Study on the Main Controlling Factors and Productivity Evaluation of Carbonate Gas Reservoir Productivity
Carbonate gas reservoirs are characterized by strong heterogeneity and a complex combination of pores, fractures, and caves. The productivity of gas wells is controlled by many factors, including geology, development, and engineering factors. The traditional one-point productivity evaluation method is insufficient to accurately characterize the productivity differences in gas wells with different reservoir types. In this paper, the carbonate gas reservoir of the MK Formation in the HS 4 block is taken as the research object. Based on core characteristics, thin sections, dolomite content, and permeability data, the two main reservoir types, vuggy and fracture-vuggy, are systematically categorized. The influence of geological, development, and engineering factors on gas well productivity is analyzed using SHAP values. At the same time, to improve the accuracy and engineering practicability of gas reservoir productivity evaluation, the one-point productivity equation was refined, and open-flow capacity prediction charts for vuggy and fracture-vuggy reservoirs were constructed by combining the improved equation with the steady-state productivity equation. The results show that the dolomitization degree of vuggy reservoirs is lower than that of fracture-vuggy reservoirs, and the reservoir space types are mainly small dissolution pores or intergranular dissolution pores. The lithology of the fracture-vuggy reservoir is dolomite. The reservoir space of this type of reservoir is mainly composed of large dissolution pores and fractures, with a good matching relationship between fractures and pores. Based on the SHAP interpretation and analysis method, it is clear that cumulative water production, total acid fracturing fluid volume, gas-layer thickness, and porosity are the main factors affecting gas well productivity. Among them, the influence of cumulative water production and total acid fracturing fluid volume is the most significant, indicating that changes in the gas-water relationship and the effect of acid fracturing factors during development have a greater impact on gas well productivity. Based on the improved one-point productivity equation and the steady-state productivity equation, the open-flow prediction chart of vuggy and fracture-vuggy reservoirs is established. In predicting the gas well productivity of medium-deep reservoirs, with reconstruction scale and geological conditions similar to those of the HS 4 block, the chart’s predictions are in good agreement with field gas test and production test results. The single-well prediction error ranges from 2.2% to 6.5%, with an average error of 4.3%, indicating that the established chart has good applicability and predictive reliability. The research results can provide a theoretical and technical basis for reservoir classification evaluation, reasonable production allocation optimization, and new-well productivity prediction for carbonate gas reservoirs in the HS 4 block.