Optimal Design of Rational Drainage Schedule for Deep Coalbed Methane Wells:Taking the Shenfu Block in Eastern Ordos Basin as An Example
In recent years,the development of deep coalbed methane has achieved significant breakthrough in China. The initial production of deep coalbed methane can reach up to 10,000 m3 per day,but the production decreased rapidly. It is therefore urgent to optimize the design of a reasonable drainage schedule in order to ensure long-term stable production of deep coalbed methane and increase the ultimate recoverable reserves. Taking the deep coalbed methane reservoir in the Shenfu Block on the eastern edge of the Ordos Basin as an example,this work determined the productivity evaluation indicators for deep coalbed methane wells based on the history matching and prediction results of 19 wells in the block. The relationship between various parameters and productivity evaluation indicator was analyzed,and identification charts for high,medium,and low productivity wells were provided. A multi-objective optimization design method for drainage schedule was proposed,and reasonable drainage schedule templates for high,medium,and low productivity wells in the Shenfu Block were developed. The drainage schedule templates for three types of wells were applied to new wells. Results show that the second year's average daily gas production,which can indicate the short-term and long-term productivity changes of gas wells,can be used as an evaluation index for the productivity of deep coalbed methane wells. There is a positive relationship between the average daily gas production in the second year and nine parameters,including coal reservoir thickness,coal reservoir temperature,original coal reservoir pressure,critical desorption pressure,Langmuir pressure,coal reservoir permeability,supply radius,half-length of fracture,and critical flow rate of pulverized coal damage. The pore compressibility,fracture stress sensitivity coefficient,and permeability damage rate resulted from pulverized coal are negatively correlated with the average daily gas production in the second year. The intersection diagram of critical desorption pressure and permeability can effectively distinguish high,medium,and low productivity wells. High,medium,and low production wells have their own optimal drainage schedules,and faster drainage speed cannot yield better development effect. Especially,low productivity wells are significantly affected by pulverized coal,so it is recommended to drainage slowly. This research can provide a basis for the optimization of new well locations and the formulation of drainage schedule for deep coalbed methane wells in the Shenfu Block,which will lay a foundation for the long-term stable and efficient development of deep coalbed methane.
deep coalbed methanehistory matchingproduction forecastingdrainage scheduleShenfu BlockOrdos Basin
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