Abstract
The Lower Paleozoic Ordovician strata within the Ordos Basin harbor dolomite gas reservoirs are characterized by low porosity (0.98% to 14.2%) and low permeability (0.001 mD to 2.8 mD). Gas extraction from these reservoirs is frequently impeded by water lock due to the intrusion of water-based drilling fluids and the accumulation of formation water, which increase water saturation near the wellbore and significantly decrease gas permeability. This research is pivotal in elucidating water-lock mechanisms and developing water-unlocking strategies for such tight gas reservoirs. Comprehensive analysis through wettability tests, spontaneous imbibition, high-speed centrifugal drainage, and nuclear magnetic resonance (NMR) revealed that Jingbian gas field rocks are predominantly water-wet with a spontaneous imbibition water saturation of 60% to 80%, indicating a high propensity for water lock. The pore structure, mainly within the 200 to 300 nm range, presents challenges as high-speed centrifugation achieves only 70% to 80% water saturation displacement, with a drainage rate of about 20% to 30% and a drastic decline in gas permeability by several orders of magnitude. This study identifies the surfactant sodium dodecyl benzene sulfonate (SDBS) as an optimal agent for enhancing water displacement and gas production. At a 0.1% concentration, SDBS improves drainage rate and permeability by 58.5% and 69.42%, respectively, demonstrating its efficacy in mitigating water lock and enhancing recoverability in tight dolomite reservoirs. These findings serve as a scientific guide for augmenting production in similar geological settings.
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