Reactive Transport Modeling of Reflux Dolomitization of Carbonate Platforms: Enlightenment from Yingshan Formation in Shunnan Area, Tarim Basin

Dolomite plays an important role in carbonate reservoirs. The topography in the study area creates conditions for reflux dolomitization. The northeastward paleogeomorphy during the deposition of the Yingshan Formation was favorable for reflux dolomitization. Furthermore, the petrological and geochemical evidence indicated that the formation of finely crystalline dolomites was penecontemporaneous to sedimentation. The content of powder crystal dolomites increases from grainstone, to packstone, to mudstone. Previous studies only analyzed the origin of dolomites based on traditional geological methods, but did not analyze the spatial influence of reflux dolomitization on the reservoir quality. In this study, the reflux dolomitization of platform carbonate sediments was evaluated using three-dimensional reactive transport models. The sensitivity of dolomitization to a range of intrinsic and extrinsic controls was also explored. The reflux dolomitization involves replacement dolomitization and over-dolomitization. The porosity change is the result of the abundance change of dolomite and anhydrite. The fluid flow pattern in the model is related to the injection rate and geothermal gradient. According to the spatial and temporal change of mineral, ionic concentration, and physical property, the reflux dolomitization could be divided into five stages. From the sensitivity analysis, high permeability promotes dolomitization only in the initial stage, while low permeability and high porosity means stronger dolomitization. Besides, the injection rate, reactive surface area (RSA), geothermal gradient, and brine salinity are all proportional to the dolomitization. Differently from porosity change, the permeability change is concentrated in the upper part of the numerical model. The location of “sweet spot” varies with the locations of change centers of porosity and permeability. In the stage-1 and 4 of dolomitzation, it overlaps with porosity and permeability growth centers. While in the stage-2, 3 and 5, it lies between the porosity and permeability growth/reduction centers.

Study on The Change Law of Wave Velocity and Porosity of Triaxial Loaded Coal

The mechanical properties of loaded coal time to time, so that the voice wave propagation in coal changes with it. In order to study the relationship between the wave velocity of coal and its mechanical properties.The ultrasonic testing system was used to calculate the wave velocity of loaded coal at different times through laboratory triaxial loading test.The relationship between the wave velocity of loaded coal-body and the confining pressure of different coaxial pressures was analyzed.The results show that the wave velocity is positively correlated with the axial pressure under the same confining pressure. By calculating the variation of wave velocity of different loaded coal bodies. it is concluded that the variation of wave velocity of coal bodies can reflect the mechanical state of coal bodies undergo the compaction stage, the elastic stage and the new stage of crevasse. At the same time, the change of wave velocity in coal is affected by the porosity of the pressure-bearing coal-boby. Porosity was calculated using stress-strain.It was found that porosity and axial pressure presented a good negative correlation under the same confining pressure, and the change in wave velocity is exactly right opposite to the porosity change trend diagram.It can be obtained that the larger the amplitude of wave velocity change is, the smaller the porosity change is, and the smaller the amplitude of wave velocity change is, the larger the porosity change of coal body is, which is consistent with the actual analysis. It verifies the different the pressure-bearing coal-boby change in wave velocity to the feasibility of analyzing the mechanical condition of the loaded coal body. making the analysis more persuasive. It provides basis for coal body bearing and safe mining.

Modelling frost weathering processes and related stresses in Alpine rockwalls

<p>The periglacial areas of the European Alps are characterised by rugged peaks and steep rockwalls with adjacent scree slopes that reflect high rates of rockfall activity. The current state of knowledge regards ice segregation as the dominant mechanism responsible for the disintegration of rock and associated destabilization of rockwalls. In the present work, we (1) monitored rock temperature in Alpine rock walls, (2) determined rock properties in the laboratory and (3) simulated frost weathering using purely temperature-driven models (Hales and Roering, 2007; Anderson et al., 2013) and physical-based models (Walder and Hallet, 1985; Rempel et al., 2016).</p><p>(1) We monitored rock temperature in 9 rockwalls in the Hungerli Valley and 10 in the Gaisberg Valley at altitudes between 2400 m and 3000 m between 2016 and 2019. Mean annual rock temperature is between -2.8 and 7.9°C and is strongly affected by snow cover, which ranges between 3 and 283 days.</p><p>(2) Lithologies comprise Mica Schist in the Gaisberg Valley and Schisty Quartz Slate with inclusions of Aplite and Amphibolite in the Hungerli Valley. Rock density, seismic and strength properties were quantified in the lab (Draebing and Krautblatter, 2019) to be included in physical-based frost weathering models.</p><p>(3) Frost weathering due to ice segregation can be expressed as cracking intensity, crack growth and porosity change. Our model results show that an annual maximum of cracking intensity, crack growth and porosity change within the first meter of rock depth in the study areas’ rockwalls. Although frost weathering is highly dependent on the thermal distribution inside a rock mass, our data demonstrate that lithological parameters strongly determine frost weathering due to their influence on water migration and fracture toughness. Furthermore, the results suggest that there is no relationship between average annual rock temperature, frost weathering and exposure, a tentative conclusion that is broadly contrary to prevailing consensus.</p><p>In conclusion, rock walls are exposed to strong thermo-mechanical stresses due to ice segregation, which leads to a disintegration of rock and lowering of stability. The present work lends support to other studies, which regard frost weathering as the dominant mechanism responsible for rockfall in mountain periglacial settings.</p><p> </p><p>Anderson, R. S., Anderson, S. P., & Tucker, G. E.: Rock damage and regolith transport by frost: an example of climate modulation of the geomorphology of the critical zone, Earth Surface Processes and Landforms, 38(3), 299-316, 2013.</p><p>Draebing, D., & Krautblatter, M.: The Efficacy of Frost Weathering Processes in Alpine Rockwalls. Geophysical Research Letters, 46(12), 6516-6524, 2019.</p><p>Hales, T. C., & Roering, J. J.: Climatic controls on frost cracking and implications for the evolution of bedrock landscapes. Journal of Geophysical Research-Earth Surface, 112, F02033, 2007.</p><p>Rempel, A. W., Marshall, J. A., & Roering, J. J.: Modeling relative frost weathering rates at geomorphic scales. Earth and Planetary Science Letters, 453, 87-95, 2016.</p><p>Walder, J., & Hallet, B.: A Theoretical-Model of the Fracture of Rock During Freezing. Geological Society of America Bulletin, 96(3), 336-346, 1985.</p>