Characterization of the pore system in an over-mature marine shale reservoir: A case study of a successful shale gas well in Southern Sichuan Basin, China

Abstract

During the past two years the shale gas exploration in Southern Sichuan basin received some exciting achievements. Data of a new appraisal well showed that the gas producrtions of vertical well and horizontal well are ∼1.5 × 104 m3/day/well (with maximum ∼3.5 × 104 m3/day/well) and ∼12.5 × 104 m3/day/well (with maximum ∼40 × 104 m3/day/well), respectively, indicating a good gas potential in this area. Eight core samples from the reservoir were investigated by using a carbon sulfur analyzer, microphotometry, x-ray diffractometry, field-emission scanning electron microscopy (FE-SEM), mercury injection porosimetry (MIP), and low-pressure nitrogen adsorption to obtain a better understanding of the reservoir characteristics of the Upper Ordovician–Lower Silurian organic-rich shale. Results show that the total organic carbon (TOC) content ranges from 0.5% to 5.9%, whereas the equivalent vitrinite reflectance (VRr) is between 2.8% and 3.0%. Pores in the studied samples were observed in three modes of occurrence, namely, interparticle pores, intraparticle pores, and intraparticle organic pores. The total porosity (P) ranges from 1.6% to 5.3%, and MIP data sets suggest that pores with throats larger than 20 nm contribute little to the pore volume. Low-pressure N2 adsorption isotherms indicate that the total specific surface area (SBET) ranges from 9.6 m2/g to 18.9 m2/g, and the pore volume (V) ranges from 0.011 cm3/g to 0.020 cm3/g. The plot of dV/dW versus W shows that the fine mesopores (pore size(BJH) 2% are more than 3.0 m3/ton. The observation can be a good reference for the future exploration and evaluation of reservoir in this area.

Keywords

• Pore system
• SEM imaging
• Mercury injection porosimetry
• Low-pressure nitrogen adsorption
• Wufeng–Longmaxi gas shale