Abstract:

Some shale gas sweet spots preferentially selected based on traditional selection criteria are still of low production， the reason for which has been attributed to poor pore connectivity according to many researchers and engineers. Although hydraulic fracturing can enhance the effective exploitation area of shale reservoirs， the migration of shale gas from the matrix to hydraulic fractures depends solely on the pore connectivity within. For shale reservoirs in the Luzhou block of the Sichuan Basin， revealing the pore connectivity of the reservoirs and establishing reservoir identification and assessment model are crucial to guiding the pore connectivity assessment and selecting optimal target zones. In this study， we analyze typical shale samples collected from wells in the Luzhou block using experiments and tests， including nuclear magnetic resonance （NMR）， mercury intrusion porosimetry （MIP）， focused ion beam-scanning electron microscopy （FIB-SEM）， and large-field splicing scanning electron microscopy （MAPS）. Based on connected porosity and the proportion of connected pores， the shale reservoirs in the Wufeng-Longmaxi formations in the Luzhou block are categorized into types A， B， and C in terms of pore connectivity. Type A reservoirs with connected pore volume exceeding 0.006 7 cm³/g and connected porosity going beyond 1.75 %， generally exhibit trimodal NMR-derived pore size distribution curves， suggesting high connectivity among small pores， mesopores， and macropores， together with well-developed connected， gas-expansion-type， organic matter-hosted pores， and inorganic mineral-hosted pores. Type B reservoirs， as characterized by connected pore volume ranging from 0.005 7 to 0.006 7 cm³/g and connected porosity from 1.55 % to 1.70 %， exhibit approximate bimodal NMR-derived pore size distribution curves， reflecting high connectivity among small pores and mesopores but low connectivity among mesopores and macropores. The pores in these reservoirs are dominated by isolated， sponge-like organic matter-hosted pores， with the presence of substantial inorganic mineral-hosted pores. Type C reservoirs， featuring connected pore volume of less than 0.005 7 cm³/g and connected porosity below 1.55 %， manifest unimodal NMR-derived pore size distribution curves， reflecting an absence of connectivity among pores with different sizes. In these reservoirs， organic matter generally contains no pores， and inorganic mineral-hosted pores are also poorly developed. The results of this study indicate that a reasonable configuration of pores with varying sizes contributes to pore connectivity improvement in shales. The extensively distributed inorganic mineral-hosted pores play a role in connecting locally connected organic matter-hosted pores， ultimately forming the interconnected pore networks in the shales. Among various shale facies， clay-rich siliceous shales demonstrate the highest pore connectivity. The chart for the qualitative and quantitative identification of the pore connectivity of shale reservoirs， established based on reservoir connectivity characteristics， and MAPS and FIB-SEM data， along with the three classes of pore connectivity pattern， provide a basis for determining the pore connectivity of shale reservoirs and offer support for selecting the optimal target zones of high-quality shale reservoirs in the future.

Key words: connectivity pattern, pore connectivity, shale gas reservoir, Wufeng-Longmaxi formations, Luzhou block, Sichuan Basin