Downhole PVT Samples
Downhole samples are primarily acquired by the customer directly from oil and gas wells located offshore. The sampling tools are often conveyed on wireline and activated from the surface to capture representative fluids. Advanced single-phase chambers keep the fluids above reservoir pressure, despite cooling during transfer, helping to avoid pressure drop induced asphaltene precipitation.
Once received in our laboratory, the collected fluid samples are restored via agitation at the original reservoir pressure and temperature. Initial validation checks provide basic PVT properties and a measure of oil-based mud contamination common to open-hole samples. More in-depth analyses provide additional information about specific hydrocarbon behaviors.
Data in Context
GeoMark maintains a database of thousands of offshore PVT reports (www.rfdbase.com) for easy reference. This data provides excellent context when considering the behavior of new fluids. For example, fluids with complex geochemical sourcing usually have different PVT properties than those with more straightforward pedigrees.
Are the captured samples reliable?
This question is answered by the opening pressures and initial flash validations. Opening pressures should correspond to the reported pressures from the field. The flash validations should show consistency between fluids from the same depth and, ideally, low levels of oil-based mud contamination.
How do the fluid properties compare to nearby wells?
This question is best addressed by accessing GeoMark’s Rock and Fluid Database (www.rfdbase.com).
Do the fluid properties make sense?
GeoMark offers a substantial advantage by incorporating geochemical data into our PVT studies. Geochemical markers help identify certain characteristics, like fluid thermal maturity and mixing, which are key drivers of eventual PVT behavior.
Standard enhanced oil recovery (EOR) and (PVT) testing programs include experiments, such as solubility/swelling, multi-contact, slim tube, vapor-liquid equilibrium (VLE), and fluid property measurements.
These tests are designed to determine the extent of gas miscibility and mixture phase behavior during gas injection into hydrocarbon reservoirs. While common in conventional reservoirs for decades, EOR programs have also found recent applicability in unconventional plays.
Surface oil and gas samples are captured and recombined to recreate downhole reservoir fluids. Injection gases may be separator or sales gas derived from a nearby installation, or synthetically prepared gas based on a composition provided to our laboratory.
Data in Context
GeoMark has conducted a multi-client EOR consortia study in the Eagle Ford shale. The behavior of new systems is compared to the industry trends established by this relevant Eagle Ford study. Contact us to get more information about this study.
Will a specified gas be miscible with an oil?
This question is answered through a PVT and EOR testing program. Compositional data is combined with EOR tests, like swelling and slim tubes, to determine the miscibility pressure.
What if a reservoir fluid is already saturated (at bubble point)?
This is a difficult situation. By definition, the reservoir fluid is already saturated with gas and won’t “accept” any more to change it properties. In such cases, the most applicable test is a multi-contact equilibrium test; whereby, repeated contacts of fresh injection gas are used to vaporize some of the light components of the oil, while also reducing its viscosity.
Why is the bubble point pressure so critical in unconventional oil wells?
Dropping below the bubble point pressure dramatically reduces oil production in unconventional wells. PVT studies are used to determine this pressure and identify any corresponding gap to the reservoir pressure.
GeoMark follows industry-standard best practices for all PVT laboratory testing. Our overriding mission for the 15 years we’ve been in business is to “technically do the right thing!” That means adhering to the highest scientific standards of laboratory processes and controls, taking the utmost care with each and every sample we receive. To give you the full historical picture of your reservoir, we combine geochemistry data with PVT data. Not only do you find out the bulk properties of your oil sample, you also learn why it is the way it is…the original hydrocarbon source.
Downhole oil samples are collected by a third-party service provider and sent to GeoMark’s lab. For surface oil testing purposes, GeoMark sends a highly experienced sampling engineer to collect samples at your site. We follow a strict, step-by-step procedure when collecting and transporting the samples, including type of cylinders and equipment specifications to ensure valid sampling.
Once samples are received in our lab, we pay attention to the smallest details to ensure testing accuracy. During all phases of the test program, we stay connected with you so you are part of an interactive process:
- Validation phase – The first thing we do is validate your sample and share preliminary general results to you via email or phone, rather than waiting until the entire testing cycle is completed. This extensive validation also includes checking for any leaks or errors that may have occurred in sample collection or transportation.
- In-depth testing phase – We use the latest technology to accurately measure oil components and PVT parameters. In fact, we don’t just measure once, we measure twice! But, before doing any tests, we communicate with you to ensure the right tests are ordered, making adjustments if needed. Our lab technicians stay in touch with you during the course of the testing program, so there are no surprises.
- Final reporting phase – Depending on the scope of the project, you will receive a final written report typically in 4-6 weeks, containing all the laboratory findings. This fast turnaround time is made possible by our lab’s 24-hour operation. We offer to discuss the results, if desired. In some cases, things may not seem reasonable; that’s where our extensive PVT database with similar cases can add a level of comfort that all is within the norm.