fax 01-972-952-9435. AbstractThrough-Tubing Rotary Drilling (TTRD) is a technique which has been developed to extend the production life of older fields by substantially reducing drilling costs to render smaller reserves of oil economically recoverable 1 . However, the small wellbore dimensions required to utilise drill-pipe inside production tubing place some unusual demands on the drilling fluid and when these are compounded by the challenges of drilling into a formation containing a large range of pressures in the reservoir and high pressure shales, a radical approach to drilling fluid design is required.The requirement for a low viscosity fluid (to minimise ECD) which would still meet all other drilling requirements meant that conventional fluids were all unsuitable and a review of the options available resulted in the selection of a novel weighting material, manganese tetraoxide, for use in a mineral oil-based mud as the system most likely to provide the desired properties. The physical properties of commercially available manganese tetraoxide (very small, spherical particles) were known to result in very low plastic viscosities in muds made with this material when compared to muds weighted with barium sulphate. A much-reduced potential for weight material sag was an added bonus.This paper details the design considerations of the fluid used to drill three TTRD side-track wells in Shell's North Cormorant field. Modelling of hydraulic parameters is presented, together with field results from the first well and how these led to modifications of fluid design and engineering practice which were incorporated into the subsequent wells.

Through-Tubing Rotary Drilling (TTRD) is a technique which has been developed to extend the production life of older fields by substantially reducing drilling costs to render smaller reserves of oil economically recoverable1. However, the small wellbore dimensions required to utilise drill-pipe inside production tubing place some unusual demands on the drilling fluid and when these are compounded by the challenges of drilling into a formation containing a large range of pressures in the reservoir and high pressure shales, a radical approach to drilling fluid design is required. The requirement for a low viscosity fluid (to minimise ECD) which would still meet all other drilling requirements meant that conventional fluids were all unsuitable and a review of the options available resulted in the selection of a novel weighting material, manganese tetraoxide, for use in a mineral oil-based mud as the system most likely to provide the desired properties. The physical properties of commercially available manganese tetraoxide (very small, spherical particles) were known to result in very low plastic viscosities in muds made with this material when compared to muds weighted with barium sulphate. A much-reduced potential for weight material sag was an added bonus. This paper details the design considerations of the fluid used to drill three TTRD side-track wells in Shell's North Cormorant field. Modelling of hydraulic parameters is presented, together with field results from the first well and how these led to modifications of fluid design and engineering practice which were incorporated into the subsequent wells. Introduction 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. Originally, coiled tubing drilling (CTD) was considered as a way to address this need. However, CTD limits the size, and therefore strength, of drill-string that can be used, does not allow pipe rotation, and requires a lot of surface equipment. For these and other reasons, the technique was abandoned in favour of TTRD. In the most general terms, TTRD is a method of sidetracking wells by rigging up BOPs on top of the christmas tree, leaving the completion in place and sidetracking as deep in the well as possible. Many of the problems associated with CTD are resolved by being able to use a larger, stronger drill-string that can be rotated while drilling. TTRD wells are challenging from a drilling fluids point of view primarily as a result of the small annular clearances throughout the length of the well. These challenges are:High kick sensitivityHigh equivalent circulating densitiesHigh pump pressures Kick sensitivity can be addressed by upgrading the accuracy and sophistication of the surface equipment but the other challenges could only be addressed by re-engineering the drilling fluid to have the lowest possible plastic viscosity for the fluid density required.