This photo shows a DSP section for the RDT that is typically used in low-permeability carbonate formations. Also shown are retraction rods that apply a tension to the packer elements when they are deflated to minimize any residual budging.

Obtaining representative fluid samples with mixed-phased fluids that are encountered in transition zones or when sampling oil with water-based muds (WBM) has been a challenge for pumpout wireline formation testers. Fluid sensor data can appear erratic and irresolvable, making accurate estimates of contamination and reservoir fluid properties difficult.

New advancements for formation sampling using pumpout wireline formation testers have been introduced to the oil and gas industry, and they are dramatically improving the accuracy of samples collected. In turn, this improved accuracy is reducing the frequency of erroneous sample collection decisions that occur from the use of inaccurate fluid contamination analyses.

Among these new advancements is a new vibrating tube fluid density sensor that provides valuable data for assessing wireline formation pumpout tester fluid contamination and type while pumping and sampling.

Additionally, a new dual-port straddle packer section has been introduced that provides clean samples quickly by manipulating selectable inlet port valves, thereby taking advantage of gravitational segregation of fluids occupying the packed-off interval. In many cases, zero contamination samples are obtainable in less time than a traditional probe tool.

These advancements can help reduce the frequency of erroneous sample collection decisions that can occur from the use of inaccurate fluid contamination analyses.

The pumpout wireline formation tester is one of the tools that operators routinely use for collecting pressure-, volume-, temperature (PVT)-quality reservoir fluid samples. When using a formation tester, a downhole pumping system drives fluid from the reservoir through the tool and into the borehole. Once an acceptable contamination level is exhibited by the fluid flowing into the tool, a sample of the fluid is captured. Despite widespread use, the sampling of mixed phases continues to present challenges when a formation tester is used for fluid identification.

Several of the latest advancements in sampling technology were developed for Halliburton’s Reservoir Description Tool (RDT) to solve mixed-phase and other sampling challenges presented by certain types of formations. Included among these advances are a high-resolution fluid density sensor and an inflatable dual-port straddle packer section.

Reservoir description tool

The RDT uses multiple technologies to reduce sample contamination; provide accurate, reliable hydrocarbon/fluid typing; deliver improved permeability estimates; and provide high reliability assurance through built-in redundancies. The tool consists of multiple sections that can be connected in numerous configurations for testing and sampling applications (Figure 1).

Vibrating tube fluid density sensor

The key to multiphase fluid detection is the use of sensors that can identify the fluids being pumped and estimate contamination. Sensor types currently used are resistivity, dielectric, optical, and nuclear magnetic resonance (NMR). While these sensors can accurately detect fluid properties, multiphase flow has been challenging, and the sensor readings can appear erratic. It was not until a new vibrating tube density sensor was developed that it became possible to determine the nature of this apparent erratic sensor behavior.

The sensor uses the tuning fork principle in which the fluid flowing through a vibrating tube causes the natural frequency to change in proportion to the fluid mass. This method of density measurement was developed for Coriolis mass flow meters, which are very accurate and robust, having been used for decades in process control applications.

One of the main advantages of this sensor design is that it is sensitive to the entire fluid volume contained within the flow tube.

One of the problems with other fluid sensors is their limited volume of investigation. Unlike the vibrating tube density sensor, these other sensors can be pathway-dependent, in which case the measurement can depend on the fluid phase in contact with the sensor probe or window. If they become coated with oil, the sensor is unable to investigate beyond the oil coating even when the water dominates the fluid flow volume.

In contrast, because the vibrating density sensor is sensitive to the entire volume of fluid flowing through it, a coating of oil or filtrate solids has only a very small effect on measurements. And because the sensor is a straight tube with the same diameter as the tool flow line, it tends to be self-cleaning, thereby further minimizing measurement concerns. Furthermore, new processing methods provide a clearer understanding of flow behavior, enabling more accurate estimates of fluid contamination. Data from the tool can also be used to observe changes in fluid properties over depth intervals and aid in identifying fluid interfaces and compartments.

Dual-port straddle packer section

One advantage of straddle packers is that they can isolate a large borehole interval, typically a meter or more. However, when using a single-port straddle packer for sampling, the phases can mix where they enter the tool (i.e., whole mud, water filtrate, formation water, and oil). Because a fluid contact can form at the inlet port, the volume fraction of the fluids entering the tool may not accurately represent the formation.

To improve straddle packer sampling performance, a new dual-inflatable straddle packer section (SPS) was developed for use with the RDT to take advantage of the oil and water phase separation that naturally occurs in the packed-off borehole annulus interval during pumping and sampling. In order to reduce sampling times and collect cleaner samples, it incorporates a dual inlet port design.

When the SPS is operating, fluid is drawn from the formation into the packed-off interval and then enters the flow line through upper or lower inlet ports. These ports are located at different heights separated by approximately 17 in. within the packed-off interval.

Normally, a density contrast exists between the reservoir fluid and the invading formation mud filtrate. The denser phase gravitates to the bottom of the interval, and the lighter phase floats to the top. For example, whole mud is denser than filtrate, so it settles to the bottom of the packed-off interval. When sufficient volume is pumped, the height of the dense mud column recedes slightly below the bottom port, as depicted in the left image of Figure 4.

A fluid contact or interface exists within the packed-off interval if two immiscible fluid phases are present. Even when fluids are miscible or emulsified, gravity can segregate the fluids between the ports. After initially pumping through both inlet ports and detecting reservoir fluid with the fluid density and other sensors, the top port can be closed to draw the heavier fluid from the lower port, causing the fluid contact to be lowered. When the fluid contact reaches the lower port, the lighter fluid is detected, the upper port is opened, and then the lower port is closed. The purpose of this step is to assess the contamination of light fluid, i.e., reservoir oil contaminated with WBM filtrate flowing through only the upper probe after lighter fluid was detected flowing through only the lower port.

The fluid volume occupying the space between the upper and lower ports in a vertical 81?2-in. borehole is approximately 13 liters. When the lighter reservoir fluid flowing through the upper probe is clean, there is potentially an additional 13 liters of clean reservoir fluid available for pumping through the upper probe and collecting multiple PVT quality samples. If the lighter fluid drawn through the upper port is not yet clean, the lower port can be reopened and the upper port closed to deepen the fluid contact to the lower inlet port. At this time, the upper port is reopened, and the lower port is closed. The lighter fluid then flows through the upper port, and contamination is determined.

Properly manipulating the dual ports and taking advantage of naturally occurring fluid segregation in the packed-off interval can provide very low and even zero-contamination samples faster than comparable samples attainable with a single-ported straddle packer section.