Experimental circulation loss study
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Circulation losses could occur during any operation that involves pumping into a well. As of today, it is recognized as one of the most costly drilling problems. In some situation it might be hard to stop, and usually takes precious rig time to deal with the problem. In order to mitigate the risk of circulation loss solid particles are used in the active drilling fluid, known as lost circulation materials (LCM). These materials have a tendency to increase the fracture gradient of the well. Circulation losses occur in different ways, however, the type of loss that is treatable with LCM are those related to fractures in the wellbore wall. LCMs in the active drilling fluid will create bridges at the fractures; seal them off and stop/reduce the losses. Numerous of studies have been conducted for water-‐based drilling fluids, but not so many on oil based fluids. One of the big differences between water and oil based fluids is that the friction between the LCM particles tend to be less for oil based fluids opposed to water based fluids. Due to this reason, the bridges created with an oil-‐based fluid are, somewhat, more unstable than bridges formed by a water based fluids. Experiments in this thesis are divided into two parts. Part I attacks the problem of finding a suitable particle size distribution (PSD) for bridging. A theoretical PSD was proposed, but test results showed that the theoretical PSD was not very suitable for bridging purposes. However, an interesting observation was made. As the concentration of smaller particles increases, the fluid seemed to perform better in terms of bridging. The bigger particles form some sort of a framework at the fracture mouth, whereas the smaller particles fills the voids between the bigger particles. This indicates that smaller particles are also important in order to achieve good bridging properties. The bridging properties of four different materials are tested as LCM in oil based drilling fluid in part II of the experimental part of this work. CaCO3, LC-‐Lube, Feldspar and Quartz were tested in a bridge apparatus. A couple of different observations were made. Firstly, CaCO3 mixed with LC-‐Lube (Graphite) has shown to be suitable for creating bridges in water-‐based systems. In this study, CaCO3 and LC-‐Lube did not show promising results, not separately or in a mix. On the other hand, Quartz and Feldspar had good results, and was able to withstand a high average pressure for a wide spectrum of fracture openings. This was the case even for reasonably small concentration of the materials mixed in the drilling fluid. Both of these materials had good bridging capabilities with a concentration of 39kg/m3. At a fracture opening of 500-‐micron, Quartz proved to be 400% better in terms of average test pressure than CaCO3. Generally, particles at the upper part of Mohs scale of hardness performed better than particles at the lower part of Mohs scale. This observation is in line with the recently developed Elastoplastic fracture model from the University of Stavanger. Synergy between the materials was also tested. No good combinations were found. All materials acted better as LCM separately opposed to being in a mixture with any of the other materials.
Master's thesis in Petroleum engineering