What are the different fluid sampling techniques used in formation evaluation?

MDT is one of famous procedure for the collection of formation samples, it can be performed either on wire line or on pipes as TLC to collect fluid sample and get formation pressures to evaluate the available Hydro carbons in the formation and book the reserve.

Fluid sampling in oil and gas is crucial for assessing reservoir properties and is broadly divided into surface and downhole methods. Surface Sampling: - Collects samples at the surface, encompassing wellhead and separator sampling. Downhole Sampling: - Involves tools deployed through wireline or coiled tubing. - Classified into open-hole sampling and cased-hole sampling. - Various downhole techniques include Modular Formation Dynamics Tester (MDT), Wireline Formation Tester (WFT), Repeat Formation Tester (RFT), Pump-Out Testing, etc. These methodologies are essential for gaining insights into reservoir fluid composition and properties, contributing to effective reservoir management in the oil and gas industry.

Even DST is giving more fluid samples to analyze, WL/MDT method is cost friendly and faster option. Besides, DST is bringing complex risks that are required detailed engineering design and proper assessment.

Sampling techniques for formation evaluation, also can be applied while survey and seismic evaluation by doing exploration well. Formation evaluation start from stratigraphic structure, rock properties and fluid analysis.

Open-hole sampling offers a valuable opportunity to obtain high-quality, representative samples of reservoir fluids directly from the formation, providing crucial insights into the reservoir's composition. This technique, often conducted using wireline or DST tools, allows for precise control of pressure and temperature conditions during sample collection. However, the benefits of open-hole sampling come at a cost, both in terms of finances and time. Balancing the advantages of accurate reservoir fluid data against the costs and risks involved is a critical consideration in deciding whether open-hole sampling is the most suitable method for a given well or reservoir.

Fluid sampling using coiled tubing most especially in well intervention scenarios not limited to increase well production, opening the perforated zone and taking samples. In cased hole sample collection with coiled tubing are done with a specific BHA that allows for nitrogen lift which in turn allows for flow-back to the flowback tank where samples are collected

This method is particularly useful when sampling needs to be conducted in wells that have already been drilled and cased. It allows engineers to assess the reservoir properties and fluid composition in a controlled manner. Cased hole sampling can provide valuable information for reservoir management, production optimization, and decision-making without the need for additional drilling

It is more safe compared to the open hole sampling . To achieve good results it is better executed few weeks after the initial well start up to get initial fluids condition . The parameters changes for the aging formations

There are three ways to sample formation fluids; at the wellhead, in the borehole by a downhole sampler, and at the sand face downhole by a formation sampler. Each one has pros and cons. The wellhead sampling is the least expensive and can be taken anytime there is a need, but sampling at reservoir P and T may be difficult, if it’s possible. It is also average of the well produced fluids in case of multiphase production. The downhole sampling can be taken at specific depths sample fluids below the sampling depths, check out the commercial tool Armada which is combinable with a PLT. The formation sampling is the most expensive and typically requires good mud cake to ensure good sealing.

Downhole fluid sampling in PVT test can measure density, viscosity, bauble point, factor volume formation oil, fluid behavior etc. The live crude can use for relative permeability test.

"The main purpose of surface sampling is to monitor the flow status and correct the flow measurement data for calculating the net oil that is produced. Surface sampling is easier than downhole sampling, but it is more important after bringing a well to production. It needs to be done on a daily/weekly/monthly plan depending on the production status."

Surface sampling from the wellhead, separator, and stock tank is performed during most surface well tests. Successful PVT analysis requires fluid samples that represent the original hydrocarbon in the reservoir.

Surface sampling is not recommended for oils with asphaltene or wax precipitation problems. In these cases, pressure-compensated bottom-hole samples are recommended. This is especially important when collecting fluid samples for flow assurance studies.

This is very widely used method . It is the cheapest compared to downhole analysis and it can be done at any time . Yhe draw back is the drop in pressure and temperature of the samples.

This technique involves collecting samples of fluids produced at the surface, such as oil and gas, directly from the well. These samples are crucial for understanding the chemical composition of hydrocarbons and other substances present in the formation. Surface fluids captured during production undergo a detailed analysis in the laboratory, where key properties such as density, viscosity, hydrocarbon composition, and the presence of impurities are determined. This analysis provides valuable information for decision-making in reservoir management, allowing for an accurate assessment of oil quality, reservoir pressure, and other fundamental properties.

For the detailed fluid analyses, several techniques can be used. Wireline formation testing is such an important process to get samples by dropping the tools into the downhole. For sampling at multiple depths, a Modular formation dynamic tester can be very vital. For the fluid analysis, sidewall coring, assessing gas samples and PVT Pressure Volume Temperature Testing can be employed.

Drill-Stem Test (DST): The drill string is RIH with an empty section at the bottom end, say 1000 ft., A disk is ruptured from the bottom side and the vacant space is filled with a fluid sample. A valve is closed to trap the sample. Aslo, a valve such as an OMNI may be opened to capture the sample, and closed to trap the sample, at DH conditions. At the surface the sample is transferred to bottles (also at DH conditions). This is an expensive operation (especially offshore) due to rig time, equipment and specially trained personnel required. Also there is some contamination as other well fluids may be captured along with the sample.

Wireline Formation Testing (WFT) is a specialized sampling technique used in the oil and gas industry to obtain downhole fluid samples without the need for wellbore fluids to be brought to the surface. It involves lowering a tool, known as a formation tester, into the well on a wireline cable. The formation tester contains a probe that penetrates the reservoir rock and collects fluid samples directly from the formation. Once in the desired depth zone, the probe is deployed, and the tool creates a sealed environment to capture representative samples of the reservoir fluids.

The method of taking samples , experiance of the manpower and the quality of the devices used can play a big role in the samples accuracy. It is recommended to spend more money in aquiring good data rather than cutting the cost for cheaper options