Sampling Geopressured Fluids: Some Contributions from the Properties of the H20-CH4 System

Authors: Eduardo R. Iglesias
Conference: Stanford Geothermal Workshop Session: Geopressured Systems
Year: 1980 Language: English
Abstract: The geopressured formations of the United States Gulf Coast are being probed for methane recovery feasibility. One of the critical variables involved is the amount of methane actually dissolved in the pore brines. Sampling and subsequent analysis of these geopressured fluid s is therefore important for the economic assessment of the resource. Thus, interest in use of conventional downhole fluid samplers and, recently, in development of samplers especially designed for geopressured environments, has been stimulated.

The purpose of these tools is to obtain fluid samples at reservoir conditions and to bring them to the surface, preserving the ir integrity, for subsequent chemical analysis. The sampling process may be envisioned as the following simplified sequence. Firstthe sampler is lowered along the wellbore to the reservoir depth (hereafter the bottomhole). At the bottomhole the sampler is filled with formation fluid. The valve(s) are then closed and the sampler is pulled back to the surface. There it is housed in the wellhead lubricator. A valve isolating the lubricator from the wellbore is then closed. The next stage is to dispose of the hot, high pressure fluid contained in the lubricator in order to reach the sampler. The sampler is then recovered. Finally, the fluid enclosed in the sampler is transferred to suitable containers for subsequent chemical analysis.

Pore fluids in geopressured formations are subjected to very high (up to 1400 at m do000 psi ) pressures, and moderately high (up to 2000C) temperatures ( e.g., Dorfman and Fisher, 1979); significant amounts of methane, sodium chloride and lesser chemical species are dissolved in the formation waters. During the sampling process described above, the temperatures of the fluid in the sampler in the wellbore may depart significantly from the common bottomhole value; furthermore, the pressure of the fluid surrounding the sampler decreases with decreasing depth. Correspondingly, various effects associated with the thermodynamic properties of the fluids would be induced. These effects may include; gas exsolution, both in the sampler and in the wellbore; vertical compositional changes along the wellbore due to slippage between the gas and the liquid phase; temperature-induced pressure drops in the fluid sample; substantial changes of the differential pressure exerted on the sampler walls, which relate to possible leaks; and formation in the sampler of a solid methane hydrate.

Prediction of such effects, and estimates of the ir magnitudes are useful for both users and designers of downhole geopressured fluid samplers. For example, this knowledge may help interpret field results, be used in assessment of sampling conditions to avoid those that favor leakage of the sampler, and suggest safer procedure for handling the sampler in surface operations, as will be shown in this paper.

This paper is devoted to predicting and quantitatively estimating geopressured fluid behavior during sampling of reservoirs in the ir natural, unperturbed conditions. Both the fluid in the sampler and the wellbore fluid are considered. To that end, I have developed a simple model ( an ''equation of state") to estimate thennophysical properties of geopressured fluids. This model is briefly described in Section 11; full details are given elsewhere ( Iglesias, 1980).

In Section III the "equation of state" is applied to compute and discuss fluid properties associated with the different stages of the sampling process. Questions explored include: the probable range of CH4 content of the samples; pressure, phase transitions, fraction of total volume corresponding to each phase, and composition of each phase present in the sample, over the expected range of temperatures; whether and under what conditions the fluid collected at wellhead in a flowing well provides a representative sample of the bottomhole fluid composition; the expected range of fluid pressures in the lubricator; and the expected range of differential stresses on the sampler. Bottomhole temperatures and pressures generally increase with depth in the geopressured formations of the Gulf Coast (e.g., Dorfman and Fisher, 1979). Thus, two well depths, representing approximatley the to p and the bottom of the geopressured zone, were considered in detail to assess effects associated with depth.
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