Measuring and testing – With fluid pressure – Porosity or permeability
Reexamination Certificate
1999-12-15
2001-10-16
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Porosity or permeability
C073S152040
Reexamination Certificate
active
06301953
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to substantially real-time field analysis of drilling mud cuttings for determining the permeability of the formations through which drilling has occurred in order to assess zones for potential of hydrocarbon production.
BACKGROUND OF THE INVENTION
One of the challenges during drilling is to identify zones of interest in the formation. Zones of interest are usually distinguished by their permeability relative to the entire formation through which the drilling is occurring. Zones of high permeability contain highly porous rock, having interconnections between the pores, which allow the hydrocarbons residing within the rock to flow through and out of the rock. Suitable reservoir rock is therefore porous, permeable and contains sufficient hydrocarbon to make it economically feasible to drill and produce them.
Ideally, the identification of zones of interest should occur at the time of drilling, so as to be most efficient and cost effective, however most methods of determining permeability are rarely determined while drilling.
One prior art method is to perform a drill stem step in which the drill bit is tripped out, a tool is run in and pressure over time is monitored. Dril stem tests, while usually effective and time consuming and expensive. Others apply qualitative geological analysis of the drilling mud cuttings for a tough yet often very inaccurate assessment of the formation characteristics.
It is known to pump drilling mud downhole through the drill string to the bit to flush cuttings and hydrocarbons from around the bit. The mud is flowed up the annulus between the wellbore and the drilling string, to the surface for removal of solids and cuttings in an active mud system. The recovered mud, containing the cuttings, is flowed across a shale shaker where large solids are removed and can be sampled. A mud tank containing cleaning systems, such as sumps or centrifuges, are used to remove the fine particulates. The cleaned mud can then be recirculated downhole.
The solids in the mud returning from the wellbore are representative of the formation and can be analyzed for a number of characteristics indicative of it's hydrocarbon producing ability.
Some quantitative values for drilling have already available through observation of drilling performance, including such parameters as force of the bit (FOB) and bit revolutions per minute (RPM). Drilling through highly porous and possibly permeable rock results in an increased RPM and a decrease in the FOB. Additionally, the rate of penetration (ROP) is determined and is much faster through porous rock. Other methods and equipment exist for the measurement of hydrocarbon content in porous and permeable rock and require sophisticated on-site equipment and monitoring devices.
In its simplest form, conventional analysis has been performed qualitatively, largely by visually assessing characteristics known to occur in suitable reservoir rock. Such characteristics include the size of particles present, the angularity of the particles, the degree to which the particle sizes are similar (sorting), the degree to which the particles bind together (cementation), and the porosity of the rock and have been individually assessed by many different means. Industry standards have been set for these qualitative analyses, however they remain, to a large degree, subjective.
As sampling is normally done at defined drilling depths, characteristics can be indexed to the drilling depth using the lag time required to bring the mud to the surface. The zones of interest can then be identified on well logs coordinated with the depth and lithology of the sample.
Because of the subjective nature of visual analysis, even though attempts have been made to set standards, it would be advantageous to provide an easily calculated quantitative or semi-quantitative index that utilizes all of the simply assessed characteristics of the cuttings to determine a relative index of permeability.
SUMMARY OF THE INVENTION
Applicant has determined that conventional analysis of drilled cuttings can be rendered or transformed from the merely qualitative (e.g. coarse) to a semi-quantitative (Index=23) values through application of the method of the invention. In one preferred aspect of the invention, and in contradistinction to the prior art which takes visual analysis of a cuttings sample and applies a statement of its quality, a quantitative grain size analysis is implemented to apply a more representative semi-quantitative assessment of a cuttings sample's of grain size.
In a broad aspect, a method of quantitating relative porosity of a drilled formation from a cuttings sample is performed, the sample having constituents which may be classified into plurality of discrete grain size divisions, and comprising the steps of:
assigning numerical weighting factors to each of the discrete grain size divisions having larger values for larger grain sizes, preferably in the range of 1-5;
classifying the proportions of the cuttings' constituents between the discrete grain size divisions, preferably totaling 10;
multiplying the classified proportions by the corresponding weighting factor for establishing values representative of the relative contribution to the formation's porosity; and
summing the relative contributions for each grain size division for establishing numeric values corresponding to the porosity of the formation.
Applicant has determined that other conventional qualitative characteristics of a cuttings sample can be advantageously combined for transforming them into a more useful semi-quantitative value of relative permeability. Such additional characteristics include angularity, sorting, cementation and porosity. Accordingly, in another aspect of the invention, a method is provided for quantitating relative permeability of a drilled formation comprising the steps of:
assigning numerical values for the proportion grains in each of a plurality of grain size divisions within a sample and a weighting factor for each division corresponding thereto for establishing a environmental index value EnviroNdx;
assigning a numerical value to the degree of angularity of grains within a sample for establishing an angularity index value AngNdx;
assigning a numerical value to the degree to which the grains within a sample are the same for establishing a sorting index value SrtNdx;
assigning a numerical value to the degree to which the grains within a sample bind together for establishing a cementation index value CemNdx;
assigning a numerical value to the degree of porosity of the sample for establishing a porosity value Por; and
determining the relative permeability index PermNdx of the sample as being substantially proportional to EnvNdx, AngNd, SrtNdx and Por and inversely proportional to CemNdx.
Most preferably the relationship is as follows:
PermNdx
=
EnvNdx
×
(
AngNdx
)
2
×
(
Srt
Ndx
)
2
(
CemNdx
)
2
×
Por
.
REFERENCES:
patent: 5205164 (1993-04-01), Steiger et al.
patent: 5285692 (1994-02-01), Steiger et al.
patent: 5844136 (1998-12-01), Marsala et al.
Goodwin Sean W.
Politzer Jay L.
Williams Hezron
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