Analysis of Ultimate Gas Recovery in Marcellus Shale Reservoirs
Authors: Zhang Yiming , Kleit Andrew , Morgan Eugene , Wang John 
Published: October 2023

Abstract
Horizontal wells with multistage fracture treatments enabled economic gas production from Marcellus shale gas reservoirs. However, recovery factors inside and outside the stimulated reservoir volume (SRV), especially the variations spatially and temporally, are not yet fully understood. Previous studies suggest the inclusion of region outside the SRV can mitigate gas production decline rates for shale reservoirs and that 63% of available natural gas are recovered from the SRV zone compared to 17% from outside the SRV for shale gas reservoirs in Sichuan Basin. These studies, however, do not differentiate recovery factors in different areas inside and outside the SRV.
In this study, a single-phase 2-dimensional dual-porosity, dual-permeability model was built to investigate ultimate gas recovery and its spatial variations throughout the production period. We then conducted parametric studies to evaluate the effect of matrix permeability, fracture conductivity, fracture half length, and operating conditions on recovery factors. We carried out systematic and comprehensive numerical experiments using Markov chain Monte Carlo (MCMC) to generate a probability distribution of ultimate recovery factors.
More-in-depth knowledge regarding how recovery factors change spatially and temporally can help us improve well development strategies, fracture treatment designs, and optimizing operating conditions to increase economic recovery in Marcellus shale gas reservoirs. In addition, the results and analysis from the parametric studies provide information on key parameters to consider when estimating recovery factors in reservoirs with these characteristics. These insights will provide guidelines such as optimized locations for infill wells and accommodate for the associated SRV size based on the recovery factor spatially. The methodology model in this study provides a foundation to develop more-reliable models in accurately forecasting ultimate gas recoveries spatially in shale gas reservoirs.
The recovery factor in the SRV rapidly decreases through the first part of the SRV. Our estimate is that recovery is about 68% next to the wellbore, falling all the way to 15% halfway across the SRV (150 feet from the wellbore). The recovery factor then falls gradually from 15% to 11% at the end of the SRV (300 feet from the wellbore). The recovery factor then decreases sharply to 0% once out of the SRV. The recovery outside the SRV is sensitive to matrix permeability as recovery factor increases significantly outside the SRV from 0% to 20% when matrix permeability is increased from 1×10-7 md to 1×10-3 md. Fracture conductivity affects the recovery in the SRV significantly as an increase from 0.001 md∙ft to 2 md∙ft increases the recovery from 2% to 50%. The recovery of the well is increased from 7% to 19% by increasing the fracture half length from 30 ft to 510 ft. The recovery from the SRV is increased from 11% to 20% by lowering the bottom hole pressure from 2,700 psi to 300 psi. Recovery factor in SRV is within the range of 12% to 31% with a most likely recovery value of 20% and a percent standard deviation of 21%.