关键词: 呼图壁储气库, 时变微重力, 介质密度, 注采压力

Abstract:

In this paper, based on microgravity time-varying signals, the gravity field and underground medium density change of Hutubi gas storage were simulated and calculated, and the response relationship between gravity change and injection-production pressure was analyzed. By using the 7 phases of mobile gravity data of Hutubi underground gas storage, adopting the classical adjustment method and selecting the absolute gravity points of HKPN, HKPS and Urumqi(BJ00) and Shihezi gravity point(BJ06) of CMONOC around the gas storage area as the calculation basis, the relative gravity variation of each monitoring point in the study area was obtained with the precision ranging (3~5)×10^-8m/s² for each point in each phase. Combined with the relevant data of gas storage injection-production pressure, the response relationship image between the spatial-temporal variation characteristics of gravity field and injection-production pressure in this area was acquired. The research shows that the gravity change in the entire survey area exhibits zoning characteristics. The gravity change in the outer area of the gas storage south of Hutubi Fault is relatively small, and the gravity change in the gas storage area increases and decreases alternately. Especially in the east side of the reservoir area, the gravity change shows obvious characteristics of decreasing in spring and increasing in autumn, which causes the natural gas in the gas storage to basically drop to the lowest in March, thus resulting in the minimum internal stress in the gas storage. According to the theory of crustal stress equilibrium, when the pressure inside the gas storage tends to increase or decrease, the stress outside the gas storage will be adjusted correspondingly. When the gas injected into the gas storage spreads between the rocks and their gaps in the gas storage, it will exert a certain pressure on the rocks, causing the medium density in the underground gas storage cavity to vary in different degrees, thus resulting in the changes in the gravity values of the surface measuring points in the gas storage area. Finally, based on the dynamic change data of gravity field observed on the surface of Hutubi underground gas storage, the constraint of depth weighting function was added in the calculation process to eliminate and weaken the multi-solution and skin effect, and the compact gravity inversion algorithm of spatial distribution of underground density variation anomaly body was adopted to simulate and calculate the underground material density change image of Hutubi gas storage and the morphological structure distribution characteristics inside the gas storage. In this paper, according to the structural framework of about 1km/layer in Hutubi gas storage, all slices are constructed in the vertical direction of 1km to the crust, and a total of 9 layers are cut into them. That is, they are divided from the surface to the interior of the gas storage from 0 to 9km. Based on the change amount of gas injection and production in Hutubi gas storage, combining with the density images of underground media in different periods, it can be clearly seen that the internal cavity shape distribution inside the gas storage is irregular, so the stress on each point in the gas storage will be uneven, resulting in different density changes of the medium in different depths. The density distribution of underground medium in this gas storage varies with time, and the density variation is relatively different, but it has a certain change rule. Most density variation images show four quadrant distribution characteristics, especially at the depth of about 3000~4000m of the gas storage, where the migration degree of underground medium substances is the largest, resulting in the largest density variation in this area, with the maximum density variation of about 0.7kg·m^-3. At this stage, the gas storage is just at the peak points of gas injection and production, that is, the maximum and minimum peak points of stress. In addition, the density change image has showed that the internal structure of the gas storage is in NW-SE direction, which is basically consistent with the geological structure distribution characteristics of Hutubi gas storage. Therefore, using gravity data, the structural form of Hutubi underground gas storage and the whole process of medium density changing with injection-production pressure can be clearly explained.

Key words: Hutubi gas storage, time-varying microgravity, medium density, injection-production pressure