The temporary plug and abandonment (P&A) of old wells, and the following slot recovery, will be important for increasing the average rate of recovery from the Norwegian Continental Shelf (NCS). However, these operations faces challenges like high costs, safety concerns, environmental issues, and rapidly growing demand. Maintaining well integrity may be difficult when re-entering old wells. The demand for increased efficiency may lead operators to compromise on safety to finalize projects in time. This thesis tries to give a broad understanding of the well integrity issues on the NCS, and then tie these ndings around the term slot recovery operations. It will be important to understand the aspects which aects the lifetime of a well. Especially the long-term pressure, temperature and chemical effects on casings/tubings are important aspects to be understood. If this is done, one can increase the material lifetime in a well, and thus be able to re-use more of the casing strings in a slot recovery. These measures will help to keep marginal fields protable for a longer period. The thesis has also kept a strong focus on the challenges regarding the planning phase of a slot recovery operation. Of the essential factors in a slot recovery is to verify the old barrier envelope, and based on these findings create a robust operational plan. One of the mistakes which has been done in several slot recoveries is that the plan is created before any tests have been done. Once the plan is set and signed by the management, changes are harder to implement. Testing of the well often reveals unexpected factors which needs to be taken into consideration when planning an operation. Two separate slot recovery operations were also studied. They were carefully chosen to highlight typical issues with a slot recovery, and thus show that the theory fits with the reality. The requirements and guidelines for wells on the NCS also needs to be more customized for slot recovery operations.

Drilling in deep water is requiring more advanced technology as fields at greater depths are being discovered. Managed Pressure Drilling and Dual Gradient Drilling are both offering different techniques for navigating through the narrow pore pressure and fracture gradient window during an operation. Along with different drilling and development systems they are introduced as possible solutions to many of the challenges associated with deepwater drilling. The industry is looking into the possibility of doing highly deviated extended reach wells in deep water environments. Before doing so different simulations are done to investigate which factors will limit the maximum well trajectory and to figure out of far it is theoretically possible to drill in horizontal and vertical direction. With the help of the WELLPLANTM software a reservoir located in the Gulf of Mexico is chosen as a well candidate to run simulations on. Case study shows that for both directional extensions buckling of the drillpipe is what keeps us from drilling further. In terms of torque and pump capacity both rig candidates used for the study are well within their maximum capacities. Equivalent circulating density (ECD) would have been the main problem for the case study, but can easily be compensated for assuming we have the potential to control the pressure profile. With conventional drilling we would not be able to handle problems associated with ECD, meaning that DGD or other methods are required. From the sensitivity study we learn the importance of having access to accurate wellbore data, as a reduction in friction factor has the potential to extend the well trajectory even further and a potential dogleg severity would make us unable to reach target depth.

Every year kick incidents occur, maybe best remembered by the Macondo blowout in April 2010 resulting in devastating oil spills throughout the Gulf of Mexico. Well control is one of the most important factors in any drilling operation, preventing disastrous blowouts where people and the environment will be affected. The development of new technologies has increased significantly, lowering the risks of blowouts, mostly because of the reliability of blowout preventers. Better hardware systems have been developed and better materials has increased the performance during critical parts of an operation. There are several causes why we encounter kicks; not keeping the hole full, lost circulation, swabbing, underbalanced pressures, trapped fluids/pressures and mechanical failures. Before an actual kick, there are warning signs that might occur and knowing how to interpret positive indicators of kick is very important. Pit gain, increase in return flow rate and abnormalities in drillpipe pressure are all signs that formation fluid has entered the well. When experiencing a kick, procedures to reduce the danger and the non productive time have to be started. Firstly the well has to be shut in by either the hard shut-in method or the soft shut-in method. Then the influx has to be circulated out of the well by the use of either the Driller’s Method or the Wait and Weight Method. To better understand and visualize the behavior of formation fluid entering the well, the simulation program Drillbench Kick has been used. The soft shut-in has been compared against the hard shut-in and the Driller’s Method has been run against the Wait & Weight Method. The simulations have been performed with both oil based mud and water based mud.

Wells that are drilled today are becoming deeper and more complex, and actions to make these wells economically justifiable are desired by oil companies. One method is to reduce the non-productive time during drilling by analysing the hook load response to restrictions in the well. The goal is to recognize abnormal behavior as early as possible to prevent stuck pipe or other incidents that may lead to non-productive time. In present thesis a mathematical model for simulating hook load during normal tripping conditions is made. The model is based on theory about forces acting in the borehole, and properties of the drill string and the wellbore are inserted into it. The goal is to make good hook load simulations for simple tripping conditions, as restrictions most often cause problems during tripping operations. If restrictions were to occur, the changes in hook load during tripping would be identified quickly, and remedies can be started early. A laboratory that simulates tripping out of a nearly horizontal well is buildt and experiments are done. The laboratory experiments are carried out with and without mud flow, to identify also the forces that depends on mud circulation. Washout has been imitated in order to see how restrictions affect the hook load. The model consists of mathematical equations that describe all forces acting on the drill string. The laboratory results were used to adjust the equations, so they match hook load behavior even better. From comparison with the laboratory results it was found that the model creates accurate hook load simulations under simple conditions. Simulations with mud flow during tripping were inaccurate because the forces that occur during circulation were poorly mathematically described in the model. It is not possible to run real time drilling data in the model to see if the model is realistic because block position measurements are missing and some measurement rates are too low, so the equations in the model is not general enough. With improved position measurements and following adjustments in the model, it is likely that the hook load behavior model could be used to reduce the non-productive time during drilling.

To cover the world’s future consume of hydrocarbons, technological improvements are needed, turning currently unreachable and unprofitable reservoirs into the opposite. The main focus of this thesis, Dual Gradient Drilling (DGD), is a drilling technology that is ideal for drilling in ultra-deep waters, and could prove vital in drilling the reservoirs that are currently undrillable. This will increase the possible supply of hydrocarbons available for consumption. DGD is not fully accepted in the drilling industry, and DGD is still considered unconventional. In this thesis, simulations investigating pressure control in a DGD system are done, showing possibilities and limitations when using DGD. Also, it is done a study of how the challenges faced on the Macondo prospect in the Gulf of Mexico could have been solved better by using DGD instead of the conventional drilling methods. MATLAB was used for mathematical simulation of the control of the hydraulic pumping system. The program reads field- and equipment specific input data from an excel sheet. The different input parameters are changed separately, and simulations are run for each parameter change, showing each parameter’s effect caused to the system abilities. Criteria for approved well control are set, and by trial and error with the program, requirements for pump rates, well bore design and pressure safety margins needed are found, presented and discussed. It is here shown that the currently available pumping rates are not able to control the pressure as agile as other available drilling technology can, using the marine riser size currently developed. A subsea pump producing rates of more than 13000LPM, the double of what can be produced today, is needed. Utilizing a narrower marine drilling riser, with an inner diameter (ID) of 12¼” instead of the conventional riser with 19½” ID, can increase the speed of a pressure change with 171%; from 0.045bar/s to 0.121bar/s, when other parameters are kept constant. Because of linearity, a doubling of pumprates will result in a doubling of pressure change speed. It is here concluded that sufficiently agile wellbore control, depends on the development of a narrower riser and higher subsea pump rates. Other unconventional drilling currently available, like Managed Pressure Drilling (MPD), is far superior to DGD when it comes to quickness and accuracy in keeping the BHP at a desired level.

Many accident investigation techniques and other methods used by the petroleum industry today list a set of underlying human related causes and subsequent improvement suggestions. Do these techniques address the root cause behind the problem so that the appropriate initiatives can be implemented? The focus of the present thesis was to determine the human related root cause of two major accidents in the North Sea. This in order to give recommendations to improve the safety levels in the organisation. In order to achieve the above-mentioned goals, the IPT Knowledge Model was adapted to the given accidents. The data input into the model was based on interpreted observations from former investigation reports. The analysis of the blowout on Snorre A and the well control incident on Gullfaks C resulted in 49 and 63 observations respectively. For both accidents, the Human Factor that was indicated to have the largest affect on the accidents was Training and Competency (29% for Snorre A and 19% for Gullfaks C). Lack of competence was indicated as the majority subclass. Collectively, management and supervision, or lack thereof, was also indicated as being a contributing factor to the accidents. These final results coincide with the findings in other investigation reports. However, these are more acute, indicating a specific area of improvement within the company. By increasing the competency levels within the company and ensuring that the leaders and management have the proper tools to follow-up their employees and their operations, the safety levels and culture will improve.

Denne oppgaven omhandler teoretisk modellering av trykktap grunnet rotasjon i eksentriske borehull med Herschel-Bulkley strømning. En eksisterende modell for trykktap for konsentriske borehull ble videreført til å inkorporere borerrørets eksentrisitet ved å fremstille ett konsentrisk borehull med flere konsentriske borehull med varierende radier. Gjennom å studere numeriske resultater for trykktap for både Newtonske og ikke-Newtonske strømninger i ringrommet, ble en korrelasjon for trykktap grunnet akselerasjon av strømningen laget som en funksjon av viktige parametere. Disse inkluderer Taylor nummer, eksentrisitet og radiusforhold (borerør- mot ringrom-radius). Den nye modellen kan altså beregne trykktap i ringrommet i eksentriske borehull med effekten av rotasjon (skjær-fortynnende og akselerasjon av strømmen inkludert); eller enklere scenarioer hvis det er ønskelig. Modellen ble skrevet til et enkelt program som kan brukes i felten. Programmet ble videre validert mot eksisterende numeriske data og feltdata. Resultatene var svært gode for konsentriske borehull og trendene ble riktig regnet. Trendene for Herschel-Bulkley strømninger var ikke like gode som for de andre fluidene i eksentriske borehull, men anses fortsatt som tilfredsstillende.

En av de viktigste operasjonene i en brønnkonstruksjon er primærsementering. En dårlig utført primærsementering kan være en fare for både mennesker og miljøet rundt. En reduksjon i produktivitet, dyre sementering løsninger for å reparere skader og tap av brønnen kan komme av en dårlig primærsementering. Migrering av olje og gas grunnet mangel på isolasjon kan føre til forurensninger, i verste fall kan mangel på isolasjon føre til en farlig utblåsning. I dag er det slik etterspørselen etter olje og gass økes mens oljen og gassen som er ”lett tilgjengelig” minkes fort. Dette fører til at operatører blir tvunget ut mot dypere vann, arktiske områder, reservoarer med høytrykk og høytemperaturer og andre ukonvensjonelle resurser blir oppsøkt. Konsekvensen av dette er at de ulike operasjonene som er involvert blir mer komplisert, spesielt primærsementeringen. Dagens sementeringsteknologi gir ikke tilstrekkelig isolasjon over lenger tid. For å forbedre på primærsementering har alternative materialer med egenskaper som gir bedre langtidsisolering blitt evaluert. En vurdering av ThermaSet og Sandaband som primærsementerings materiale ble gjennomført, forslag til plassering av materialet og evaluering metoder ble også diskutert. Materialene ble sammenlignet opp mot konvensjonell sement. Vurderingen av materialene viste at det kan være mulig å bruke både Sandaband og ThermaSet som erstatning for sement i en primærsementerings operasjon. Begge materialene hadde egenskaper som gjorde dem passende for en langtidsvarende primærsementering, men sement vil fortsatt være det eneste materialet for primærsementering i nær fremtid. Dette er på grunn av den overlegne evne til å støtte foringsrøret, mangfoldet av sement å velge mellom, og de lave kostnader av sement. Bruken av Sandaband vil være begrenset til enklere brønnløsninger, et eksempel på bruk av Sandaband kan være vertikale brønner i nærliggende gassreservoarer. Når det gjelder ThermaSet så er det fortsatt en del tester som per dags ikke er blitt utført. Et forslag til å redusere svakheten av sement og best mulig forbedre på primærsementeringen er å benytte seg av forebyggende tiltak, et eksempel på et forebyggende tiltak er ”swellable elastomer packers”. SEP i bruk sammen med sement kan styrke sementen. En vurdering av muligheter til å kombinere ulike sement systemer sammen for å oppnå en bedre og langvarig sement design kan også være en løsning til å styrke svakheten med sement.

The main objective of drilling a well is the potential economic profit. The total cost of a drilling operation result from a complex interplay of many factors (Graham and Muench, 1959). Work that can optimize some of these factors is desirable, and the need for information that can eliminate unfortunate incidents resulting in increased operational time and make the operation more efficient is constant. Well cemented layers and the composition of the cemented materials are the main causes of hard stringers. Hard stringers may lead to severe downhole problems like washouts, severe local doglegs and keyseats. Such downhole problems are often related to the unexpected appearance of hard stringers and may cause the drill string to stick and result in lost time. Hard and soft stringers may be detected through real-time drilling data. This is achieved by comparing different drilling parameters, like rotary speed (RPM), weight on bit (WOB) and block position (BPOS). A decrease in RPM followed by an increase in WOB indicates hard formations. However, manual analyses are time consuming and inefficient. A modified and improved version of the hardness detection program previously developed by Solberg (2011) is presented in present thesis. The model is based on a simplified version of the rate of penetration (ROP) equation proposed by Bourgoyne and Young (1986). Drillability is the desirable result from the equation, as drillability is the inverse of hardness. Not all the functional relations that initially are part of the ROP equation have been assumed relevant for the detection of soft and hard stringers. Some of them alter only gradually, either with depth or as the drilling operation progresses, and do not affect the sudden change in drillability as the well is being drilled. WOB, RPM and ROP (ΔBPOS/Δt) are drilling parameters chosen as relevant for present work. They are obtained through real-time drilling data and utilized in the calculations of drillability. Exponents related to the WOB and the RPM functions will frequently vary with the hardness in the formation. However, frequent manipulation of the exponents is difficult to achieve and only one soft and one hard formation exponent related to each exponent type have been applied. The final result shows a plot of hardness variation with depth. The plot has been proved to correlate well with experienced hard stringers stated in the Final Well Report. The program has been proved to be able to detect lithology transitions. The boundaries of particularly the Utsira Fm are evident on the hardness curve. The detection of Utsira Fm is enhanced by both gamma ray and sonic log from the same formation in wells nearby. Correlations between the two logs and hardness have been established. However, due to absence of sonic and gamma ray data from Well 0, the trustworthiness of the establishment is hard to evaluate.

AbstractUnderbalanced drilling has received more and more attention in recent years. The reason for that may be because many oil fields, especially on the Norwegian continental shelf, have started to show signs of aging. By that I mean that the production is going down, and reservoir pressure is more and more reduced. Underbalanced drilling is a technique that is suitable for dealing with these challenges. The major benefits like for example reduced reservoir damage, and the ability to drill through narrow pressure windows, both help in prolonging the life of mature fields. To be able to perform a successful underbalanced drilling operation control over the pressure in the well is crucial. The bottom hole pressure must be controlled and adjusted as the operation goes on to make sure that underbalanced conditions are maintained at all times. My main goal with this thesis have been to demonstrate how the bottom hole pressure can be controlled by different parameters, and the investigation of different scenarios that can occur during an underbalanced operation and how they affect the bottom hole pressure.The Drillbench package contains two multiphase flow simulators that are specialized for underbalanced drilling. The Steadyflodrill is a steady-state simulator and the Dynaflodrill is a dynamic simulator. The dominant simulator in my work has been the Dynaflodrill.During drilling the bottom hole pressure is adjusted and controlled with three different parameters. These are the liquid injection rate, gas injection rate and the choke opening. Which of these parameters that will change the bottom hole pressure the fastest will vary with the type of well being drilled. Doing a connection during underbalanced drilling can be problematic, but this will vary with the type of reservoir being drilled. Separation of fluids in the wellbore when circulation is halted can cause severe fluctuations in the bottom hole pressure. By using the choke these fluctuations can be reduced to an acceptable level. Tripping underbalanced also offer challenges regarding separation of fluids which will kill the well after some time since the circulation is stopped for a long period. Also reservoirs containing heavier oil will be killed eventually if the reservoir is under pressured. Drilling through narrow pressure windows can be problematic if a pump failure should occur. This failure can lead to a sudden drop in the bottom hole pressure which can cause it to drop under the collapse pressure limit of the formation and cause for instance a stuck-pipe situation. Hitting an unexpected high pressure zone can lead to a well control situation due to an increased influx of formation fluids which exceeds the limits of the separator.Automation of underbalanced drilling has been difficult due to low data transportation rate from the bottom to surface, and difficulties with correct modeling of the multiphase flow in the well. A possibility can be to make use of the pump pressure and make an estimation of the bottom hole pressure based on that and a model of the friction in the drill pipes.