MDT Sampling - Pipe-conveyed (TLC) MDT Sampling in Gas Wells

This article assumes you are familiar with open-hole (wireline) logging and the Schlumberger Modular Dynamic Tester (MDT) tool.

How do you plan a mega-combo MDT sampling job in a gas well? First and foremost, we need to clearly understand our requirements as the stakeholder.

Typical questions we would ask are:

1. How many sampling points are planned?

2. How many samples will be taken per point, i.e. how many bottles or sample chambers?

3. Are the samples for PVT and/or H2S analysis?

4. Would SPMCs and/or MPSRs be required?

5. Are there any water samples required (in wells with a water leg below the hydrocarbon)?

6. Is wellsite transfer and analysis required?

Most sampling are now done using low-shock sampling method but its worth asking.

These questions will help determine the MDT modules which will be run in the well.

The decision making process regarding when to take a sample should also be clearly defined. The decision is usually taken by the company representative in consultation with the service company expert/reservoir engineer.

The MDT tool-string consists of several different modules, each designed for specific purposes. Each module added to the toolstring should be selected to meet the objective of obtaining PVT quality samples. Modules for analyzing the quality of gas are the CGA (Condensate-Gas Analyzer) and the LFA (Live Fluid Analyzer). Also for pumping out the gas from the formation, the MRPO (MDT Reservoir Pump-out) is required. Other modules required as standard for operating the MDT tool will be specified by the service company. SPMCs (Single Phase Multi-sample Chamber) and MPSRS (Multi-Phase Sample Receiver) are sample bottles (250cc and 450cc respectively) typically used to take PVT quality gas samples. Where wellsite analysis and transfers (SPMC) are required the service company needs to be informed so that the necessary equipment is mobilized to the wellsite.

Pipe-conveyed MDT sampling jobs can go on for more than 2 days therefore personnel from the service companies need to be prepared for 24 hour coverage.

Additional considerations not covered in this article include specifying equipment for tight reservoirs. This covers the type of probe to be used and even using dual-packers. This is a summary of planning for a pipe-conveyed MDT sampling operation, more information should be obtained from in-house experts like the operations petrophysicist, from the service company or from consultants (wireline witnesses).