RFT & MDT: Wireline Formation Testing

What For?

The main objectives of RFT (Reservoir Flow Test) and MDT (Modular Dynamic Testing) are to measure reservoir pressure, assess mobility, and determine the fluid content at specific reservoir intervals.

When Do We Need It?

RFT and MDT are beneficial during both the exploration and development phases of a reservoir. These tests are primarily performed in open hole using a cable-operated formation tester and sampling tool after determining the appropriate depth from logs. They are commonly conducted following petrophysical analysis results and serve as a quick and inexpensive alternative to DST (Drill Stem Test) tests.

Objectives of RFT/MDT

These pressure measurements are essential for:

• Variation in Pressure Among Formations: Identifying pressure differences between various geological formations.
• Fluid Pressure Gradient: Determining fluid pressure gradients within a formation to indicate fluid contacts.
• Fluid Contacts Identification: Locating gas-oil or oil-water contacts.
• Formation Permeability: Assessing the permeability of the formation.
• Controlled Local Production: Managing controlled local production and vertical interference.

General Tool Evolution

FT (Flow Tester)

• Earliest Tool: Specifically designed to collect reservoir fluid samples but limited to one sample per trip.

RFT (Reservoir Flow Tester)

• Multiple Pressure Measurements: Provides comprehensive pressure profiles and mobility assessments.
• Sampling Capacity: Capable of sampling with up to two chambers.
• Gauges Used: Utilizes strain gauges, offering limited pressure range and quality fluid samples.

MDT (Modular Dynamic Tester)

• Enhanced Capabilities: Supports multiple pressure measurements, profiles, and mobility assessments.
• Live Fluid Analyzer (LFA): Integrated fluid analysis capabilities.
• Increased Sampling Capacity: Can sample with up to six chambers, allowing for a dozen or more fluid samples per run.
• Advanced Gauges: Utilizes highly accurate quartz gauges with a resolution of 0.1 psi.
• Versatile Configurations: Available in various configurations for different applications (e.g., SRFT, CHDT, XPT, Saturn Dual Pocket).

Main Components of MDT Tool

1. Formation Pressure Gauges (Strain & Quartz): Accurate pressure measurements.
2. Formation Resistivity Gauges: Measures resistivity to assess fluid content and saturation.
3. Pump Out & Live Fluid Analyzer: Removes fluids and analyzes them in real-time.
4. Sample Chamber: Collects fluid samples from the reservoir for analysis.

Potential Problems During Testing

• Tight Test: Occurs when the formation is very impermeable, causing the sampling pressure to drop near zero.
• Stuck Tool: Happens when the tool remains set at a given depth for an extended period.
• Plugging: Sand grains from the formation may enter the tool, blocking flow lines, especially in unconsolidated samples.
• Seal Failure: If the packer fails, drilling mud may be sampled instead of reservoir fluids, and mud pressure will be recorded.
• Supercharge: Occurs in very low permeability formations where the near wellbore area is overpressured due to mud filtrate invasion that hasn't dissipated during pre-test.

RFT/MDT Applications

• Formation Pressure and Mobility Assessment: Evaluating the pressure behavior and fluid mobility within the reservoir.
• Downhole Fluid Typing & Sampling: Identifying and collecting different fluid types present in the reservoir.
• Reservoir Properties Analysis: Determining properties such as permeability, skin factor, and anisotropy.

RFT/MDT Limitations

• Reservoir Conditions: Provides good results only in reservoirs with good porosity and permeability.
• Reservoir Type Restrictions: Limited effectiveness in shaly sand and carbonate reservoirs.
• Geological Considerations: Requires thorough geological analysis to determine appropriate sampling points.