Production Data Analysis of Naturally Fractured Reservoirs: A Density-Based Approach

Open Access
Author:
Zhang, Zhenzihao
Graduate Program:
Petroleum and Natural Gas Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 09, 2014
Committee Members:
  • Luis F Ayala H, Thesis Advisor
Keywords:
  • Gas
  • Oil
  • Naturally Fractured Reservoir
  • Dual-Porosity
  • Decline Curve Analysis
  • Density-Based Approach
  • Pseudo-Functions
Abstract:
Significant amounts of oil and gas are trapped in naturally fractured reservoirs, a phenomenon which has attracted growing attention as the shale boom has evolved. The dual-porosity concept has been commonly used in modeling these naturally fractured reservoirs. In this model, the fluid flows through the fracture system in the reservoir, while matrix blocks are segregated by the fractures and act as the fluids sources for them. This model was originally developed for liquid in naturally fractured systems and therefore inadequate for capturing pressure-dependent effects in gas systems. This study presents a rigorous derivation of a gas interporosity flow equation that accounts for the effects of pressure-sensitive properties. A numerical simulator using the gas interporosity flow equation is built and demonstrates a significant difference in system response from that of a simulator implementing a liquid-form interporosity flow equation. For this reason, rigorous modeling of interporosity flow is considered essential to decline curve analysis for naturally fractured gas reservoirs. State-of-the-art approaches to decline curve analysis have typically used pseudo-functions, yet these approaches remain limited in utility as demonstrated in many previous comparisons between analytical results and production data that revealed discrepancy. In this study, we show the gas interporosity flow equation eliminates the discrepancy at the decline stage and enables rigorous decline curve analysis for production at constant bottomhole pressure. We investigate the applicability of a density-based approach for decline curve analysis for production at constant bottomhole pressure in dual-porosity gas systems. This approach relates gas production profiles to their liquid counterparts by decoupling pressure-dependent effects from pressure depletion. This study further demonstrates the process of rigorous derivation for density-based decline curve analysis in dual-porosity gas systems. The interporosity flow equation for gas is used, and a deliverability equation for dual-porosity systems is rigorously derived in the process. In light of density-based approach for production at constant bottomhole pressure in dual-porosity gas systems, a density-based, rescaled exponential model for variable pressure drawdown/variable rate production was developed for dual-porosity gas systems. We also explore straight-line analysis for convenient prediction of OGIP and production rate at variable pressure drawdown/rate production. This density-based model was tested in a variety of scenarios to showcase its validity. Furthermore, based on Warren and Root’s model, a density-based exponential model for variable pressure drawdown/rate in dual-porosity liquid systems is proposed and verified. Then, a straight-line analysis is proposed to enable explicit OOIP prediction and convenient future production calculation. Aside from these, we develop a double-exponential decline model under constant BHP for liquid which is not only applicable to both decline stages but also convenient to implement.