Coiled tubing drilling (CTD) technology has been widely adopted as a cost-effective re-entry strategy to sidetrack from the existing wellbores, and drill high angle/horizontal wells to maximize recovery from remaining oil columns in the Alaska Prudhoe Bay field. The oil rim in this giant field is overlain by a gas cap and has been producing for over 36 years by various recovery mechanisms such as gravity drainage, water flood and miscible injection and hence, holds a complex fluid distribution with locally variable oil-water and gas-oil contacts.

Determining current reservoir fluid contacts using multidetector pulse neutron logging (MDPNL) technology is a crucial component in the overall strategy to maximize production from each well and identify future drilling targets. This technology has been widely used in the industry for reservoir evaluation and surveillance in open- and cased-hole environments using conventional wireline techniques in standard borehole sizes. However, logging tool conveyance in CTD wells has been challenged due to small hole size and high angle/horizontal/U- shape wells.

An innovative deployment assembly was developed by BP Alaska and Baker Hughes to acquire pulsed neutron data while tripping out of the hole during the wellbore cleanout operation, eliminating the need for a dedicated logging run, which in turn, reduces rig time costs. This deployment assembly consists of a non-magnetic stainless steel carrier and a memory adaptor that is attached to the MDPNL tool. The carrier does not limit tripping operations or wellbore circulation, and requires no wireline for data acquisition.

Another major advantage of this assembly is the additional compressional strength provided to the tool, which reduces failure rates and provides a cost-effective well logging solution in highly deviated wells.

This paper presents case studies in which this coiled tubing logging technology was applied in the Alaska Prudhoe Bay field. Various aspects of the technology are discussed, including the memory-enabled, three-detector pulsed neutron tool and carrier assembly configuration. The paper also discusses identification of reservoir fluid contacts and other petrophysical properties using the nuclear attributes extracted from MDPNL data, which ultimately provides the critical information needed for optimal perforation strategy and maximize oil production in CTD wells.

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