Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-11-27
2004-02-17
McElheny, Jr., Donald E. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C702S012000
Reexamination Certificate
active
06694262
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to evaluation of geological information and more particularly relates to analysis of geophysical data to determine the porosity of a geologic formation.
BACKGROUND OF THE INVENTION
Fracture detection in coal seams is critical for the recovery of economic quantities of methane. Coal is a dual porosity medium, comprising a matrix containing abundant micro-scale pores intersected by larger macro-scale fractures. The micro-scale pores are of the size that gas movement occurs via diffusion, resulting in a very slow rate of gas exchange per unit volume. The larger macro-scale fractures act as the conduits for connecting the gas-diffusing matrix to a well bore. For economic quantities of methane to be recoverable, extensive well-developed macro-scale fractures must be present to connect a large enough volume of coal matrix such that the total volume of gas diffused becomes significant. Thus, the detection of subsurface fracture systems is critical for delineating desirable locations for methane exploration.
During the drilling of a wellbore, drilling mud is pumped into the well in order to flush rock chips and other unwanted debris from the well bore as it is being drilled. The drilling mud is introduced into the wellbore under pressure, where the mud pressure is slightly greater than the pressure of a formation traversed by the wellbore to prevent escape of material from the formation through the well bore, a phenomenon known as well blowout. The resultant differential pressure between the mud well bore column pressure and the formation pressure forces mud filtrate into the permeable formation, and solid particles of the mud are deposited on the wellbore wall, forming a mudcake.
The mudcake usually has a very low permeability, and once developed, considerably reduces the rate of further mud filtrate invasion into the wellbore wall. In a region very close to the wellbore wall, most of the original formation may be flushed away by the mud filtrate. This region is known as the “flushed zone” or the “invaded zone”. If the flushing is complete, the flushed zone pore space contains only mud filtrate.
Further out from the wellbore wall, the displacement of the formation fluids by the mud filtrate is less and less complete. This results in a second region, this region undergoing a transition from mud filtrate saturation to original formation water saturation. The second region is known as the “transition zone”. The extent or depth of the flushed and transition zones depends on many parameters. Among them is the type and characteristics of the drilling mud, the formation porosity, the formation permeability, the pressure differential and the time since the well was first drilled. The undisturbed formation beyond the transition zone is known as the “uninvaded, virgin or uncontaminated zone”.
FIGS. 1 and 2
show prior art representations of an invasion and resistivity profile in a water-bearing zone.
FIG. 1
illustrates a cross section of a wellbore showing the locations of the mud cake
8
formed on the inner surface of the well bore by the drilling mud, the flushed zone, the transition zone and the uninvaded zone extending radially from the wellbore wall.
FIG. 2
illustrates the formation of a mud cake
8
by the drilling mud along the well bore wall and a radial distribution of formation resistivity extending radially from the wellbore wall, into the flushed zone, into the transition zone, and into the uninvaded zone. Sometimes, in oil and gas bearing formations, where the mobility of the hydrocarbons is greater than that of the water, because of relative permeability differences, the oil or gas moves away faster than the interstitial water. In this case, there may be formed, between the flushed zone and the uninvaded zone, an “annular zone or annulus”, with a high formation water saturation. Annuli probably occur in most hydrocarbon bearing formations; and their influence on measurement depends on the radial location of the annulus and its severity.
The existence of these zones (the flushed, transition, annular and uninvaded zones) influence resistivity log measurements and therefore the accuracy of the resistivity log itself. In it's conventional use, the resistivity log is used to determine if oil exists in the formation traversed by the wellbore. The main interest of the resistivity log, shown in the upper portion
10
of the graph of
FIG. 2
, is to obtain the true and correct value of the resistivity (reciprocal of the conductivity) of the uninvaded zone, Rt in the graph. High values of Rt indicate the presence of an insulator, possibly oil, in the formation. Conventionally, it is therefore desirable to correct for the effect of mud filtrate invasion on formation resistivity.
Conventionally, mud filtrate invasion analysis from resistivity logs is attempted by qualitative inspection of the separation between measurement displays representing different depths of investigation. The purpose of this analysis is to determine the radial geometric function of the logging tool response in order to correct for invasion and generate a more accurate value of Rt. It is desirable to have a method for determining geological formation fracture porosity that does not rely on values of Rt.
Conventional log analysis techniques require correction for hydrocarbon saturation in the void spaces of the geologic formation, and are complicated by depth based variation in the hydrocarbon saturation gradient through the flushed zone/uninvaded zone interface that may confuse invasion character. Variations in drilling mud properties between wells that change the radial resistivity profile and differences in the properties of the formation water can cause errors in conventional interpretation. As well, laboratory measurements of fracture porosity in coal may not be applicable to the bulk reservoir properties due to sampling error, the inherent friability of coal, and the sensitivity of coal to changes in stress regime. It is desirable to have a basis for analysis that does not require correction for hydrocarbon saturation or depth based variations.
After an exploration well is drilled, specialized tools are lowered down into the bore hole to test and record the responses of the different rock formations to various electrical, acoustic and radioactive stimuli. This process is termed geophysical logging, and the recorded data are termed geophysical logs. In one petroleum producing region of the world, the Western Canada Sedimentary Basin, approximately 280,000 wells have been drilled to date, and geophysical logs exist for virtually all of them. Geophysical logs have been used extensively in the past in conventional oil and gas exploration, but little data exist on their use in fracture detection in coal.
Some highly specialized geophysical logs are able to detect fractures in coal under very specific conditions, but the data are prone to error and the logging techniques have seen limited use. Advancement in the art delineated by the disclosed invention is that a large portion of previously unused data can now be processed for a new and useful result.
Various geophysical techniques exist to detect mud filtrate invasion and/or mudcake. One such device is an electrical pad containing regularly spaced electrodes. As the pad moves across the target formation, variations between the voltages are recorded, detecting the existence of mudcake on the borehole wall. This device relies on a solid contact with the bore hole wall and any variations in the size of the hole can disrupt its operation. This is significant as, over time, coals tend to cave-in resulting in rugose and irregular bore holes, thus limiting the utility of the pad contact type device.
Other types of electrical logging devices exist, but all have the goal of determining the rock properties away from the invasive and damaging effects of the well bore. In general, most of these devices are able to detect the depth accurately that the drilling fluid has invaded. However, because of the complex geometry of the
Blake Cassels & Graydon LLP
Leier Terry L.
McElheny Jr. Donald E.
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