There is free water in the reservoir. Because of the difference of the production pressure, the liquid water in the formation flows from the reservoir into the wellbore and then outputs from the ground. This type of water that leaves the ground is called edge (bottom) water (Hao et al., 2010). The gas testing of this kind of well shows that the capacity of daily water production is large, and the capacity of daily gas production is generally not attained to the standards of industrial gas flow. This kind of well has obvious characteristics of water layer on the well logging curves, and is generally located in an area rich in water or an edge. The water saturation of the production layer and the water yield during production process are large.

The edge (bottom) water is commonly found in structural gas reservoirs, which are a conventional aquifer and locate in the lower part of a sand body with contiguous development (Dou et al., 2010). It is generally distributed in the low structure part of the reservoir, and comes into contacting with the high part of the gas layers in relationship to edge (bottom) water. The sand body has a unified interface between gas and water. The edge (bottom) water shows low natural gamma values, a high acoustic time difference, low resistivity and obvious differences with the gas layer. The interpretation shows that the porosity and thickness of permeable sand layers are large; thus, the water energy is strong. Its water yield is large during the gas testing, belonging to formation water with high salinity.

The Y44 well shows low natural gamma values (the average value is 26.5API) and low resistivity (the average value is 82.3Ω·m) in the section of 2663.00m~2667m, which was comprehensively judged as the aquifer (see Fig. 3). There are low natural gamma values (the average value is 26API) and higher resistivity (the average value is 207.5Ω·m) in the upper part of the aquifer of the section of 2657~2663m. The results of gas testing indicate that the production of gas is 3.58×10⁴ m³/d, the production of water is 46.6 m³/d. Integrating the above judgment, the upper part generally is a gas layer; the lower part is generally a water layer. Combined with the overall recognition of this area, there is a gas-water layer in the Shan 2³ member of the Y44 well which constitutes a contact relationship with edge (bottom) water.

Fig. (3) Results of the comprehensive interpretation for the Y44 well.

Affected by heterogeneity, some isolated sand bodies containing water are interrupted or wrapped by non-permeable layers (Dai et al., 2012). Thus, natural gas cannot displace this part of the original water and the isolated water body with high water saturation forms. The sand body around the water layer pinches out. And it is not developed in adjacent wells in horizontal directions, and no gas layers are connected to it in the vertical direction. Therefore, it is a sand body with high water content and isolated development. Because of the limited volume of the isolated lenticular water, the producing characteristics of the drilling well are that the daily water yield is large in the early stage but drops rapidly.

Against the background of low porosity, low permeability and strong heterogeneity of the gas reservoir, the dense sand bodies are distributed in the lenticular sand body. In the early stage of sedimentation, the sand bodies in the reservoir are saturated by formation water. When the source rocks are in the peak period of hydrocarbon generation, due to the effect of hydrocarbon-generating pressurization, the natural gas is desolate to form free gas and migrate to the sand bodies of reservoirs. Then the water is discharged into a low position. First, the gas passes through the reservoirs with a relatively large channel and good connectivity, but for sand bodies with poor physical properties, the gas cannot displace the formation water in fine pore-throat or low-quality reservoir beds, which forms ‘tight lenticular water’ because of the insufficient filling energy of gas. The physical properties of this kind of aquifer are poor, and the logging characteristics are similar to that of a dry layer. The production capacity of the gas well is small.

A typical well that has the lenticular water layer is Y39 (see Fig. 4). In the section of 2365.9~2380.6m of the Shan 2³ member of this well, the average value of natural gamma is 40.4API, the average value of deep lateral resistivity is 20Ω·m, and the deep and shallow laterologs have basic coincidence or show a small positive amplitude difference (3Ω·m). The average value of the density logging is 2.5g/m³, and the average values of acoustic logging and neutron logging are 225.1µs/m and 10.1%, respectively. The interpretation of this layer shows that the average porosity is 7.5%, the average permeability is 0.32 mD, and the average water saturation is 72%, which is comprehensively judged as an aquifer based on the log interpretation. The interpretation of the Shan 2³ member of the west direction of the M1 well and the northeast direction of the M19 well shows no sand body (see Fig. 5). The sand body around the water layer pinches out. In addition, the sand body is not developed in adjacent wells in horizontal directions, and no gas layers connect to it in the vertical direction. Therefore, it is a sand body with high water content and isolated development. The comprehensive judgment is that the Shan 2³ member in the Y39 well belongs to the isolated lenticular water layer.

Fig. (4) Results of the comprehensive interpretation for the Y39 well.

During the process of gas accumulation in gas wells that produce formation water residue in the gas reservoir, when gas migrates into a reservoir, the saturation of the formation water remaining in the gas reservoir is slightly higher than that of critical water due to the strength of the natural gas discharge not being sufficient (Hou et al., 2010). Its typical characteristics are that the gas saturation is higher, and the information about the water layer is not obvious in the well logging curves. However, during production, there is a small quantity of formation water, and following the production, this part of the water is produced. The logging characteristics of the formation water residue in the gas reservoir are as that of the gas layer in the well logging curves, and there is no obvious water layer information.

Fig. (5) Typical profile of the lenticular water.

This water type is relatively common in lithologic gas reservoirs but rare in structural gas reservoirs (Wood, 2012). The logging response characteristics are low natural gamma, high resistivity and low water saturation, which are comprehensively judged as a gas layer by log interpretation. During the process of gas testing and production testing, the water yield of the gas well is small and slightly higher than theoretical condensate water volume. Due to the influence of condensate water, the salinity of the produced water is small, belonging to desalinated formation water. In the process of production, the water yield decreases.

A typical well that produces ‘the formation water residue in gas reservoir’ is Z16-19 (see Fig. 6). The produced layer of this well is the Shan 2³ segment, and the log interpretation shows that it is a gas layer. The perforation interval is the section of 2709.5 m~2715.5 m, and the results of gas testing show that the gas production is 2.0342×10⁴ m³/d, with no water production. The well was put into production in August 29, 2007, the volume of daily water production was up to 1 m³ in September 2007, but dropped rapidly, which was judged to be the result of fracturing fluid in the early stage. From December 2007 to April 22, 2008, the gas production was maintained at approximately 1×10⁴ m³/d, the water production was approximately 0.4 m³/d, and the maximum volume of condensate water was calculated to be 0.157 m³. It was judged that formation water was produced. Compared with the logging curves of this well, the average porosity and permeability of sandstone are 5.8% and 0.167 mD in the perforation layer. The output water was judged to be a mixture of condensate water and formation water residue in the gas reservoir. The proration increased from 1×10⁴ m³ to 2×10⁴ m³ after April 22, 2008, but daily water production decreased from 0.4 m³ to 0.1 m³. The degree of mineralization of the produced water was analyzed to be 3664.08mg/L, which is obviously condensate water.

The similarities between the ‘formation water residue in gas reservoir’ and ‘the lenticular water in tight reservoir’ are that they belong to the retention water in the formation and have limited energy in the water body. In the process of testing or the production of gas wells, with a small amount of formation water, the degree of mineralization of the water sample in the separator of gas well decreased after the mixture of the formation water and condensate water, which demonstrates the characteristics of desalted formation water.

Fig. (6) Results of the comprehensive interpretation for wells 16-19.

The differences between both of them are that the porosity logging curves of the section of ‘lenticular water in tight reservoir’ shows poor physical properties and the gamma curve is higher, which is similar to the logging response characteristics of the dry layer, but the formation water residue in the gas reservoir is shown as a gas reservoir in the well logging curves.

The similarities between the isolated lenticular water and edge (bottom) water are that their logging characteristics are similar, and the logging curves shows good physical properties with a low gamma curve and deep lateral resistivity, which shows the characteristics of water layer. In the process of layer testing, the water yield is larger, which shows the characteristics of high-salinity.

The differences between them are that the edge (bottom) water is located in the lower part of the sand layer, the high position of the same connected sand layer has developed a gas layer, and both of them have a uniform interface between gas and water, which belongs to the same pressure system. In contrast, the sand body around the isolated lenticular water pinches out. The sand body is not developed in adjacent wells in horizontal directions, and no gas layers are connected to it in a vertical direction. Therefore, it is the isolated sand body with the high water content.

Study show that the types of formation water of the section of the Shan 2³ in the Zizhou gas field are mainly edge (bottom) water and lenticular water. Based on the various types of formation water recognized by the well logging response characteristics, combined with predecessors’ research results on the distribution characteristics of sand bodies and structures, a superimposed plan of the water bodies, sand bodies and superface structures of the Shan 2³ member in the study area is drawn (see Fig. 9). From this figure, we can see that the sand bodies of the Shan 2³ segment are more developed, and the physical properties are relatively good. The gas layers and water layers are mainly distributed in the Shan 2³ segment. In the study area, the gas and water in the same sand body follow the principle of the upper part being gas, and the lower part being water. Because of that, the reservoirs are influenced by the lithology, the physical properties and the structural height, and local edge (bottom) water and lenticular water are developed in this area.

The distribution of gas and water in the Zizhou area is controlled by the structure, and the water is mainly distributed in the Y47-Y29-Y43 well area, Y64-Y40 well area and Y69 well area with low structures in the west, south and southwest. This is determined to be edge (bottom) water after differentiation during the process of migration and accumulation of gas and oil. During this process, water migrates to nearby lower parts of the edge based on gas and oil, namely, the west and southwest of low part of structure. Then, relatively water-rich areas form in the west. In addition water-rich points of smaller area form in the east. For instance, the Y47-Y42-Y44-Y43 well area is a large-scale water-rich area, whereas the Y54-Y64 well area and the Z28-43—Z30-44 well areas in the east form relatively small water-rich areas. In the lower part of the sand body in the southwest, controlled by the relative height of the reservoir, the water bodies with a larger extended range appear. The water bodies are independent of each other, and there is no uniform interface between gas and water. In the relatively low structure part of the northeast, water-rich areas with relatively small extended ranges appear due to the shielding effect of the pinchout zone and the bend zone of the sand body. More lenticular water exists in the central and southern parts of the study area, and the water yield is small (less than 4m³/d). The scale of water body is very limited, mainly controlled by the change in reservoir conditions (reservoir heterogeneity), and there is no uniform interface between gas and water. In addition, due to the influence of reservoir characteristics, such as poor physical properties of the local area, there are a few residual areas of formation water in the well area, such as the Z16-19 well area.

Fig. (9) Superimposed plan of water bodies and sand bodies and the superface structures of the Shan 23 member in the Zizhou gas field.

Based on the fine logging interpretation of 191 wells in the study area, and combined with the analysis of dynamic production, a statistical table of the types of water-producing wells in the Shan 2³ member of the study area is shown in Table 1. This table shows that the Shan 2³ member in the study area mainly produce edge (bottom) water, followed by lenticular water. The formation water residue in gas reservoirs is relatively small.

Fig. (9) and Table 1 clearly describe the well areas with the distribution of formation water in study area. Based on these research findings, the gas reservoir development and the well placement can be easily developed.

The above research shows that the edge (bottom) water is clearly controlled by the structure, especially the low-amplitude structure; thus, the structure is more important for the control of edge (bottom) water. Structural characteristics also have a certain impact on lenticular water and the formation water residue in gas reservoirs. The lower part of the structure is the distribution area of the main water-rich area. In the relatively independent area with gas, the relatively low position of the micro structure is also a common distribution area of water bodies. In addition, the distribution of sand bodies also determines the distribution of formation water, and the physical properties of reservoir affect the size and the connectivity of formation water bodies.

Table 1
Statistical table of types of water-producing wells in the Shan 2³ member of study area.