Heat-Induced Pore Structure Evolution in the Triassic Chang 7 Shale, Ordos Basin, China: Experimental Simulation of In Situ Conversion Process

Abstract

The reservoir properties of low–medium-maturity shale undergo complex changes during the in situ conversion process (ICP). The experiments were performed at high temperature (up to 450 °C), high pressure (30 MPa), and a low heating rate (0.4 °C/h) on low–medium-maturity shale samples of the Chang 7 Member shale in the southern Ordos Basin. The changes in the shale composition, pore structure, and reservoir properties during the ICP were quantitatively characterized by X-ray diffraction (XRD), microscopic observation, vitrinite reflectance (Ro), scanning electron microscopy (SEM), and reservoir physical property measurements. The results showed that a sharp change occurred in mineral and maceral composition, pore structure, porosity, and permeability at a temperature threshold of 350 °C. In the case of a temperature > 350 °C, pyrite, K-feldspar, ankerite, and siderite were almost completely decomposed, and organic matter (OM) was cracked into large quantities of oil and gas. Furthermore, a three-scale millimeter–micrometer–nanometer pore–fracture network was formed along the shale bedding, between OM and mineral particles and within OM, respectively. During the ICP, porosity and permeability showed a substantial improvement, with porosity increasing by approximately 10-times and permeability by 2- to 4-orders of magnitude. Kerogen pyrolysis, clay–mineral transformation, unstable mineral dissolution, and thermal stress were the main mechanisms for the substantial improvement in the reservoir’s physical properties. This study is expected to provide a basis for formulating a heating procedure and constructing a numerical model of reservoir properties for the ICP field pilot in the Chang 7 shale of the Ordos Basin.

Pressure Analysis of Onshore and Offshore Shale Gas Reservoirs under Constant-Rate Condition Considering Thin Sandstone Layer and Interlayer Cross-Flow

“…Firstly, the capacity/resistance ratio (CRR) and the time of arrival at the boundary (t BDF ) can be calculated by Equations ( 5) and (7). Meanwhile, the given constant rate is q c to be this value.…”

“…where m(p) is the pseudo-pressure, ki is the intrinsic permeability, * ( ) z p is the pressur dependent compressibility factor. Based on the above definitions and equations, a new analytical calculation metho can be obtained to perform pressure analysis of onshore and offshore shale gas reservoi under constant-rate condition considering interlayer cross-flow and thin sandstone laye Firstly, the capacity/resistance ratio (CRR) and the time of arrival at the boundary (tBD can be calculated by Equations ( 5) and (7). Meanwhile, the given constant rate is qc to this value.…”

“…Production characteristics of shale gas wells containing thin sandstone interbeds are different compared to conventional shale gas reservoirs [5,6]. Therefore, pressure analysis and performance prediction of shale gas reservoirs with thin interbedded sandstones is challenging and crucial, and have attracted the attention of many scholars at home and abroad [7][8][9][10][11][12].…”

Evaluation of Recoverable Hydrocarbon Reserves and Area Selection Methods for In Situ Conversion of Shale

“…China has focused geological studies on shale oil since 2010, especially on key technological breakthroughs and exploration dimensions [9][10][11][12]. Moreover, in 2023, there were over 2500 horizontal wells, and the shale oil yield reached 342 × 10 4 t. However, global shale oil exploration and development practices have revealed that the existing horizontal well and hydraulic fracturing techniques are not able to achieve the cost-effective development of immature to low-moderate-maturity shale hydrocarbon reserves [13][14][15][16][17]. The main reasons for this are the low maturity of the organic matter in such shale layers, the low hydrocarbon conversion rate of the convertible organic matter, undeveloped organic pores, poor reservoir connectivity, and a low gas-oil ratio (GOR).…”

“…Hubbert proposed a peak oil production theory in 1956, which led many countries and companies to target immature to low-moderate-maturity shale oil to increase hydrocarbon production [18]. In this respect, a number of international oil companies, such as Shell, ExxonMobil, and Total, have initiated research on the in situ conversion of shale oil and have conducted field experiments on various in situ conversion techniques [14,15,19,20]. In addition, the in situ electrical heating conversion technology proposed by Shell in the 1960s is the most prominent technique to date.…”