Modeling of gel deformation
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Drilling fluids can build a gel structure under periods with ceased circulation. Pumping or drill string rotation can be used to break the gel. In some cases the measures might fail, e.g. gel breaking pressure might exceed the available pump pressure, or drill string rotation breaks the gel in the annulus, but not inside the drill string. The question arises about how high the drill string must be hoisted up in order for gravity to break the gel.A simple model for the hoisted drill fluid level height exist. It predicts the height to be linear and requires gel strength measurement as an input. Previous experimental work by Kronborg was an attempt to verify the model. The lack of advanced measuring techniques proved it difficult, and the results by Kronborg were inconclusive. This thesis aims to prove the model by the use of modern rheometer measurements. A series of amplitude sweeps in oscillatory tests are performed in order to determine the shear stress at flow point as the manifestation of the gel strength. The new measurements are used as the input to the model. The modelled, theoretical values of drill string height are compared to the experimentally measured. Also, previous measurement procedure is repeated. The results show a better coherence between the height values measured experimentally and the theoretical values with the newest gel strength measurements as an input. Still, the deviation between those values is too high to conclude the model to be correct. There might be several reasons for that. The model did not account for mud compressibility, thixotropy or temperature variation of the fluid. Also, sources of error are discussed and potential improvements suggested.The attempt to develop a mathematical correlation for the gel strength versus temperature and gelling time was made (the Gandelman correlation). It failed due to poor measurements results at elevated temperatures and long gelling times.