A New Empirical Method for Constructing Capillary Pressure Curves from Conventional Logs in Low-Permeability Sandstones

INTRODUCTION

Geothermal resources can provide clean green energy and have recently received considerable attention. The Niutuozhen Geothermal Field is located in the Niutuozhen Uplift in North China (Fig. 1) and its geothermal resources have been developed for more than 30 years (Chen, 1988). Geothermal water is extensively used for space heating in Xiongxian, Gu'an and Baxian to construct smoke-free cities. There is an urgent need for further research on geothermal energy potential, reservoir temperature and water chemistry because of rapidly increasing demand (Dotsika et al., 2010; Wu and Ma, 2010; Gao et al., 2009; Liu H et al., 2009; Zhang et al., 2007; Liu J R et al., 2002; Giggenbach et al., 1995; Wang and Shen, 1993; Wang J Y et al., 1993; Wang Y X et al., 1993; Giggenbach, 1992, 1988; Dai et al., 1988). Chemical and isotopic components record the origin and evolution of geothermal water which reflect geothermal energy potential (Shen and Wang, 2002; Bi, 1998; Chen, 1998; Wang and Shen, 1993; Wang Y X et al., 1993). Twenty years ago discussion of the origin and evolution of geothermal resources throughout the north China Basin was based on relatively few geothermal water samples (Yao, 1995; Chen, 1988; Zhou, 1987; Geothermal Group, 1983) and did not achieve modern levels of accuracy. The present work includes new sampling sites and presents modern analyses for major ions, δ²H, δ¹⁸O, tritium and ¹⁴C ages of 43 samples from geothermal wells, three samples from groundwater wells and one rain water sample along with 14 rain water analyses obtained from the Global Network of Isotopes in Precipitation (GNIP).

Figure  1.  Location of the Niutuozhen uplift.

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GEOLOGICAL SETTING AND GEOTHERMAL RESOURCES

Geological Settings

The Niutuozhen Geothermal Field is located in the Niutuozhen uplift zone in the northern part of the Jizhong graben in the North China Basin bounded by the Yanshan Mountains and the Taihang Mountains. The basin began developing by vertical crustal movement before the Late Triassic Period, followed by mountain building from Jurassic to Early Tertiary and Late Miocene subsidence. NE-SW striking horsts and grabens formed alternately from west to east. The Niutuozhen uplift (or horst) is bounded by the Niudong fault, Niunan fault, Rongcheng fault and Daxing fault initiated during Late Jurassic to Cretaceous earth movements and finally during Himalayan movements. The uplift continued to move up to the surface until the Late Oligocene when it became mature and was eroded. The Minghuazhen Formation was deposited over the entire horst by the Late Miocene (Fig. 2) and it was finally buried locally under Quaternary sediments.

Figure  2.  Pre-Cenozoic geological map and cross-section of the Niutuozhen geothermal field.

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The Neogene Minghuazhen Formation sandstone with an average effective thickness of 225 m and Jixianian Wumishanian Formation dolomite with a thickness more than 1 000 m are the main reservoirs in the Niutuozhen geothermal field. Quaternary clay with a thickness of 400 m is the main caprock of the Minghuazhen Formation reservoir, and mudstone of the Minghuazhen Formation and Quaternary clay more than 493 m thick are the caprocks of the Wumishanian dolomite reservoir.

The geothermal gradient in the cap rock of the Niutuozhen uplift is more than 3.0 ℃/100 m, reaching a maximum of 11.5 ℃/100 m in the axis of the uplift (Chen, 1988). The gradient in the Wumishanian reservoir is generally between 1.65 and 2.55 ℃/100 m (Chen, 1988; Geothermal Group, 1983). Maximum wellhead pressure was measured at 3.74×10⁵ Pa in 1970s (Chen, 1988), but has decreased to its present level of less than 1.01×10⁵ Pa with drastic development of the geothermal water and no reinjection. Wellhead temperature ranges from 59 to 84 ℃. The permeability of the Minghuazhen Formation Reservoir is between 3.36×10^-14 and 2.39×10^-12 m^-2 and of the Wumishanian reservoir is between 2.10×10^-11 and 6.15×10^-10 m^-2.

METHODS

Samples were taken from a Quaternary cold water aquifer, the Minghuazhen Formation sandstone geothermal reservoir, an Ordovician limestone geothermal reservoir and the Wumishanian dolomite geothermal Reservoir and rainwater in the Niutuozhen geothermal field in order to compare geochemical characteristics between different waters (Fig. 3). The samples were taken by inserting a sampling tube against the pressure of a water discharge main. Temperature and pH were measured in-situ, other analysis were made in laboratory. K, Na, Li and a wide range of metal ions were analysed by atomic absorption; total acidity, total hardness and most anions were analysed by titration; NH₄⁺, Fe²⁺, Fe³⁺, metaboric acid, metasilicic acid, metaphosphoric acid and some halide ions were analysed by spectrophotometry; and NO₃^- was analysed by ultraviolet spectrophotometry; selenium, metaarsenic acid and mercury were analysed by atomic fluorescence; and total dissolved solids were analysed by gravimetry.

Figure  3.  Sampling sites.

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Stable isotopes (¹⁸O, ²H) were analysed by isotope ratio mass spectrometry and Picarro L1102-i liquid water isotope analyzer and ¹⁴C samples were analyzed by Liquid Scintillation Counter.

RESULTS AND DISCUSSION

Hydrogeochemistry

Geothermal waters from the Wumishanian and Ordovician reservoirs are of Cl-Na type, and those of the Minghuazhen Formation reservoir are of Cl-Na type and HCO₃-Na type (Table 1, Fig. 4). Wumishanian geothermal water has higher total dissolved solid (TDS) ranging from 2 300 to 3 000 mg/L, than Ordovician reservoir water (2 206 mg/L) and Minghuazhen Formation (561–1 427.7 mg/L) geothermal water.

Table  1.  The chemical components of the samples

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Figure  4.  Piper diagram of compositions of geothermal water and groundwater.

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Symbolic Components

Silica (SiO₂) is the most significant symbolic component of geothermal water (Liu et al., 2002) and all water samples from Wumishanian geothermal reservoir are plotted in Fig. 5. The concentration of silica in Wumishanian geothermal water increases from 40 to 80 mg/L with temperature increase from 60 to 85 ℃ demonstrating a good linear relationship (R²=0.842) between silica concentration and temperature. The linear relationship indicates that temperature instead of lithology probably enhances the enrichment of SiO₂ in such a low-temperature geothermal system because the geothermal water is stored in the Jixianian dolomite geothermal reservoir (Li, 1982; Browne, 1978).

Figure  5.  Relationship between well head temperature and SiO₂ concentration

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Origin of geothermal water

Craig (1961) found that there was δ¹⁸O shift when geothermal water is compared with the Global Meteoric Water Line, which was brought about by ¹⁸O exchange between geothermal water and rock under high temperature conditions (>300 ℃). He found that change of δ²H due to depletion of H in the rock was negligible compared with δ¹⁸O. Wen et al. (2010), Ma et al. (2008), Zhang et al. (2006), Pang (2004), Zhao et al. (2001), Wang Y X et al. (1993), Sun et al. (1992) and Pang et al.(1990a, b) have also used isotopic methods to study origin and mixing of geothermal water in different geothermal fields.

Stable isotope data for all geothermal waters (Fig. 6) show a δ¹⁸O range from -8.66‰ to -11.90‰. It is obvious that the geothermal water from the Ordovician and the Minghuazhen Formation geothermal reservoir and from the Quaternary groundwater fall near the GMWL. However, samples from the Wumi- shanian geothermal reservoir demonstrate an average 1.57‰ ¹⁸O shift, slight compared with samples from a Tibet high temperature geothermal system (Zhou et al., 2009). Truesdell and Hulston (1980) found that the mineral components of reservoir rock are a significant factor affecting δ¹⁸O shift and that maximum shift occurred in carbonate rock systems. Exchange of ¹⁸O between deeply circulating meteoric water and hot carbonates in deep-seated aquifers for a long time (Zhou et al., 2004, 2001) may explain the slight O¹⁸ shift.

Figure  6.  δ¹⁸O and δ²H of different groundwaters.

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Evolution of geothermal water

In order to study the evolution characteristics of geothermal water along a flow line, Well O1 in the northernmost part of the geothermal field was taken as origin point. A plot of ¹⁴C age along the flow line shows a progressive increase indicating age increasing from north to south (Fig. 7a). A similar trend was also found in the studies of Chen (1988) and Geothermal Group (1983). There is a significant positive linear correlation between ¹⁴C age and δ¹⁸O (Fig. 7b) which is good evidence that the extent of ¹⁸O exchange between geothermal water and reservoir rock was enhanced with the increasing residual time of geothermal water.

Figure  7.  The relationship of ¹⁴C age with distance and δ¹⁸O.

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DISCUSSION

The high concentration of Cl and Na in the Wumishanian and Ordovician geothermal water indicates that the water experienced long time lixiviation from the recharge area during the process of flowing from north to south in the geothermal field (Wang et al., 1995), similar to the geothermal water in the geothermal fields of Tianjin and Beijing (Hu et al., 2007) and confirmed by the ¹⁴C age of the geothermal water.

The remarkably differences in geochemical characteristics between Quaternary, Minghuanian, Ordovician and Wumishanian geothermal waters suggests that they derive from different groundwater systems (Zhou, 1987). The Quaternary groundwater system is mainly recharged by local modern rain and groundwater recharge from mountains to the north and west (Chen et al., 2010). By contrast, geothermal water of the Minghuazhenian, Ordovician and Wumishanian reservoirs was recharged by paleo- precipitation at least 12 000 years ago (Table 1).

Its tritium isotope concentration 67.8 TU indicates that some Quaternary groundwater receives modern rain recharge. The δ¹⁸O and δ²H values of -8.79‰ and -66.36‰ of Quaternary geothermal water are close to modern rain water but its tritium isotope value < 0.5 TU indicates that this water has not been recharged by modern rain, in agreement with the ¹⁴C results (Table 1).

¹⁴C ages indicate that the Wumishanian geothermal water flows from north to south at an average velocity of 3.6 m/a in the Niutuozhen geothermal field (Fig. 8).

Figure  8.  A conceptual evolution model of Wumishanian geothermal water.

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With increasing time for water-rock ¹⁸O exchange processes, ¹⁸O in the geothermal water is progressively enriched along the flow line from north to south as demonstrated by the good linear relationship between δ¹⁸O and ¹⁴C age in Fig. 7b.

According to research by Yan and Yu (2000) and Zhou (1987), the Niutuozhen geothermal field receives recharge both from the Yanshan Mountains in the north (about 100 km from the northern part of the geothermal field) and the Taihang Mountains in the west (about 50 km from the west boundary of the geothermal field) and moving Wumishanian geothermal water from the Yanshan Mountains may be diluted by water from the Taihang Mountains. Water from the Taihang Mountains is generally younger than water from Yanshan Mountains because of relative shorter flow distance. It could also be inferred that the water in the south of the geothermal field should be older than 33 000 year if it was only recharged by water from the north without being diluted by water from the west.