Study On The Mechanism And Control Of Roof Leakage In Water-soluble Cavern Of Horizontal Connected Well Group

In this paper,based on the background of the rapid development of underground salt rock reservoir construction,combined with the basic national conditions of layered salt rock in China,the roof leakage mechanism and control of water-soluble cavern in the horizontal connected well group of underground layered salt rock are studied.This paper takes the fact of horizontal well cavern leakage in a salt mine in Jiangsu province as the breakthrough point,investigates and analyzes the geological and stratigraphic data of the salt mine,carries out a series of experiments on the basic physical properties of on-site core samples,and reveals the roof leakage mechanism of horizontal well cavern.Combined with geological data and rock parameters measured by experiment,a two-dimensional numerical model of roof leakage of water-soluble cavern in horizontal connected well group was established,and the roof leakage rule of horizontal well cavern under different roof loss plane and time was studied,and the key factors of roof leakage control were analyzed.To facilitate the research on the contour expansion law of the water-soluble cavern of the horizontal connected well group under different production processes,a physical simulation experiment platform with large-size molded rock salt was designed and built.Finally,the production process suggestions for controlling roof leakage in horizontal cavern are put forward.The main conclusions are as follows.After the investigation of geological data,it is found that the strata of water-soluble cavern in the horizontal connected well group of salt mine have good tightness and with no obvious fault.The roof of the cavern is salt-gypsum interbed,and above the roof is clay siltstone layer.Dissolution,SEM,EDS,XRD experiments were carried out on the core samples,and it was found that the salt rock cavern dissolved rapidly in the top area,the gypsum mudstone,clay siltstone and the rock interface had more pores,gypsum mudstone and clay siltstone contained a large amount of clay minerals or gypsum.Then we speculated that roof leakage may occur in the cavern.Immersion experiment and nuclear magnetic experiment were carried out,and the water absorption characteristics and pore permeability parameters of gypsum mudstone and clay siltstone were obtained.And the mechanism of cavern roof leakage is revealed,which was that the gypsum mudstone layer of the roof was exposed to the high-pressure brine,as the result of the rapid dissolution of the cavern roof,and then,the gypsum mudstone absorbs water,expand and being soften,and collapses.Then,the high-pressure brine directly contacts with the permeable clay siltstone layer,eventually,the roof leakage occurs.At the same time,numerical simulation experiments were carried out for different roof leakage surfaces and different leakage time of the cavern.It was found that the leakage of horizontal well cavern is positively correlated with the roof leakage surface and leakage time.As a result,the control mechanism of roof leakage was analyzed,and found that the key to control roof leakage was to control the rapid top dissolution of the cavern.In this paper,we independently designed the mould for pressing the supersize molded rock salt and then successfully pressed the large-size molded rock salt.Based on the similarity theory,a physical simulation experiment platform was built for the chamber water-soluble cavitation of horizontal connected well group with lager-size molded rock salt.In addition,four simulated cavitation experiments with different combination of cavitation techniques were designed in combination with the on-site water-soluble cavitation technology.It was found that with the technology of ‘injection with incline well and production with vertical well’ used by the site,salt rock dissolution of the horizontal cavern was mainly concentrated at the slant well side,and there was a wide range of ’horizontal insoluble zone’ in the cavern,which has a bad effect on controlling roof dissolution and preventing roof leakage.At the same time,the effects of different production processes and combinations,such as the flow rate,the oil-blanket and the transform of the injection well,on the contour expansion rule of the cavern in the process of water-soluble mining of horizontal connected well group were studied.Finally,the technological suggestions for controlling the roof dissolution of cavern and preventing and controlling roof leakage were put forward.For example,when the salt layer was thin,the low-flow with oil-blanket mining method could be adopted,and after a certain extent of the cavern,transform of the injection well could be better,and oil-blanket as well as transferred injection well could be used when built the bottom of the cavern,and so on.