Strong Reservoir Wettability Heterogeneities of an Eocene Tight Oil Play from the Bonan Sag, Bohai Bay Basin as Revealed by an Integrated Multiscale Wettability Evaluation

Properly determining reservoir wettability is crucial for enhancing oil recovery and optimizing production strategies; this is particularly the case for tight oil reservoirs. The lower-fourth member of the Shahejie Formation (Es4x) in the Bonan Sag of the Bohai Bay Basin is a typical tight sandstone play, which is characterized by a low permeability and poor fluid mobility and quite variable daily production. Despite efforts in reservoir stimulation, the incremental production remains negligible. A detailed investigation of the reservoir wettability was carried out using four representative tight sandstone samples from Es4x to better understand the production behavior of the tight oil reservoir. We employed a suite of analytical methods, including Environmental Scanning Electron Microscopy, contact angle measurement, spontaneous imbibition, and Nuclear Magnetic Resonance, to comprehensively evaluate the wettability characteristics of the reservoir at different scales. Two samples, C1 and C2, exhibit oil-wet characteristics at both pore and macro scales, with Sample C1 showing weak oil-wet behavior and Sample C2 demonstrating strong oil-wet behavior. In contrast, the other samples, C3 and C4, display strong water-wet characteristics across different scales. The pore size threshold between water-wet and oil-wet conditions for samples C1 and C2 is 0.1 μm, while that for samples C3 and C4 is 1 μm. Grain-coating chlorite and grain-coating illite are the primary clay minerals contributing to the oil-wet pore walls of the reservoir, whereas dispersed sheet-like chlorite and rosette chlorite predominantly exhibit water-wet characteristics. The tight oil reservoir in Es4x exhibits pronounced wettability heterogeneities, with distinct regions displaying either oil-wet or water-wet characteristics, significantly impacting the mobility and producibility of the tight oil in this reservoir interval.