Title:

The Beijing Geothermal System, P.R. China: Natural State and Exploitation Modelling Study of a Low Temperature Basement Aquifer System

Authors:

M.P. Hochstein, Y. Zhongke

Geo Location:

Beijing, China

Conference:

Stanford Geothermal Workshop

Year:

1988

Session:

Field Data Analysis

Language:

English

File Size:

613KB

View File:

Abstract:

An extensive (>8OO km2) basement aquifer confined to the top 300m of Sinian dolomites occurs beneath the Beijing Graben at 3OOm to 300Om depth (Fig. 1). The Beijing Geothermal Field is associated with a broad horst structure (called from now on the Beijing sub-uplift) under the SE flank where the aquifer lies typically between 600 to 1200m depth covering an area of about 70 km2 (Fig. 2). Host of the 44 wells drilled between 1971 and 1982 were brought down on this structure and produce thermal waters at wellhead temperatures between 45 to 55?C; a few we116 also produce from the deeper part of the Beijing Graben with wellhead temperatures of up to 70'C from 2600m depth. Fluid production began in 1972 and vas increased to about 130 kg/s in 1985; the production rate varies significantly during the year as about 40% of the produced water is used for heating and about 60% for baths and industrial purposes (base load). As a result of this abstraction the reservoir pressure has decreased by about 3 bars. In terms of total production the Beijing Field ranks as the third largest lov temperature field presently exploited in China; higher production rates are documented only for the Tianjin Field (Ouyang et al., 1986) and the Fuzhou fracture zone reservoir in Fujian Province (Huang and Goff, 1986). A summary description of the characteristics of the Beijing Field and its utilization has been given by Zhang, Z.G. (1981), Hochstein et al. (1984), and in the final technical report of a UNDP-sponsored project (UNDP-Ministry of Geology, 1984). There was great uncertainty as to the origins of the thermal anomaly of the Beijing Field. Most of the sediments which infill the Beijing Graben have a very low permeability; temperature gradients in wells are almost linear for the sedimentary section. If one plots the temperature in the basement aquifer and the heat flow in the sediments above versus basement level, one finds that there is a higher than normal heatflow (80-90 mW/mZ) over the sub-uplift and a lower than normal flux (less than 45 rnW/mZ) in the deepest part of the graben (see Fig. 3); the! heat flux changes by more than a factor of two over a distance of less than 8 km (Hochstein et al., 1984). The thermal waters are of meteoric origin (Zheng, K.. 1981; Zheng, K. et al., 1982). There is still some uncertainty about management of the resource. Although the production has been moderate. the resulting pressure drop in the basement aquifer has been noticed throughout the whole production area. The permeability of the aquifer is very high as indicated by the immediate response to seasonal variations in production noticed in wells throughout the whole sub-uplift area (see Fig. 4). For economic production expensive deep well pumps cannot be used, and the presently installed shaft pumps have a lifting capacity of only about 50-6Om. If production increases at the rate as measured during the period 1982-85 (i.e. from 3.5 Mt/yr to 5 nt/yr), the data in Fig. 4 indicate that some production problems will probably be encountered when the total pressure drop reaches, say, 6 bars in the aquifer. It wss uncertain whether significant natural recharge had occurred, whether re-injection was required on a larger scale to stabilize the existing pressure drop, and whether colder fluids from shallower levels were moving into the reservoir. Some reservoir modelling was therefore required to obtain a better understanding as to which processes in the past caused the thermal anomaly of the Beijing Field and t o analyze heat and mass transfer induced by past production.


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