The Sichuan-Chongqing region in China has become one of the fastest growing regions in the world for shale gas production, following the successful development of shale gas in North America. By the end of 2019, the total proved original gas in place (OGIP) of shale gas in China is 17865 × 10⁸ m³, and the shale gas production capacity has reached 200 × 10⁸ m³/a, with an annual shale gas production of 154 × 10⁸ m³ in 2019. The production growth of marine shale gas in the Sichuan Basin will become the main contributor of China's natural gas production growth in the future^[1,2,3].

The “Shale Gas Revolution” in the United States has had a huge impact on the world’s energy landscape. Although many scholars have analyzed the experience of successful development of shale gas in U.S. in terms of technology, economy, policy, and management, there are few analyses of the development practices and well productivity dominating factors of individual shale gas play^[4]. Take the Fayetteville shale gas play, the second commercial shale gas play in the United States, as an example, its rapid production boom cannot be achieved without a deeper understanding of the geology, the progressive development strategy from sweet area to the inferior area, the continuous improvement of drilling and completion technology, and large number of shale gas wells^[4,5,6,7,8]. Compared with shale gas play in North American^[9,10,11,12], the marine shale in the Sichuan Basin has low TOC value, low porosity, low gas saturation and smaller thickness of high- quality reservoir, but high degree of thermal evolution. The shale gas accumulation conditions in China are more complex^{[13,14,15,16,17]}. Located on the slope of Sichuan Basin, Weiyuan shale gas play is different from Changning-Zhaotong located in the depression and Fuling located on the box faulted anticline in China^[14,18]. The Weiyuan shale gas play is characterized by small thickness (1-7 m and an average of 3.8 m) of high-quality reservoir, large horizontal stress difference (7.7-18.7 MPa), and large variation in well productivity. In order to improve the production rate of shale gas wells, it is of great significance to analyze the dominating factors on well productivity and development strategies optimization for commercial development of shale gas in South Sichuan Basin.

In this paper, based on systematic evaluation of shale gas reservoir geology and well logging interpretation in more than 20 appraisal wells, a correlation has been built between the test production rate and the high-quality reservoir length drilled in the horizontal wells, high-quality reservoir thickness and the stimulation treatment parameters in over 100 horizontal wells. The dominating factors on horizontal well productivity are identified, and the development strategies optimization is proposed. The research results effectively guide the development of shale gas under complex geological conditions.

The Weiyuan shale gas play, in the southwest of the Sichuan Basin, is located in the central Sichuan lower uplift structural belt with shale gas area of 1520 km² (Fig. 1). The organic-rich marine shale in the Wufeng Formation of Ordovician and Longmaxi Formation of Silurian, which was deposited in the deep-water continental shelf, is the favorable reservoir for shale gas exploration and development ^[19,20]. Controlled by the ancient uplift of Leshan-Longnüsi in Caledonian period, the thickness and burial depth of the shale strata gradually increase from northwest to southeast, with total thickness of 180-600 m and burial depth of 2000-4000 m (Fig. 1a). The favorable shale is located in the Wufeng and Longyi 1 subsection. While the Longyi 1 subsection can be divided into four layers named as Longyi ₁¹, Longyi ₁², Longyi₁³ and Longyi ₁⁴ from bottom to top (Fig. 1b).

The reservoir properties of shale in Weiyuan are similar to those in the rest area of Sichuan Basin. But faults and fractures are more developed, high-quality reservoirs are thinner and horizontal stress difference is larger in Weiyuan.

1.2.1. Reservoir properties

The reservoir properties in Wufeng-Longyi 1 are generally high, with TOC value of 2.1%-8.1% and average of 3.2%, porosity of 2.1%-9.6% and average of 5.9%, brittle mineral content of 60%-82% and average of 74%, gas content of 3.3-8.5 m³/t and average of 5.5 m³/t.

1.2.2. Characteristics of the study area

(1) The curvature distribution map by three-dimensional seismic (Fig. 2a) shows that the fractures, especially the microfractures, are well developed in Weiyuan shale and a few small faults (with larger curvature) are found locally. Natural fractures are more developed in the north of W202 area and in the middle and east of W204 area, followed by Z201 area. It can also be seen from the imaging logging that the fractures in the shale reservoirs are developed (Fig. 2b).

(2) The thickness of high-quality reservoir is smaller and varies greatly in Weiyuan. It can be seen from Fig. 3 that the thickness of type I + II reservoirs is 29-47 m, and the thickness of high-quality reservoir (Type I_a reservoirs, definition see Section 2) is small. The high-quality reservoirs are mainly distributed in Longyi ₁¹. The thickness of high-quality reservoir in W202 area and W204 area is relatively large, but the average thickness is only 3.8 m in Weiyuan, which makes it difficult to improve the drilling length of high-quality reservoir in drilling.

(3) The horizontal stress difference is large. The horizontal stress difference in W202 area and W204 area is significantly higher than that in Z201 area. It is more difficult to create complex fracture network by hydraulic fracturing. The maximum horizontal principal stress is 70.0-88.3 MPa, the minimum horizontal principal stress is 54.0-69.6 MPa, and the horizontal stress difference is 16.0-18.7 MPa in W202 area and W204 area. In Z201 area, the maximum principal stress is 84.0 MPa, the minimum principal stress is 76.3 MPa, and the horizontal stress difference is 7.7 MPa (Table 1).

By the end of June 2019, 188 horizontal wells have been put into production, with a cumulative gas production of 53.87 × 10⁸ m³. The lateral length of drilled horizontal well is between 816 m and 2200 m with an average of 1595 m, and the well spacing is between 300 m and 400 m.

(1) Test production rate. 173 wells have been tested with a total well test production rate of 3126.85×10⁴ m³/d. The test production rate is 1.81-71.22×10⁴ m³/d, with an average of 18.1×10⁴ m³/d. While the well head pressure is in range of 5.7-36.7 MPa, with an average of 19.0 MPa during the well test.

(2) Average daily production rate in the first year. The average daily production rate in the first year of shale gas wells is used to evaluate the gas productivity, which is similar to that of North America. The average daily production rate of the producing wells in the first year is (1.0-32.0)×10⁴ m³, with an average of 8.2×10⁴ m³.

(3) Decline of the production rate. There are some differences in production decline rate of horizontal wells between W202 area and W204 area. The production decline rate in the first five years is 56%, 34%, 26%, 21% and 18% respectively for W202 area, and 59%, 32%, 23%, 18% and 14% respectively for W204 area.

(4) Estimated Ultimate Recovery (EUR). The EUR is in range of (0.12-2.34)×10⁸ m³, with an average of 0.78×10⁸ m³. The average EUR of Type I wells is 1.21×10⁸ m³, and 0.75×10⁸ m³ for Type II wells and 0.47×10⁸ m³ for Type III wells.

It is necessary to obtain industrial gas flow through hydraulic fracturing for shale gas wells due to the shale has extremely low permeability. The well productivity is controlled by reservoir quality, reservoir thickness and the effectiveness of stimulation. Based on the analysis of the reserves in the shale gas reservoir, this paper makes it clear that the deployed reserves of high-quality reservoir is the dominating factor on well productivity through integrated analysis of the geology and engineering in the Weiyuan shale gas play. In other words, the production rate of shale gas horizontal wells is controlled by the thickness of high-quality reservoir, the drilling length of high-quality reservoir and the effectiveness of stimulation.

Shale gas reserves are mainly composed of free gas and adsorbed gas^[21]. In overpressure areas with best preservation conditions, free gas is the main contributor for shale gas wells. According to the equation (1), free gas reserves are mainly related to effective porosity, gas saturation and gas volume factor. High quality gas reserves refer to the free gas contained in the high-quality reservoirs. Vertically, the high-quality reservoir shale has the characteristics of high effective porosity and high gas saturation. As the organic matter content can reflect the development of pores and brittle mineral content means that the shale is more easy to be fractured in South Sichuan Basin^[22,23,24], the TOC content and brittle mineral content are also used to evaluate the high-quality reservoirs. According to Fig. 4, the shale with high TOC (greater than 4%), high effective porosity (greater than 6%), high gas saturation (greater than 70%), high brittle mineral content (greater than 75%) is defined as high-quality reservoir (shale with high-quality gas reserves), also known as Type I_a reservoir. In W202 area, high-quality reservoirs are mainly distributed in the middle and lower part of Longyi ₁¹. In W204 area, high-quality shale is mainly distributed in Longyi₁¹. The high-quality reservoir is characterized by medium-high natural gamma, low density, high resistance and low gamma ray without uranium. The siliceous mineral content is high and the beddings is well developed in high-quality reservoir as well (Fig. 4).

The production logging result is consistent with the above conclusions. The Longyi ₁¹ (high-quality reservoir) has the highest gas production rate, which is usually 2-5 times of Longyi ₁² (overlying stratum, thickness no more than 5m), and more than 10 times of Longyi ₁³ (Fig. 5). Although the thickness of high-quality reservoir is small with an average thickness of only 3.8 m, it is difficult to fully deplete the reserves of high-quality reservoir if another zone such as Longyi ₁² is taken as the target of the horizontal well.

In addition, based on the average daily production rate in the first year with the reservoir drilling length of 6 wells in H2 platform of W202 area, it can be seen that the average daily production rate in the first year is positively correlated with the drilling length of the high-quality reservoir of each horizontal well, and the correlation coefficient can reach as high as 0.93 (Fig. 6).

The high-quality shale gas reservoirs are mainly developed near the Weiyuan County, with a maximum thickness of over 7 m, and gradually thinned outward (Fig. 7). Compared with Changning (N201) and Zhaotong (Z104), the thickness of high-quality reservoirs in Weiyuan is significantly reduced (Fig. 8).

As the developed bedding restricts the longitudinal extension of the hydraulic fracture, temperature logging shows that the proppant fracture height of hydraulic fracture is only 6-7 m, and it is easier to extend upward^[25]. The closer the horizontal well target is to the bottom of the high-quality reservoir, the higher the deployed reserves of high-quality reservoir (Fig. 9). According to the combination of equation (1) and Fig. 9, the greater the thickness of high-quality reservoir (h_1a), the more free gas reserves there are, and the higher the gas well production rate under a certain fracturing treatment. The drainage area of a shale gas well (A_gw) is mainly determined by L and L_f, where L is the drilling length of high-quality reservoir and L_f is the half length of hydraulic fracture in horizontal wells.

It can be seen from Fig. 10 that the test production rate is positively correlated with h_la·L. Under certain stimulation treatment, the thickness and drilling length of high-quality reservoir are the domination to the deployed reserves of high-quality reservoir. In the complex geological conditions, some wells are affected by the casing deformation, and some wells are tested with new technology, so the production rate are not so high. The red dots in Fig. 10 represent the wells with the best productivity under the same geological conditions but eliminating the negative influence of engineering factors. The relationship between the ideal productivity (q_gt) of a well and the h_la·L under current technical conditions obtained by regression of red dots is shown in equation (2). According to equation (2), if the ideal gas production rate and the thickness of high-quality reservoir are given, the needed drilling length of high-quality reservoir can be estimated, and vice versa (Fig. 11).

The fracture network, including both natural fractures and hydraulic fractures, is the guarantee of deployed reserves of high-quality reserves.

Natural fracture is the fracture network formed by in-situ stress in shale gas reservoir, which can not only extend the storage space, but also is helpful to connect isolated pores during stimulation. Natural fractures are well developed in Weiyuan, which provides favorable flow path to improve the production rate (Fig. 2).

The effectiveness of hydraulic fracturing is one of the key factors to dominate well productivity^[26]. The number of fracturing stage, proppant volume per length, fluid volume per length, pump rate and number of clusters are the key parameters for hydraulic fracturing. In Weiyuan, the average frac length per stage is 68.2 m, the average frac stages are 22.4, the average proppant volume is 1.61 t/m, the average liquid volume is 27.4 m³/m, the average pump rate is 13.5 m³/min, and the number of clusters is mainly 3-4. Since the second half of 2019, 5-7 clusters have been tested in some wells. The geological characteristics of Haynesville shale gas play in the United States are similar to that of the Weiyuan shale gas play. The fracture length in Haynesville is 30-50 m, with proppant volume of 2.36-5.17 t/m and liquid volume of 19.9-43.5 m³/m. On the whole, the fracturing parameters in the Weiyuan shale gas play are much lower than that of Haynesville with similar geological conditions in North America.

Increasing pump rate, high proppant volume per length, shorter frac length and more clusters per stage are the main technical means to improve well performance. It is found that increasing the intensity of fracturing has a significant effect on improving the production rate of shale gas wells in Weiyuan. Through the statistics analysis of fracturing parameters of gas wells with test production rate over 30×10⁴ m³/d, it can be seen that the frac length per stage is 52-77 m, the frac stages is 23-33, the proppant volume is 1.57-2.00 t/m, the liquid volume is 26.4-32.1 m³/m, and the pump rate is 13.1-15.3 m³/min, which all are higher than that of low productivity wells.

In W202 area, considering the facts such as the high clay content, high calcium content and low physical properties at the top of Longyi ₁¹, the target zone is located in the middle and lower part of Longyi ₁¹ with high TOC, high brittle mineral content, high gas saturation and more bedding developed. In W204 area, the longitudinal variation of Longyi ₁¹ is minor, and the target zone is located in Longyi ₁¹.

Horizontal wells are preferentially arranged in the surrounding area of Weiyuan County where the thickness of high-quality reservoir is relatively thick, and gradually step out to the surrounding area where the thickness of high-quality reservoir is relatively thin. The continuous progress of technology can make up for the negative influence of thin high-quality reservoir.

Based on the results of fracturing parameters of high-performance wells in Weiyuan and best practice in Haynesville, aiming at greatly increasing the production rate of shale gas wells, it is recommended that the frac length of the horizontal wells is more than 1800 m, the frac length per stage is less than 60 m, the proppant volume is more than 2.0 t/m, the liquid volume is about 30 m³/m, and the pump rate is more than 14 m³/min.

By the end of 2019, many horizontal wells or platforms in the Weiyuan shale gas play have achieved high production rate after the implementation of development strategies optimization in April 2018. For example, the total test production rate of the four- well-platform H15 in W202 area is 192.67× 10⁴ m³/d, the average test production rate per well is 48.17× 10⁴ m³/d, and the maximum well test production rate is exceeded 70×10⁴ m³/d. Since the second half of 2018, the average daily production rate in the first year for new production wells has reached 11.7×10⁴ m³, an increase of 85.7% compared with that of 6.3×10⁴ m³ in the first half of 2018, and an increase of 63.4% compared with that of 7.16×10⁴ m³ of all production wells before the implementation of development strategies optimization (Fig. 12a). The average EUR of new production wells is 1.11×10⁸ m³, 85% higher than that of production wells in the first half of 2018, and 63.2% higher than that of all production wells before the implementation of development strategies optimization (Fig. 12b). It should be noted that the average EUR of Blasingame method and analytical method are taken for all production wells to ensure the reliability of the EUR.

Based on the analysis of the geological characteristics and dynamic performance of the Weiyuan shale gas play, evaluation criteria of high-quality reservoir are built, and the reservoir characteristics and distribution of high-quality reservoir are clarified. High-quality reservoirs are characterized by high TOC value (more than 4%), high effective porosity (more than 6%), high gas saturation (more than 70%) and high brittle mineral content (more than 75%). They are mainly distributed in the middle and lower part of longyi ₁¹ (W202 area) or longyi ₁¹ (W204 area), with high gas production contribution. It is clear that the well productivity of shale gas horizontal wells is mainly controlled by the thickness of high-quality reservoir, drilling length of high-quality reservoir, the development of natural fractures and the effectiveness of stimulation. Suggestions on target zone of horizontal wells, optimized deployment of horizontal wells and fracturing parameters are put forward. After the implementation of development strategies optimization, a breakthrough in well production rate has been achieved, and the average EUR of new wells is 85% higher than that in the first half of 2018. In the future, we should improve the stability of technology and the application of advanced technology, which include improving the effectiveness of stimulation (focus on high proppant volume per length), reducing the casing deformation, and reducing the frac hit.

A_g—reservoir area, km²;

A_gw—drainage area of single well, km²;

B_gi—volume factor of original shale gas;

G_x—total free gas volume, 10⁸ m³;

h—thickness of reservoir, m;

h_la—thickness of high-quality reservoir, m;

L—drilling length of high-quality reservoir, m;

L_f—half length of fracture, m;

q_gt—test production rate, 10⁴ m³/d;

S_gi—original gas saturation, %;

ϕ—effective porosity, %.