CA2508404A1

CA2508404A1 - Preferred embodiment

Info

Publication number: CA2508404A1
Application number: CA002508404A
Authority: CA
Inventors: James William Wheeler
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-05-31
Filing date: 2005-05-31
Publication date: 2006-11-30

Description

Description of the Preferred Embodiment The present invention comprises an improved subsurface tool 1 which is adapted to be included integrally in the bottomhole assembly of a drill string, as near to the bit 2 and the bit saver sub 3, as is practicable, which is capable of identifying non-framework rocks in real time. The tool 1 is suspended in the bore hole 4, by means of the drill string 5. The tool 1 is provided with a telemetry means 6, which transmits data from a plurality of sensor devices within the toot 1, to a telemetry receiving means 7 at the surface.
Data from the surface receiver 7 is transferred to a data processing apparatus 8. Data is simultaneously transferred from the driller's console 26 to the data processing apparatus 8.
The processing apparatus 8 integrally includes a visual display means 9, which provides the operator with various selectable images or pages. The primary page includes a plurality of plotted lines that are representative of the various geophysical and drilling parameters available to the processing apparatus 8 of the invention. The processing apparatus 8 further provides an operator with aids to data analysis, interactively through means of an integral keyboard device 10 and a mouse and cursor control system 11. The processing apparatus 8, incorporates an operating system running a dedicated computer program that automatically provides the operator with, for example, continuous statistical information of each available physical parameter over a selected interval, including variance, maximum value, minimum value, mean value, and the magnitude and direction of any continuous slope defined by the data. The processing apparatus 8 of the invention aids the operator in judging the degree of symmetry of a given physical parameter over a given interval and about a given midpoint by means of an inversion overlay function.
The ultimate objective of the analyses is to compute the degree to which a given shale, identified and analyzed during the time of its penetration by the bit, conforms to the characteristics of a non-framework shale. The degree of conformity is presented as a simple mathematical probability. The presence of a non-framework rock informs the driller, in a timely manner, that an abnormally high pore system pressure is about to be encountered. The data received by the processing apparatus 8 and the assessment of

-2-conformity are recorded on a permanent medium such as a compact disk writer 27, and presented as a paper log by a dedicated plotting apparatus 12.
The tool 1 of the invention illustrated in Figure 1 is substantially a modified version of known logging while drilling tools, comprising a cylindrical body of high density metal alloy, provided with an axial passageway, permitting the movement of drilling mud through the core of the tool 1. Specific mod~cations of the tool 1 include three strain gauges. One strain gauge 13 is mounted parallel to the long axis of the tool 1 so as to continuously measure the vertical strain experienced by the body of the tool 1. The tool 1 is further provided with a second strain gauge 14 mounted at a right angle to the axis of the tool 1 so as to continuously measure the lateral strain experienced by the body of the tool 1. The tool 1 is further provided with a third strain gauge 15 mounted at forty five degrees to the axis of the tool 1 so as to continuously measure the torque experienced by the body of the tool 1.
Another modification of the tool 1 of the invention is related to the means provided to measure the sonic velocity of the rock formations exposed in the borehole 4.
The measuring means are modified from known devices in two aspects. Firstly, the vertical separation of the acoustic source 16 and the acoustic receivers 17 is greatly reduced on the body of the tool 1 of the invention in order to increase vertical resolution. This permits the tool 1 of the invention to more reliably detect the very thin central high sonic velocity spike that is peculiar to a non-framework shale. Secondly, the receivers are of a type and disposition that enable the collection of shear wave data as well as compressive wave data. These two data streams permit some measure of the plasticity of the shale to be calculated by the processing apparatus 8. Certain portions of non-framework shales are much more plastic than common shales.
The tool 1 of the invention is further provided with means to continuously measure natural gamma radiation emanating from the rocks of the open borehole 4. The spectral gamma radiation detector 19, counts separately, the gamma rays in the three energy ranges that coincide with the three common elements that emit gamma radiation; potassium, uranium, and thorium.

-3-The tool 1 of the invention is further provided with a neutron source 20 and a neutron detector 21 which continuously measure neutron capture cross-section in the wall rocks of the open borehole 4.
The tool 1 of the invention is further provided with a gamma ray source 22 and two gamma ray detectors 22 and 24 to continuously measure the electron density and photoelectric absorption factor of the wall rocks of the open borehole 4.
At present, the measuring of hydrocarbon gas saturations of the drilling fluid takes place at the surface, specifically at the mud tank. Because the mud exiting from the jets of the bit 2 takes hours to travel back to the surface, the mud saturation data can lag many meters behind the advance of the borehole. To eliminate lag time, the tool 1 of the invention is provided with a pair of identical mud resistivity meters. The electrode pairs are sheltered in two small concavities, one pair on the internal surface and one pair 25 on the external surface of the tool 1. The electrodes are supplied with identical electric potentials. The pair of electrodes on the inner surface of the tool 1 is exposed to the mud flow as it travels down the interior passage of the tool. The pair 25 on the outer surface of the tool 1 is exposed to the mud flow just after it has swept past the cutting face. If there are pores in the rock, and they contain formation water that is more saline than the mud, the electrode pair on the outer surface will register a lower resistivity than the pair on the interior surface.
If, however, there are pores in the rock that are hydrocarbon filled, the electrode pair 25 on the outer surface will register higher resistivity than the pair on the interior surface. The comparative resistivity method of measuring hydrocarbon saturation in the mud system is superior to the current technology because it provides instantaneous information, and the hydrocarbon saturation profile of a given rock unit is not distorted and degraded by rising several kilometers through a fluid mud column in the annulus 18 of a well that is of a complex and irregular shape.
OPERATION OF THE INVENTION

-4-In the preferred embodiment of the invention, the processing apparatus 8 of the invention incorporates a computer with an operating system running a dedicated computer program that provides a plurality of automated calculations and graphic displays. For example, the display apparatus 9 can be directed, by way of on-screen controls such as drop down menus, to provide graphic renderings of line curves that represent real-time geophysical data such as vertical acoustic velocity, or neutron capture cross-section, combined with real-time drilling data such as torque stress on the tool 1 of the invention, as depicted in Figure 2. The line curves can be of various colours and can be displayed against well depth, which can be depicted along the vertical axis of one page, on the display apparatus 9.
The units of display for the various line curves can be controlled by the operator by means of drop down menus. For example, sonic velocity can be displayed in units of meters per second or feet per second, or as travel times such as microseconds per meter or microseconds per foot, or as sonic porosity. The calculation of sonic porosity within a non-framework shale is theoretically meaningless, however, a display in this format facilitates the comparison of sonic velocity data from the subject well with similar data from other wells that have used the porosity unit as an industry convention.
In the case of the natural spectral gamma ray data as captured by the gamma radiation detector 19, the operator may choose to display the combined radiation of all energy levels so as to define the extent of the shale mass under study. The operator may choose to allow the processing apparatus 8 to calculate a default shale line, or he may enter an alternate value by means of the keyboard device 10, or he may choose to determine the position of the shale line visually, manipulating the shale line interactively by means of the cursor and mouse device 11. As an aid to interpretation, the processing apparatus 8 can fll the area between the shale line and the line curve with a transparent colour or a pattern.
Likewise, the operator can control the unit of display for the neutron capture cross-section data as derived from the neutron source 20 and neutron detector 21. The operator can select a line curve representing either neutron flux or neutron porosity. As in the case of sonic porosity, neutron porosity is meaningless with respect to non-framework shafes but is _5_ helpful in comparing neutron flux data with similar data available from the conventional logs of other welts.
Furthermore, the operator can choose to display the gamma ray flux data derived from the gamma ray source 22 and detectors 23 and 24, as photoelectric effect, electron density, bulk formation density, or density porosity after having either assigned a value for the grain density of the shale or accepting a default value provided by the processing apparatus 8 of the invention.
One of the most definitive characteristics of non-framework shales is the extreme homogeneity in physical attributes such as breaking strength. The data from the three strain gauges 13, 14, and 15, which are mounted on the tool 1, can be displayed separately, or in combination on the display means 9 of the processing apparatus 8.
While drilling, the variations of strain suffered by the tool 1 of the invention are largely the result of the friction between the teeth or buttons of the drill bit 2, and the rock face that it is working on. In typical clastic rocks, including shales with typically well developed mineral frameworks, the mechanical integrity of the rock is relatively variable. The homogeneity of non-framework shales leads to very low statistical variance in "bit bounce" or vibration.
The strain gauge data coming from the tool 1 is analyzed by the processing apparatus 8, and the statistical variance of the strain data is presented on the display means 9, both numerically and graphically. The graphic display can consist of the line curve of the original data, enclosed by two straight lines representing the standard deviation of the data, which is the mathematical inverse of variance. The space between the two dashed lines can be filled with transparent colour to visually emphasize the degree of variance.
Furthermore, the operator can choose to display the rate of penetration (ROP) data at a sampling interval that is much reduced from that normally available directly from the driller's console. The operator of the processing apparatus 8 of the invention is provided with the option of displaying the ROP in units of feet per minute, metres per minute, minutes per foot, or minutes per metre. There is no range of ROP data values that is directly indicative of a non-framework shale, rather the statistical variance in ROP data diminishes markedly in such rocks. In addition, ROP data occasionally reveals a very thin anomaly of a reduced rate of penetration at the axis or hydraulic mid-point of a non-framework shale. The metre thick anomaly or spike is a representation of the dense, impermeable central membrane of a non-framework shale that constitutes an actively functioning pressure seal.
If time permits, and there is some doubt that a given candidate shale is a sealing non-framework shale, drilling can be interrupted and the mud system circulated until the lowermost mud in the annulus reaches the surface. This procedure is referred to as circulating bottoms up. By this means, the mud gas concentration profile in the shale under study can be obtained from the standard mud tank sampling system. Again, there is no range of gas readings that is indicative of a non-framework shale, rather there is sometimes a special case of symmetry visible in the gas profile. Centered over the hydraulic mid-point or axis of the shale is a symmetric curve that appears to be parabolic in form and reaches its maximum value at the mid-point of the non-framework shale. The two lows on either side of the axis closely approach the zero gas line. This pattern is unique to non-framework shales. With conventional mud pit technology this gas pattern is rarely preserved. Furthermore, it normally can only be seen some hours after the penetration of the shale. The rarity of the pattern is due to poor preservation of the pattern as the gas moves several kilometers up the annulus. Any turbulence in the mud flow will degrade the pattern as it rises. Both the delay problem and the degradation problem are avoided by adopting the dual resistivity method of the invention.
Many of the data streams coming from the various sensors on the tool 1 display a symmetry about the central axis of the shale. To aid the operator in recognizing symmetry, the processing apparatus 8 can automatically display a second copy of any given line curve immediately adjacent to the original line curve, which is differentiated from the original by rendering it in a dissimilar colour. The second curve line is also rendered vertically inverted in relation to the original line curve and can be moved about the display means 9, by way of the cursor and mouse device 11. The operator can readily judge the symmetry of a given parameter by visually assessing the degree of correlation between the original and the inverted line curves. The operator has the option of referring to a calculated coefficient of con-elation that is automatically provided by the processing apparatus 8 of the invention.
The visual display means 9, can be toggled by a command key on the keyboard device 22 or by means of an on-screen cursor command, to display muttiple pages. One page provides the operator with an image of plotted line curves as well as several aids to analysis that include statistical variance, the magnitude and direction of slope, and the degree of symmetry, Another page, as depicted in Figure 3, can accept input from an operator. The input page consists of several arrays of radio buttons than can be selected or unselected by the operator, either by means of the cursor and mouse device 11, or by means of the keyboard device 10. Each button has a predetermined numeric value assigned to it. The operator can select only one of the radio buttons in any particular array.
The selection of any radio button automatically provides the associated numeric value to a simple arithmetic accumulator. The accumulator calculates the product of the numeric values arising from the multiple button arrays. The product in the accumulator can represent the probability that any given shale is a non-framework shale that is functioning as a pressure seal.
As an example, one array, consisting of a single set of 3 radio buttons can be reserved for the evaluation of the variance in the torque data derived from the strain gauge 16 mounted at forty five degrees to the axis of the tool 1. Only one button can be selected by the operator to represent a judgment that the torque data displays low, medium, or high variance within the shale bed. The accumulator can receive a value that diminishes as the degree of variance increases. As a further example, another array consisting of a single set of 3 radio buttons can be reserved for the evaluation of the central sonic spike seen in many non-framework shales. If no central spike is visible, a low value can be sent to the accumulator. If a well developed central spike is present, a larger value can be sent to the accumulator than if a poorly developed spike is found. If none of the buttons in any particular array are selected, that array can automatically be excluded by the processing apparatus 8 when the overall probability is calculated.