Challenges in Fluid Sampling for Flow Assurance Testing

Accurate fluid properties are fundamental to economic evaluation, reservoir modeling, flow assurance, system selection, and process design of any deep water or high capex project. The final production installation must be able to handle the produced fluids over the life of the reservoir because subsequent intervention and redesign will dramatically impact economic viability.

Two main categories exist in sampling operations – subsurface and surface. Subsurface samples are carried out using a tool conveyed either by slick line, wireline, coiled tubing, or pipe, or as part of the drill-string in a well test at in situ pressure and temperature conditions. Surface sampling is primarily carried out at the wellhead, separator, flow line, or pipeline, and involves atmospheric conditions or moderate pressures and temperatures.

A good sampling plan must consider the input of all relevant disciplines to determine the volumes, types, and priority of the various fluid samples needed and the various analyses required. A good plan must also include redundancy for various mishaps. Planning alone is not sufficient. For a representative analysis, a quality representative sample must first be collected. There are some basic requirements for a representative formation fluid sample: a) the flowing and sampling pressure must remain above any saturation or solids onset pressure, b) the amount of contamination in the sample must be low enough to enable accurate estimation of reservoir fluid properties, and c) the sample must remain intact through transfer and all the way to the lab. For flow assurance, samples should ideally remain in single phase and above reservoir pressure all the way to the lab.

This talk will focus primarily on down hole samples as these are the most suitable for flow assurance work, especially in deepwater. It will give examples of the real life challenges involved in the planning and execution of representative down hole sampling such as those caused by low permeability formations, unconsolidated sands, critical fluids, etc.