Mechanical analysis and failure modes prediction of composite rock under uniaxial compression

Composite rocks are easily encountered in a wide range of geotechnical construction projects. Understanding their mechanical properties and failure modes is very important to ensure project quality and safety. This study conducted a mechanical analysis to assess the stress distribution in composite rock with a horizontal interlayer and predicted the possible failure modes. Uniaxial compression tests were carried out on the composite rock samples to reveal their mechanical properties. It was concluded that a composite rock with a thick interlayer failed more easily than a composite rock with a thin interlayer. Four potential failure modes were related to the internal stress distribution under compression and the differences in deformation capacity and strength among the constituent components. The stress distribution derived from the mechanical analysis could explain the failure mechanism very well. These results verified the validity of the mechanical analysis results and improved understanding of the mechanical properties of composite rock with a horizontal interlayer.

Optimization Technique and Practice of Stratified Injection in Low and Medium Permeability Thin Interlayer Reservoirs

Reservoir S is a typical medium and low permeability thin interlayer reservoir. It is developed by co-production of directional Wells and separate injection of directional Wells. The comprehensive water content of the oilfield is 54%. Therefore, it is urgent to optimize the injection method of stratified injection distribution in this reservoir, so as to improve the degree of water driving and recovery. In this paper, the reservoir engineering and numerical simulation methods are adopted to introduce the impact factor of injection-production correspondence rate, modify the previously used fine injection matching model, and use the model to quantitatively analyze the sensitivity of key parameters such as longitudinal splitting coefficient, pressure recovery rate and injection-production correspondence rate. The results show that it is very important to select the influencing factors for the injection and production response ratio of medium and low permeability thin interlayer reservoirs, which directly affects the effect of stratified water injection. At present, this research technology has been widely applied in S oilfield, and the water injection effect is very significant, effectively improving the recovery rate of the implementation area by 2%, which provides experience for the water injection development of similar reservoirs.

Occurrence, Classification and Formation Mechanisms of the Organic-Rich Clasts in the Upper Paleozoic Coal-Bearing Tight Sandstone, Northeastern Margin of the Ordos Basin, China

The detailed characteristics and formation mechanisms of organic-rich clasts (ORCs) in the Upper Paleozoic tight sandstone in the northeastern margin of the Ordos Basin were analyzed through 818-m-long drilling cores and logging data from 28 wells. In general, compared with soft-sediment clasts documented in other sedimentary environments, organic-rich clasts in coal-bearing tight sandstone have not been adequately investigated in the literature. ORCs are widely developed in various sedimentary environments of coal-bearing sandstone, including fluvial channels, crevasse splays, tidal channels, sand flats, and subaqueous debris flow deposits. In addition to being controlled by the water flow energy and transportation processes, the fragmentation degree and morphology of ORCs are also related to their content of higher plants organic matter. The change in water flow energy during transportation makes the ORCs show obvious mechanical depositional differentiation. Four main types of ORC can be recognized in the deposits: diamictic organic-rich clasts, floating organic-rich clasts, loaded lamellar organic-rich clasts, and thin interlayer organic-rich clasts. The relationship between energy variation and ORCs deposition continuity is rarely studied so far. Based on the different handling processes under the control of water flow energy changes, we propose two ORCs formation mechanisms: the long-term altering of continuous water flow and the short-term water flow acting triggered by sudden events.