for this thesis. The case histories of the test wells show that the limitation may be the low flow rate circulated through the liner, which may cause cuttings accumulation on top of the liner where the flow area suddenly expands. A flow diverter is therefore developed to have more flexibility in the choice of flow rate above the liner. The flow diverter and other solutions to improve the hole cleaning have been evaluated in this thesis. The high annular circulating pressure is another issue with the liner drilling technique. Many of the factors which improve hole cleaning also increase the annular pressure. The ECD must also be taken into consideration when optimizing the hole cleaning of the candidate wells. The simulations of hole cleaning and annular circulating pressure for the SDL system have been performed in WellPlan. The recommended flow rate for hole cleaning in the sections above the liner is at least 500 lpm higher than the maximum possible flow rate through the liner. By circulating this recommended rate through the liner, the equivalent circulating density increases. The circulating pressure exceeds the fracture pressure of the formation for the 7” SDL system. It is therefore not an alternative to come up with solutions to be able to circulate with a higher flow rate through the SDL system. A flow diverter above the liner is the best solution to increase the flow rate in the critical part of the well. With the flow diverter above the liner, the flow rate can be increased to the minimum required rate for hole cleaning and at the same time keep the equivalent circulating density at an acceptable level. The increased flow rate above the flow diverter will have minimal effect on the annular circulating pressure below the previous casing shoe. The simulations also showed that this minimum rate can be significantly reduced by changing the drilling fluid parameters. If the flow rate is kept constant, the bed height of cuttings can be reduced by optimizing the fluid parameters. Optimization of the drilling fluid is also important to make sure the cleaning in the annulus outside the liner is good. The required rate could be reduced further by increasing the rotation of the string and increasing the size of the drillpipe above the liner, and these solutions have been evaluated. Future work should include the development of a simulation program that can include all the downhole tools and components required for the Steerable Drilling Liner system, including two flow split components. The simulation program should also be able to handle the combination of a pilot bottom hole assembly and an underreamer to ream the hole to full size, and then calculate the correct amount of cuttings in the annulus. The drilling fluids should also be optimized to improve the hole cleaning in liner drilling operations further.