Ground Subsidence Prediction Model and Parameter Analysis for Underground Gas Storage in Horizontal Salt Caverns

Due to a great demand of natural gas or oil storage in these years, horizontal caverns were proposed to fully use bedded salt formations of China. Under the same geological and operating conditions, the horizontal cavern would shrink more than traditional pear-shaped cavern, which might bring larger ground subsidence and affect the safety of storage facilities. A new prediction model was proposed in this paper for the time-dependent ground subsidence above horizontal caverns. The proposed model considered the impurity of bedded salt formations and simplified the horizontal cavern to an ideal cylinder. The shape of the subsidence trough was determined by the probabilistic integration method, and corresponding calculation formulas for the tilt, curvature, horizontal displacement, and horizontal strain were derived. Based on the assumption that the subsidence volume at the ground was proportional to the reduced volume of horizontal cavern, a formula for the reduced volume over time was established. FLAC3D was introduced to simulate the ground subsidence, and the results show that the proposed prediction model agreed well with the simulation results. Finally, the proposed prediction model was used to analyze the impacts of different stratigraphic parameters and design parameters. The results mainly show that, as the draw angle increases, the subsidence trough becomes deeper and narrower; as the depth of the cavern increases, the maximum subsidence first increases and then decreases, and the subsidence trough gradually becomes round; with the increase of the purity, the subsidence gradually decreases; with the increase of the creep properties and the stress exponential constant, the maximum subsidence first increases rapidly and then slowly approaches the limit; increasing the brine extraction velocity can shorten the cavern construction period and then reduce excessive ground subsidence; the subsidence decreases nonlinearly with the increase of internal pressure; with the increase of the cross section diameter and length of the horizontal cavern, the subsidence presents a significant nonlinear increase. In addition, unlike the traditional pear-shaped cavern, under the same conditions, the ground subsidence above the horizontal cavern according to this newly proposed model is much larger, and the ground subsidence contour line is no longer a standard circle. The findings of this study can help for better understanding of the prediction of ground subsidence above salt caverns and also provide a reference for the design and construction. However, the proposed prediction method is ideal and theoretical and should be further improved by engineering practice in the future.

Justification of engineering safety criteria for undermining of water-proof layer in the Upper Kama Salt Deposit

Safety of a water-proof pillar between the stoping void and the aquifers defines the key feature of water-soluble mineral mining. In this regard, the most important element of geomechanical supervision of mining operations, especially, at the mine project stage, is the engineering safety criteria aimed at the adequate valuation of safe undermining of water-proof strata (WPS). The WPS safety procedures now in force calculate only maximal sagging of undermined beds and disregard deformation in the edge area of WPS. In the meanwhile, the edges of WPS are the areas of localization of maximal horizontal strains, and the hazard of vertical jointing is the highest in these areas. In this connection, in the capacity of the index of the manmade load on WPS, it is proposed to use the maximal slope of the edge area of the subsidence trough. The evaluations were carried out in 6 Uralkali’s mine sites selected from the mathematical modeling and geophysical survey data which exhibited considerable damage of WPS. Based on the implemented research and justifications, it is recommended to use the generalized safety criterion for undermining of WPS as a maximal subsidence/mining depth ratio which is directly proportional to the ground slope. This engineering procedure describes more adequately the fracture mechanism in WPS rocks, in particular, damage localization in edge areas of the subsidence trough, or influence of mining depth on WPS stability and, which is main thing, is based on the criteria derived from the long-term observations and measurements performed in the Upper Kama deposit. The study was supported by the Russian Science Foundation, Grant No. 19-77-30008.

INFLUENCE OF UNDERMINING ROCK MASS ON THE STATE OF MINE WORKINGS

Purpose. Research of influence of shifts of rocks on a condition of mine workings at their undermining by longwalls. Methods. The study used a computer simulation method. The simulation was performed by simulating the process of rock shift. The initial coordinates of the points of the rock mass turned into the final ones. This allowed to build sections of the rock mass after undermining, which characterized its deformed state, as well as graphs of displacements and deformations in the mine workings. Results. The performed simulation showed that during the movement of the longwall, the mine working (drift), which is undermined, consistently falls into the zone of horizontal deformations of tension, compression and complete displacements. After stopping the longwall, part of the mine working near the boundaries of the displacement area will be constantly in the zones of stretching and compression. It is established that at a distance of 40 m in front of the moving face, the mine working is in the least favorable conditions with maximum vertical compression deformations. Under the given conditions the drift for the most part after completion of clearing works is in a zone of full subsidence. This zone is characterized by the maximum possible subsidence and the absence of deformations. The performed researchers showed that the conditions of undermining of the southern field haulage drift of seam l7 proved to be quite favorable. Shifts and deformations did not cause a significant deterioration in the state of mine working, except for the increase of slopes on the edge sections of the subsidence trough, which makes it difficult to carry out electric locomotive rolling. For any other purpose, after minor repairs, the mine working may continue to operate. Scientific novelty. The study of the process of shear by computer simulation allowed to establish the patterns of shear and deformation during undermining of the mine working. This is the first time for the conditions of the Ukrainian Donbass. Practical significance. The obtained results can be used to predict the condition of mine workings that are undermined during the cleaning works in similar conditions. Key words: undermining overlying mine workings, shifts, deformations, zone of full subsidence, subsidence trough.

Dynamic Surface Subsidence Characteristics due to Super-Large Working Face in Fragile-Ecological Mining Areas: A Case Study in Shendong Coalfield, China

The dynamic subsidence characteristics due to super-large working face (SLWF) are the basis for further understanding of land ecology damage in fragile-ecological mining areas. In order to acquire the evolution characteristics of dynamic subsidence parameters and surface cracks, a series of field monitoring and comparisons with previous studies were conducted. The results indicate that (1) the subsidence trough is characterized with self-healing characteristics, including rapid formation of subsidence trough, the convergence of deformation, a steep trough edge, the smaller range of surface cracks; (2) the dynamic curves of dynamic subsidence parameters conformed to the exponential function curve with an inflection point when the SLWF advanced ca. critical dimension, which is the commonality of the dynamic subsidence characteristics; and (3) the optimized monitoring strategy for land ecology damage is recommended, and more attention should be paid to the quantitative prediction of root damage due to coal mining. The research results would benefit mining damage control and civil engineering protection in fragile-ecological mining areas.

ANTHROPOGENIC INFLUENCE OF THE KOMSOMOLSK OIL AND GAS CONDENSATE FIELD ON MODERN DEFORMATION PROCESSES

The authors created a geodynamical polygon on the Komsomolsk oil and gas condensate field to ensure the industrial safety of oil and gas production facilities. The aim of its creation is mul-tiple repeated observations of recent deformation processes. Analysis and interpretation of the results of geodynamical monitoring which includes class II leveling, satellite observations, radar interferometry, exploitation parameters of field development provided an opportunity to identify that the conditions for the formation of recent deformations of the earth’s surface is an anthropogenic factor. The authors identified the relationship between the formation of subsidence trough of the earth’s surface in the eastern part of the field with the dynamics of accumulated gas sampling and the fall of reservoir pressures along the main reservoir PK1 (Cenomanian stage).

Determination of Elastic Parameters of Near-Surface Layers Over Subsidence Trough Development During Longwall Exploitation

Seismic and geodetic studies were carried out before, during, and after underground exploitation of a coal bed in Katowice - Kleofas Coal Mine, located in the Upper Silesia Coal Basin, Poland. Development of a subsidence trough was completed approximately 3 months after passage of a longwall exploitation in the coal seam. This was the time required for the subsidence trough to appear on the surface, which was confirmed by levelling measurements. Sharp changes in the elastic parameters were observed on each profile during subsidence trough development. This observation can result from changing tension and compression forces caused by increase and/or decrease of the elastic parameters of the rock mass. After completion of subsidence trough development, the rock mass appeared to return to its isotropic state and the observed changes ceased. Some minor fluctuations were noted, but they probably resulted from changes in groundwater levels, which might have affected the measured parameters.

Simulation of Rock Mass Horizontal Displacements with Usage of Cellular Automata Theory.

In the article there was presented two dimensional rock mass model as a deterministic finite cellular automata. Used to describe the distribution of subsidence of rock mass inside and on its surface the theory of automata makes it relatively simple way to get a subsidence trough profile consistent with the profile observed by geodetic measurements on the land surface. As a development of an existing concept of the rock mass model, as a finite cellular automaton, there was described distribution function that allows, simultaneously with the simulation of subsidence, to simulate horizontal displacements inside the rock mass model and on its surface in accordance with real observations. On the basis of the results of numerous computer simulations there was presented fundamental mathematical relationship that determines the ratio of maximum horizontal displacement and maximum subsidence, in case of full subsidence trough, in relation to the basic parameters of the rock mass model. The possibilities of presented model were shown on the example of simulation results of deformation distribution caused by extraction of abstract coal panel. Obtained results were consistent with results obtained by geometric-integral theory.

Analysis of Horizontal Displacements Measured over the Mining Operations in Longwall No. 537 at the Girondelle 5 Seam of the Bw Friedrich Heinrich-Rheinland Coal Mine

The article presents an analysis of the horizontal land surface displacements taking place as an effect of coal exploitation on the Girondelle 5 seam in the Friedrich Heinrich-Rheinland mine in Germany. The paper presents changes in displacement vectors measured on the surface following the progress of the exploitation face and an analysis of horizontal displacement measurements assuming that their magnitude is proportional to the profile of subsidence trough slope. Then, the values of horizontal displacement coefficient B are assessed for selected longitudinal and transversal calculation cross-sections of longwall No. 537.