A formation testing portfolio provides complete reservoir characterization.

A recently launched portfolio of reservoir fluid characterization services provides operators with a complete package of fluid characterization modules and services to fully characterize their reservoirs. The portfolio of reservoir fluid characterization services, RESolution, is available with Baker Atlas' Reservoir Characterization Instrument (RCI). The portfolio is divided into segments, each providing reservoir pressure, in situ fluid samples and fluid characterization technologies designed to improve the efficiency and data quality of wireline testing and sampling programs. The segments are:
• Pre-Job Modeling and Planning;
• Precision Pressures;
• Smart Samples;
• Downhole Fluid Analysis; and
• Post-Processing and Advanced Deliverables.
One of the primary functions of a wireline formation fluid sampling and testing tool is to collect multiple reservoir fluid samples to evaluate potentially productive reservoir units. The determination of accurate produced fluid phase behavior and composition is fundamental in production facilities' design and flow assurance programs. The final production installation must be able to handle the produced fluids over the life of the reservoir because subsequent intervention and redesign often affects economic viability. Fluid samples offer operators a narrow window of opportunity to make the requisite analyses to get the most value out of their investment.
If a pressure, volume and temperature (PVT) analysis is required, collected fluid samples are transported to a laboratory facility after the field operations. The thermodynamic data (PVT data) of a new field is often the first in-depth study of a reservoir to determine its future completion design and associated production facilities. It is imperative to ensure that a high quality "in situ representative" sample be collected to obtain the highest data quality.
Sample quality, to a great extent, is dependent on the sample pressure in the cylinder relative to the reservoir condition. If the sample remains at or above reservoir pressure, then it is considered to be of a very high quality. However, if the sample is subjected to one or more pressure drops during the field operation or the transportation to the laboratory, it may or may not be of further use for PVT or solids deposition studies. In particular, the study of asphaltene deposition is directly dependent on the sample quality and the method by which the sample was collected. Over-pressured samples, a service offered by the portfolio, compensates for any pressure loss due to volume change resulting from cooling effects as the sample is retrieved to the surface or transported to the PVT lab.
The testing and sampling services of the new reservoir characterization portfolio offer a range of advancements that are incorporated into the RCI formation fluid testing and sampling instrument. The service portfolio is designed to improve the efficiency and data quality of wireline testing and sampling programs.
Impact on efficiency
The portfolio's many fluid characterization services using the RCI tool with a fluid sample monitoring system design to ensure minimum sample contamination promote wellsite efficiency. Several of the portfolio's modules improve efficiency over previous sampling instruments.
Smart Samples is a term given to the new generation of sampling capabilities and recovered samples. Up to 30 samples in a variety of combinations can be recovered in a single trip into the well. All samples are pressure, volume and temperature (PVT) quality and shock/surge protected. Single-Phase 1 (SPT 1) or single-Phase 2 (SPT 2) samples are recovered by using a nitrogen gas pre-charge in the tanks to compensate for sample cooling as the sample is retrieved from the borehole.
SPT 2 tanks are similar to SPT 1 tanks except they have two floating pistons with a pre-determined nitrogen charge behind the second piston (nitrogen piston). The nitrogen piston has a 2:1 ratio in surface area, effectively doubling the compensation over that of a SPT 1 tank. The pistons compress the nitrogen charge and overpressure the sample (Figure 1).
The portfolio also has a fluid characterization module for assessing fluid types and quality in real time while the sampling instrument is deployed downhole (Figure 2).
Pressure testing is another segment of the portfolio that provides a number of capabilities and mathematical modeling techniques that enable real-time validation of recorded pressure data.
A straddle packer with an observation snorkel enables fluid sampling in reservoirs previously not testable via wireline testers (Figure 3). Low-permeability and fractured reservoirs can now be tested by incorporating inflatable packers in the tool string to isolate a 3-ft (1-m) section of formation. Vertical permeability anisotropy (Kv/Kh) testing is also possible, and the packer elements are compatible with oil-based mud.
The final module is a method of predicting petroleum fluid properties for each reservoir unit of interest. It provides in situ estimates of critical PVT parameters such as saturation pressure and viscosity. Real-time downhole PVT simulation at the wellsite provides users with estimated PVT parameters that can be used to expedite reservoir and production planning, saving the days or weeks that normal lab PVT analysis takes.
Reservoir characterization
From a wireline point of view, reservoir characterization is divided into two major segments: formation pressure evaluation, which encompasses the RCI functionality, and the formation fluid evaluation, which encompasses both the RCI and fluid characterization functionality.
Normally pressure testing is done using a snorkel or straddle packer, depending on the formation and its flow characteristics. A straddle packer is required in fractured, laminated or tight formations. During pressure testing three main tasks should occur in real time:
1. Optimizing the drawdown rate;
2. Optimizing the drawdown volume; and
3. Pressure test validation.
Drawdown rates and volumes are infinitely variable and controlled from the surface by the engineer conducting the testing.
Pressure test validation is accomplished using a mathematical technique called Formation Rate Analysis (FRA). One of the most important outputs of the FRA is an accurate pressure gradient, which can be used to predict fluid density. Fluid density, reservoir pressure and temperature are key inputs to an updated prediction calculation that is used to determine pump-rate and pressure maintenance during sampling.
For formation fluid evaluation there are three primary factors under consideration:
1. Reservoir fluid type;
2. Minimum contamination; and
3. Maintaining sample integrity.
To collect the best possible sample the reservoir fluid must be identified, including an estimation of saturation pressure (bubble point or dew point, depending on the reservoir fluid type). This is accomplished using the property prediction calculation in conjunction with the gradient that was determined by the pressure data. This software uses pressure gradient and geochemical parameters to estimate PVT parameters. For real-time use there are only two parameters determined, the saturation pressure and the viscosity. These two parameters are used to optimize the pump rate based on the in situ mobility. Other PVT parameters such as bubble point and the FRA while pumping provide additional opportunity to collect a higher-quality single-phase sample by adjusting the pump rate to the changing mobility as the clean-up occurs. A plot of mobility vs. time is generated from this software.
The contamination levels are monitored employing near-infrared sensing technology (Figure 2). The fluid characterization module also provides optical density, fluorescence and refractive index values, which allow the engineer to look at the clean-up process from different perspectives. The sample is collected at the minimum relative contamination observed during the clean up process.
Testing and sampling
The RESolution testing and sampling process begins with an in-depth pre-job planning process. Pre-job planning typically involves modeling and estimation of the pressure and rate responses during pressure testing and sampling with either a snorkel, straddle packer or both, pumping volumes to minimum contamination as well as reservoir fluid characteristics. Once the pre-test and sampling conditions are modeled, the optimum tool configuration can be determined and the various conveyance methods can be analyzed to determine the most efficient.
Operations commence when the RCI is positioned within the interval of interest. Hydrostatic pressure of fluid in the borehole is measured before the hydraulically actuated pad (or packer) extends from the tool and seals against the borehole wall. The purpose of the packer is to isolate the hydrostatic pressure in the borehole from the pressure in the formation.
A small quantity of fluid is drawn into the tool to confirm a seal has been established. The formation pressure is then recorded using a fast-response, highly accurate quartz gauge. As fluid flows into the tool, pressure builds up to a final stabilized value, the pressure build-up profile, providing information about the mobility of the formation fluid.
A double-action piston pump draws formation fluid in repeated drawdowns and discharges fluid into the well bore or into a sample chamber. Numerous repeat pressure tests may be conducted without having to reset the packer against the borehole wall and without forcing fluid back into the formation. A wide range of mobility measurements may be made using a controlled drawdown with the engineer controlling the drawdown rate and volume from the surface. Pressure testing offers a disciplined formation testing procedure aimed at improving the accuracy of the measured formation fluid gradients. Once a fluid gradient is established, a simulated PVT and compositional analysis can be generated.
If a formation fluid sample is required, additional fluid can be pumped out of the formation into the well bore. A variety of sensors, including near-infrared spectroscopy, refractometry, bubble point, resistivity/capacitance and fluorescence, are available to measure the properties of the fluid to distinguish mud filtrate from formation fluid and further characterize the fluid being pumped. When the engineer is confident that mud filtrate contamination is at a minimal level and a representative formation fluid is being sampled, the sample can be directed into a sample tank for future analysis.
These new sampling methods capitalize on the drawdown pressure control capability of the RCI. High-quality samples require the sampling pressure to remain above the bubble or dew point pressure and the asphaltene or paraffin deposition envelope. This is accomplished by estimating these critical parameters prior to sampling and maintaining the sampling pressure above the critical pressure by controlling the fluid withdrawal rate during pumping.

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