Simulation of multi-period paleotectonic stress fields and distribution prediction of natural Ordovician fractures in the Huainan coalfield, Northern China

Highlights

• The characteristics and formation time of fracture in the Ordovician are examined.

• The stress states of the Indosinian, Early Yanshanian and late Yanshanian periods are determined.

• Numerical simulation of multi-period paleotectonic stress fields in Huainan is conducted.

• Distribution characteristics of multi-period fractures in the Ordovician are predicted.

Abstract

Fractures are effective seepage channels and storage spaces for oil, gas, and groundwater. Understanding the distribution characteristics of fractures can improve the efficiency of water production and the control of water-related geohazards in fractured aquifers. The development and distribution of natural fractures are usually controlled by paleotectonic stress fields. In this study, the data obtained from outcrop, cores, and thin sections were used to determine the development characteristics of such fractures. By doing an analysis of burial history, tectonic evolution, acoustic emission test, rock mechanics test, and conjugate joint analysis, two-dimensional (2D) heterogeneous geomechanical models were established by using a finite element method (FEM) stress analysis approach to simulate paleotectonic stress fields during the Indosinian, Early Yanshanian and Late Yanshanian periods. The effects of the variations of paleotectonic stress fields, hydrodynamic fields, structures, and rock mechanical parameters on the development of fractures could then be identified. A calculation model for the rock rupture rate was established to determine the quantitative development of fractures in different periods. Favorable areas for natural fracture development were determined by examining the relationship between the fracture linear density and pumping capacity of eleven wells. The simulation results indicate that the distributions of the maximum and minimum principal stresses and the maximum shear stress were nonuniform during the Indosinian, Early Yanshanian and Late Yanshanian periods, and high stress and/or low stress were mostly distributed in the fault and fold belts. During the Indosinian period, the tectonic and dissolution fractures were not developed. During the Early and Late Yanshanian periods, the tectonic and dissolution fractures were primarily developed around the fault and fold belts in the NNW-SSE and NWW-SEE directions. Those fault and fold belts are the locations with the greatest development of the natural fractures, which are also the main channels for the migration and accumulation of karst groundwater in the Ordovician formation in the Huainan coalfield. This study provides an important reference for the prevention and control of the Ordovician karst water damage in the study area, and proposes a new method for the prediction of fractured carbonate reservoirs.

Introduction

Natural fractures usually include tectonic fractures and dissolution fractures, which are effective seepage channels and storage spaces for oil, gas and groundwater (Tang et al., 2017, Wang et al., 2016, Yu et al., 2009, Zhang et al., 2021a). Many studies have shown that the natural fractures developed in most coalfields in the world are closely related to multi-period tectonic movements and corrosive fluid dissolution (Liu et al., 2017b, Wang et al., 2016, Ye et al., 2022, Zhang et al., 2021b, Zhang et al., 2011). Understanding the distribution characteristics of natural fractures can improve the efficiency of water production in fractured aquifers and provide an important reference for the prevention and control of water-related geohazards in fractured carbonate aquifer during coal resource mining. Therefore, quantitative prediction of degree of natural fracture development and its distribution characteristics has attracted increasing attention from the coalfield hydrogeological community.

Prediction of natural fractures development has gone through three stages up to present: qualitative, semi-quantitative, and quantitative prediction. Qualitative prediction of natural fractures was primarily based on the relationship between the degree of fracture development and its structural position or lithology (Hunter and Young, 1953, Nelson, 1985, Gong et al., 2021). Price (1966) proposed that the degree of fracture development was proportional to the elastic strain energy saved in the rock. Based on this principle, the energy method and fracture rate method were used to predict natural fractures, and the prediction of natural fractures has gradually entered the semi-quantitative stage. Murray (1968) was the first researcher to use the curvature method to predict reservoir fractures in a small island oil field in the United States; this method is suitable for tectonic fracture prediction in brittle rocks (Shaban et al., 2011, Suo et al., 2012). Lisle (1994) introduced the curvature attribute to the geological field, and the technique has been applied in fracture prediction. With the continuous discovery of more fractured reservoirs, scientists have gradually realized that the formation of tectonic fractures was not only related to physical and mechanical properties of rocks, but also closely related to the paleotectonic stress fields (Gale et al., 2014, Gong et al., 2019, Guo et al., 2019, Tuckwell et al., 2003, Su et al., 2014). Methods such as rock mechanics test, acoustic emission test and numerical simulation were used to restore and reconstruct the paleostress field to quantitatively predict the tectonic fractures (Guo et al., 2016, Guo et al., 2019, Huang et al., 2019, Liu et al., 2017a), signaling that fracture prediction has entered the quantitative stage. The finite element method (FEM) is generally used to analyze tectonic stress fields, and its validity has been confirmed (Fang et al., 2017, Guo et al., 2019, Li et al., 2020, Liu et al., 2017a). Numerical simulation of the stress field is based on the mechanism of fracture development, considers the actual geological information, and combines numerical simulation results with the Coulomb-Mohr criterion and the Griffith criterion. Afterwards, the distribution of fractures can be predicted or evaluated.

Up to present, most stress field simulations were based on simple geological models, treating the geological unit as a homogeneous body with the same mechanical parameters for the entire geological unit, without considering the effects of faults and folds (Guo et al., 2016, Hashimoto and Matsu'Ura, 2006, Jiu et al., 2013, Wu et al., 2017). The parameters in those simulations were often inconsistent with the actual geological conditions, leading to substantial biases of stress field simulation and fracture prediction. In the prediction of tectonic fractures, petroliferous basins and coalfields in China, especially in the north, have undergone multi-period tectonic movements, and it is difficult to accurately predict fracture development using a single period stress field simulation because of the superposition of multiple tectonic fractures in reservoirs. In addition, the previous studies mostly focused on the prediction of tectonic fractures in the sandstone or shale formations (Fang et al., 2017, Guo et al., 2016, Ju and Sun, 2016, Liu et al., 2021), but did not pay too much attention on the distribution of tectonic fractures and dissolution fractures in the carbonate rocks. Unlike sandstone and shale, tectonic fractures in carbonate rocks are easily dissolved and expanded by corrosive fluids (e.g. meteoric water, groundwater, and hydrothermal fluids), thereby forming dissolution fractures, caves and even underground rivers (Chen et al., 2022, Li and Cai, 2017, Ye et al., 2020, Zhang et al., 2021b). Therefore, the study of tectonic fractures and dissolution fractures in carbonate rocks is helpful to understand the development and distribution of fractured carbonate aquifers/reservoirs.

The Ordovician carbonate is an important oil, natural gas and groundwater reservoir in China, and is also a key target for the prevention and control of water-related geohazards during deep coal mining (Fang et al., 2017, Guo et al., 2019, Li and Cai, 2017, Liu et al., 2017b, Zhang et al., 2019b). Like other coalfields in Northern China, the Ordovician karst water in the Huainan coalfield is an important ecological resource, and a major water inrush source that threatens mine safety (Zhang et al., 2019a, Zhang et al., 2020a, Zhang at al., 2020b). Drilling data showed that tectonic fractures and dissolution fractures in the Ordovician strata in the Huainan coalfield are well developed, which are the main storage spaces and migration channels for groundwater (Li et al., 2010, Zhang et al., 2021a). Previous studies mainly described and counted the fractures in the cores and carbonate outcrop areas but did not investigate the overall distribution characteristics and degree of fracture development from a macroscopic perspective (Zhang et al., 2021a), resulting in that the water inrush accidents of the Ordovician aquifer in the study area occurred quite frequently and the effect of grouting and blocking groundwater inrush was poor and unsatisfactory. To obtain accurate prediction of the distribution of natural fractures in the Ordovician strata in the Huainan coalfield, the FEM was employed to simulate the paleotectonic stress fields in different periods (the Indosinian, Early Yanshanian and Late Yanshanian periods) in this study. We will also predict the distribution of tectonic fractures based on the Coulomb-Mohr criterion and Griffith criteria, and eventually to predict the distribution of tectonic and dissolution fractures combined with the tectonic fracture fields and hydrodynamic fields. The accuracy of the prediction results was verified by cores, fracture linear density and pumping tests.