ABSTRACT Development of an accurate reservoir model and prediction of reservoir behaviour associated with geothermal exploitation using that model are fundamentally important for sustainable geothermal utilisation. Gravity observation for a few tens of days after a field-wide shut-in of geothermal wells potentially provides valuable constraints to improve reservoir models. We investigated the applicability of superconducting gravimeters (SGs) to short-term observations after shut-ins, as a first step towards the development of a method for reservoir model improvement using short-term gravity data. Results from a reservoir simulation of a shut-in at a geothermal field in Japan showed that gravity change at 20 days after the shut-in was 10 nm/s2. To detect such changes with SGs, locations of SG stations should be chosen to minimise noise sufficient to resolve the signal. Considering that field operations and traffic in geothermal fields can make SG data noisy, laboratory gravity measurements were carried out with a SG to investigate the influence of noise level on signal detection. Test masses, which produced a gravity change (7.7–9.0 nm/s2) comparable to the simulation result, were repeatedly placed and removed above the SG during both day and night times with different ambient noise levels. Test results showed that noise level greatly affected signal detection. The signal was only detected twice, under low-noise conditions with the standard deviations of 1.1 and 5.3 nm/s2. In addition, tests in which the SG was operated without a cryogenic refrigerator revealed that the absence of the refrigerator did not increase the noise level. Enhancing the portability of a SG by reducing its weight can make multi-location noise surveys feasible. Our method, of combining reservoir simulations and preliminary noise surveys, would enable successful SG observations to be made in relatively short periods after shut-ins, which would possibly contribute to the development of a method to obtain additional constraints to be used for reservoir model calibration.

中文翻译：

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没有低温冰箱的超导重力仪的实验室测试：对地热场噪声调查的影响

摘要 开发准确的储层模型和使用该模型预测与地热开采相关的储层行为对于可持续地热利用至关重要。在地热井全场关闭后数十天的重力观测可能为改进储层模型提供有价值的约束。我们研究了超导重力仪 (SG) 对关井后短期观测的适用性，这是开发利用短期重力数据改进储层模型方法的第一步。日本地热田关井的储层模拟结果表明，关井后 20 天的重力变化为 10 nm/s2。为了检测 SG 的此类变化，应选择 SG 站的位置，以尽量减少足以解析信号的噪声。考虑到地热田的野外作业和交通可能会使SG数据产生噪声，因此使用SG进行了实验室重力测量，以研究噪声水平对信号检测的影响。产生与模拟结果相当的重力变化 (7.7–9.0 nm/s2) 的测试质量，在白天和晚上以不同的环境噪声水平反复放置和移除在 SG 上方。测试结果表明，噪声水平极大地影响了信号检测。该信号仅在低噪声条件下检测到两次，标准偏差为 1.1 和 5.3 nm/s2。此外，SG 在没有低温冰箱的情况下运行的测试表明，没有冰箱不会增加噪音水平。通过减轻 SG 的重量来增强 SG 的便携性可以使多地点噪声调查变得可行。我们的方法将油藏模拟和初步噪声调查相结合，可以在关井后相对较短的时间内成功进行 SG 观测，这可能有助于开发一种方法，以获得用于油藏模型校准的附加约束条件.