This paper discusses the planning and preparation process for drilling and completing a 10km departure well at the Wytch Farm Oilfield in Southern England. An 8km departure well was drilled at Wytch Farm during 1995. At the time, this was at the limit of drilling operations. Some fundamental changes involving new technology, operational practices and techniques have been necessary to enable this well to be drilled. More specifically, the paper will describe the directional drilling techniques necessary to enable the reservoir interval to be accessed. Research and development work performed by the drilling team to identify mechanical limitations in drilling operations at this departure are also examined. This includes a comprehensive review of offset well data to determine friction factors for all operations and concomitant measures adopted to limit rotary torques and drags to workable levels. The other major focus of this paper is to appreciate the mechanism by which drilling muds are lost to the permeable reservoir and describe methodologies employed to overcome this problem. The paper will also discuss recent experience with the application of partially floating casing, rotary steerable drilling systems and running and cementing long liners. Completion and intervention challenges are also examined with an analysis of future requirements. Introduction The development of the offshore extension of the BP operated Wytch Farm Oilfield, located on the South coast of Britain, utilising extended reach (ER) wells into the offshore part of the Sherwood sandstone reservoir has been extensively documented in a series of technical papers [1], [2] and [3]. (Figure 1). The current phase of the development is to tap into significant reserves located at reservoir departures in excess of 10km. The Drilling Challenge. The section of the reservoir targeted by this Ultra-Reach well (Measured Depth : True Vertical Depth ratio >5), M-11, lies between 8km and 10km departure from the 'M Wellsite' where the Deutag T-47 rig is located. This site, specially constructed to access the offshore portion of the reservoir currently contains 10 extended reach wells and yields over half of the total 110,000 bopd production at Wytch Farm. The challenge is to safely and efficiently drill and complete a 10km departure well into the target area to access oil in place of 24mm STB. A single horizontal well into this target is expected to recover some 25–35% of these reserves. Although this is likely to be the most expensive well drilled at Wytch Farm, the significant reserves that it will access are critical to the long term profitability of the field. The well design is the culmination of over a year's evaluation, design and planning, providing an excellent test of the industry's capabilities. Successful delivery of this well will have a significant impact on the industry to access reserves that a decade ago would have required major capital expenditure, in the shape of new facilities, to recover. P. 191^
The application of particle sealing technology, field monitoring techniques and improved operational practices has been used to minimise downhole lost circulation while drilling extended-reach wells. These techniques have been applied at the Wytch Farm Development in Southern England and in the Pompano Development in the Gulf of Mexico. Using particle sealing technology, losses were reduced from an average 5,000 bbls of low toxicity mineral oil to zero while drilling the 8" section in offset Wytch Farm wells M-07, M-09 and down hole synthetic mud looses were significantly reduced on Pompano A-l8. Pressure While Drilling (PWD) data have contributed to the improved understanding of how changes in downhole pressure influence lost circulation. P. 871
Drilling rigs were traditionally designed and built to drill vertical wells. Their horsepower rating and capacity is still quoted in terms of vertical depth. Such rigs and associated tubulars have limitations when drilling ERD wells and require modifications to give them the necessary capacities to drill to 8 km departure and beyond. This paper will initiate the conceptual design of an ERD drilling rig capable of reaching these departures. Introduction It is often not possible to build the ideal new rig for a particular drilling programme, we will therefore consider the limitations of existing (conventional) rigs for drilling ERD wells and consider how to extract the maximum potential from such existing equipment and the necessary additions or modifications to give them the required capacities. While drawing heavily on the experience with Deutag's rig T-47 at BP's Wytch Farm oilfield (Fig. 1), where one well in the current ERD programme has already been drilled to an 8 km departure at 1,600 m TVD and one is planned to reach 10 km in 1997 (Fig. 2), information has been drawn from other such projects and also from those that are still as yet in the conceptual stage. This paper looks primarily at the surface and downhole equipment supplied traditionally by the Drilling Contractor. Due to the ongoing advances in ERD technology any recommendations for rig design or modifications made today are unlikely to be valid in a year's time. From experience at Wytch Farm it would be prudent to build a "redundancy factor" of, say, 25% into all aspects of rig design to allow for future developments. But, wherever possible, extra equipment and associated power requirements should be minimised by looking at ways in which to reduce loads through mud pump pressure and drillstring torque-and-drag reduction. Conventional hole sizes have been used throughout since there is currently limited small-hole equipment available with which to drill these wells. Smaller hole sizes might be considered for a mature ERD application where the risks are better understood. The attached Figures consider a theoretical well with 33,000 ft (10 km) of 12 1/4" hole and a final measured depth of 49,500 ft (15 km) in 8 1/2" hole. 1. Drilling Mud and the Mud System Hole cleaning is of critical importance when drilling the high angle tangent sections of ERD wells. To obtain the necessary annular velocities higher flow rates result in a higher demand on the mud pumps. Experience at Wytch Farm shows that above a tangent angle of 80 degrees in 12 1/4" hole flow rates of at least 1,100 gpm are required to clean the hole as it is being drilled. On an existing rig it is therefore often necessary to:increase the number of mud pumps from two to three or moreincrease the power rating of the pumps from 1,600 to 2,000 or 2,200 HPincrease the pressure rating of the pumps and surface system from 5,000 to 6,000 or 7,500 psi. Apart from the capital cost of option "c" the maintenance costs can be expected to double for the higher pressure equipment. It is unforeseen that the full output from even three 2,200 HP mud pumps would be required simultaneously but at the anticipated higher surface pressures consumable parts replacement occurs more frequently and the extra pump(s) may allow less disruption to the drilling programme from pump repairs. The extra capacity may also allow only one liner or plunger size to be used for the complete well programme. While there may be constraints on the maximum number of mud pumps installed, the chosen solution will inevitably be a compromise between mud pump capacity and drillpipe size(s). P. 225^
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