Limited Entry Formation Sampler For High Pressure Sour Gas Wells
Authors: Hyde W.E. , Farley D.L. 
Published: December 2007

A new technique for sampling sour gas crudes is being developed for use in deep holes where pressures may be as high as 20,000 psi (137.9 MPa) and temperatures as high as 450 degrees F (232 degrees C).

A dual sampler testing string is run in on pipe allowing the pipe to fill up as it is lowered. Before setting a compression pipe to fill up as it is lowered. Before setting a compression type packer which seals off the formation to be tested, a light gravity fluid is pumped through surface control valves and down the inside of the pipe, displacing the heavier mud inside the pipe.

After the packer is set and the two samplers opened by application of pipe weight, the light gravity fluid can be released back into surface tanks. This creates the necessary differential for the formation fluid to flow into the samplers. The well is under control at all times during the test.

After trapping the samples, residual formation fluid inside the drill pipe may be pumped back into the formation, or reverse circulated.

As the samplers are withdrawn to the surface, the volume in each expands 43%. This reduces the pressure for safer handling.

At the surface one sampler can be examined at the well site and the other sent to a laboratory for sample analysis if desired. The volume of each sampler is 0.66 gal. (2.5 litres). Materials throughout the tool string meet NACE specifications for sour gas service.

Introduction

The energy situation is constantly changing. Not only are costs rising but depletion and scarcity of more conventional petroleum reservoirs are causing oil companies to look at potential petroleum reservoirs are causing oil companies to look at potential deep production that, until a few years ago, would have been impossible with the existing technology of that time.

Development of new metallic and elastomeric materials along with the higher prices for crude oil has made it feasible to produce deeper and hotter reservoirs containing very hostile produce deeper and hotter reservoirs containing very hostile environments of H2S. These reservoirs may have static pressures to 20,000 psi (137.9 MPa) at temperatures above 450 degrees F (232 degrees C) with H2S in excess of 35% by volume.

Very costly completion techniques are required. Sub-surface tubulars, surface controls and gathering systems must be planned for where sour gas is present. All of this material must meet the applicable federal and state regulations. This is not a federal criteria for the industry because the oil industry itself formulated these regulations.

In unproven reservoirs it is very beneficial to know whether or not sour gas is present. Material and techniques demanded in an H2S environment could be several hundred thousand dollars more than required for the same purpose in a well environment where sour gas is not present.

A device is desirable which can obtain fluid samples under reservoir conditions during the drilling phase of the well. Such a sampler should be operated under controlled conditions in the open hole section of the reservoir. Samples obtained could be analyzed with laboratory procedures and should be made entirely safe for transporting under pressure to such laboratories while still in the samplers.

The content of the fluid samples would then influence the type of well completion.

The Limited Entry Formation Sampler was designed for this preliminary reservoir evaluation. Several major oil companies and preliminary reservoir evaluation. Several major oil companies and suppliers were consulted in development of the tool string and techniques of operation.

The following development goals were established and included:The tool string should be able to obtain formation fluid samples at in situ pressures of 20,000 psi (137.9 MPa) and at 450 degrees F (232 degrees C) with maximum safety.