Borehole Stability in Coal-Bearing Formations on Heidrun
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The objective of this master thesis is borehole stability in the Åre Formation of the Heidrun reservoir, in connection with open hole sand screen completions. Due to operational limitations this type of completion will be more sensitive for borehole stability problems, and each failure or misrun will cost a lot of money in lost equipment, downtime, side tracking and delayed production. To find a best practice for drilling and completing these wells will therefore be of key importance for the further field development. The purpose of this Master Thesis was to perform literary studies on coal, borehole stability of fractured formations and previous coal experience, and tie this back to the experienced problems with running screens in the Åre reservoir. Some of the main results are shown below: o Coal is a combustible, organic rock with large heterogeneities and anisotropydue to a very characteristic cleat system. With confining strengths Heidrun coal are quite strong with a UCS of 37 MPa from rock mechanical laboratory testing performed by SINTEF. However, due to the cleat system coal appearsas an easily friable rock, which can cause significant problem downhole whenrunning equipment like open hole screen completions, as is the case on Heidrun. o The presence of coal in the Heidrun reservoir has been investigated, as has the coal appearance in Åre problem wells. The coalbed thickness is mostly quite small, about 0,5 mTVD, and this would most likely not cause a problem inlow to medium angle wells. Almost horizontal wells on the other hand canhave severe differences with this thickness, well illustrated by well A-24 T2. o Due to the unpredictable nature of the coal abundance and cleat orientation some uncertainties can not be eliminated in the planning phase. o Even though there are at least 4 of the problem wells that seemingly has nocoal in them, the general assumption from CPI logs is that coal is the mainproblem geology regarding hole enlargements and possible ledge problematic. o Due to the cleat system, coal can be particularly sensitive to pressure transients, caused by surge/swab and ECD variations. This also applies to other geologies, but to a much smaller degree. The solution can prove to be implementing a good mud system that can prevent pressure equalization through the cleat system of coal. Also LEDO drilling and “finesse” during drilling operations are recommended. o The increase in failure wells in the recent years can possibly explained by an increase in ECD and other pressure transients. In addition, depletion in thereservoir can in case of a permeable borehole wall and high angles have a negative effect on stability.