Volumetric Analysis of Two-Phase Flowback Data for Fracture Characterization

Abstract

Abstract Analysis of early-time production data obtained during the ‘flowback’ period presents the earliest opportunity to characterize a stimulated reservoir volume (SRV). Previous studies have developed analytical models/methods to analyze two-phase flowback data for fracture characterization. However, the mechanisms responsible for the early-time gas production in shales are poorly understood. The objective of this paper is to understand the mechanisms responsible for early gas production and develop a mathematical model to estimate effective fracture volume. The study incorporates a comprehensive field data analysis from 8 wells of a single well pad completed in the Horn River Basin. Firstly, we develop several diagnostic plots [production rates, cumulative gas production (Gp) vs. cumulative water production (Wp), GWR vs. Gp] to identify the early time trends/signatures. The Gas-Water-Ratio (GWR) plots from the wells considered indicate a V-shape trend, dividing the flowback data into two regions: 1) Early Gas Production-EGP and 2) Late Gas Production-LGP. Water rate increases during EGP, resulting in a decreasing GWR curve. Production then ’rolls over’ to the gas dominant phase with a positive GWR slope. Secondly, we introduce a method to estimate effective fracture volume by assuming a simplistic two-phase tank model for the fracture system. Conventional p/Z analysis shows that the fracture network can be approximated by a tank model during the EGP phase and that the dominant main mechanisms during EGP include the expansion of initial free gas in fractures (IGIF), fracture closure and the expansion of residual frac fluids (water). Effective fracture volume is calculated using a modified material balance approach for the two-phase system. The material balance approach enables the estimation of effective fracture volume regardless of the fracture geometry. Finally, the proposed model is validated by numerical simulation and is applied to estimate effective fracture volume using field production data.