2.1

Structure, reservoir and gas reservoir characteristics

A certain well area is located in the Xujiaweizi fault depression in the southeastern part of the Songliao Basin, with a Shengping Xingcheng nose shaped structure. It mainly produces volcanic and conglomerate rocks, with explosive and fan delta facies as the main lithofacies. The lithology is mainly rhyolite, tuff, conglomerate, and sandstone. The reservoir belongs to a medium low porosity and low permeability reservoir, with poor physical properties. The gas water relationship in the well area is complex, and it belongs to a normal pressure and temperature system. The natural gas properties are mainly methane, which is a typical dry gas feature. The formation water is NaHCO₃ type.

2.2

Development status

Production time of a certain well area: Starting from July 2008; Total number of wells: 12; Average daily gas production per well: 6.3×10⁴ m³; Average daily water production per well: 1.7 m³; Daily production capacity: 76×10⁴ m³; Annual production capacity: 2.2×10⁸ m³; Gas extraction speed: 4.7%; Accumulated gas production: 13.4×10⁸ m³; Extraction degree: 28.3%.

3.1

Capacity characteristics

Define abbreviations and acronyms the first time they are used in the text, even after they have been defined in the abstract. Abbreviations such as IEEE, SI, MKS, CGS, sc, dc, and rms do not have to be defined. Do not use abbreviations in the title or heads unless they are unavoidable.

The production capacity of gas wells varies greatly⁵, with a higher proportion of Class II gas wells (Table 1).

Table 1.

Gas well classification evaluation and composition.

The production characteristics are characterized by a rapid decrease in production capacity in the early stages of production^6-7, which is currently stabilizing (Figure 2).

Figure 2.

Changes in calibrated production capacity of a certain well area over the years.

3.2

Formation pressure

The formation pressure decreased rapidly in the initial stage of production and is currently stabilizing (Figure 3), while the gas production per unit pressure drop is relatively low (Figure 4).

Figure 3.

Formation pressure variation curve in a certain well area.

Figure 4.

Variation curve of gas production per unit pressure drop in a certain well area.

Pressure monitoring shows that there is a connectivity relationship between wells M6-101, M6-102, M6-202, and M603 (Figures 5 and 6).

Figure 5.

M6-101 volcanic rock pressure profile.

Figure 6.

Volcanic rock and structural overlay in block M.

3.3

Reserve characteristics

The dynamic reserves of single well control are low (Figure 7), and the degree of reserve utilization in the well area is low (Figure 8).

Figure 7.

Schematic diagram of single well controlled reserves in a certain well area.

Figure 8.

Schematic diagram of reserves utilization in a certain well area.

3.4

Produced water characteristics

At present, the main type of produced water in the well area is fracture type weak water channeling, with a water to gas ratio ranging from 0.1 to 0.9 m³/10⁴ m³. Some gas wells have low production and are difficult to carry liquid. Currently, there are three wells producing layer water, all located in lower structural areas, adjacent to the same layer of gas and water (Table 2).

Table 2.

Production situation and water quality analysis of a certain well area.

4.1

Well network encryption

From a planar perspective, the well network is encrypted for the unused area. A certain well area has strong heterogeneity in the reservoir and low degree of reserve utilization. By calculating the current well control area and remaining reserves between wells of the gas wells that have been put into operation (Table 3), the degree of reserve utilization can be improved by adjusting the well network encryption. The remaining reserves under well control between well groups are 5.1×10⁸ m³, with an area of 0.51 km². It is recommended to select a favorable location in the distribution area of remaining reserves and arrange a horizontal well to fully utilize the remaining reserves.

Table 3.

Calculation of single well control area in a certain well area.

4.2

Perforation and fracturing

Vertically, for gas wells with unused reservoirs, perform perforation and fracturing, summarize the experience of implementing measures in the well network, and select potential wells for the next step of measures based on actual gas well production performance, reservoir conditions, process technology, and other aspects^5-8 (Table 4).

Table 4.

Situation of unused reservoir gas wells in a well area.

The drilling and fracturing of the gravel layer in well M1-4 have achieved increased storage and production of the gas well. After the measures, the well’s controlled reserves have increased by 1×10⁸ m³, and the initial production has reached 10×10⁴ m³, with a cumulative increase of 2430×10⁴ m³.

The M6-308 well in the well area is used to extract the Shahezi Formation reservoir, while the adjacent M6-309 and М6-301 wells are used to extract the volcanic rock reservoir, which has good physical properties.

From the perspective of production dynamics, the stable production capacity of M6-308 well is 6×10⁴ m³/d, the daily production capacity of M6-301 well and M6-309 well are both above 10×10⁴ m³, and the peak shaving capacity can reach 18×10⁴ m^3/d. The effective thickness of the unused volcanic rock reservoir in M6-308 well is 46.8 m, and it is predicted that the daily gas increase of the well after perforation and fracturing can reach 10.3×10⁴ m³.

It is predicted that the total increase in gas production through perforation and fracturing of unused reservoir gas wells in a certain well area is 86.2×10⁴ m³/d (Table 5).

Table 5.

Prediction of gas increase in unused reservoir gas wells.

4.3

Repeated fracturing

In terms of precision, for gas wells with inconsistent dynamics and insufficient reservoir utilization, repeated fracturing was carried out. The M6-Xiang201 and M6-102 wells were affected by edge and bottom water, with an average perforation of only 15.4% of the effective reservoir thickness. The amount of sand added was less than 50 m³, and the amount of liquid added was less than 400 m³. It is predicted that the effective thickness of the unused reservoir is 52.6 m (Table 6).

Table 6.

Insufficient utilization of gas wells in the edge and bottom water gas reservoir.

The M1 Ping 1 well underwent repeated fracturing to improve the degree of reservoir transformation. During the gas testing, only 44 m³ of sand was added, resulting in low fracturing fluid backflow rate and large leakage. In addition, there was a 448 m reservoir that was not used, and the daily gas production before the measures was only 3.2×10⁴ m³. In order to improve the gas well production capacity, 13 stages of fracturing transformation were carried out in 2017, with a sand addition of 372 m³ and a liquid volume of 8260 m³. After the measures, the well control reserves increased by 2.6×10⁸ m³, and the production increased by 6×10⁴ m³. The cumulative gas increase has been 2808×10⁴ m³.

It is predicted that the total amount of gas increase from repeated fracturing in a gas well with insufficient reservoir utilization in a certain well area is 3.1×10⁴ m³/d (Table 7).

Table 7.

Prediction of gas well increment for underutilized gas reservoirs in the edge and bottom water gas reservoirs.

4.4

Reservoir unblocking

There is a discrepancy between the dynamic and static conditions, and there is reservoir contamination in the gas well. Reservoir unblocking is carried out, and the M6-102 well is adjacent to the same gas and water layer, producing below the critical liquid carrying flow rate. The wellbore is prone to liquid accumulation, with a large production pressure difference, and there is “water lock” pollution in the reservoir. From the perspective of production dynamics, the daily gas production and water production of the well are low, with low wellhead pressure and a recovery rate of 34.5%. The remaining well control reserves are 2.03×10⁸ m³. It is recommended to perform reservoir unblocking on the well⁹.

Taking the M6-103 well reservoir unblocking as an example, the daily production capacity of the well was low before the measure, and it could not continue normal production. After the measure, continuous production was achieved, with an initial daily gas production of 1.8×10⁴ m³ and a daily water production of 5.3 m³. For reservoir unblocking wells (Table 8), timely acquisition of dynamic data, evaluation of production increase effect, summary of mature practices, identification of shortcomings, and reliable reference for the next batch of well measures are provided. It is predicted that the daily gas increase of M6-102 well after the measures will be 1.5×10⁴ m³.

Table 8.

Statistical table of reservoir unblocking wells.

4.5

Composite drainage

In terms of precision, for the liquid gas well, composite drainage is carried out. The physical properties of the reservoir in M6 Xie201 well are poor, the daily gas production during gas testing is low, and the skin coefficient reflects that the reservoir has no obvious pollution. Small layer data shows that there is no potential layer above and below. It is recommended to improve the recovery rate of the gas well by implementing composite drainage measures on this well^10-12.

M6-101 has eddy current tools underground, with a large pressure difference in the oil casing, up to 3.5 MPa, and the wellbore is prone to fluid accumulation. The control measures of adjusting production and carrying liquid are taken for the well. When the inlet pressure is below 6 MPa, the instantaneous production is increased by 1000-1500 m³/h until the inlet pressure rises to about 8 MPa and is adjusted to production allocation. Through the control, the cumulative gas increase of M6-101 well is 148.72×10⁴ m³ so far, and it is predicted that the daily gas increase of M6-XE201 well combined drainage is 0.7×10⁴ m³.