Innovative Application of Wireline Formation Testing Technology in Naturally Fractured Buried Hill Reservoirs—Case Studies from China Bohai Bay

Abstract With conventional shallow resources depleting in the Bohai Bay Basin, production decline intensifies as deep buried hill reservoirs (metamorphic/volcanic) gain prominence due to their vast distribution near the hydrocarbon-rich Bozhong Sag. Hydrocarbons sourced from Shahejie Formation migrate vertically via faults, accumulating in fracture networks. Metamorphic reservoirs feature distinct vertical zoning: a weathered zone, the main-pay inner fracture zone (100-500m thick), and basement, with ~4% matrix porosity where fractures—controlled by multi-phase tectonics—constitute primary storage. Volcanic reservoirs exhibit weak dissolution with 4.8% porosity dominated by tectonic and tectono-dissolution fractures. These fractured reservoirs face extreme heterogeneity (permeability spans 3 orders/well), complex pore systems, unreliable fracture assessment, and high exploration risks/costs, making Wireline Formation Testing (WFT) the most economical and accurate method to identify fluid properties, evaluate fracture effectiveness, and characterize storage space. Traditional WFT uses single probes or straddle-packer to seal off the target layer by pumping from formation and evaluates well productivity based on the interpretation of pressure buildup data simultaneously. However, there are few successful cases of sampling from buried hill reservoirs domestic and abroad due to their complexly structured heterogeneous reservoirs often characterized by dual-porosity media with pores and fractures, consequently, WFT faces three primary challenges: First, with fractures serving as the main fluid storage space, sampling via straddle packers requires effective fracture sealing, yet fracture effectiveness assessment remains unreliable, leading to inaccurate sampling interval determination. Second, while borehole imaging partially assesses near-wellbore fractures, fracture networks in remote zones may connect beyond the sampling interval, compromising sealing integrity. Third, in fracture-underdeveloped reservoirs, poor formation deliverability causes low pumping efficiency, where prolonged mud extraction from the packer-sealed annulus severely reduces operational efficiency. To address these challenges and improve WFT success rates, three innovative pre-test preparations and tool design optimizations are implemented: ①Integrated Acoustic/Electrical Imaging Logging comprehensively evaluates fracture openness and fracture clusters per meter, establishing regional classification criteria to determine optimal sampling intervals. ②Fracture-Scale Modeling 3 permeability orders) due to unreliable sealing and low efficiency; integrating near/far-field fracture imaging (acoustic/electrical + 50m 3DFF) with ORA's gravity-segregated dual-flowline and 6.4m straddle packer (6.4× conventional) enables effective fracture sealing, while predictive simulations boost sampling success by 40-65% and permeability data self-calibrates fracture benchmarks.