Plenty of productivity test data were collected from 20 wells of the Devonian strata in the Marsel Block, among them, 4 wells showed no dry layer and the remainder wells all had different degrees of gas shows or gas flow, as shown in Fig. 4. The test data mainly comprised three monitoring operations, namely, production capacity tests, pressure recovery tests, and gradient tests, relevant parameters are shown in Table 1.
Figure 4. Plane distribution of exploratory well testing for gas in Devonian strata in the Marsel Block (Pang et al., 2014a).
Well name Structure Physical log interpretation Productivity test Porosity (%) Permeability (mD) Gas saturation (%) Test interval (m) Fluid situation Gas yield (104 m3/day) Water West Oppak 1-G West Oppak 3.98–8.46 0.13–5.46 52–62.5 1 809–1 878 7 Water shows West Oppak 2-G 3.37–11.06 0.46–8.13 55.13–81.21 1 903–2 030 0.13 Water shows West Oppak 3-G 2.62–8.79 0.18–8.93 50.38–79.28 1 997–2 242 No fluid Oppak 1-G Oppak 7 74 1 224–1 372 No fluid Water shows Kendyrlik 3-G Kendyrlik 2 238–2 384 Water layer Kendyrlik 5-RD 2.63–9.34 0.18–1.35 55.44–86.22 1 679–1 680 No hydrocarbon Water layer Ortalyk 1-G Ortalyk 4.32 0.1–0.78 57.49–60.42 2 606–2 690 10 Ortalyk 2-G 5.28–8.7 0.41–2.19 55.42–82.44 2 118–2 614 Dry layer Ortalyk 3-G 5.24–7.92 0.37–5.33 64.39–78.56 2 500–2 680 3.0 Pridorozhnaya 3-G Pridorozhnaya 2 380–2 435 102 Pridorozhnaya 4-G 2 225–2 450 7.4 Pridorozhnaya 5-G 2 526–2 690 Weak hydrocarbon Water Pridorozhnaya 6-G 2 522–2 605 Dry layer North Pridorozhnaya 1-G North Pridorozhnaya 7.61–15.7 0.45–11.1 58.8–65.23 2 644–2 798 1.2 Water shows South Pridorozhnaya 17-G South Pridorozhnaya 2 876–3 048 0.3 Water shows Assa 1 Assa 4.5–14.3 0.1–3.2 50.2–67.82 2 413–2 580 38.4 Nainman 1-P Nainman 1 407–1 690 Water layer Bulak 2-G Bulak 2 940–3 015 Dry layer
Table 1. Parameters related to the analysis and testing of the Devonian gas reservoir in Marsel Block
Well logging curves from 17 wells were also obtained, consisting of HSGR, NPHI, RHOB, DTCO, RLA5, etc. According to the logging curves processed by the appropriate software, we can determine the parameters such as shale volume, calcium volume, porosity, permeability, and water saturation, some of them are presented in Table 1. There are three wells with obvious thick aquifers, namely, Naiman 1-P, Kendyrlik 5-RD, and Kendyrlik 3-G.
Seven wells including West Oppak 1-G, West Oppak 2-G, West Oppak 3-G, Oppak 1-G, Pridorozhnaya 3-G, Pridorozhnaya 5-G, and Pridorozhnaya 6-G, were organized by the former Soviet Union, and other wells were organized by Condor and the Marsel company.
Although the distribution of the gas reservoir in the Marsel Block is limited, the distribution characteristics of natural gas still obey some obvious rules, which are analyzed in detail in this part.
The testing gas deposits by exploratory wells generally show that the accumulation of natural gas in the Devonian strata in the Marsel Block has the characteristics of continuous gas accumulations, which means that not only there is much gas present at the apexes of structures, but also there are gas accumulations on slopes and in depressions (Li et al., 2016; Wandrey et al., 1997).
According to the testing gas deposits by exploratory wells (Fig. 4), we can see that in general some exploratory wells had gas shows located both at the apexes and sides of structures. Three gas fields, namely, West Oppak, Ortalyk, and Pridorozhnaya, were found that are located in the center of the lower part of the block. However, there are also two local structures, namely, Oppak and Kendyrlik, which have gas shows located at relatively high positions in the block, whereas other areas were not found to contain natural gas in the Devonian strata. Regarding local structures, gas shows can be found both inside and outside the structural traps. For example, among the wells drilled in the Pridorozhnaya gas field, Pridorozhnaya 4 is located in the upper part of the structure and its gas production rate is 7.31 m3/day, however, although Pridorozhnaya 5 is located outside the structural traps, it still has faint gas shows. The above analysis shows that gas in the Devonian strata in the Marsel Block can not only accumulate at the apexes of structures but also at their bases, slopes, and depressions. This phenomenon makes the study area possess the characteristics of continuous gas accumulations.
In the process of well logging interpretation, by combining logging features with the results of gas logging and testing, carrying out a comprehensive analysis of the lithology, physical properties, and electrical properties of the reservoir, and then selecting appropriate processing parameters, we can generate a resistivity-porosity intersection diagram and a gas saturation-resistivity intersection graph to obtain the logging interpretation of the porosity of the target layers. Results show that the main reservoir, namely, the Devonian Famennian reservoir, has generally low porosity, which is generally less than 12% and mostly in the range of 4%–9%, with an average of 6.28%. However, there are still some logging interpretations of reservoir porosity that are greater than 12% (Fig. 5). For example, some gas reservoirs fractures and cavities have developed in individual well sections, which resulted in a logging interpretation of high porosity. It can be concluded that the gas-bearing reservoir of the Devonian system in the Marsel Block can be classified as a tight reservoir, but locally develops layers with high porosity and permeability (Zhao et al., 2018; Qiu et al., 2017; Pang et al., 2014a).
We tested the pressure in three gas-bearing structures in the Upper Devonian strata in the Marsel Block, namely, the Oppak West, Pridorozhnaya, and Kendyrlik structures. Then, we analyzed the pressure data for the formation and calculated the formation pressure, the hydrostatic pressure, and the corresponding pressure coefficient of each well in each depth segment (Table 2), the relationship between the actual formation pressure and the hydrostatic pressure was also established (Fig. 6). Results show that the actual formation pressure in the Devonian system is generally lower than the hydrostatic column pressure. Eleven test points in five wells were used for data collection. The measured data points are all to the left of the line representing the hydrostatic column pressure and corresponding to negative pressure, as their pressure coefficient is generally less than 1. For example, the Assa 1 well in the Assa structure was tested for gas deposits in the two layers of the Upper Devonian in 2 402–2 426 and 2 530–2 580 m segments. Daily gas production was 0.81–6.14 million m3 and 21.5–38.35 million m3, respectively, the formation pressures in the two gas-bearing strata were 24.58 and 26.01 MPa, respectively, and the pressure coefficient was 0.87, which indicated negative pressure characteristics. There were also some points where the pressure was close to normal, for example, the average depth of the Kendyrlik 4 well was 2 236 m, its formation pressure was 26.10 MPa, and its pressure coefficient was 0.998, which is close to the hydrostatic pressure, belonging to the normal pressure. It can therefore be concluded that the characteristics of negative or low-pressure in the gas reservoir are obvious, and some of the wells have pressure that is close to normal in the target layer (Li et al., 2018; Pang et al., 2014a, b; Roy and Ryan, 2013).
Well name Mid-depth (m) Layer Formation pressure (MPa) Hydrostatic pressure (MPa) Pressure coefficient Kendyrlik 4 2 236 D3 26.10 26.16 0.998 Kendyrlik 4 2 420.5 D3 25.78 28.32 0.910 Pridorozhnaya 4 2 401.5 D3 25.58 27.54 0.929 Pridorozhnaya 4 2 420.5 D3 25.26 28.32 0.892 Pridorozhnaya 4 2 246.5 D3 25.58 26.28 0.973 Pridorozhnaya 4 2 250 D3 25.26 25.80 0.979 West Oppak 1 1 861 D3 18.40 21.77 0.845 West Oppak 1 1 842 D3 15.33 21.55 0.711 Assa 1 2 555 D3 26.01 29.89 0.870 Assa 1 2 414 D3 24.58 28.24 0.870 Tamgalytar 1-G 2 294.5 D3 22.43 26.31 0.853
Table 2. Well pressure tested in the Marsel Block
Results show that the characteristics of water deposits are not obvious in the Devonian system in the Marsel Block. At the base of the depression, we can see that there is essentially no water present on testing. Only the Kendyrlik 5-RD and Kendyrlik 3-G wells, which are located in the northern part of the study area, and the Naiman 1-P well, which is located in the southern part of the uplifted anticline, contain an obvious water layer in the Upper Devonian. In the former two wells, the water-type is sodium chloride, and in the latter it is sodium sulfate and potassium sulfate. The Assa 1 well, which is located on the slope in the study area, contains an obvious gas-bearing formation. The logging interpretation of the porosity of oil-, gas-, and water-bearing reservoirs in the Devonian also shows that the relationship between gas and water deposits in the Marsel Block is complex, and the characteristics of the water deposits located beneath the gas deposits are not obvious (Fig. 7).