Impacting factors on horizontal coal seam gas well production and proxy model comparison

Abstract

This paper investigated the impact of geological and engineering factors on coal seam gas production in horizontal wells. The results were then used to compare the performance of proxy models based on linear and quadratic response surfaces and Universal Kriging (UK) to models based on Polynomial Chaos Expansion (PCE). A simple reservoir model was created using a commercial reservoir modelling software package which includes the capability to construct proxy models. The simple model was used for the dynamic modelling of cumulative gas production and peak gas rate under uncertainty in the input variables, e.g. the reservoir principal permeability and permeability in orthogonal directions varied as ratios of the principal permeability, porosity, gas content, coal saturation and the angle between the horizontal well and the principal permeability direction. The simulation results for cumulative gas production and peak gas rate were used to generate training data for proxy models, which were then used to predict the simulated output for other combinations of input parameters. Error analysis was conducted for each proxy model and used to compare and contrast the different modelling techniques. In addition, we investigated the sensitivity of the model to changes in the input variables. The results indicate that, for the given study, proxy models based on linear regression were not good estimators for cumulative gas production and peak gas rate. However, when enough training points were utilized, all other techniques provided good estimates. The best performing proxy model was a cubic PCE. In addition, the cubic PCE proxy model provided direct access to the sensitivity of the gas production to changes in the values of the input variables. Considering the main effects, the changes in the principle permeability and the porosity were predominant factors for the cumulative gas production and peak gas rate, respectively. As for the pairwise interactions, the combined effect of the principle permeability and the coal saturation had the most impact on the cumulative gas production, followed by the combined effect of the drilling angle and the ratio of the permeabilities in the y-direction and the x-direction, indicating that gas production can be improved by optimizing the orientation of the horizontal well. In addition, the combined effect of the coal saturation and the porosity had the most impact on the peak gas rate, an interesting result that warrants further investigation in future studies.