Abstract
This paper presents a new five-step method to evaluate shale gas formations with intricate pore networks. The method overcomes challenges posed by traditional workflows through an improved workflow and a new unconventional petrophysical model: (a) the new model accurately defines components of shale gas formations, including effective and isolated pores occupied by free and adsorbed gas; (b) total organic carbon (TOC) is evaluated using three techniques using conventional well logs to determine which techniques are more accurate; and (c) the improved method provides integrated evaluation of geomechanical properties, resources in place, and selection of stimulation candidate. Our field case study on Marcellus shale shows that (a) density-based TOC technique is more accurate and average TOC in the study area is 2.8%; (b) density porosity model is more reliable and average porosity in the study area is 8.5%. Positive correlation between TOCs and porosities in the upper and lower Marcellus suggests that effective pores contribute more significantly to the pore network than isolated pores; (c) gas in place is 120 Bcf with 60% free gas for a drilling unit of 640 acres. Large contrasts in Young's modulus (1 million psi) and minimum in-situ stress (893 psi) along the upper Marcellus-Stafford limestone boundary, and resistivity separations in the lower Marcellus member, show that the upper Marcellus is a good stimulation candidate. This method and field case study provides valuable insights to evaluate Marcellus shale reservoirs and improve economic recovery of the resources in place.
