Pyrolytic oil-productivity index method for predicting...

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Reexamination Certificate

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Reexamination Certificate

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06823298

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the use of data derived from the pyrolytic oil-productivity index, or POPI, to further predict other characteristics of the oil-bearing reservoir rock and the characteristics of the oil in the reservoir.
BACKGROUND OF THE INVENTION
A method for characterizing reservoir rock from the pyrolytic analysis of rock samples known as the Pyrolytic Oil-Productivity Index Method, or “POPI method”, is disclosed in U.S. Pat. No. 5,866,814. The disclosure of U.S. Pat. No. 5,866,814 is incorporated herein in its entirety by reference.
In the practice of the POPI method, the quality of the reservoir rock at a given location and depth is characterized as (a) oil-producing; (b) marginally oil-producing; or (c) non-reservoir or tar occluded. These relative characterizations are based on a comparison of the value of POPI
X
for a given rock sample X with the value of POPI
o
that has been previously determined from either (1) oil-stained reservoir rock samples similar to the drilling target that are known to be of good reservoir quality; or (2) a sample of oil that is similar to the expected composition of the well's target zone. The principal advantage of the POPI method is its ability to provide data in real time based on cutting samples taken from the drill rig, so that on-the-fly changes can be made, e.g., in horizontal drilling directions, to keep the bit in oil-producing reservoir rock. The POPI method can also be used to amass a body of comparative data for a given region or an oil field that can be used in planning further exploration and production.
The analytical procedures for determining the values for POPI are described in U.S. Pat. No. 5,866,814 (Jones and Tobey), and in view of the relationship of the present invention to the POPI method, the following summary is provided to facilitate an understanding of the terminology and significance of the data points.
1. Definitions
As used in this specification and claims, the following terms have the meanings indicated:
HC means hydrocarbons.
ln means natural logarithm.
LV is the weight in milligrams of HC released per gram of rock at the static temperature condition of 180° C. (when the crucible is inserted into the pyrolytic chamber) prior to the temperature-programmed pyrolysis of the sample.
TD is the weight in milligrams of HC released per gram of rock at a temperature between 180° C. and T
min
° C.
TC is the weight in mg of HC released per gram of rock at a temperature between T
min
° C. and 600° C.
LV+TD+TC represents total HC vaporizing between 180°-600° C. A low total HC indicates rock of lower porosity or effective porosity. A low value can also indicate zones of water and/or gas.
POPI
o
is the value of the pyrolytic oil productivity index as calculated for a representative sample of crude oil of the type which is expected to be found in good quality reservoir rock in the region of the drilling and chosen as a standard.
T
min
(° C.) is the temperature at which HC volatization is at a minimum between the temperature of maximum HC volatization for TD and TC and is empirically determined for each sample. Alternatively, a temperature of 400° C. can be used for samples where there is no discernable minimum between TD and TC. The latter sample types generally have very low total HC yields.
Phi is the average porosity of the rock.
Sxo is the saturation of drilling mud filtrate and represents the amount of HC displaced by the filtrate, and therefore, movable HC.
Phi*Sxo vs depth plot—the area below the curve represents the proportion of porosity which contains movable HC.
Phi vs depth plot—the area between the Phi curve and the Phi*Sxo curve represents immovable HC, or tar.
Gamma—the naturally occurring gamma rays that are given off by various lithologies while measuring directly in the well bore by the prior art petrophysical tools and are reported in standard API (American Petroleum Institute) units.
Caliper—the measured diameter of the well bore taken at the time of running petrophysical logs.
Density porosity—the porosity calculated by prior art methods from the petrophysical bulk density tools using an assumed fluid and grain density.
Neutron porosity—the porosity measured by prior art methods from petrophysical neutron tools.
Deep resistivity—the resistivity measured by deep invasion (long spacing between source and receiver), lateral log or induction petrophysical tools which is used as a measurement of undisturbed formation resistivity.
Medium resistivity—the resistivity measured by medium invasion (medium spacing between source and receiver), lateral log or induction petrophysical tools which is used as a measurement of resistivity of the formation that has been flushed by mud filtrate from the drilling fluid.
Shallow resistivity—the resistivity measured by shallow invasion (short spacing between source and receiver), lateral log or induction petrophysical analytic techniques which is used as a measurement of the resistivity of the mud filtrate from the mud cake that forms on the interior of the well bore during drilling operations.
Neutron-density cross-plot porosity (N-D Phi)—the porosity determined from a common prior art method which compensates for the effects of lithologic and fluid changes that lead to inaccuracies in employing either density or neutron porosity measurements by themselves.
Core plug permeability—the permeability measured by prior art methods from cylindrical rock samples that are cut from cores taken from the drilling process that is reported in units of millidarcys (md).
2. Pyrolysis Analytical Procedure
The analytical method used to quantitatively determine the presence of hydrocarbons in reservoir rock samples is known as open-system pyrolysis. In the practice of the POPI method of the invention the following expression is used to provide one or more data points:
ln(
LV+TD+TC
)×(
TD÷TC
)=
POPI
  (I)
In the above expression, the term “ln(LV+TD+TC)” means the natural logarithm of the value and the term “POPI” is used as shorthand for Pyrolytic Oil Productivity Index. The term POPI is also used more broadly hereinafter as a reference to the method of the invention.
In the POPI method for pyrolysis, a time and temperature-programmed instrument heats a small amount of ground rock sample from a starting temperature of 180° C. (held for 3 minutes) to 600° C. at a rate of increase in temperature of 25° C. per minute. During the programmed heating, the hydrocarbons driven from the rock are recorded as a function of temperature.
FIG. 1
shows a typical instrument output plot, which is known as a “pyrogram”. A typical analysis results in three peaks. The first is composed of hydrocarbons that can be volatized, desorbed, and detected at or below 180° C. while the temperature is held constant for the first 3 minutes of the procedure. These are called light volatile hydrocarbons, or “light volatiles” (LVHC, or LV). The next phase of the pyrolytic analysis consists of a programmed temperature increase from 180° C. to 600° C. that usually results in two more distinct peaks. The first of these peaks occurs between 180° C. and about 400° C., and corresponds to thermal desorption of solvent-extractable bitumen, or the light oil fraction. These are called thermally distilled hydrocarbons (TDHC, or TD). The second peak in this phase (third peak overall) occurs after about 400° C., generally after a minimum in pyrolytic yield is observed and extends typically to about 550° C. The temperature corresponding to the minimum in pyrolytic yield between TD and TC is referred to as T
MIN
. This peak is due to the pyrolysis (cracking) of heavier hydrocarbons, or asphaltenes. The materials that thermally crack are called thermally cracked hydrocarbons or “pyrolyzables” (TCHC, or TC).
As will be understood by those familiar with the art, many other types of data are employed in the characterization of reservoir rock and the oil in the reservoir for the purposes of modeling exploration and production. It is therefore an

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