The logging of geological formations is, as is well known, economically an extremely important activity.
Virtually all commodities used by mankind are either farmed on the one hand or are mined or otherwise extracted from the ground on the other, with the extraction of materials from the ground providing by far the greater proportion of the goods used by humans.
It is extremely important for an entity wishing to extract materials from beneath the ground to have as good an understanding as possible of the conditions prevailing in a region from which extraction is to take place.
This is desirable partly so that an assessment can be made of the quantity and quality, and hence the value, of the materials in question; and also because it is important to know whether the extraction of such materials is likely to be problematic.
The acquisition of such data typically makes use of techniques of logging. Logging techniques are employed throughout the mining industry, and also in particular in the oil and gas industries.
In the logging of oil and gas fields, specific problems can arise. Broadly stated this is because it is necessary to consider a geological formation that typically is porous and that contains a hydrocarbon-containing fluid such as oil or gas or (commonly) a mixture of fluids only one component of which is of commercial value.
This leads to various complications associated with determining physical and chemical attributes of the oil or gas field in question. In consequence, a wide variety of logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that is lowered into a borehole formed in the formation by drilling.
It is commonplace in the mining and oil/gas industries to refer to the elongate dimension of the borehole as its “depth”; and to talk of the logging tool as being moveable “vertically” in the borehole. This is true even in the case of boreholes that do not extend directly vertically into the ground or indeed are substantially horizontal, or extend in complicated paths only parts of which are vertical. Such terminology is used herein and is not to be construed as limiting.
Typically, the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question). A log can be considered as the result of a mathematical convolution of (a) the geological data along the formation penetrated by the well-bore with (b) the spatial response function of the well-logging tool.
One known aspect of log processing is known as inversion. In this technique, certain data that may pertain—e.g., to a log or to measured characteristics of a logging tool—are used to improve the spatial, and especially the vertical, response characteristics of a logging tool used to produce another log. Generally, as indicated, log inversion in the past has been applied commonly to electrical (resistivity/conductivity) logging.
Broadly speaking, the vertical response function (vrf) of a logging measurement represents its sensitivity to an attribute or property of a geological formation over a certain distance corresponding to the resolution of the tool. The vrf may be represented as a plot of weighting factors such that the measurement at any depth may be generated from the product of the factors and the attribute or property of a formation. It is known to pre-compute vrf plots for a given logging tool design consisting of the output curves of the tool in a range of rock types that exhibit a range of values of the property the tool is intended to measure. Log analysts use the vrf plots to assist in the interpretation of log data since they indicate the accuracy with which the tool can be expected to identify specific features in a particular type of formation.
More specifically, depending on the tool design and the character of the formation the tool resolution may range from a relatively small distance to several tens of centimeters. The response function of a tool has the effect of “smearing” the measurements in accordance with the weighting factors mentioned, such that it is only possible to resolve features in the formation that extend for a greater distance along the borehole than the aforesaid tool resolution. Features the depths of which are less than the tool resolution cannot readily be distinguished owing to the data smearing effect of the response function.
As a result of the vertical response function therefore a feature that extends over a relatively short part of the borehole depth, being less than the tool resolution, and that exhibits a measured characteristic relatively strongly may produce a closely similar log measurement to another feature that extends for a greater distance along the depth of the borehole nonetheless also less than the tool resolution and that exhibits the measured characteristic less strongly. In consequence, it is impossible to a good level of accuracy to determine from study of the log whether the feature is a relatively short one that exhibits the characteristic somewhat intensely, or is a feature extending over more of the depth of the borehole while exhibiting the characteristic less concentratedly over its depth.
The main aim of inversion is to process acquired log data in an attempt to eliminate this data smearing effect and thereby permit a log analyst to discriminate between two or more features that otherwise would be indistinct owing to the vertical response function.
The problems deriving from poor tool resolution pertain in the case of nuclear logs in which the tool is capable of resolving only to an accuracy of, depending on the exact tool design, about 0.6-m. This means that geological features of a depth of less than 0.6-m potentially may be wrongly identified or categorized, or may not even be detected at all.
This is an especially acute difficulty when prospecting for e.g. coal bed methane or shale gas. The formations in which such commodities are found typically are heavily stratified, with distinct beds (also referred to herein as zones or layers) often being only a few centimeters deep. Clearly in such cases, the existing nuclear logging techniques may be largely unsuited to the correct identification of hydrocarbon-bearing strata.
Moreover, the techniques of inversion that are known in relation to resistivity logging for one reason and another are computationally complex. Aside from the fact that they do not readily transpose to the nuclear logging environment, the complexity of the resistivity inversion algorithms means that, depending on the speed of the logging tool in the borehole, they cannot be performed in real time.
On the contrary, it is commonly necessary to acquire resistivity data and subsequently invert it; and this means that the prior art techniques of inversion are not generally useable when it is required to carry out so-called “logging while drilling” (LWD) and “geosteering” operations, the nature of which will be known to the worker of skill in the art.