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

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 & Pumping Simulation models fracture development levels and emulates pumping under varying scales, guiding precise interval selection. ③Next Generation Formation Testing(FTNG, also called ORA) Technology utilizes dual-flowline/dual-inlet design leveraging gravitational segregation to accelerate hydrocarbon entry. Its 6.4-meter straddle length (vs. conventional WFT 1-meter) significantly enhances sealing reliability by bridging distant fracture networks. This study firstly presents diverse WFT failure cases with root-cause analysis, then implements tool optimization and fracture-reservoir pumping simulations based on integrated acoustic/electrical imaging for fracture effectiveness assessment. These approaches significantly enhance wireline formation testing success rates in naturally fractured reservoirs. While WFT primarily evaluates formation fluid properties and acquires samples, its permeability assessments concurrently verify fracture effectiveness accuracy, thus establishing critical calibration references for future image-log interpretation. With advancements in 3D Far-Field (3DFF) acoustic technology enabling fracture characterization within 50 meters of the wellbore—including orientation and spatial distribution—WFT feasibility assessment in highly networked reservoirs now integrates borehole imaging (acoustic/electrical) and 3DFF data. This combined approach facilitates comprehensive near-wellbore to far-field analysis, providing critical inputs for optimizing straddle-packer length design to ensure sealing integrity. Conventional WFT fails in Bohai's heterogeneous fractured buried hills (>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.