Evolution features of in-situ permeability of low-maturity shale with the increasing temperature, Cretaceous Nenjiang Formation, northern Songliao Basin, NE China

Abstract

Temperature-triaxial pressure permeability testing at the axial pressure of 8 MPa and confining pressure of 10 MPa, closed shale system pyrolysis experiment by electrical heating and scanning electron microscopy analysis are used to study the evolution mechanism of in-situ permeability in the direction parallel to bedding of low-maturity shale from Member 2 (K2n2) of Cretaceous Nenjiang Formation in northern Songliao Basin with mainly Type I kerogen under the effect of temperature. With the increasing temperature, the in-situ permeability presents a peak-valley-peak tendency. The lowest value of in-situ permeability occurs at 375 °C. Under the same temperature, the in-situ permeability decreases with the increase of pore pressure. The in-situ permeability evolution of low-maturity shale can be divided into 5 stages: (1) From 25 °C to 300 °C, thermal cracking and dehydration of clay minerals improve the permeability. However, the value of permeability is less than 0.01×10−3 μm2. (2) From 300 °C to 350 °C, organic matter pyrolysis and hydrocarbon expulsion result in mineral intergranular pores and micron pore-fractures, these pores and fractures form an interconnected pore network at limited scale, improving the permeability. But the liquid hydrocarbon, with high content of viscous asphaltene, is more difficult to move under stress and more likely to retain in pores, causing slow rise of the permeability. (3) From 350 °C to 375 °C, pores are formed by organic matter pyrolysis, but the adsorption swelling of liquid hydrocarbon and additional expansion thermal stress constrained by surrounding stress compress the pore-fracture space, making liquid hydrocarbon difficult to expel and permeability reduce rapidly. (4) From 375 °C to 450°C, the interconnected pore network between different mineral particles after organic matter conversion, enlarged pores and transformation of clay minerals promote the permeability to increase constantly even under stress constraints. (5) From 450°C to 500 °C, the stable pore system and crossed fracture system in different bedding directions significantly enhance the permeability. The organic matter pyrolysis, pore-fracture structure and surrounding stress in the different stages are the key factors affecting the evolution of in-situ permeability.