Drilling in Salt Formations and Rate of Penetration Modelling
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As the industrialization of the world is growing, both energy consumption anddemand are steadily increasing. Hydrocarbons have been an important energyprovider for several decades, but the production from mature oil and gas producersis now declining. In order to meet this rise in energy demand, new hydrocarbondeposits must be found and produced. Because large oil and gas reservoirs areassociated with salt formations, this may be an important energy source for thefuture. Salt s low permeability and ability to deform under stress and temperature,makes it an ideal hydrocarbon-trap.The initial objective of this thesis was to establish which challenges are related todrilling for pre-salt hydrocarbons, and propose solutions for how to overcome thesechallenges. When drilling towards these oil and gas reservoirs, the first problemsmay occur already in the formations above the salt structures. The density of saltdoes not increase with burial depth. When its density becomes lower than of thesurrounding formations, salt will start to migrate and push through the overlyingrocks. Due to the combined effect of compaction disequilibrium and salt tectonics,complex stress patterns can be created in the formations surrounding the saltstructures. This may lead to many hazardous scenarios, created by rubble zonesand recumbent beds. When drilling inside salt formations, it is salt s ability tocreep and flow that may cause problems. Salt flow may cause borehole deformation that impedes with the drilling and casing operations. When a wellbore has beendrilled, salt can creep into the removed rock volume. This may cause situationssuch as stuck-pipe, hole instability, and high levels of shock and vibration whendrilling. Salt flow is a positive function of time, so minimizing the time factorwill decrease the possibility of salt flow related problems. One of the measures tominimize exposure time is to perform drilling operations quickly. Hence, a highrate of penetration (ROP) is beneficial when drilling in salt.As part of proposing solutions to the drilling challenges in salt, two new drillbittechnologies have been evaluated. These bits may be beneficial in order to overcomemany of the problems related to salt drilling. Based on the results obtainedin previous studies, these bits are capable of reducing the shock and vibrationlevels while drilling. This is because these bits are able to drill smoother thanconventional PDC bits. Further, the reduced shock and vibration levels will allowan increase in WOB and rotary speed. Based on previous studies, these arethe two of the most important parameters to ROP. Thus, these two new drillbittechnologies might be able to increase ROP when drilling in salt.Another goal for this thesis was to establish which parameters have the most effecton ROP when drilling in salt formations. Knowing this could help minimize thechallenges faced due to salt creep and flow. Therefore, a modelling attempt wasperformed using Bourgoyne and Young s ROP model. This model uses multiplelinear regressions to calculate a straight line that best fits the data used in themodel. In this attempt, data acquired from a well drilled in salt formation wasused. Due to lack of variation in the drilling data, several parameters had tobe discarded from the model in order to obtain physically meaningful results.This led to only three variables being used in the model. This was weight on bit(WOB), rotary speed of the drillstring, and jet impact force. It was found thatthe parameter that had most effect on ROP in salt was WOB, followed by rotaryspeed, and last, jet impact force.