TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractToday the cement job is the most common technique used to achieve the critical hydraulic seal between casing and formation to isolate individual zones and prevent annular flow. As such, it is often considered one of the most important operations during construction of the well. However as an isolation mechanism, it can fail, and may not provide the down hole zonal isolation needed during the life of the well.
Shell Exploration & Production in Aberdeen, Scotland has implemented through tubing rotary drilling (TTRD) as a cost effective method of accessing small accumulations of hydrocarbons in some of its mature assets in the UK sector of the North Sea. Working together with its contractors, the implementation of TTRD has taken place over the past 6 years. The most recent experience is a 5 well campaign on the North Cormorant platform during the past year. This paper gives an overview of the implementation process including a history of the work done, highlights and lowlights of drilling operations that have taken place to date. However, it focuses on the issues that were addressed in relation to the current campaign. Topics that will be covered are: Planning Process: An evaluation was done leading up to the current campaign to identify the areas of improvement that would yield the highest value. Resources were dedicated to the improvement initiative, beyond what would normally be assigned for conventional drilling operations. Novel Drilling Fluid Application: The conventional weighting agent (barite) was replaced by manganese tetra-oxide to give a low rheology fluid with enhanced anti-sag characteristics. Drilling Performance Optimisation: Work was done to reduce the torque requirement of the drill bits used for TTRD and match those requirements to the capability of the slim hole drilling motors for the required hole size. Equipment Upgrades: Surface equipment and downhole tools were developed and existing tools improved in order to improve performance and increase overall capability. Critical equipment that was either developed or significantly improved included: kick detection equipment, directional drilling bottom hole assembly, perforating guns, and swelling elastomer packers. The benefit of TTRD, demonstrated by the successful campaign on the North Cormorant, is being carried forward to the rest of the Shell Exploration & Production organisation through the establishment of Global Implementation Team specifically for through tubing drilling. Shell is looking forward to realizing the benefit of this technology worldwide. Introduction1 The North Cormorant Field was discovered in 1975 and brought onto production in 1982. The field is located 120 miles (190 km) north-east of the Shetland Islands in the northern North Sea. Production from the field peaked at just over 20,000bbls oil/day in the mid 1980s and then dropped slowly to its current level of below 5,000bbls oil/day. Throughout the life of the field, there have been over 100 wells drilled. One of the main sections of the reservoir is characterized by a high degree of faulting and poor connectivity. The faulting combined with the requirement for water injection makes production from this part of the reservoir difficult to achieve. The combination of the following factors:Large number of penetrationsFaulted and isolated nature of the reservoirSmall size of the individual fault blocks has led to a situation with:A reasonable understanding of the stratigraphyLarge numbers of wells required to access the remaining reservesSmall remaining accumulations. All of the above highlighted the need for a drilling technique that would make it possible to drill low cost, short sidetracks without the requirement for a full Logging While Drilling (LWD) capability.
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