(Central Laboratory of Planning and Design Institute of Northwest Petroleum Bureau, Urumqi 8300 1 1)

According to the principle of statistics, the oilfield water of different pay zones in Tahe Oilfield is preliminarily studied, and the following understandings are obtained: ① Each block of Tahe Oilfield has the same source rock conditions as Sidari Ya area; The relationship between the main ion content and ion concentration of oilfield water in different blocks and pay zones in Tahe shows that oilfield water comes from marine facies. (2) There is a good linear relationship between density and density. The older the stratum, the greater the slope of the straight line. ③ The newer the pay zone age in Tahe Oilfield, the greater the total salinity, density, Cl- and Na+ concentration of oilfield water, but the opposite of Ca2+ concentration. The graphic results of main characteristic ions show that the distribution area of oilfield water in marine limestone reservoir is obviously different from that in marine sandstone reservoir and continental sandstone reservoir, and it is easy to distinguish. However, the oil fields and water bodies of Triassic and Carboniferous sandstone reservoirs are closely related, so it is difficult to distinguish them.

Keywords characteristic ionic solubility of aqueous solution in Tahe Oilfield

Tahe Oilfield is located at the junction of luntai county and Kuqa County in Xinjiang, north of Tarim River, and its structural position is in the south of Akkule Uplift in Shaya Uplift of Tarim Basin.

Since 1990 Sha23 and Sha29 wells were tested in Carboniferous and Triassic respectively, through further exploration, Triassic oil and gas reservoirs have been discovered in Tahe 1 block and block 2, and Ordovician and Carboniferous oil and gas reservoirs have been discovered in blocks 3, 4 and 6. The oil source of Tahe Oilfield is Cambrian-Ordovician, and the oil and gas mainly come from the southeast Maingard craton depression basin and platform margin slope. Due to the influence of tectonic movement, the oil and gas reservoirs in Tahe Oilfield have the characteristics of multi-stage accumulation, and the properties of crude oil in different accumulation periods are quite different. The proportion of crude oil accumulated in the early period (late Hercynian-Indosinian period) is relatively large, and such reservoirs include Ordovician in Tahe 4 and 6 blocks, Lower Ordovician in Tahe 3 block, and Lower Triassic in Tahe 1 block. The crude oil accumulated in the middle period (Yanshan-early Himalayan) is of medium density, and the oil and gas reservoirs include Triassic in Tahe Area 2, Carboniferous in Tahe Area 3 and 4, and Middle-Upper Ordovician in Tahe Area 3. The crude oil accumulated in the late period (after the early Himalayan period) is light oil, and the oil and gas reservoirs include Triassic Zhong You Formation in Tahe 1 block.

The pay zones put into development and trial production in Tahe Oilfield are Ordovician carbonate rocks and Carboniferous and Triassic sandstones. The chemical composition and content of Tahe oilfield water change greatly, mainly in the same layer of water in the same well, and its chemical analysis results change greatly. The chemical analysis results of water from different wells in the same layer also vary greatly. Through sorting out and studying the analysis data of oilfield water in Tahe Oilfield, it is considered that the oilfield water in different pay zones in Tahe Oilfield has high overall salinity and density and good sealing conditions, and it is Surin CaCl2 _ 2 type water. By comparing the chemical composition and content of oilfield water, it is easy to distinguish, and it is closely related to the lithology of pay zones and pay zones and the properties of crude oil.

In oil and gas exploration, the change of chemical composition and content in oilfield water can be used to describe layered reservoirs in the same stratum, qualitatively evaluate the preservation conditions of oil and gas reservoirs, study the migration and accumulation direction of oil and gas, and indicate potential stratigraphic traps. In primary oil recovery and secondary oil recovery, the source of invaded water can be identified according to the chemical characteristics of oilfield water in different pay zones. It can provide guidance for the design of brine treatment scheme. The concentration of soluble solids and the composition of interstitial water have great influence on electrical logging data. Therefore, in electrical logging interpretation, the logging interpretation formula or interpretation chart can be corrected according to the formation water characteristics in different regions and different horizons. Therefore, it is of great significance to study the characteristics and distribution of oilfield water for oil and gas exploration and development.

1 experimental analysis method

The water quality analysis data of Tahe Oilfield cited in this paper are all obtained by chemical analysis in the central laboratory of Planning, Design and Research Institute of Northwest Petroleum Bureau of Xinxing Oil Company according to the requirements of the specification.

Sampling method: formation water samples are mainly taken from the sampling valve of wellhead separator, or formation water samples such as drill pipe sampling cavity and drill pipe reverse circulation.

Experimental analysis method: For Cl-, Ca2 ++ and Mg2++, titration analysis method is adopted; Fe(T) (total iron), Fe2+, I- and Br- were determined by visual colorimetry. The sum of Na+ and K+ is calculated by anion-cation balance method, and the content of Fe3 ++ is calculated by subtracting Fe2 ++ from Fe(T) total iron. The reliability of chemical titration for the determination of major ions Cl-, Ca2 ++ and Mg2 ++ is much greater than that of visual colorimetry. However, the calculated Na+, K+ and Fe3+ contents have the worst reliability due to the transmission and superposition of errors.

Before analyzing the data, by analyzing the sampling time, method and test conditions of water samples, some analysis data that obviously can not reflect the chemical properties of pay zone are eliminated. In addition, in order to make the chemical properties of oilfield water representative, the water quality analysis data of wells with long-term stable water output in oilfield development should be selected as far as possible. For Ordovician pay zones, the results of oilfield hydrochemical analysis obtained in the early stage of testing and production are likely to be transformed by drilling fluid and acid due to lost circulation during drilling acidification and fracturing reconstruction of reservoirs. Therefore, in the data screening, the hydrochemical analysis results of wells with long water production time and relatively stable water production are selected.

2 chemical characteristics of Tahe oilfield water

The crude oil and natural gas in Tahe No.3 and No.4 structures are mainly produced in carbonate rocks or clastic rocks, and the well depth is 4300~5500m m. The water in Tahe Oilfield is Surin CaCl2 _ 2 water with good preservation conditions, which is weakly acidic and has a pH value of 5.5 ~ 5.7.

The statistical results show that the average content of main ions in oilfield water in Tahe Oilfield has little change, and the total salinity is195.2×103 ~ 222.1×103 mg/L, with an average of 208.65×103 mg/L. Mg/L; The density is1.126 ~1.147g/cm3, and the average value is1.137g/cm3; The range of Ca2+ content is 173000 ~ 9 100mg/L, with an average value of13200 mg/L; The content of Mg2 ++ is 2138 ~ 51mg/L, with an average of 2105 mg/L; The contents of Na+ and K+ ranged from 74,000 to 56,600 mg/L, with an average of 65,300 mg/L; Cl- content is 13600 ~ 12500 mg/L, with an average value of127mg/l; The content was 187 ~ 44 1 mg/L, with an average of 243 mg/l; Br- content is 13.8 ~ 4.2 mg/L, with an average of 9 mg/l; The content of I- is 37 1 ~ 89.3 mg/L, with an average of 302 mg/L; The content is 138 ~ 258 mg/L, with an average of 2 18mg/L (see table 1).

The average values of total salinity, density and main ion content of Tahe oilfield water are compared with those of other oilfields in northern Tarim, and have the following characteristics: ① The average values of total salinity, density and main ions such as Cl-, Ca2 ++ and Mg2 ++ of oilfield water in various blocks of Tahe oilfield are close to those of Dalia oilfield water in the west, but quite different from those of Yakela Lower Cretaceous oilfield water, Bashituo oilfield water and Yasendi oilfield water. For example, the total salinity, density and average contents of major ions such as Cl-, Ca2+ and Mg2+ in oilfield water of Ordovician limestone pay zones in Tahe Area 3 and Area 4 are much higher than those of dolomite pay zones of Subongdi in Bachu Uplift and Xiaohaizi Formation in Bashituo Carboniferous. ② The total salinity, density and average content of major ions such as Cl-, Ca2 ++ and Mg2 ++ in Carboniferous sandstone reservoirs in Tahe 3 and 4 areas and Triassic sandstone reservoirs in Tahe 1 and Tahe 2 areas are much higher than those in Yakela Lower Cretaceous sandstone reservoirs (see table 1). The total salinity and density of Tahe oilfield water and Sidari Yayou oilfield water are basically consistent with the average contents of major ions such as Cl-, Ca2+ and Mg2+. The above comparison results show that Tahe Oilfield and Sidari Ya Oilfield have the same oil and gas source conditions (including lithology, lithofacies, oil and gas generation period, etc. ) and the preservation conditions of oil and gas reservoirs are basically similar. The obvious difference between Tahe oilfield water and Yakela Lower Cretaceous oilfield water and Bachu oilfield water may be related to the difference of sedimentary facies belts of source rocks on the one hand, and the relatively mature source rocks of Yakela Lower Cretaceous oilfield water and Bachu oilfield on the other hand. The oil and gas reservoirs are condensate gas reservoirs, and the samples taken contain a certain amount of condensate water, which reduces the main indicators of water quality analysis results.

Average mass fraction of main ions in oilfield water of different blocks and pay zones in Tahe Oilfield.

Cause analysis of water in Tahe oilfield

Oil and gas migrate from source rocks and accumulate in traps. Whether through pressure difference or diffusion, water is the main carrier of oil and gas migration. Although the carrier of oil and gas migration is influenced by oil and gas reservoirs and water bodies in oil and gas accumulation layers along the migration route, oilfield water still retains the hydrochemical characteristics of source rocks to a great extent. By studying the percentage of soluble matter in oilfield water, the sedimentary facies of source rocks in oil and gas reservoirs can be identified qualitatively (MaSon, 1952).

Comparing the average content of soluble components in Tahe Oilfield with the statistical results of soluble components in river water and seawater, it is found that the chemical characteristics are as follows:

(1) Although the concentrations of the three components are quite different, the salinity of seawater is about 30.9× 103mg/L and that of Tahe oilfield water is about 2 10× 103mg/L, but the percentage content of some ions in Tahe oilfield water and seawater is similar.

Table 2 Comparison of Soluble Matter in Tahe Oilfield Water, River Water and Seawater Table 2 Correlation of Soluble Matter Quality and Fraction in Tahe Oilfield Water, River Water and Seawater

(2) The chemical characteristics of water, seawater and river water in Tahe No.3 and No.4 oilfields are almost opposite.

Tahe Oilfield Water: Cationic Anions

Seawater: cation anion

River water: cation anion

From the content of ionic components, the main components of water-soluble substances in Tahe Oilfield are consistent with seawater, mainly NaCl. The content of Na+ in seawater is about11000 mg/L. The content of Na+ in water in Tabei Oilfield is 56 600 ~ 74 000 mg/L. The content of Na+ in seawater and water in Tabei Oilfield is much higher than that in river water and river water without circulating salt. In addition, the Na+ content in seawater and water of Tahe Oilfield itself is much higher than that of other oilfields. Cations Na+, Ca2+, Mg2+ and anion Cl- in the water of Tahe Oilfield have the same change characteristics of ion content, which is contrary to the change characteristics of river water. In addition, according to a large number of statistical data, both seawater and Tahe 3 and 4 oilfield waters contain I- and Br-, with little or no trace elements. Therefore, it is considered that the water in Tahe Oilfield is marine.

4 The relationship between the change of main ion concentration in water and strata in Tahe Oilfield

Although the source rocks in Tahe Oilfield are the same, and the oilfield hydrochemical composition is similar, the oilfield hydrochemical characteristics are different in different blocks and pay zones due to different reservoir-forming periods and later reconstruction processes, and different lithology and sedimentary facies of pay zones.

It can be seen from the table 1 that the Ordovician limestone oilfield water in Tahe is obviously different from the Carboniferous and Triassic sandstone reservoirs, and its average total salinity, density and contents of Cl-, Na+ and K+ are low, while the contents of Ca2+, Mg2+, I- and Br- are high, especially Ca2+, Br- and I- are high. The content of water ions in sandstone reservoirs (Carboniferous and Triassic) is similar. Judging from the total salinity, density, Cl-, Na+ and K+ concentrations, the newer the stratum, the higher the ion concentration, while the Ca2+ concentration is just the opposite.

The content changes of various soluble substances in oilfield water are inevitably related to the formation conditions and environment of oilfield water. This paper mainly discusses the relationship between the main ion compositions (Ca2++, Mg2++ and * * * oil layers in different horizons of Tahe oilfield water.

We know that the solubility of ionic compounds in water mainly depends on ① the size of their crystal lattice; ② Hydration energy of ions.

A water-soluble salt whose attraction between ions and water molecules is greater than that between opposite charges. Slightly soluble salts are characterized by firm crystal lattice and small ionic hydration tendency. The solubility of common substances in formation water and the solubility product of insoluble substances are listed as follows (Table 3).

Table 3 Solubility of common substances in formation water and solubility product of some insoluble substances, Basil, calculation manual for chemists. The temperature not indicated in the table is 25℃.

Table 3 Solubility of common substances in formation water and solubility products of some insoluble substances

The solubility of some substances listed in Table 3 and the solubility product constants of insoluble substances are in pure water. However, due to the high salinity of the solution and the complexity of the formation environment, there are many factors that affect the solubility or solubility product Ksp in oilfield water, such as salt effect to increase solubility, common ion effect to decrease solubility, pH value of the solution, redox state Eh, high temperature and high pressure, etc. That is to say, the composition and concentration of water-soluble substances in oilfield are pH, Eh and high pressure. When all kinds of influencing factors are determined, the dynamic balance is reached. For different equilibrium states, it is difficult to describe them with a simple and fixed mathematical formula, and only statistical methods can be used to describe the characteristics of main ions in formation water.

According to Ksp in Table 3, CaCO3, MgCO3 and FeCO3 are insoluble substances, and there are a lot of Ca2+ in the formation water of Tahe Oilfield in different periods, with the concentration range of 9.1×103 ~17.3×103g/L (theoretically, the concentration of Ca2+is greater than 6. Therefore, it does not exist in the water of Tabei Oilfield, which is consistent with the measured results. Because the sum of Na+ and K+ is an estimate with a large error, it is not considered in the statistical process. But in fact, because K+ easily reacts with clay minerals, K+ only accounts for a small part, and the main cation is Na+. However, the proportion of K+ content in surface water is relatively high. Because of the complexity of Fe3+ and Fe2+, Fe2+ is easily oxidized to Fe3+ in air, and its content and state are closely related to sampling method and sample analysis time. In addition, the true value of Fe3+ caused by pipeline corrosion and other factors is difficult to measure, so the relationship between Fe and formation is not carefully studied here. The information about the amount and type of iron compounds in oilfield water is mainly used to estimate the corrosion degree of production system and the measures to be taken when it is used for water injection. According to the concentration ratio of ferrous iron to ferrous iron, the redox environment of water body can also be inferred.

4. 1 Relationship between concentration and density and stratum

In the process of oil recovery, organic matter will be oxidized and decomposed, producing CO2 and CO2 as the main by-products of bacterial life, which will be produced after being dissolved in water.

In aqueous solution, the following equilibrium relationship exists:

Essays on exploration and development of oil and gas fields in northern Tarim basin

That is, the content is directly related to the concentration of H+ in the solution. As can be seen from the equilibrium relationship, when the pH is around 6.3, the equilibrium is reached. If the acidity increases (the pH value decreases), it will be balanced in the direction of generating water and carbon dioxide; If the acidity decreases, it will be converted into H+,and if the pressure of CO2 gas in the closed system increases, the above reaction will move to the right. With the participation of organic components, the equilibrium equation changes, and the pH range can be increased from 2 and 3 to 12. The measured pH value of Tahe No.3 and No.4 oilfield water is generally between 5 and 6, which is favorable for existence under this acidity. Because the acidity of oilfield water is closely related to sampling time and method, its relationship with rigor is not easy to determine. In addition, the factors that affect the concentration of oilfield water are soluble matter concentration and solution composition. The concentration of the solution is proportional to the density; The change of solution composition is directly related to the chemical composition of formation minerals. There is a good linear relationship between water and density in Tahe Oilfield, and the correlation coefficient is 0.9949 ~ 1. The linear slopes of different formations are different (Figure 1, 2, 3).

Essays on exploration and development of oil and gas fields in northern Tarim basin

4.2 Relationship between Ca2+and Mg2+ concentration and stratum

Ca2 ++ and Mg2 ++ in oilfield water are converted into water-soluble Ca(HCO3)2 and Mg(HCO3)2 by CO2 dissolved in water after weathering by insoluble minerals CaCO3 and MgCO3, and their concentrations depend on the pH value and concentration of the solution, on the other hand, they are related to the chemical composition of reservoir minerals and the chemical properties of other ions in the solution. Insoluble calcium carbonate reacts with +2 HCO 3- in carbon dioxide aqueous solution. When it comes into contact with calcium-containing minerals such as limestone, dolomite, gypsum (CaSO4 2H2O) or gypsum-containing rocks, the Ca2+ content in the solution will increase. Magnesium is dissolved in the process of chemical weathering, mainly in the form of chloride and sulfate. Iron and magnesium minerals in igneous rocks and magnesium carbonate in carbonate rocks are usually considered as the main sources of magnesium in natural water systems. Magnesium is dissolved from silicate and carbonate minerals, and carbon dioxide plays an important role. At this time, magnesium is dissolved in the form of magnesium bicarbonate Mg(HCO3)2. Because both calcium carbonate and magnesium carbonate are closely related to carbon dioxide dissolved in water, in a certain formation water balance system, because the dissolved carbon dioxide is limited, there is a certain restrictive relationship between the concentrations of Ca2+ and Mg2+ in the solution. Generally, dissolved Mg2+ decreases and dissolved Ca2+ increases.

Essays on exploration and development of oil and gas fields in northern Tarim basin

Essays on exploration and development of oil and gas fields in northern Tarim basin

The concentration of Mg2+ in the water of Tahe Oilfield is between 511~ 2138 mg/L, which is far less than the concentration of Ca2+.

The content of sulfate in formation water is affected by bacterial activities, and sulfur-oxidizing bacteria can oxidize H2S to provide sulfur source for organisms. Another kind of sulfate-reducing bacteria can reduce sulfate in water and pore water to H2S, creating a strong reducing environment, which is beneficial to the preservation of organic matter. In addition, the sulfate content in formation water is also affected by the existence of Ca2++, Sr2 ++ and Ba2+. If the concentration of these three cations is quite high, the concentration of sulfate is low. Ca2++, Mg2 ++ and 3 restrict and influence each other. The concentrations of Ca2+ and Mg2+ in different oilfield waters in northern Tarim are quite different. See Figure 4 for Ca2+, Mg2+ and their concentrations in different formations.

Figure 4 Relationship between sulfate ion concentration and calcium and magnesium ion concentration in formation water of Tahe Oilfield Figure 4 Relationship between sulfate ion concentration and calcium and magnesium ion concentration in formation water of Tahe Oilfield

Fig. 5 Relationship between calcium and magnesium ion concentration and density in Tahe oilfield water Fig. 5 Relationship between calcium and magnesium ion concentration and density in Tahe oilfield formation water.

As can be seen from Figure 4, Ca2+, Mg2+ and their concentrations are closely related to strata, mainly related to lithology. The contents of Ca2+ and Mg2+ in Ordovician limestone are relatively high, and the range of concentration change is small, while the changes of Carboniferous and Triassic oilfield water are small, but the concentration changes greatly. The graphic analysis of oilfield water between Ordovician limestone and Carboniferous and Triassic sandstone reservoirs shows that the graphic analysis results of Ca2 ++ and Mg2 ++ concentration and density of oilfield water between Ordovician limestone and Carboniferous and Triassic sandstone reservoirs are more obvious (Figure 5).

4.3 Characteristics of Oilfield Water in Different Strata of I-Tahe Oilfield

As a characteristic ion of oilfield water, I- mainly comes from algae and other marine organisms. Its concentration in formation water reflects the content of algae and other marine organic matter in ancient seawater of this formation. For Tahe oilfield water, although it is all seawater, the content of I- ions varies greatly due to the difference of formation lithology and oil and gas migration path. The highest content is Ordovician, and the I- content ranges from 8 to 20 mg/L, with an average value of13.8 mg/L; The Carboniferous changed greatly, and the content of I- was 1.5 ~ 9 mg/L, with an average of 4.2 mg/L; The I- content in Triassic changed little and was low, ranging from 2.5 to 6 mg/L, with an average of 4.3 mg/L. According to the statistical results and the corresponding oil and gas field crude oil properties and formation lithology, the I- content was related to formation lithology and sedimentary facies, and the I- content in marine limestone was high. The I- content of oilfield water in carboniferous sandstone reservoirs with alternating land and sea changes greatly. The I- content of tidal flat sandstone closely related to the ocean is higher, and the I- content of delta sandstone closely related to the land is lower. The I content of continental Triassic sandstone has little change and is low. The graphic analysis of I- ion concentration and Ca2+ concentration in the water of Tahe Oilfield shows that Ordovician limestone can be easily distinguished from Triassic and Carboniferous sandstone oilfield water, but Triassic and Carboniferous oilfield water cannot be distinguished (Figure 6).

Fig. 6 Relationship between calcium ion and iodine ion in water of Tahe Oilfield Fig. 6 Relationship between calcium ion and iodine ion in information water of Tahe Oilfield

5 conclusion

From the above statistical analysis and research, we can draw the following conclusions:

(1) The ion composition characteristics of Tahe oilfield water show that Tahe oilfield water is close to that of Siddaria oilfield water, but quite different from that of Yakela Lower Cretaceous oilfield water and Bachu oilfield water, indicating that all blocks in Tahe oilfield have the same source rock conditions as that of Siddaria oilfield water; The relationship between the main ion content and ion concentration of Tahe oilfield water shows that it comes from marine phase.

(2) There is a good linear relationship between concentration and density, and the slope of the straight line is different in different strata. The older the stratum, the greater the slope of the straight line.

(3) The newer the pay zone age in Tahe Oilfield, the greater the total salinity, density, Cl-, Na+ and K+ concentration of oilfield water, while the Ca2+ concentration is the opposite.

(4) The graphical results of Ca2+, Mg2+ and Ca2+, Mg2+ and density, Ca2+and I- all show that the distribution area of oilfield water in marine limestone reservoir is obviously different from that in marine sandstone reservoir and continental sandstone reservoir, and it is easy to distinguish. However, the oil fields and water bodies of Triassic and Carboniferous sandstone reservoirs are closely related, so it is difficult to distinguish them.

refer to

[1] Collins a.g. oilfield hydrochemistry. Beijing: Petroleum Industry Press, 1984.

[2] (Japan) Masahiro Omori, Akio Mao Mu, Tomono Hoshino. Shallow sea geology. Beijing: Science Press, 1980.

Wang Qijun, Chen Jianyu. Geochemistry of petroleum and natural gas. Wuhan: China Geo University Press, 1988.

[4] Na Bogell ·W·H· Zhu, translated by Chen Fu. General Chemistry (2,3,4), Beijing: People's Education Press, 1979.

[5] Ideal solution, W.A. Oates, Journal of Chemical Education, 46,501(1969)

[6] Reaction under pressure, W.J.Le Noble, J.Chem, Educ, 44,729 (1967).

Chemiluminescence reaction in solution, J.W.Haas, Jr., Journal of Chemical Education, 44,396 (1967).

Relationship between hydrochemical characteristics and strata in Tahe Oilfield, Tarim Basin

Zhou Xiaofen

(Planning and Design Institute of Northwest Petroleum Bureau of Urumqi 8300 1 1)

Abstract: The hydrochemical characteristics of various strata prove that the relationship between hydrochemical characteristics of Tahe Oilfield and its corresponding strata is complex, and it is difficult to describe it with mathematical formulas. Through the graphic analysis of the oilfield water in Tahe pay zone, the following results are obtained: Tahe Oilfield (1) block has the same source rock conditions as Sidari Ya; The characteristics of water ions in each block and pay zone prove that they all come from marine phase; (2) There is a good linear relationship between 3 and density, that is, the older the stratum, the greater the linear slope value; (3) For the total salinity, density, concentration of Cl-, Na+ and K+ of water, the younger the stratum age, the greater they are, while the concentration of Ca2+ is the opposite. The main characteristic ion diagram proves that the field water distribution of marine limestone reservoir is very different from that of land-sea interaction and continental sandstone reservoir, which is easy to distinguish. Triassic gas field water is closely related to Carboniferous reservoir gas field water, which is difficult to distinguish.

Key words: characteristic ion solubility of aqueous solution in Tahe Oilfield