Variable Pressure-drop/flow-rate System Analysis of Natural Gas reservoirs: A Density-based Approach

Open Access
Zhang, Miao
Graduate Program:
Energy and Mineral Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 21, 2013
Committee Members:
  • Luis F Ayala H, Thesis Advisor
  • decline analysis
  • natural gas reservoir
  • density-based approach
Conventional analytical methods analyzing gas well production data during boundary-dominated flow (BDF) largely rely on the application of concepts such as pseudo-pressure, pseudo-time, and material balance pseudo-time. Recently, Ayala H. and Ye (2012, 2013) introduced a density-based analytical approximation that used a rescaled exponential model that proved successful in explicitly estimating original-gas-in-place and forecasting constant bottomhole pressure (BHP) gas well BDF decline without pseudo-variables. In this study, the rescaled exponential model is verified through a rigorous derivation based on fundamental physical principles applied to BDF conditions and that leads to a rate-time equation modeling gas rate decline in wells produced against a constant BHP specification. We further investigate the applicability of this rate-time equation to the analysis of production data under varying BHP conditions. The validity of rescaled exponential and density-based gas decline models for the analysis of variable rate/pressure-drop is also verified and its successful performance against numerical data is presented. We also demonstrate that the constant rate and constant BHP solutions given by the proposed density-based decline models for gas wells under BDF are interchangeable. We propose a density-based methodology that enables production data analysis of variable BHP/rate gas wells under BDF using density variables and dimensionless viscosity-compressibility ratios. The proposed model uses actual material-balance time and density variables and eliminates the need for the calculation of material balance pseudo-time. Convenient and reliable OGIP calculations are successfully achieved via harmonic decline type-curve or straight-line analysis. A number of field and numerical case studies are presented to showcase the capabilities of the proposed approach. We finally extend the applicability of the rescaled exponential and density-based decline models to high-pressure/high-formation-compressibility gas reservoir systems. We formally derive the density-based analytical techniques rigorously capturing formation compressibility effects during for the analysis of gas well BDF production data. The proposed formulation proves to provide reliable gas rate forecasting for gas well under constant pressure-drop specification and enable accurate calculation of OGIP by incorporating formation compressibility and the change of reservoir pore volume effects applicable to both constant BHP, variable flow-rate/pressure-drop, and constant rate systems.