Good reservoir management depends to a great extent on decisions made using high quality reservoir information. Much of that information is obtained from the capture and subsequent analysis of representative reservoir fluid samples. The preferred method for obtaining a representative reservoir fluid is to collect downhole fluid samples from the reservoir. In a cased-hole environment these are usually collected using slick/wire line conveyed samplers or tubing conveyed sample carriers. The use of a sample carrier allows multiple samples to be collected simultaneously.
This need for representative sampling was particularly true in a field in West Africa being developed by a Texas-based company who wanted quality data in order to manage the field’s wells and reservoir more efficiently. Reservoir fluid sampling requires special tools and expertise to ensure the collected fluid samples are free of contamination, and are preserved in a representative state during recovery for later analysis. Traditional contamination takes the form of oil- based drilling fluids that penetrate a formation of interest and are then produced when a sample is collected, making it non-representative. In such a situation, flowing the reservoir to lower the concentration of the contaminant in the produced fluid is the best approach.
Occasionally however, the nature of the reservoir fluids can require chemical intervention in the form of additives to address specific flow assurance issues. In these instances, the volume, and the location and methodology of the chemical injection process can significantly impact the amount of contamination in the flowing oil stream. In this particular case the nature of the reservoir fluid indicated that there might be flow assurance issues during recovery. To mitigate any complications during flow to the surface, the client needed to inject flow chemicals downhole. The presence of these chemicals would make for a non-representative sample downstream of the chemical injection point. Furthermore, the client was convinced that only taking large numbers of samples would improve the odds of collecting a representative sample.
Fortunately, the client had previous experience with Halliburton’s ability to deliver large numbers and volumes of high quality samples. Specifically, the client had prior experience running Halliburton’s Armada® sampling system on an earlier job in West Africa where eight out of a possible nine samples were collected. However, whereas in the past a single run to collect nine samples was considered more than adequate, in this instance the client decided to collect significantly more than nine samples.
“This decision by the operator was absolutely unprecedented,” said Derek Glassey, Africa region manager, Halliburton Completion Tools. “We knew when we started the job we were going to be involved in an unprecedented sampling effort that would present a formidable challenge.”
The Armada® System
The Armada sampling system is Halliburton’s new state-of-the-art tubing conveyed cased- hole sampling tool. It was specifically designed to consistently deliver large volumes of monophasic samples even after extended exposure to routine and most high-pressure, high-temperature (HPHT) environments. As such, it is the ideal sampling option for the growing number of instances where wire in the hole can be problematic or is unacceptable.
The tool uses nine samplers rated to 400°F (204°C) and 20,000 psi. Each sampler can collect a 400 cc monophasic sample. The samplers are located on the outside of the sample carrier, thus presenting a smooth bore in the tubing string through which wireline or coiled tubing can pass without risk of entanglement or damaging the samplers. This is an important consideration in the tool’s performance and reliability. The sample carrier is also equipped to carry a three-gauge set. The gauge set can include a gauge for measuring tubing pressure, a gauge for measuring annulus pressure, and a gauge for measuring the common nitrogen pressure.
A number of innovative features have been engineered into the design to facilitate improved sample quality and timely interpretation of the tool’s performance. For example, the sample piston has a junk chamber for isolating the initial surge of potentially non-representative fluid during sample capture. The piston also has a position location feature embedded in it which allows its location to be tracked by non-invasive means. If a sampler has successfully collected and retrieved a monophasic sample to the surface, then the change in sample piston position will signal this fact. Consequently, within minutes of the recovered sampler being set down on the rig floor, the client can have a reasonable understanding of the success of his/her sampling program. Furthermore, the Huntington Inconel® samplers have been ceramic coated to eliminate hydrogen sulfide adsorption, so that accurate hydrogen sulfide readings will be measured even at levels of 10 ppm or less.
However, perhaps the most significant aspect of the Armada system is the safety element it helps bring to the client’s operation. Because of its unique design, the Armada sampler can be completely assembled and pressure tested at the Halliburton facility. It is then shipped to location. At location all that is needed is for the nitrogen section to be charged and the sampler is ready to run in the hole. Therefore, only one field engineer and a couple of hours are required for the Armada system to run.
The Armada® Sampling System is a full bore sampler with an OD of 5.38”. Each of the nine Inconel samplers is rated to 400°F (204.4 C) and 20,000 psi and will collect a 400 cc sample.
Reservoir Sample Integrity
Maintaining fluid samples in a truly representative state requires preserving the original reservoir temperature and pressure during recovery. Attempts at temperature control during sample recovery have not been very successful, and consequently, the current approach is to try to maintain the pressure of the sample as high as possible during recovery to the surface.
The Armada system is specifically designed to require minimal rig-up and run-in time and operate in normal or hostile cased- hole environments for extended periods of time. Because samples are maintained in a single phase, the sampler also helps eliminate the need for lengthy surface reconstitution. These key design features help enable operators to benefit from optimizing their rig time by helping to reduce expenses while helping to increase the accuracy of the reservoir performance model.
In standard practice the carrier is loaded with nine samplers which releases in sets of three using annulus pressure triggers. However, the design is completely flexible and any number of samplers from one to nine can be run. Because one common nitrogen section services all the samplers, for any given initial nitrogen charge the final return pressure is very much a function of the number of samplers run.
“We are committed to providing our customers with a safer and more economical alternative to conventional sampling,” said David King, President, Halliburton’s Completion and Production Division. “The Armada carrier conveyed sampling system helps provide operators with a comprehensive understanding of key reservoir attributes as well as an assessment of their asset’s net present value.”
West Africa Field and Other Results
“Accommodating the client’s desire to retrieve more than two dozen samples in one run required us to become very creative,” stated Glassey. “We ended up stringing together a number of tools in tandem to obtain a total of 30 samplers. The string incorporated two Armada sampling systems, two Simba® sample carriers, and four additional conventional sample carriers. Twenty-six out of thirty possible samples were successfully collected and transferred to the client. The customer was extremely pleased with our overall performance and indicated they expected to use us again to gather reservoir samples from future wells in the field.”
The Armada system is also delivering exceptional performance in a number of international locations including Latin America and Europe/Eurasia. Keys to the significant growth in Halliburton’s sampling activities have been the simplicity and reliability of the Armada system and the move from country to regionally based operations. Focusing on the region rather than a single country enables Halliburton to better utilize equipment and personnel to meet specific needs in an optimum fashion.
In early 2007, the system was used to capture bottomhole samples of reservoir fluid in a North Sea well. The pipe-conveyed system was loaded with nine samplers to run a drillstem test at 228°F (109°C) and 3,500 psi well conditions. All nine samples were collected and transferred in a controlled environment on location. A second well was also successfully sampled at bottomhole temperature of 260°F (127°C) and 10,170 psi bottomhole static pressure.
During both applications, operational requirements outside the scope of the system required that the bottomhole assembly be run into and out of the hole three times to 13,000 ft (3,963.4 m). The system retained its integrity between trips and required no redress. The goal was to optimize the reliability of reservoir evaluation by gathering samples uncontaminated by drilling fluids while expediting turnaround time and helping to enhance safety.
A cross-section view of the sampler clearly shows the smooth internal bore, which allows for wireline or capillary tubing to pass through without damaging the samplers.
The Armada system represents a significant improvement in the industry’s ability to capture representative reservoir fluid samples in a tubing conveyed cased- hole environment. The samplers are rated to 400° F (204° C) and 20,000 psi and ceramic coated, allowing them to collect HPHT samples in corrosive environments without any change in the samples’ integrity. The Armada system can be fully assembled and pressure tested at the Halliburton base before shipping to location, resulting in fewer personnel and fewer hours required at location for the system to be deployed, which translates to significant savings for the client. Its unique design provides prompt information on the success of the sampling operation when the tool comes out of the hole. The Armada system was specifically designed to provide extended service in hostile environments that challenge the capabilities of existing systems.
By Cyrus Irani, Peter Royce, and Jerry Apperley, Halliburton