Analysis of Water Flowback and Gas Production Data for Fracture Characterization in the Horn River Basin

  • Author / Creator
    Xu, Yanmin
  • Shale gas is one of the most promising energy resources due to its wide distribution, abundant reserves, and low pollutant emissions to the environment. Although shale gas plays usually have very low permeability and porosity, the use of horizontal drilling and hydraulic fracturing technologies has made their economic production possible. This is achieved by creating complex fracture networks underground, through which the trapped gas flows from the rocks to the wellbore. Since these induced fracture networks are essential for hydrocarbon recovery forecast and future operation optimization, the industry is very interested in their characterization. The data recorded immediately after opening the wells during flowback present the earliest opportunity to characterize the stimulated reservoirs. The objectives of this study include: 1) investigating flow regimes and understand fluid flow physics during flowback period, and 2) quantitatively characterizing the induced fracture network by analyzing the flowback rate and pressure data. The study focuses on an eight-well pad completed in the Horn River Basin, and aims to develop a protocol for flowback data analysis in gas shales. The main steps and key results are summarized in subsequent paragraphs below.Step 1 constructs a series of diagnostic plots for investigating flow regimes in target shale gas wells. The rate plots show two-phase production at the very beginning of flowback period. The Gas Water Ratio plots separate the flowback period into two regimes: an early-time flow regime characterized by decreasing Gas Water Ratio trend, and a late-time flow regime characterized by increasing Gas Water Ratio trend. Step 2 builds a numerical model to validate the flow signatures observed in field data using a commercial reservoir simulation software. The numerical model simulates the fracturing, shut-in, and the flowback processes. The results suggest that the gradual build-up of gas in the fractures during shut-in is responsible for the immediate two-phase flowback. The results also suggest that the early-time flow regime indicates fracture depletion with negligible fluid support from the matrix; while the late-time flow regime suggests significant fluid and pressure communication between the matrix and the fracture systems. Step 3 develops three material balance models for quantitatively characterizing the effective fracture network. These models include a closed-tank model, a closed-tank flowing model, and an open-tank model. Both closed-tank models estimate the initial volume of the effective fracture network from the early-time flowback data, while the open-tank model estimates the effective fracture-matrix interface area from the late-time flowback data. Step 4 conducts a comparative volumetric analysis by using the estimated fracture parameters, total injected volume, pressure and water production profiles during flowback. The objectives of this step are to understand the hydraulic fracturing efficiency and to investigate the change in effective fracture volume with time during flowback period. The results show that most of the fracturing fluids are used in creating effective fracture volume. However, there is severe fracture volume loss during early-time flowback due to excessive pressure drop. The severe fracture closure is a key drive mechanism for early-time two-phase flowback. The results also imply that part of the induced fracture network may not contribute to long-time production. Step 5 develops a mathematical model to estimate fracture compressibility, which is a key parameter to evaluate fracture closure in material balance analysis. The results show that fracture compressibility comprises two parts: the rate of fracture aperture change and the rate of fracture porosity change with respect to the change in effective pressure. The results show that proppants play a dominant role in resisting fracture closure and reducing the fracture volume loss. The results also indicate that the severe fracture volume loss during early-time flowback is mainly due to the closure of unpropped fractures. Overall, this research demonstrates the feasibility of flowback data analysis for fracture characterization in shale gas reservoirs. Although this study focuses on an eight-well pad completed in the Horn River Basin, the methodology and some results could be extended for applications in other shale gas reservoirs.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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