Evaluation of main factors affecting fractured horizontal well productivity of tight sand gas reservoir in Sichuan Basin

Hydraulic fracturing horizontal well is the key technology of tight gas development. After fracturing, the gas well shows the characteristics of great difference in production energy and many factors affecting production capacity. Therefore, taking 10 fractured horizontal wells in JQ gas field in Sichuan Basin as an example, comprehensively considering the influence of geological and engineering factors, and based on the statistical analysis of gas well productivity and the laws of various index parameters, 17 geological and engineering parameters were selected, and the main factors controlling productivity were studied by grey correlation method. The results show that the tight gas fracturing horizontal well can obtain high production, which is jointly determined by geological and engineering factors. The five main control factors affecting the productivity of tight gas fracturing horizontal well are as follows: proppant type, preflush, φꞏh. Sand volume and reservoir volume.


Introduction
The world is rich in tight gas resources, with geological reserves of about 201×1012m3. With the progress of hydraulic fracturing technology, the production of tight gas is higher and higher, which occupies a more and more important position in the world energy pattern. However, The porosity, permeability and abundance of tight gas are lower than those of other gas reservoirs, hydraulic fracturing technology improves the productivity and economic benefits of tight gas wells, but there are many factors affecting productivity and it is difficult to evaluate productivity. However, traditional theoretical methods and numerical simulation methods are difficult to evaluate and analyze the factors affecting production capacity. Based on the statistics of productivity influencing factors of 10 fractured horizontal wells in JQ gas field, Sichuan Basin, 17 engineering and geological parameters are analyzed by grey correlation method, and the main factors affecting the productivity of fractured horizontal Wells, namely productivity controlling factors, are obtained. Through this study, we have a better understanding of development sweet spots, sweet spots, fracturing and development mode formulation in tight areas.

Productivity characteristics and influencing factors of gas wells
Jinqiu gas field in Sichuan Basin is dominated by fractured horizontal wells, and the length of horizontal section is between 400-1200 meters. Multiple clusters of perforations and continuous sand addition process are adopted in the fractured section of horizontal wells. The productivity of Jinqiu gas field varies greatly. The minimum absolute open flow rate of single well is 1.14× 10 4 m 3 /d and the maximum is 214.05×10 4 m 3 /d. The relationship between geological parameters and open flow rate of gas well and engineering parameters and open flow rate of gas well are obtained according to the productivity data obtained from the test in the early stage of gas well production. As shown in Figure 1, the absolute open flow of gas well has a strong positive correlation with reservoir thickness, ϕꞏh, reservoir volume and pressure, so the physical properties of reservoir have a great impact on the productivity of gas well.  According to the statistical relationship between geological and engineering parameters and absolute open flow of gas wells, the productivity of fractured horizontal wells in tight gas reservoirs is comprehensively affected and restricted by many factors. It is very important for gas well drilling and gas reservoir efficient development to clarify the main controlling factors of productivity.

Grey correlation analysis
Grey correlation analysis method is an analysis method based on multi-factor comparison to get the main control factors, and the strength of correlation between factors is represented by grey correlation degree.The closer the change of grey correlation degree between two factors is, the stronger the correlation is. The geological parameters analyzed include porosity, permeability, water saturation, energy storage coefficient, pressure coefficient, reservoir thickness and reservoir volume; Engineering parameters are: number of clusters, number of perforations, amount of preflush, amount of sand carrying fluid, total amount of fracturing fluid, 70/140 proppant dose, 40/70 proppant dose, total amount of proppant, fracture length and SRV. The process is as follows: (1) The invalid water injection ratio is selected as the reference sequence, and the geological and production parameters are the comparison sequence, expressed as (1); Where,X 0 -reference sequence; X i -comparison sequence; j=0,1,2…,m; k=1,2,…,n。 (2) In the process of grey correlation analysis, the influence of dimension and weight relation among all factors should be comprehensively considered. All factors should be unified in the same level and equal weight condition for calculation and analysis, and the dimensionless data should be carried out through the initial value method. First, select the optimal sequence and record it as Where, Y j (k) is the dimensionless sequence.
(4) Find the maximum and minimum difference of two levels: Where, ∆ max -maximum difference of two stages; ∆ minminimum difference between two levels. (5) The correlation coefficient is calculated as follows: Where, ρ-Resolution coefficient. Its function is to magnify the difference between correlation coefficients numerically, ρ ∈ (0,1), usually 0.5, ρ The smaller the value, the greater the difference between the correlation coefficients.   (1) There are 17 factors influencing the productivity of fractured horizontal Wells in tight gas. Based on the grey correlation analysis of these influencing factors, five main controlling factors are obtained, which are proppant type, pre-fluid, ϕꞏh, sand volume and reservoir volume in order.
(2) Whether tight gas wells can obtain high production is determined by geological and engineering factors, which complement each other. High reservoir quality is the material basis for shale gas wells to obtain high production, and high engineering quality is its technical guarantee.