Wells – Processes – Sampling well fluid
Reexamination Certificate
2002-08-01
2004-06-08
Bagnell, David (Department: 3672)
Wells
Processes
Sampling well fluid
C166S100000, C166S167000, C175S058000, C175S059000, C073S152230, C073S152240, C073S152260
Reexamination Certificate
active
06745835
ABSTRACT:
BACKGROUND OF INVENTION
The invention relates generally to formation fluid sampling. More particularly, the invention relates to a method and an apparatus for obtaining a fluid sample from a subsurface formation traversed by a borehole while controlling the flow rate and/or pressure.
Fluid samples from subsurface formations are typically collected from a reservoir for analysis at the surface, downhole or in specialized laboratories. Information obtained from analyzing formation fluid samples often plays a vital role in the planning and development of hydrocarbon reservoirs and in the assessment of a reservoir's capacity and performance.
FIG. 1
shows one example of a conventional formation testing tool
100
which may be used to obtain a sample or conduct tests in a subsurface formation. Sampling operations are typically conducted in “overbalanced” boreholes, wherein the hydrostatic pressure of the borehole fluid is greater than the formation pressure. Overbalancing typically prevents the formation fluid from breaking through the walls of the wellbore and causing either “blowouts” or undesired pressure at surface.
In a typical sampling operation, the formation testing tool
100
is lowered into an overbalanced borehole
109
on a wireline
111
and positioned adjacent the subsurface formation
103
to be sampled. The formation testing tool
100
makes physical contact with the inside surface of the borehole
109
by engaging a probe
104
of a probe assembly
102
with a wall
112
of the borehole
109
. One or more stabilizer pads
115
also extend from the formation testing tool
100
to stabilize the formation testing tool
100
in the borehole
109
.
As shown in
FIG. 1
, the formation testing tool
100
includes a pump module
105
which is used to induce fluid flow from the formation
103
into the formation testing tool
100
. An analyzer module
106
may also be provided to analyze fluid obtained from the formation. A plurality of sample tanks (not shown) are also disposed in a sample tank module
118
of the formation testing tool
100
to enable the collection of formation fluid samples in the tool
100
.
Contact between the probe
104
of the formation testing tool
100
and the borehole wall
112
enables pressure communication with the formation
103
. A seal is disposed around the probe
104
to isolate the inner parts of the formation testing tool
100
from the borehole fluid. In openhole boreholes, mudcake is typically disposed on the borehole wall
112
to isolate the formation fluid from the borehole fluid. In cased boreholes, casing and cement are disposed in the borehole to isolate the formation fluid from the borehole fluid.
Once the formation testing tool
100
is positioned and set as described above, one or more formation fluid samples may be obtained from the formation
103
. Fluid communication is established between the formation testing tool
100
and the subsurface formation
103
by contacting the probe
104
to the subsurface formation
103
. Because the formation
103
is at a lower pressure than the borehole
109
, and the formation testing tool
100
is in communication with the higher borehole pressure, formation fluid may then be drawn into the formation testing tool
100
by using a downhole pump module
105
. A downhole pump is used to create a desired pressure differential between the formation testing tool
100
and the subsurface formation
103
to induce flow from the formation
103
into the formation testing tool
100
.
Other prior art formation testing tools and sampling methods have been developed as described in detail in U.S. Pat. Nos. 4,860,581; 4,936,139 (both assigned to Schlumberger); U.S. Pat. No. 5,303,775 (assigned to Western Atlas); and U.S. Pat. No. 5,934,374 (assigned to Halliburton). The formation sampling methods and tools in these cases disclose formation sampling operations carried out by flowing fluid into the formation testing tool with a downhole pump that creates a desired pressure differential. U.S. Pat. No. 5,377,755, assigned to Western Atlas International is another example of a formation testing tool used for sampling. This patent describes a formation testing tool including a bi-directional pump adapted to control the pressure differential in sample tanks. Valves are disposed in flow lines between the pump and the sample tanks to allow for the selective communication of fluid therebetween.
The prior art downhole testers and sampling techniques utilize pumps to collect samples and maintain the samples in “single phase.” In single phase sampling operations, the pressure drop experienced by the formation fluid must be minimized to avoid drawing the formation fluid sample at a pressure below its bubble point pressure or asphaltene precipitation point. This is achieved in prior art formation testing tools by providing flow control during sampling. The flow control is largely dependent on the operation of one or more downhole pumps. As formation fluid is drawn out of the formation, the pressure drop experienced by the formation fluid and the rate of flow are regulated by the speed of the pump.
In a sampling operation, the initial drawdown of formation fluid from the formation is often contaminated by mudcake, filtrate, or debris. Pumps are used to remove a sufficient amount of formation fluid before collecting a formation fluid sample to purge these contaminates from the fluid stream. This initial formation fluid removal operation is referred to as the clean-up phase. When a sampling operation includes a clean-up phase, flow control is provided downhole by initially running a downhole pump as fast as possible to reduce the clean-up period and then lowering the downhole pump speed to maintain the formation fluid sample in a single phase during collection or downhole analysis of the sample. If the speed required by the downhole pump is below a certain operating threshold, the pump motor may stall causing the pump to fail. Therefore, the operating range of the downhole pump must be optimally designed or selected prior to a sampling operation. If failure of the downhole pump occurs during an operation, either another pump is required or the tool must be pulled to the surface and the existing pump fixed or replaced before a single phase sample may be acquired.
To minimize or avoid problems associated with the use of downhole pumps during sampling operations, a method is desired which allows for a formation fluid sample to be obtained and that allows for control of the flow rate and/or pressure disturbance experienced by the formation fluid during sampling. A method is also desired which permits sampling in a wellbore which does not require the use of a downhole pump. It is further desired that such a method may provide a technique for obtaining single phase samples.
SUMMARY OF INVENTION
In one aspect, the present invention relates to a method for sampling a subsurface formation traversed by a borehole. In one embodiment, the method comprises positioning a formation testing tool in a borehole having borehole fluid therein with a pressure less than formation pressure such that a pressure differential exists between the formation and the borehole. The formation testing tool includes a sample chamber having a first side, a second side and a movable fluid separator disposed there between. The method further includes establishing fluid communication between the formation testing tool and the formation and inducing fluid flow from the formation to the formation testing tool by exposing an interior of the formation testing tool to the pressure differential. The method also includes capturing a sample of the formation fluid in a sample tank associated with the formation testing tool by exposing the sample tank to the pressure differential.
In another aspect, the present invention relates to a method for performing a controlled pretest on a subsurface formation traversed by a borehole. In one embodiment, the method comprises positioning a formation testing tool in a borehole having borehole fluid therein with a pressure
Bagnell David
Collins Giovan
Jeffery Brigitte L.
Ryberg John
Salazar J.L. Jennie
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