Characteristics of in-situ stress and its controls on coalbed methane development in the southeastern Qinshui Basin,North China

In-situ stress is a key reservoir parameter to evaluate reservoir permeability,hydraulic fractures,coal seam deformation and coalbed methane(CBM)recovery.With limited CBM test wells in the Zhengzhuang field,southeast of the Qinshui basin,North China,the in-situ stress data is inadequate for CBM exploration and development.It is necessary to find a method to predicate in-situ stress through other exploration data such as geophysical well loggings.In this study,we provide a new well-logging databased model to predicate the in-situ stress based on 17 sets of well test data and comprehensive logging data.As a distinguished characteristic of this model,different structural compartmentalization of CBM reservoirs was considered.A regional adaptive residual strain index was introduced to the model.Based on the model,the in-situ stress distribution in the Zhengzhuang field were evaluated systematically,and the influences of in-situ stress on permeability and the propagation of hydro-fractures were discussed.Results indicate that the magnitude of the maximum(S_(Hmax),14.19e45.40 MPa)and minimum horizontal stresses(Shmin,10.62e28.38 MPa)and the gravitational stress(Sv,9.58e30.82 MPa)all show positive correlations with burial depths.The in-situ stress fields in the study area are characterized by 1)SHmax>Sv>Shmin in shallow layers(<700 m),indicating a dominant strike-slip faulting stress regime;2)Sv z S_(Hmax)>S_(hmin)and S_(Hmax)>Sv>S_(hmin)in the depth of 700e1050 m,suggesting a transformed regime;and 3)S_(Hmax)>Sv>S_(hmin)in deep layers(>1050 m),indicating a strike-slip faulting stress regime.The S_(Hmax)in the study area is orientated by NE-SW,with a trend of 40e49.Resulted from the change of the in-situ stress regimes from shallow(500 m)to deep layers(~1000 m),the reservoir permeability variation shows a typical increase followed by decrease.The presence of natural fractures significantly affect the propagation pattern of hydraulic fractures,and the length difference between the major and branch fractures increases with increasing stress anisotropy in the Zhengzhuang field.

Multi-stage gas diffusion and its implications for the productivity of coalbed methane in the southern Qinshui Basin, north China

The behavior of coalbed methane(CBM)diffusion considerably influences gas productivity.Based on the multi-porous diffusion model and on-site CBM desorption data of coal cores,the behavior of CBM diffusion and its implications on the gas productivity of No.3 coal seam in the southern Qinshui Basin(SQB)were elaborately analyzed.Results indicate that CBM diffusion of No.3 coal seam demonstrates noticeable three-stage characteristics,including the fast diffusion,transitional diffusion,and slow diffusion stages.During the gas diffusion process,the gas content and/or the degree of developed pores and fractures/cleats in coal seams can affect the desorption of CBM and the amount of diffused CBM by influencing the changes in gas pressure in pores,thus controlling the behavior of gas diffusion in different stages.Because gas content and the developed degree of pores and fractures/cleats are closely associated with the deformation degree of the coal seams,variably deformed coal seams exhibit unique characteristics of gas diffusion.The low-deformation degree of the coal seams have a relatively uniform distribution of gas production over the history of a well.By contrast,the moderate-deformation degree of the coal seams have a relatively high rate and amount of gas diffusion in the fast and transitional diffusion stages,producing most of the gas in the early-to-intermediate stages of the wells.Finally,the high-deformation degree of the coal seams has a high rate and amount in the fast diffusion stage,indicating that most of the production stage occurs during the early stage of the gas production history of a well.In summary,the behavior of gas diffusion can be used for predicting gas production potential.

A method for predicting the probability of formation of complex hydraulic fracture networks in shale reservoirs: development and application

Shales can form a complex fracture network during hydraulic fracturing, which greatly increases the stimulated reservoir volume (SRV) and thus significantly increases oil or gas production. It is therefore important to accurately predict the probability of formation of the hydraulic fracture network for shale gas exploration and exploitation. Conventional discriminant criteria are presented as the relationship curves of stress difference vs. intersection angle. However, these methods are inadequate for application in the field. In this study, an effective and quantitative prediction method relating to the probability of complex fracture network formation is proposed. First, a discriminant criterion of fracture network was derived. Secondly, Monte Carlo simulation was applied to calculate the probability of the formation of the complex fracture network. Then, the method was validated by applying it to individual wells of two active shale gas blocks in the Sichuan Basin, China. Results show that the probabilities of fracture network are 0.98 for well JY1 and 0.26 for well W204, which is consistent with the micro-seismic hydraulic fracturing monitoring and actual gas production. Finally, the method was further extended to apply for the regional scale of the Sichuan Basin, where the general probabilities of fracture network formation are 0.32–1 and 0.74–1 for Weiyuan and Jiaoshiba blocks, respectively. The Jiaoshiba block has, therefore, an overall higher probability for formation of fracture network than the Weiyuan block. The proposed method has the potential in further application to evaluation and prediction of hydraulic fracturing operations in shale reservoirs.

Brief overview and perspective on Advances in CO_(2)Geological Storage and Utilization(CGSU)

1 Introduction All over the world,with the intense longing for carbon emission reduction and the demand for clean energy(Feng et al.,2020;Iddphonce et al.,2020;Zhao et al.,2022),a promising and far-reaching technology called CO_(2) geological storage and utilization(CGSU)has attracted increasing attention(Iddphonce et al.,2020;Ma et al.,2020).This CGSU technique enjoys huge potential and broad prospect,because it usually enables dual rewarding consequences–to sequester CO_(2) and to acquire energy from geological formations,such as geothermal rock,oil and gas reservoirs(Iddphonce et al.,2020;Klewiah et al.,2020;Liu et al.,2020).In fact,the CGSU has been followed by interest of scientists all around world since geological storage was first proposed in the 1970s as a way to dispose of the CO_(2).Meanwhile,the CGSU technique is not mature enough and usually involves a complicated THMC coupled process,making it necessary to deploy more insightful investigations to boost the development of CGUS technique in a robust,safe and cost-effective manner(Liu et al.,2021b;Zhao et al.,2021;Liu et al.,2023).Basically,this theme is expected to enhance the knowledge about the crucial role of CGSU in achieving the carbon emission reduction–an issue concerned to the world(Liu et al.,2021a;Xie et al.,2021;Zhao et al.,2021;Zhao et al.,2022).Therefore,this special issue is organized and tends to present newest achievements regarding the CGSU,such as the theory expansion,economic analysis,experimental investigation and numerical simulation.