Analysis of early-time production data obtained during the "flowback" period presents the earliest opportunity to characterize a stimulated reservoir volume. Previous publications have developed analytical models/methods to analyze two phase flowback data for fracture characterization. However, there is little understanding of the mechanisms responsible for the early-time gas production in shales and the impact of the completion strategies and fracture design on this early-time gas production.
The objective of this paper is to develop a mathematical model to estimate effective fracture volume and understand the mechanisms responsible for early gas production. The study analyzes comprehensive field data analysis from 2 well pads (26 wells in total) completed in the Horn River shales. Firstly, we develop several diagnostic plots [Production rates, cumulative gas production (Gp) vs. cumulative water production (Wp), GWR vs. time] to identify the early time trends/signatures. Secondly, we conduct a volumetric analysis on the water and gas production data. Finally we introduce a method to estimate effective fracture volume by assuming a two phase tank model for the fracture system.
The GWR plots indicate a V-shape trend dividing the flowback data into two regions: 1) Early Gas Production- EGP 2) Late Gas Production-LGP. Water production dominates during EGP, with a decreasing GWR curve. Production then ‘rolls over’ to the gas dominant phase with a positive GWR slope. There is a strong correlation between WP vs. Gp, which can be approximated by a cubic equation. The instantaneous slope of this curve is used to model the V-shaped trend, allowing the estimation of the transition time to the LGP region. Conventional gas material balance analysis shows that the main drive mechanism during EGP is the expansion of initial free gas in place, indicating a closed system in EGP. Volumetric analysis of flowback data during EGP suggests that the fractures are initially saturated with both gas and water, thus early time gas production is from gas saturation within the hydraulic fractures and pre-existing natural fractures.