The measurements available to estimate reservoir parameters are numerous, yet most wells are completed in various shales without traditional log measurements. Horizontal wells continue to be drilled, and while the number of stimulation stages pumped per lateral length continues to increase, many questions remain: Is there an increase in production commensurate to the added cost, or will it soon become unsustainable? Would better characterization of the effective surface area after hydraulic fracture stimulation help explain the reservoir potential?
Analysis of production data from fractured shale gas wells is difficult. Operators try to estimate fracture and reservoir properties for a horizontal well with multiple hydraulic fractures by using pressure transient testing, even though in reality it could take 10,000 years for the actual reservoir pressure to be measured. Alternatively, others model the production of fractured shale gas reservoirs from a zone-altered permeability area, which may be quite limited in areal extent but is surrounded by a low-matrix-permeability reservoir to account for the well productivity. Ultimately a simplistic reservoir model for production forecasting uses whatever data is available and our basin experience.
How do we validate these models? Traditionally we look at case studies to find an analogous situation to validate and identify the dominate production drivers. Existing approaches to model shales require years of production data, and even then they cannot uncouple reservoir properties from completion parameters to help optimize flow efficiency. When production is measured on a stage-by-stage basis, and laboratory and log analysis data are presented for reservoir and fluid characterization, solving for the created effective surface area should be straightforward. By better characterizing along the wellbore and by discriminating the contributions of RQ and CQ to the reservoir production, we will be able to better predict long-term well production and better understand the reservoir potential.
This paper discusses the current status of production prediction for shale gas reservoirs and provides a vision of possibilities for better interpretation, i.e these production models must go hand-in-hand with hydraulic fracture models to determine the crucial parameters that drive production, thus fully optimize well and field production.