Impossible Made into Possible – New Technologies Enabled Effective and Economical Offshore Marginal Reserves Development

Abstract Developing offshore marginal reserves is an effective way of increasing production, especially for depleted oilfields. Due to the small oil-bearing area and thin reservoir thickness, those marginal reserves were not developed in the early stage of oilfield development. However, after the water production of the main target reserve shot up, they returned to the vision of the operators. Recently in North China offshore, an infill well campaign was launched to develop the marginal reserves in BZ Oilfield. The oil-bearing area varies from 0.5km2 to 1.5km2, with an average thickness of 7m. 13 horizontal wells were planned either toward a fault or close to a fault to maximize the lateral section length. The reservoir exposure rate was critical to meet economic requirements. However, uncertain reservoir characterization increases the difficulties of accurate trajectory control. To address the challenges, an ultra-high-definition resistivity (UHDR) multilayer mapping technology synergizes with a novel Logging While Drilling (LWD) nuclear-magnetic-resonance (NMR) technology to bring solutions. The UHDR mapping service can provide a real-time visual integrated resistivity inversion with a deep depth-of-detection (DOD) up to 6m from the borehole and a high resolution of 0.2m, which can remotely map boundaries of reservoir and thin interlayers, fluid contact and resistivity distribution, facilitating an earlier proactive strategy for the wellbore position optimization and a better understanding of the reservoir distribution. Besides the common pros of NMR, the new NMR tool is capable of measuring both T1 distribution, T2 distribution, NMR porosities, and permeabilities, providing a high-resolution automated interpretation in real-time. The factor analysis differentiates fluid components successfully to understand the near trajectory fluid distribution. The synergy of the above new technologies enabled the implementation of this infill well drilling campaign, offering significant advantages: Boundary detection, both the roof and floor of the target could be delineated during the lateral section, enabling a desired wellbore position.Reservoir exposure rate maximization, distinguishing pinch-out reservoir, avoiding unnecessary shale footage, reducing costs, and shortening well construction time.Completion design Optimization, even before the well TD. Eventually, the length of the casing and high/low water-control screen were deployed according to the NMR data, providing a proactive completion design. Furthermore, a water-flooded target reserve was identified, and plan B of sidetracking and re-landing to another target was executed to ensure production. The new technologies display the effectiveness and advantages of limited-scale reserve production, made impossible to possible. The average net-to-gross (NTG) ratio reached 94%, much higher than the original estimation, and the actual well production exceeding prognosis by 35%. The new technologies also helped improve drilling efficiency, reduce costs, and optimize completion in real-time.