Wei Ju

Prediction of natural fractures in the Lower Jurassic Ahe Formation of the Dibei Gasfield, Kuqa Depression, Tarim Basin, NW China

The Lower Jurassic low porosity and low permeability Ahe Formation is the major reservoir of Dibei Gasfield in the Kuqa Depression, Tarim Basin. Natural fractures are important spaces for storage of hydrocarbons in low permeability reservoirs and can significantly improve the fluid flow capability; therefore, predicting the location and intensity of natural fractures in the Ahe Formation are of extreme importance. In the present study, the Late Himalayan paleotectonic stress field, the period of time when the majority of natural fractures generated in the Dibei Gasfield, was simulated and investigated with a three dimensional finite element (3D FE) model, which serves as a starting point for the prediction. Based on the principle of energy conservation and simulated paleotectonic stress field, the relationship between fracture density and stress parameter was established, and hence, natural fractures in the Ahe Formation of Dibei Gasfield were predicted. The results indicated that the development and distribution of natural fractures were primarily fault-controlled. Regions with well-developed natural fractures were mainly located in fault zones and around faults. Tectonic activities and ultra-high pressures were the dominant factors for natural fractures in the Ahe Formation. Regions with higher development degree of natural fractures in the Ahe Formation usually have a larger gas production; therefore, regions among Well Y1, B3, X1 and B2 should be focused in the Dibei Gasfield.

In-situ stress distribution and coalbed methane reservoir permeability in the Linxing area, eastern Ordos Basin, China

Understanding the distribution of in-situ stresses is extremely important in a wide range of fields such as oil and gas exploration and development, CO2 sequestration, borehole stability, and stress-related geohazards assessment. In the present study, the in-situ stress distribution in the Linxing area of eastern Ordos Basin, China, was analyzed based on well tested parameters. The maximum horizontal principal stress (SHmax), minimum horizontal principal stress (Shmin), and vertical stress (Sv) were calculated, and they were linearly correlated with burial depth. In general, two types of in-situ stress fields were determined in the Linxing area: (i) the in-situ stress state followed the relation Sv>SHmax>Shmin in shallow layers with burial depths of less than about 940 m, indicating a normal faulting stress regime; (ii) the SHmax magnitude increased conspicuously and was greater than the Sv magnitude in deep layers with depths more than about 940 m, and the in-situ stress state followed the relation SHmax>Sv>Shmin, demonstrating a strike-slip faulting stress regime. The horizontal differential stress (SHmax–Shmin) increased with burial depth, indicating that wellbore instability may be a potentially significant problem when drilling deep vertical wells. The lateral stress coefficient ranged from 0.73 to 1.08 with an average of 0.93 in the Linxing area. The coalbed methane (CBM) reservoir permeability was also analyzed. No obvious exponential relationship was found between coal permeability and effective in-situ stress magnitude. Coal permeability was relatively high under a larger effective in-situ stress magnitude. Multiple factors, including fracture development, contribute to the variation of CBM reservoir permeability in the Linxing area of eastern Ordos Basin.

Effects of mechanical layering on hydraulic fracturing in shale gas reservoirs based on numerical models

Generally, induced hydraulic fractures are generated by fluid overpressure and are used to increase reservoir permeability through forming interconnected fracture systems. However, in heterogeneous and anisotropic rocks, many hydraulic fractures may become arrested or offset at layer contacts under certain conditions and do not form vertically connected fracture networks. Mechanical layering is an important factor causing anisotropy in sedimentary layers. Hence, in this study, with a shale gas reservoir case study in the Longmaxi Formation in the southeastern Chongqing region, Sichuan Basin, we present results from several numerical models to gain quantitative insights into the effects of mechanical layering on hydraulic fracturing. Results showed that the fractured area caused by hydraulic fracturing indicated a linear relationship with the neighboring layer’s Young’s modulus. An increase of the neighboring layer’s Young’s modulus resulted in better hydraulic fracturing effects. In addition, the contact between two neighboring layers is regarded as a zone with thickness and mechanical properties, which also influences the effects of hydraulic fracturing in reservoirs. The initial hydraulic fracture was unable to propagate into neighboring layers under a relatively low contact’s Young’s modulus. When associated local tensile stresses exceeded the rock strength, hydraulic fractures propagated into neighboring layers. Moreover, with the contact’s Young’s modulus becoming higher, the fractured area increased rapidly first, then slowly and finally became stable.