Wells – Processes – Sampling well fluid
Patent
1992-08-17
1994-08-16
Britts, Ramon S.
Wells
Processes
Sampling well fluid
166321, 73155, E21B 4908
Patent
active
053378229
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a well fluid sampling tool and to a well fluid sampling method.
Hydrocarbon fluids (oil and gas) are found in geological reservoirs wherein they are contained at a high pressure (relative to ambient atmospheric pressure), and usually also at an elevated temperature (relevant to ambient atmospheric temperature). At such pressures, the gas is dissolved in the oil such that the reservoir fluid initially exists as a single-phase. fluid, but the reservoir fluid will release dissolved gas to form a two-phase fluid with separate gas and oil components if the reservoir fluid has its initial pressure sufficiently reduced towards ambient atmospheric pressure. Also, the initial relatively high temperature of the reservoir fluid results in volumetric contraction of a given mass of fluid as it cools toward ambient atmospheric temperature if withdrawn from the well.
When hydrocarbon exploration wells are drilled and hydrocarbon fluids are found, a well fluid test is usually performed. This test usually involves flowing the well fluid to surface, mutually separating the oil and the gas in a separator, separately measuring the oil and gas flow rates, and then flaring the products.
It is also desirable to take samples of the oil and gas for chemical and physical analysis. Such samples of reservoir fluid are collected as early as possible in the life of a reservoir, and are analysed in specialist laboratories. The information which this provides is vital in the planning and development of hydrocarbon fields and for assessing their viability and monitoring their performance.
There are two ways of collecting these samples:
In Bottom Hole Sampling (BHS) a special sampling tool is run into the wall to trap a sample of the reservoir fluid present in the well bore. Provided the well pressure at the sampling depth is above the "Bubble Point Pressure" of the reservoir fluid, all the gas will be dissolved in the oil, and the sample will be a single-phase fluid representative of the reservoir fluid, i.e. an aliguot.
Surface Recombination Sampling (SRS) involves collecting separate oil and gas samples from the surface production facility (e.g. from the gas/oil separator). These samples are recombined in the correct proportions at the analytical laboratory to create a composite fluid which is intended to be representative of the reservoir fluid, i.e. a re-formed aliquot.
Several BHS tools are currently available commercially, which function by a common principle of operation.
A typical BHS tool is run into the well to tap a sample of reservoir fluid at the required depth by controlled opening of an internal chamber to admit reservoir fluid, followed by sealing of the sample-holding chamber after admission of predetermined volume of fluid. The tool is then retrieved from the well and the sample is transferred from the tool to a sample bottle for shipment to the analytical laboratory. As the tool is retrieved from the well, its temperature drops and the fluid sample shrinks causing the sample pressure to drop. This pressure drop occurs because the sample-holding chamber within the typical BHS tool has a fixed volume after the sample is trapped. Usually the sample pressure falls below the Bubble Point Pressure, allowing gas to break out of solution. This means the sample is now in two phases, a liquid phase and a gas phase, instead of in single-phase form as it was before the pressure dropped. In order successfully to transfer the sample from the tool to the sample bottle, it is necessary to re-pressurise the sample sufficiently to force the free gas back into solution, recreating a single-phase sample. This recombination is a lengthy procedure and thus expensive.
The phase changes which the sample experiences may also cause the precipitation of compounds previously dissolved in the well fluid, some of which cannot be re-dissolved by re-pressurisation. The absence of these compounds in the re-formed aliquot renders certain analyses meaningless.
A means by which a well fluid sample could be collected, retrieved an
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Brown Jonathan W.
Massie Keith J.
McGilvray James A.
Britts Ramon S.
Schoeppel Roger J.
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