Reservoir-Fluid Sampling and Characterization — Key to Efficient Reservoir Management
Authors: Nagarajan N.R. , Honarpour M.M. , Sampath K. 
Published: June 2014

This paper, SPE 103501, is based on paper 101517 presented at the 2006 Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, 5-8 November. Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Reservoir-fluid properties play a key role in the design and optimization of injection/production strategies and surface facilities for efficient reservoir management. Inaccurate fluid characterization often leads to high uncertainties in in-place-volume estimates and recovery predictions, and hence affects asset value. Reservoir-fluid pressure/volume/temperature (PVT) characterization begins with acquisition of adequate volumes of representative fluid samples followed by PVT-data measurement with strict quality-assurance/quality-control (QA/QC) protocols and phase-behavior modeling through best-practice methods. In this paper, key steps involved in accurate fluid characterization are discussed for a wide spectrum of fluid types ranging from extra-heavy oils to highly volatile near-critical fluids and lean gas condensates undergoing a wide range of production processes from simple depletion to complex tertiary recovery. Selection of appropriate sampling methods and tools, design of tool strings, and customizing procedures are demonstrated through these examples. Routine and special laboratory-fluid-analysis strategies for various fluid types and for different production strategies are highlighted. Fluid-modeling techniques including optimum-component selection, accurate C7+ characterization, robust Gibb's energy minimization, and gravity/chemical equilibrium calculations are demonstrated through appropriate field examples.

Introduction

Reservoir-fluid PVT properties are critical for efficient reservoir management throughout the life of the reservoir, from discovery to abandonment (See complementary paper by Honarpour et al. 2006.) Reliable PVT properties of in-situ fluids are essential for the determination of in-place volumes and recovery-factor calculations and are key input to reservoir simulations for technical evaluation of reservoir-development/-depletion plans. Fluid characterization and distribution within the reservoir help in defining reservoir continuity and communication among various zones. Interpretation of well-test data and the design of surface facilities and processing plants require accurate fluid information and its variation with time. In addition to initial reservoir-fluid samples, periodic sampling is necessary for reservoir surveillance.

Reservoir-fluid characterization consists of several key steps:acquisition of representative samples,identification of reliable service laboratories to perform PVT measurements,implementation of QA/QC procedures to ensure data quality, anddevelopment of mathematical models to capture fluid-property changes accurately as functions of pressure, temperature, and composition. The fluid type and production processes dictate the type and the volume of required fluid data. This paper outlines recommended sampling techniques, PVT-data-acquisition strategies, and modeling methods and presents field examples covering a wide range of fluid types from heavy oils to lean gas condensates and production processes such as depletion, pressure maintenance, and miscible recovery.