Measuring and testing – Borehole or drilling – Formation logging
Reexamination Certificate
2002-09-09
2004-12-21
Bagnell, David (Department: 3672)
Measuring and testing
Borehole or drilling
Formation logging
C073S152180, C073S152290, C166S250010, C166S100000
Reexamination Certificate
active
06832515
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates generally to the field of oil and gas exploration. More particularly, the invention relates to methods for determining at least one property of a subsurface formation penetrated by a wellbore using a formation tester.
2. Background Art
Over the past several decades, highly sophisticated techniques have been developed for identifying and producing hydrocarbons, commonly referred to as oil and gas, from subsurface formations. These techniques facilitate the discovery, assessment, and production of hydrocarbons from subsurface formations.
When a subsurface formation containing an economically producible amount of hydrocarbons is believed to have been discovered, a borehole is typically drilled from the earth surface to the desired subsurface formation and tests are performed on the formation to determine whether the formation is likely to produce hydrocarbons of commercial value. Typically, tests performed on subsurface formations involve interrogating penetrated formations to determine whether hydrocarbons are actually present and to assess the amount of producible hydrocarbons therein. These preliminary tests are conducted using formation testing tools, often referred to as formation testers. Formation testers are typically lowered into a wellbore by a wireline cable, tubing, drill string, or the like, and may be used to determine various formation characteristics which assist in determining the quality, quantity, and conditions of the hydrocarbons or other fluids located therein. Other formation testers may form part of a drilling tool, such as a drill string, for the measurement of formation parameters during the drilling process.
Formation testers typically comprise slender tools adapted to be lowered into a borehole and positioned at a depth in the borehole adjacent to the subsurface formation for which data is desired. Once positioned in the borehole, these tools are placed in fluid communication with the formation to collect data from the formation. Typically, a probe, snorkel or other device is sealably engaged against the borehole wall to establish such fluid communication.
Formation testers are typically used to measure downhole parameters, such as wellbore pressures, formation pressures and formation mobilities, among others. They may also be used to collect samples from a formation so that the types of fluid contained in the formation and other fluid properties can be determined. The formation properties determined during a formation test are important factors in determining the commercial value of a well and the manner in which hydrocarbons may be recovered from the well.
The operation of formation testers may be more readily understood with reference to the structure of a conventional wireline formation tester shown in
FIGS. 1A and 1B
. As shown in
FIG. 1A
, the wireline tester
100
is lowered from an oil rig
2
into an open wellbore
3
filled with a fluid commonly referred to in the industry as “mud.” The wellbore is lined with a mudcake
4
deposited onto the wall of the wellbore during drilling operations. The wellbore penetrates a formation
5
.
The operation of a conventional modular wireline formation tester having multiple interconnected modules is described in more detail in U.S. Pat. Nos. 4,860,581 and 4,936,139 issued to Zimmerman et al.
FIG. 2
depicts a graphical representation of a pressure trace over time measured by the formation tester during a conventional wireline formation testing operation used to determine parameters, such as formation pressure.
Referring now to
FIGS. 1A and 1B
, in a conventional wireline formation testing operation, a formation tester
100
is lowered into a wellbore
3
by a wireline cable
6
. After lowering the formation tester
100
to the desired position in the wellbore, pressure in the flowline
119
in the formation tester may be equalized to the hydrostatic pressure of the fluid in the wellbore by opening an equalization valve (not shown). A pressure sensor or gauge
120
is used to measure the hydrostatic pressure of the fluid in the wellbore. The measured pressure at this point is graphically depicted along line
103
in FIG.
2
. The formation tester
100
may then be “set” by anchoring the tester in place with hydraulically actuated pistons, positioning the probe
112
against the sidewall of the wellbore to establish fluid communication with the formation, and closing the equalization valve to isolate the interior of the tool from the well fluids. The point at which a seal is made between the probe and the formation and fluid communication is established, referred to as the “tool set” point, is graphically depicted at
105
in FIG.
2
. Fluid from the formation
5
is then drawn into the formation tester
100
by retracting a piston
118
in a pretest chamber
114
to create a pressure drop in the flowline
119
below the formation pressure. This volume expansion cycle, referred to as a “drawdown” cycle, is graphically illustrated along line
107
in FIG.
2
.
When the piston
118
stops retracting (depicted at point
111
in FIG.
2
), fluid from the formation continues to enter the probe
112
until, given a sufficient time, the pressure in the flowline
119
is the same as the pressure in the formation
5
, depicted at
115
in FIG.
2
. This cycle, referred to as a “build-up” cycle, is depicted along line
113
in FIG.
2
. As illustrated in
FIG. 2
, the final build-up pressure at
115
, frequently referred to as the “sandface” pressure, is usually assumed to be a good approximation to the formation pressure.
The shape of the curve and corresponding data generated by the pressure trace may be used to determine various formation characteristics. For example, pressures measured during drawdown (
107
in
FIG. 2
) and build-up (
113
in
FIG. 2
) may be used to determine formation mobility, that is the ratio of the formation permeability to the formation fluid viscosity. When the formation tester probe
112
is disengaged from the wellbore wall, the pressure in flowline
119
increases rapidly as the pressure in the flowline equilibrates with the wellbore pressure, shown as line
117
in FIG.
2
. After the formation measurement cycle has been completed, the formation tester
100
may be disengaged and repositioned at a different depth and the formation test cycle repeated as desired.
During this type of test operation for a wireline-conveyed tool, pressure data collected downhole is typically communicated to the surface electronically via the wireline communication system. At the surface, an operator typically monitors the pressure in flowline
119
at a console and the wireline logging system records the pressure data in real time. Data recorded during the drawdown and buildup cycles of the test may be analyzed either at the well site computer in real time or later at a data processing center to determine crucial formation parameters, such as formation fluid pressure, the mud overbalance pressure, ie the difference between the wellbore pressure and the formation pressure, and the mobility of the formation.
Wireline formation testers allow high data rate communications for real-time monitoring and control of the test and tool through the use of wireline telemetry. This type of communication system enables field engineers to evaluate the quality of test measurements as they occur, and, if necessary, to take immediate actions to abort a test procedure and/or adjust the pretest parameters before attempting another measurement. For example, by observing the data as they are collected during the pretest drawdown, an engineer may have the option to change the initial pretest parameters, such as drawdown rate and drawdown volume, to better match them to the formation characteristics before attempting another test. Examples of prior art wireline formation testers and/or formation test methods are described, for example, in U.S. Pat. No. 3,934,468 issued to Brieger; U.S. Pat. Nos. 4,860,581 and 4,936,139 issued to Zimmerman et al.;
Follini Jean-Marc
Pop Julian
Bagnell David
Bomar Shane
Echols Brigitte L.
Salazar J. L. Jennie
Schlumberger Technology Corporation
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