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All of them can be called oilfield water, which is a part of groundwater in oil and gas areas and forms the same flow system with oil and gas. Usually, oilfield water refers to the groundwater directly connected with the oil layer inside the oilfield, that is, the oil layer water. Groundwater can be divided into phreatic water and interlayer water (Figure 2-6).

Figure 2-6 Groundwater Types

Oilfield water exists in three forms in underground pore and fracture system, namely, adsorbed water (semi-solid), capillary water (active only when external force is greater than capillary force) and free water.

Types of oilfield water

The basic problems to be solved in the classification of oilfield water include: ① the chemical mark of oilfield water and its difference from non-oilfield water; ② Characteristics and differences of different types of oilfield water. Since 19 1 1 the first oilfield water classification scheme was put forward by Pasmel in the United States, the oilfield water classification scheme has been revised and supplemented many times, but it is basically based on the contents and combinations of Na+, Mg2+, Ca2+ and Cl-, SO42- and HCO3-. Among all the classification schemes, Surin (B.A. щулин) is a concise classification method, which is also widely used at home and abroad. So here we focus on the classification of Surin.

The classification of Surin (1946) is based on the comparison of chemical composition characteristics between ancient mainland water and seawater. Because natural water is mainly divided into continental water and seawater, water with typical components of Na2SO4 and NaHCO3 is called continental water, Xin 20 10-09- 14. Although the content of NaCl in seawater is the highest, it is MgCl2 (other components have accumulated) that is concentrated in the water after dilution, so Su Lin regards the typical components as continental water. Then according to the exact equivalence ratio of Na+ and Cl-, combined with two genetic factors (rNa+-rCl-)/rSO42- and (RCL-RNA+)/RMG 2+, groundwater is divided into four basic types (Table 2-4).

Su Lin thinks that Na2SO4 water develops near the surface, CaCl2 water is deep in the earth's crust, and MgCl2 water is typical seawater. On the oilfield profile, the upper part is sodium bicarbonate type water, and the deep part is magnesium chloride and calcium chloride type water. The hydrochemical types of oilfield water are mainly CaCl2 _ 2, followed by NaHCO3 _ 3, and Na2SO4 and MgCl2 are rare. It should be pointed out that these analyses can only be used as necessary conditions (but not sufficient conditions) for oilfield water.

Su Lin believes that in terms of its forming environment, natural water is mainly divided into two categories: continental water and seawater. The salinity of continental water is low (generally less than 500mg/l), and its chemical composition has the relationship of HCO 3-> SO 42-> Cl-, Ca 2+> Na+< Mg 2+, Na+> Cl-, Na+/Cl- (equivalent ratio) > 1. Seawater has a high salinity (generally about 35,000 mg/L), and its chemical composition is characterized by Cl-> SO4 2-> HCO 3-, Na+> Mg2+< Ca2+, Cl-> Na+, Na+/Cl- (equivalent ratio) < 1. In mainland fresh water, heavy calcium carbonate is dominant and contains sodium sulfate; However, sodium sulfate does not exist in seawater.

According to the above knowledge, Su Lin divided natural water into four basic types with three genetic factors: Na+/Cl-, (Na+-Cl-)/SO42- and (Cl-Na+)/Mg2+.

Su Lin thinks that the groundwater in the exposed geological structure may belong to sodium sulfate type, and the enclosed water isolated from the surface atmospheric precipitation mostly belongs to calcium chloride type, and the transition zone between them is magnesium chloride type. The formation water in the upper part of oil and gas reservoir profile is mainly sodium bicarbonate type; With the deepening of burial, the transition is magnesium chloride type; Finally, it becomes calcium chloride. Sometimes the sodium bicarbonate type is directly replaced by calcium chloride type, and there is no transition type. The hydrochemical types of oilfield water are mainly calcium chloride, followed by sodium bicarbonate, sodium sulfate and magnesium chloride.

Table 2-4 Classification Grade of Surin Natural Water Source Groundwater Classification Table

(According to b. a .щулин 1946) Genetic coefficient (concentration ratio in milliequivalents) Na+/Cl-(Na+-Cl-)/SO42-(Cl-Na+)/Mg2+mainland.

Genetic classification of natural water

(According to British Airways щулин, 1946)

End point A: SO4=50%, Na-Cl = 0%; End point B: SO4=50%, Na-Cl = 50%; End point C: SO4=0%, Na-Cl = 50%; End point D: Mg=0%, cl-na = 50%; End point E: Mg=50%, cl-na = 50%; End point F: Mg=50%, cl-na = 0%;

The problems existing in the classification of forest include: ① the origin of groundwater is regarded as the infiltration of surface water, and the participation of other water sources is not considered, as well as the mixing of water and various differentiation combinations of water components that often occur in nature; (2) It is too simple to simplify the original large number of inorganic components with the origin as a whole into the classification of natural water and salt components; ③ Some typomorphic components in water, such as gas components and trace elements, are ignored, and the characteristic parameters of oilfield water and non-oilfield water are not distinguished. With the development of oil and gas exploration and the in-depth study of oilfield water geochemistry, it is generally believed that the combination of salinity and anionic groups should be the basis of oilfield water chemical classification, and then oilfield water and non-oilfield water can be divided according to the characteristic parameters or signs of oilfield water. However, most of the classification schemes proposed at present are too complicated, inconvenient to use and difficult to popularize, and have not been generally accepted.

There are three important sources of oilfield water: sedimentary water, which is filled in the gap between sediment particles and remains in the process of sediment accumulation. The salinity and chemical composition of this water are closely related to the salinity of the ancient sea (lake) water that deposited the sediments and the sediments themselves. Therefore, the salinity of oilfield water is obviously different in different sedimentary environments.

Seepage water-refers to water from atmospheric rainfall that penetrates into shallow pores and saturated rock formations. Because of the low salt content of the infiltration water, it can desalt the groundwater with high salt content. Desalination is particularly important in oilfield water near unconformity surface.

Epigenetic water refers to high temperature, high salinity, saturated and homogeneous groundwater from the mantle and deep crust, including primary water, magmatic water and metamorphic water. This kind of water plays an important role in the formation of metal deposits. However, there are different understandings of the role played in the formation of oilfield water.

Diagenetic water-crystal water (structural water) separated from the diagenetic transformation of minerals and water related to the evolution of organic matter. -refers to high temperature, high salinity, saturated and harmonious groundwater from the mantle and deep crust, including primary water, magmatic water and metamorphic water. This kind of water plays an important role in the formation of metal deposits. However, there are different opinions on its role in causing oilfield water.

Oil field water can be regarded as water mixed with sedimentary water, permeable water, plutonic water and diagenetic water in different proportions, which is accompanied by a series of complex physical and chemical actions (dissolution-precipitation) of oil (gas).

Two. Chemical composition and salinity of water

Chemical constituents of oilfield water

The chemical composition of oilfield water essentially refers to the chemical composition of solute dissolved in oilfield water. Comprises inorganic components, organic components, dissolved gases and trace elements.

1. Inorganic composition: How many ions can be used to represent the inorganic composition of oilfield water in conventional water quality analysis:

Cations -Na+(K+), Ca2+, Mg2+ anions -Cl-, SO42-, HCO3-(CO32-)

The content of each ion in water can be expressed by weight method (mg/L), equivalent method (mmol/L) and equivalent percentage method. The first two represent the relative content of ions, and the latter represents the absolute content of each ion, which is conducive to comparison.

According to research, more than 60 elements have been identified in natural water, of which about 30 are the most common. The composition of oilfield water is more complicated than that of single natural water, not only because oilfield water has been in contact with oil and gas for a long time, but also because it entered the burial environment from different sedimentary environments (such as freshwater lake, saltwater lake or mainland) and experienced a long and complicated evolution history after burial. During this period, the changes of surrounding rock properties, temperature and pressure in contact with it, and the communication with surface water when it is connected with surface water will have different degrees of influence on the chemical composition of oilfield water.

In conventional water quality analysis, Na+ (including K+), Ca2+, Mg2+ and Cl-, SO42- and HCO3- (including CO32-) are usually used to represent a large number of inorganic components. Its content can be expressed by weight method, equivalent method and equivalent percentage method. The relationship between the three representations is as follows:

Mg equivalent (mg equivalent/liter) = weight (mg/liter)/equivalent value

Percentage of milliequivalents = number of milliequivalents (one ion)/number of milliequivalents of all anions and cations. The table shows the calculation results of unified analysis data expressed in three ways. Representation of hydrochemical composition

Ion mg per liter (mg/l) mg equivalent per liter (ppm) equivalent percentage anion Cl-34,930 984.0 49.0 SO42-79016.5 0.8 HCO3-280 4.60.2 The total anion is 36,0001, 005. 1 50.0 cation Na+12,281534.0005.0 50.0 The sum of anion and cation is 56,8412,01./kloc- This is a classic joke! . The reason is that the early chemical processes experienced by sedimentary water after burial are mainly biochemical processes and redox processes. Sulfate in formation water is often reduced to H2S and S2-, and the content of SO42- is obviously reduced, that is, sulfate is reduced. At the same time, Fe3+ was transformed into Fe2+, and HCO3- and CO32- increased correspondingly.

The reduction degree of SO42- and other oxides mainly depends on the abundance of organic matter. The content of H2S in groundwater varies greatly, ranging from a few milligrams per liter to several thousand milligrams. Therefore, although there is a lot of H2S in water without or with little H2S, it is considered as one of the signs of oilfield water, but when there is a lot of sulfate in water without or with little H2S, it cannot be judged as oilfield water, and it must be comprehensively analyzed in combination with other conditions.

Trace elements in oilfield water mainly include iodine, bromine, boron and strontium. The high content of iodine, bromine, boron and ammonium is another characteristic of oilfield water. Comparison table of trace elements in different water bodies

Elements and compounds Oilfield water (mg/L) Seawater (mg/L) Freshwater (mg/L) Iodine 0.05 0.003 Bromine 65 0.000 1-0.2 Boron 10-104-6 trace or no NH4 100-500-20-60 in oilfield water. The high content of bromine and boron makes groundwater almost stop, which is beneficial to oil and gas preservation.

Ammonium is the product of decomposition of primitive organic matter, which is unstable in chemical properties and easy to be converted into ammonia and other compounds. Therefore, the existence of ammonium further shows that the underground is a reducing environment, which is conducive to oil and gas preservation.

Strontium, barium and other elements are also found in oilfield water, but not all oilfield water. In a word, the existence of trace elements in oilfield water is helpful to study the sedimentary diagenetic environment related to oil and gas, which is the premise of oil and gas preservation.

2. Organic components: The common organic components in oilfield water are hydrocarbons, phenols and organic acids, which are often used as a sign to distinguish reservoir water from non-reservoir water (Table 2-3).

Table 2-3 organic indicator hydrocarbon of reservoir water The water gas and liquid hydrocarbon in benzene series phenolic organic acid reservoir are high, toluene/benzene > 1 is high, > 0. 1 mg/L, and (o-cresol) is mainly naphthenic acid.

The main non-reservoir water is only low in CH4 and toluene/benzene < 1. The main phenolic hydrocarbons in non-reservoir water are gaseous hydrocarbons and liquid hydrocarbons. Generally, oilfield water often contains dissolved hydrocarbon gases, including methane and heavy hydrocarbons, especially the existence of heavy hydrocarbons often indicates that it is related to underground oil and gas reservoirs. The content of heavy hydrocarbons is related to the distance of oil and gas reservoirs. Generally speaking, non-oilfield water usually contains only a small amount of methane.

The content of benzene series in reservoir water is relatively high, generally reaching 0.03- 1.58mg/l, and the highest reaching 5-6 mg/L. The toluene/benzene ratio is greater than 1. The content of benzene series in non-reservoir water is low, and the ratio of toluene to benzene is less than 1.

The content of phenol in reservoir water is also high, generally greater than 0. 1mg/l, and the highest can reach10-15 mg/L. The main components are o-cresol and cresol. Non-reservoir water content is low, mainly phenol. The picture shows the comparison of benzene and phenol content distribution in produced water and non-produced water of a condensate gas field in the former Soviet Union.

Oilfield water often contains naphthenic acid, fatty acid and amino acid. Naphthenic acid is a derivative of petroleum cycloalkane, which can often be used as an important hydrochemical index for petroleum exploration. Naphthenic acid content is related to the distance between separated reservoirs, and the closer the distance, the higher the content. In addition, the content of naphthenic acid is also related to the type of water. Alkaline sodium bicarbonate water is the easiest to enrich, while calcium chloride and magnesium chloride water contain little or no naphthenic acid. This is because sodium naphthenate has high solubility in water, while calcium naphthenate is difficult to dissolve in water. Therefore, it cannot be considered that water without naphthenic acid is not oilfield water, and it must be combined with the type analysis of water.

3. Trace elements: most of them are related to the geochemical and biochemical environment of oil and gas reservoirs, and oilfield water can be identified by a single element or a combination of elements. For example, the high values of F, Cl, Br and I are mostly located in oil-bearing structures in diving in Fuyu Oilfield of Jilin Province.

4. Isotopes: mainly C 13, D, O 18, etc. , you can determine the source of groundwater.

Mineralization of oilfield water

The so-called salinity refers to the total amount of various ions, molecules and compounds contained in water per unit volume, which is usually called the total salinity of water. Units are milligrams per liter (ppm), grams per liter or milliequivalents per liter. And the total salinity can be expressed by dry residue (residue left after water is heated to 105℃ and evaporated) or total ions. It should be said that this is only a rough measurement method, because when the water contains evaporated substances, the evaporation process will make the measured salinity less than the actual value; However, when the residue contains crystal water, the measured salinity will increase.

Oilfield water, especially reservoir water, belongs to high salinity water under normal circumstances, so women masturbate.

Natural water can be divided into fresh water (salinity < < 1 0,000 ppm), brackish water (1 0,000-3,000 ppm), salt water (3000-10,000 ppm) and salt water (10,000 ppm). The total salinity of seawater is relatively high, reaching 35,000ppm. The water related to oil and gas generally has the characteristics of high salinity, which is caused by the fact that the oilfield water is buried in the deep underground and has been in a stagnant state for a long time, lacking circulation and alternation. The total salinity of water in most offshore oil fields is above 50000-60000 ppm, and the highest salinity can reach 642,798ppm (calcium chloride type water in Silurian dolomite in Sallinna, Michigan, USA). There is also high salinity water in Cretaceous sandstone of Burgan Oilfield in Kuwait, which is 154400 ppm. However, the salinity of land oilfield water is much lower, generally 5000-30000 ppm, the highest is 30000-80000 ppm (China Jiuquan Basin laojunmiao oilfield water), and the highest is 148900 ppm (Shengtuo Oilfield Shahejie Formation third member oilfield water), all of which are sodium bicarbonate water.

However, whether it is the ocean or the land, there are oilfield waters with relatively low salinity, and even the opposite situation occurs. Marine oilfield water with low salinity is like Ordovician oilfield water in Kansas, USA, with salinity of 5000-35000 ppm. Others are the water from Quarunquar Oilfield in Venezuela, with the maximum salinity of only 2,300ppm and the average value of1.400 ppm. . The water in Las Cruz oilfield in western Venezuela has a salinity of only 323ppm, which is really fresh water. It is generally believed that this abnormal scene is related to the existence of unconformity. It can be seen that the salinity of oilfield water varies greatly due to different geological conditions.

3. The relationship between oilfield water and oil and gas

Deductive, the relationship between oilfield water and oil and gas is mainly manifested in two aspects:

1. According to the hydrochemical characteristics of oilfield water, oil and gas can be directly searched.

The principle is that deep fluid changes the chemical composition of shallow water through upward infiltration and diffusion, especially at the top of oil and gas reservoir structure with comparable joints and fractures. According to the dispersion degree of abnormal values of hydrochemical characteristics in all background values, the scale of underground oil and gas reservoirs can be roughly delineated, which is called hydrochemical oil exploration

2. According to modern hydrochemical data, we can judge the migration, accumulation and preservation conditions of oil and gas.

A large number of data show that the most favorable environment for oil and gas accumulation and preservation should be the environment where seepage water and seepage water alternate slowly or stagnate, that is, the environment where groundwater does not move much. According to the characteristics of modern hydrochemistry, this area is a high salinity water distribution area with Cl- and Na+ as the main components and CaCl2 _ 2 as the main components. In addition, the combination of equivalent ions such as desulfurization coefficient (rSO42-/(rCl-+rSO42-), sodium chloride coefficient (rNa+/rCl-) and carbonate equilibrium coefficient ((rHCO3-+rCO32-)/rCl-) can also be used to judge the migration direction and accumulation conditions of oil and gas.

Main reference books (recommended textbooks):

1, Fundamentals of Hydrogeology, Wang, et al., Geological Publishing House, 1995.

2. Oilfield Hydrogeology, Di Shi Xiang, Northwest University Press, 199 1 year.

3. Oilfield hydrogeology, Yang,, Petroleum University Press, inflatable doll, 1993.

4. Underground temperature and pressure environment in oil and gas-bearing areas, Yang,, Petroleum University Press, 1993.

5. [America] A.G Collins (Lin Wenzhuang, Wesley Wang), Oilfield Water Geochemistry, Petroleum Industry Press, 1st Edition 1984+00.

6. Liu, Yan. Hydrogeological principles of oil and gas fields, Petroleum Industry Press (Beijing), 1995438+0.

7. Petroleum and natural gas geology.

Friendship cohesion: lecture notes on petroleum and natural gas geology of Northwest University

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