In 1927, the Johnston brothers developed the first drillstem tester. The tester measured pressure behavior at the drill stem and provided a valuable way for engineers to obtain important sampling information on the formation fluid which helped them establish the probability of commercial production. Today, some 79 years later, determining a reservoir's fluid properties and pressures is no less crucial to optimizing well completion strategies for the reservoir. Typically, drillstem testing is accomplished by obtaining samples of the formation's fluids and pressures and then analyzing them for critical information such as gas/oil ratio (GOR), saturation pressure, density, and viscosity. Sampling is important on any drilling project, but it is especially important on large E&P projects, such as those conducted in deep- and ultra-deep waters. These projects demand that important decisions on well completions be made early, and accurate formation data is critical to those decisions. The fact that the sampling of fluids and pressure in the porous strata of a formation being drilled can yield valuable information on the formation and its ability to yield oil and/or gas is well known. But, the difficulties of obtaining formation fluid samples that are free of contamination and delivering them to the laboratory in as close to in-situ condition as possible is a formidable task. Over the years, fluid sampling techniques that utilize a sampling tool conveyed by wireline have proven to be the most reliable methods for obtaining representative reservoir fluid samples. Two techniques are commonly used for wireline sampling. The first technique uses the pressure differential between the reservoir and an evacuated chamber to move fluid from the formation and into the sampling tool's chamber. The second technique uses a downhole pumping system to drive fluid from the reservoir, through the tool, and into the borehole until an acceptable contamination level is presented by the fluid flowing into the tool. At this point, the tool captures a sample of the fluid.

Unfortunately, upon arrival at the surface, samples are often contaminated by drilling mud filtrates in spite of the measures taken to control them. This contamination can drastically reduce the quality of the samples and make any analyses of the formation's fluid properties subject to error. Additionally, lack of pressure control can flash the sample below the saturation pressure and corrupt the PVT qualities as well as cause irreversible molecular changes adding further errors to sample analysis. New developments in formation sampling technology are enabling much more precise sampling which, in turn, is reducing the frequency of erroneous decisions that occur from the use of inaccurate analyses that use contaminated fluid samples. Next-Generation Tool
Reservoir sampling has been greatly improved with the introduction of a new RDT™ (Reservoir Description Tool) tester that incorporates the latest technologies in microprocessor control to obtain clean, truly representative samples of a well's formation fluid along with a broad range of valuable reservoir data. Wireline and Perforating introduced the RDT tool in 2000. Through its advanced digital control feedback system, which makes instantaneous changes in pumpout flow rates to maintain a prescribed pressure, the tool's zero shock PVT sampling method eliminates pressure transients during pumping and sampling.

Two closely spaced probes are standard, providing a redundant packer seal. An in-situ PVT test determines the bubble point and ideal sampling control pressure. Sample chambers are filled against hydrostatic pressure; however, additional pressure can be applied to maintain its single-phase integrity against thermal gradients. Magnetic Resonance Imaging Laboratory
A Magnetic Resonance Imaging Laboratory (MRILab®) tool section is used for contamination analysis and fluid identification while pumping. The MRILab tool makes Saturation Recovery (SR) T1 measurements during the pump-out process enabling low levels of contamination to be monitored very accurately. A wealth of reservoir fluid information is available from the RDT and MRILab tool combination including viscosity, diffusivity, and hydrogen index (HI). By utilizing other sensors in the tool, up to eight fluid and formation properties can be monitored during testing. The integration of this formation tester fluid information with open-hole logs leads to improved overall fluid interpretation. FluidXpert^SM service is an analysis method that is used with several combined RDT and MRILab sensor measurements to estimate mud filtrate contamination and formation fluid type. The FluidXpert program monitors and analyzes these properties in real time to determine when a minimum filtrate contamination has been achieved during pumpout operations, and sampling can begin. Dual-Probe Section
During operation, closely spaced dual probes are simultaneously deployed for pressure testing and fluid sampling. Besides increasing tool reliability through redundancy, the two probes also allow for advanced pressure-testing techniques.

The tool's dual-probe section (DPS) is designed to detect horizontal mobility and permeability (k_h) and anisotropy (k_v/k_h) over an extended range of operation. The DPS pressure testing flow rates are precisely controlled with the advanced digital control feedback system, thus achieving steady-state drawdown pressures very quickly and reducing the testing time required. By running two dual-probe sections in tandem, the tool can determine the pressure gradient between the probes and profile permeability and anisotropy. This further enables an extended depth of investigation and detection of permeability barriers. For example, formation anisotropy can be determined with an interference test between the two probes spaced 7.25" on a single dual probe section or 10.6' apart on two stacked dual probe sections. The drawdowns can be rate- or pressure-controlled from either probe, making it suitable for pressure testing in very low to high permeability formations (0.001 to 1000 md). Oval Pad Packer
Carbonate rocks, thinly bedded sands and naturally fractured reservoirs can exhibit a very challenging logging environment when pressure testing and fluid sampling are required. The challenge is due to, at least, reservoir heterogeneity and the difficulty of sealing the probes in these reservoirs. The RDT tool utilizes a proprietary Oval Pad packer to help overcome all of these challenges. The Oval Pad covers a vertical section of the well bore, giving it the sealing advantages of a straddle packer but still maintaining the operational flexibility of a probe. In particular, the Oval Pad design ensures an effective seal for the probe during formation testing and fluid sampling in the presence of vuggy and/or fractured carbonate rocks. In addition to the increased vertical sealing area, the oval shape can reduce the sampling time due to a focusing effect the pad has on near-wellbore flow. Simulations show that when the complete testing system performance is considered, the Oval Pad reduces pumping times compared to a standard probe and in many cases a straddle packer. Straddle Packer
Sampling heterogeneous formations represents a challenge for probes, and soon after the pumpout tools were introduced, straddle packers were adapted. Dual straddle packer systems (SPS) offer advantages over probes in low permeability applications as well as heterogeneous environments. In carbonates, thinly bedded sands and naturally fractured reservoirs, most of the production occurs from small features. Such features make sampling and reservoir characterization difficult with a probe. The probe is more likely to be placed in a location that is characteristic of the rock matrix, which usually results in a tight test. The SPS typically isolates an interval of 1 meter, which is normally ample to characterize heterogeneous rock. The primary advantage of an SPS is its ability to cover a vertical interval where a probe is a pinpoint evaluation by comparison.

Advantageous Design
Because the RDT design uses a powerful pumping-system motor and an efficient hydraulic system, invaded fluids, such as mud filtrate, which initially surround the probes can be flushed 50% faster than similar wireline testing tools, thereby enabling faster and cleaner sampling of virgin formation fluids. Sample chambers can be filled against hydrostatic pressure to insure pressure-volume temperature (PVT) quality samples, and additional pressure can be applied to minimize phase changes that might otherwise occur because of temperature gradients in the borehole. The new system can also be used to perform a closed-chamber PVT test for the bubble point of the fluid sampled. The RDT tool also provides extended-range pressure sampling by employing multiple flow-control pumpout section options, configured for 4,000, 6,000, or 8,000 psi pumping pressures. Pumping configuration is flexible, multiple pumps can be run in a single descent. Where zones are tested over a wide range of overbalance pressures in a single trip, the 6,000 or 8,000 psi pumps can extend the range of sampling, thus saving rig time and providing higher-quality samples. With this feature, samples can be obtained that are not possible with other current generation tools.

(Click image to enlarge) For operators, the RDT tool offers a variety of advantages by reducing contamination through faster pump out times, ensuring sample integrity through zero shock pressure control, providing the highest-quality PVT quality samples available, reducing rig time, and providing accurate, reliable hydrocarbon/fluid typing, delivering improved permeability estimates, and offering high reliability assurance through built-in redundancies.

RDT Case Histories
Case History No. 1
An operator drilling a well in deep, highly laminated reservoir structures in the Mahakam Delta region of East Kalimantan, Indonesia wanted fluids from oil and/or gas bearing zones identified so that development decisions on the well could be made. Halliburton recommended a RDT tester fluid and pressure sampling survey and analysis be conducted on the well to identify fluids in oil and gas bearing zones. The challenges posed by the well were numerous. The tight, thin laminar reservoir structure made fluid identification difficult from conventional logs. Also, extensive pump-out formation testing and fluid identification in the laminar sands made tool placement and pressure testing in tight zones difficult and long hours of exposure for the tool to high temperatures (310°F) and a high static differential pressure (as high as 6,000 psi) also posed issues. The job was executed and several problems were encountered and overcome. A high differential tension between running-in-hole and pulling-out-of-hole made tool positioning very difficult and, as expected, the sampling tool experienced some sticking due to the high static differential pressure. Results of the survey: The RDT tool survey collected fluid samples from five hydrocarbon zones in the well. Analyses conducted on the samples identified two gas zones, one oil zone, one light oil or condensate zone and a water zone containing some gas while flowing. Case History No. 2
During the drilling of an offshore well in the Espirito Santo Basin of Brazil in 2003, the operator asked Halliburton to run a RDT tester fluid sampling survey to determine where the well's oil zone(s) were located. The well was being drilled in marine Turbidites sands, had low porosities (9-12%) and low permability (2-30 mDarcy) and was being drilled to a TD of 4,085m (13,400 ft). A KCl (60,000 mg/l NaCl equiv) mud system was being used and mud weight was 9.8 lb/gal. The RDT was run in the well and fluid samples and pressure reading were acquired from the formation. Low contamination-bearing samples were collected at 3,840m (requiring 45 liters of fluid pumping to clean up before initiating sample collection ), 3,920m (155 liters pumping required), and two samples were taken from a zone at 3,965 ft ( requiring a total of 277 liters pumping before initiating sample collection). Subsequent analysis identified a 37.6 degree API oil zone was located at 3,965.5 ft.