Method for estimating rock brittleness from well-log data

Abstract

The invention describes a procedure for determining the shale brittleness index from data obtained in the well by at least three well-logging tools measuring corresponding parameters. Three tools, namely sonic, density and deep resistivity, are selected. The time interval signals from the sonic tool are converted to the P-wave velocity. The product of signals obtained from the sonic and density tools (P-wave velocity×Bulk density=Acoustic impedance (AI)) responds in the same direction to a variation of the volume of water and organic matter (OM) volume of the rocks, whereas the third tool (Deep Resistivity) reacts very differently in response to a change of one or other of these same components, in a three-pole diagram, with rock matrix, OM and water as the three components onto an Acoustic Impedance vs resistivity ratio function plane. The resistivity ratio function is the square root of the ratio between the water resistivity and the measured formation resistivity. The position of the curved matrix-water line with OM=0 fraction by volume is fixed connecting the rock matrix point with that of the water point. The slope of the matrix-water curve is controlled by the tortuosity factor ‘a’ that is selected for a formation zone considering the pore structure, grain size and level of compaction. The data points to be analysed can be calibrated accordingly by iterating the resistivity of water (Rw) and occasionally the tortuosity factor (a) parameter to obtain the Rw value. In a graph where the parameters used depend, for example, on the sonic velocity in the rock, the rock bulk density and on the electric resistivity of the formations, the iso quartz/calcite-content lines are denoted as iso-brittleness line as with an increase in quartz/calcite content, both organic content and porosity decrease, resulting in an increase in brittleness. These iso-brittleness lines form a set of parallel curved lines intersecting the matrix-water reference curved line. Brittleness is derived from that graph corresponding to each pair of values of the parameters measured in the well.

Claims

1. A method for quantifying the brittleness index of shales within a sedimentary basin using well-logging data measured in a well, comprising: using data provided by at least three well-logging probes measuring three different parameters, selected so that: a) The product of the velocity of sound obtained from one tool with the density data obtained from the second tool, hereby called acoustic impedance develop in the same direction in response to a volumetric change of the water, clay and organic matter content in the said sedimentary rocks, characterised by b) the third probe produces measurement signals hereby modified to a resistivity ratio function developing in opposite directions to each other due to the organic matter content variation, on the one hand, and the water content, on the other, in the same sedimentary rocks, and c) the three probes being further selected so that the resulting pairs within the acoustic impedance-resistivity ratio plane correspond to an equal brittleness, associated respectively with the said rocks comprising a given percentage of organic matter, rock matrix and water, are equal represented by one pair of values of the representative parameters of the pure organic matter, creating a system of sets of pairs of values of the acquired parameters, to obtain a continuous representation of the brittleness of the formations penetrated by the well, using equation $d)\mathrm{BI}=\frac{\frac{AI}{{\mathrm{Vp}}_{\mathrm{om}}}-{\rho }_{om}-\sqrt{\frac{{\mathrm{aR}}_w}{R_t}}\left[\mathrm{AI}\left(\frac{1}{{\mathrm{Vp}}_w}-\frac{1}{{\mathrm{Vp}}_{\mathrm{om}}}\right)-\left({\rho }_w-{\rho }_{\mathrm{om}}\right)\right]}{\left[\left({\rho }_{\mathrm{ma}}-{\rho }_{\mathrm{om}}\right)-\mathrm{AI}\left(\frac{1}{{\mathrm{Vp}}_{\mathrm{ma}}}-\frac{1}{{\mathrm{Vp}}_{\mathrm{om}}}\right)\right]}$ where V.sub.pma, V.sub.pom, and V.sub.pw are the P-wave velocities of the mineral matrix, organic matter (OM), and the pore fluid (water) respectively, ρ.sub.ma is density of mineral grains, porn is the density of organic matter, ρ.sub.w is density water, R.sub.t is deep resistivity, R.sub.w is the resistivity of water, ‘a’ is tortuosity factor, AI is acoustic impedance, and BI is the brittleness index in fraction.

2. The method of claim 1, wherein the measurements made by at least three well probes are employed, adapted for measuring the electric resistivity of the formation penetrated, the transit time of sound through the same ground, and the density of the said ground.

3. The method as claimed in claim 2, a resistivity ratio function is defined as the square root of the ratio between the resistivity of water and the total resistivity values obtained from the resistivity probe.

4. The method of claim 2, wherein measurements made by a well probe measuring the electric resistivity of the zone in the sub-surface and two other well probes measuring the transit time of sound and the density through this same zone, a representation diagram is chosen as a function of the resistivity ratio function and of the acoustic impedance where said system of sets of pairs of values of the parameters acquired, each associated with the same content, may be likened to a set of parallel iso-brittleness curves, the brittleness associated with each pair of values of the acoustic impedance and of the resistivity ratio measured in the well then being determined by identifying the iso-brittleness curve passing through the point representative of said pair in the chosen representation diagram.

5. The method of claim 2, wherein the slope of the matrix-water curve is controlled by the tortuosity factor ‘a’ that is selected for a formation zone considering the pore structure, grain size and level of compaction.

6. The method of claim 2, wherein the resistivity of water is determined by iterating the resistivity of water while aligning the 100% water-saturated well data onto the acoustic impedance-resistivity ratio plane with the matrix-water reference curved line.

7. The method of claim 2, wherein measurements are used made by a well probe measuring the electric resistivity of the ground, and two other probes, one measuring the speed of sound within the ground and the other density.

8. The method of claim 1, wherein quantities from each pair of the parameters acquired in the well is demonstrated in a diagram as a function of coordinates, one measuring acoustic impedance in the rock and the other the square root of the ratio between the resistivity of water and the resistivity of rock, hereby called the resistivity ratio function, where the collection of pairs of values equivalent to a corresponding brittleness are manifested by a system of curved lines to which a given brittleness may be allocated, intersecting a reference curved line representing water-bearing matrix with zero fraction organic volume content.

9. The method of claim 9, wherein the positions of the iso-brittleness curved lines are determined between an axis with the 100% rock matrix member on one end and the 100% organic matter on the other end, both represented by the values taken by the two parameters.

10. The method of claim 1, wherein the pair of values typical of the pure organic matter, pure matrix and water are obtained from the existing literature.

11. The method of claim 1, wherein to obtain stochastic brittleness results, the distribution of input parameters comprising resistivity of water and tortuosity factor are to be fed in random fashion performing calculations using Monte-Carlo simulation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Other features and advantages of the invention will be better understood from the following detailed description and the attached drawings in which:

[0023] FIG. 1 illustrates typical wireline log data acquisition for subsurface sonic interval time, rock bulk density and resistivity determination;

[0024] FIG. 2 shows a set of iso-brittleness lines and a matrix-water reference curve in a three-pole diagram onto AI—Resistivity ratio plane;

[0025] FIG. 3 A-B illustrates the plotting of the set of pairs on the same diagram of values of the parameters acquired in a well by three well-logging probes before the Rw calibration (A), and after Rw calibration (B);

[0026] FIG. 4 depicts an example of the graph obtained experimentally, which shows continuous brittleness values obtained by the present method of the invention;

[0027] FIG. 5 is a flowchart showing elementary steps in one embodiment of the present inventive method.

DETAILED EXAMPLES

[0028] The method of the invention comprises the use of data acquired by well-logging tools making it possible to separate the influence of ductile organic matter and the brittle mineral matrix such as quartz and carbonates and, thus, to estimate its volume percentage within sedimentary rocks. The increase of volume percentage of a brittle mineral results in a decrease in organic mineral content and overall porosity. So theoretically a brittleness index can be defined with zero fraction value at 100% organic matter pole, and 1 fraction value at 100% quartz/calcite (with zero porosity) pole.

[0029] Shales and source rocks usually consist of three components: (1) the rock matrix (clay+quartz/carbonate grains), (2) the Solid organic matter, and (3) the fluid(s) within the pore space (water or oil/gas). Non Source rocks are composed primarily of only two components: the matrix and the fluid filling the pore space. In immature source rocks, organic matter and rock matrix make up the solid fraction, and formation water fills the pore space. As the source rock matures, a part of the solid organic matter is converted to liquid (or gaseous) hydrocarbons that occupy the kerogen/organic matter pore space.

[0030] Data obtained from the wellbore may include so-called “well log” data. Such data are typically recorded and presented against depth in the subsurface of various physical parameters measured by probes lowered into the wellbore. Such probes may include, for example, electrical resistivity, acoustic interval time, bulk density, neutron slowing down length, neutron capture cross-section, natural gamma radiation, and nuclear magnetic resonance relaxation time distribution, among others. The well logging procedure comprises a recording of magnitudes of various above mentioned physical properties within a bore-hole using an array of logging probes (FIG. 1, 11), attached with a logging cable (12) connected on the other end to a data recording cabin (13).

[0031] The method of the invention contains first of all the selection of three well-logging probes appropriate for predicting the magnitude of organic matter by volume within a rock matrix. The response of well-logging tools is dependent on the properties related to the components as well as their respective percentage in the rocks investigated. The tool measuring the sonic transit time through the formations is sensitive to the water, organic matter and volume of matrix content. The probe measuring the density is sensitive to water and to the organic matter and the void spaces/porosity between the matrix grains. The tool that measures the electric resistivity of the rock makes slight discrimination between the wet clay and the saline water as both are conducting agents, and no discrimination for variations in the composition of the matrix if the conducting minerals are not in a continuous phase. The product of density with sonic derived velocity is called acoustic impedance. We used acoustic impedance values as a combined augmented response of the sonic and density probes within the method of the invention. A function namely resistivity ratio function was introduced within the method of the invention. The resistivity ratio function was defined as the square root of the ratio between the resistivity of Formation water and the total resistivity measured by the resistivity tool.

[0032] In the shale containing low organic matter content, the two curves, i.e. acoustic impedance and resistivity ratio, respond to rock porosity. But in organic-rich shales both the acoustic impedance and resistivity curves respond due to two main effects: 1) the acoustic impedance curve responds to the presence of low-density low-velocity kerogen, and 2) the resistivity ratio curve responds to the porosity and formation fluid. When maturity in a high-TOC shales/source rocks is low and no hydrocarbons have been generated, both the acoustic impedance and resistivity ratio response is caused only by the porosity response to low density and/or low-velocity TOC. Conversely, when maturity is high in such organic-rich rocks, the resistivity response increases due to the generated hydrocarbons. Since the generated hydrocarbon stays within the pores of organic matter, assumption to include this hydrocarbon volume with the organic matter, and considering the porosity equal to the volume only filled by water simplifies the process of isolating the organic matter volume. In an organic-rich rock, 100% matrix content with zero porosity, or 100% organic matter with zero porosity is assumed to yield infinity resistivity, resulting in zero resistivity ratio values. On the other hand at water pole, the resistivity of water (R.sub.w) theoretically becomes equal to the total resistivity (R.sub.t) resulting in a resistivity ratio value of 1.

[0033] The two properties obtained from the well log data are chosen also so that the collection of pairs of values of acquired parameters (namely the acoustic impedance on the one hand and the resistivity ratio function on the other) at least partly correspond to the equal quartz/calcite content volume for sedimentary rocks comprising a given proportion of organic matter or water are substantially identical.

[0034] This selection of petrophysical parameters substantially simplifies the operation for estimating the brittleness. In a cross-plot of the two chosen properties, the collection of pairs of values of the said parameters are spread over iso-quartz/calcite content curves, which are akin to brittleness curves. A diagram may be drawn where the iso-brittleness curved lines represent various matrix-organic matter ratio.

[0035] The baseline represented by the X-axis along the resistivity ratio function (√{square root over (R.sub.w/R.sub.t)})=0 was assumed to be having infinity resistivity and zero porosity. If we assume the rock consists of quartz or calcite matrix at one extreme (FIG. 2, 21), organic matter (OM) at another extreme (23), and water-filling the matrix porosity then collection of pairs of values of the parameters serving as reference which is represented by the iso-quartz/calcite curved line equivalent to a given matrix percentage within a rock obtained experimentally from values of the two chosen parameters acquired along a well. In the diagram as a function of AI and √{square root over (R.sub.w/R.sub.t)} this reference line is either the curved reference line with 0% quartz/calcite content volume or zero brittleness line extending from the 100% organic matter pole (23) to which 0 fraction brittleness may be attributed.

[0036] This method of determining the R.sub.w to align the water-bearing matrix with 0% organic matter data points along the water-matrix reference line (24) implies that, among the zones crossed by the well, some is devoid of organic matter. This is possible if we assume the data pairs with the lowest resistivity ratio function values occasionally showing a trend partly parallel to the water-matrix reference line. It is also possible to verify the existence of such zones by comparison with geochemical and other petrophysical analysis results within a basin.

[0037] The water-matrix reference line that joins the 100% (or 1 fraction) matrix (21) with the water pole (22), is obtained by applying the relation:

[00001]$\begin{array}{cc}\sqrt{\frac{R_w}{R_t}}=\frac{{\rho }_{\mathrm{ma}}-\frac{\mathrm{AI}}{{\mathrm{Vp}}_{\mathrm{ma}}}}{\sqrt{a}\left[\mathrm{AI}\left(\frac{1}{{\mathrm{Vp}}_w}-\frac{1}{{\mathrm{Vp}}_{\mathrm{ma}}}\right)-\left({\rho }_w-{\rho }_{\mathrm{ma}}\right)\right]}& \left(1\right)\end{array}$

where V.sub.Pma and V.sub.Pw are the P-wave velocities of the mineral matrix and the pore fluid (water) respectively, ρ.sub.ma is density of mineral grains, ρ.sub.w is density of pore fluids that is water in this case, R.sub.t is deep resistivity, R.sub.w is the resistivity of water, ‘a’ is tortuosity factor and AI is acoustic impedance. The tortuosity factor ‘a’ controls the slope of the water-matrix curved line and may be selected in a formation zone depending on pore structure, grain size and level of compaction. The relevant constants may be taken from Mavko et al (2009) and vendors' logging chart books.

[0038] From the following function (equation 2) we are able to define a set of lines corresponding to brittleness from 0 to 1 fraction in the Acoustic impedance-resistivity ratio function plane (FIG. 2)

[00002]$\begin{array}{cc}\sqrt{\frac{R_w}{R_f}}=\frac{\frac{AI}{{\mathrm{Vp}}_{\mathrm{om}}}{\rho }_{om}-\mathrm{BI}\left[\left({\rho }_{ma}-{\rho }_{om}\right)-\mathrm{AI}\left(\frac{1}{{\mathrm{Vp}}_{\mathrm{ma}}}-\frac{1}{{\mathrm{Vp}}_{\mathrm{om}}}\right)\right]}{\sqrt{a}\left[\mathrm{AI}\left(\frac{1}{{\mathrm{Vp}}_w}-\frac{1}{{\mathrm{Vp}}_{\mathrm{om}}}\right)-\left({\rho }_w-{\rho }_{om}\right)\right]}& \left(2\right)\end{array}$

where V.sub.pom is the P-wave velocities of the organic matter (OM), ρ.sub.om is the density of organic matter and BI is the brittleness index in fraction. Rearranging the equation, brittleness can be calculated in fraction using the following equation:

[00003]$\begin{array}{cc}\mathrm{BI}=\frac{\frac{AI}{{\mathrm{Vp}}_{\mathrm{om}}}-{\rho }_{om}-\sqrt{\frac{{\mathrm{aR}}_w}{R_t}}\left[\mathrm{AI}\left(\frac{1}{{\mathrm{Vp}}_w}-\frac{1}{{\mathrm{Vp}}_{\mathrm{om}}}\right)-\left({\rho }_w-{\rho }_{\mathrm{om}}\right)\right]}{\left[\left({\rho }_{\mathrm{ma}}-{\rho }_{\mathrm{om}}\right)-\mathrm{AI}\left(\frac{1}{{\mathrm{Vp}}_{\mathrm{ma}}}-\frac{1}{{\mathrm{Vp}}_{\mathrm{om}}}\right)\right]}& \left(3\right)\end{array}$

[0039] The data is plotted using some initial value of R.sub.w and still, the Rw is unknown. Iterate the value of R.sub.w making the upper right part of the data representing the matrix to fall on the matrix-water with 0% OM reference line (FIGS. 3A&B). This obtained Rw is not the actual resistivity of water, but rather is apparent R.sub.w that compensates for the deviation from initial assumptions since the actual rock consists of water, clay, matrix, possible hydrocarbon in pores, and OM. The rock's resistivity responds to a more complex function depending on the porosity, R.sub.w, the volume of shale (V.sub.sh) and resistivity of shale (R.sub.sh) (Bessereau et al. 1991). Finally inputting the 325 value of Rw in equation 3, calculate the brittleness index (BI) as a continuous property along the penetrated depth (FIG. 4).

[0040] The procedure workflow which is followed so as to obtain the plot of the brittleness against depth is shown in FIG. 5.

[0041] In the case where geochemical analysis is available from the well, a comparison may be made between it and the logging data to obtain an average value of the resistivity of formation water ‘R.sub.w’ and subsequently the tortuosity factor 335 ‘a’ to apply on the data from other wells.

[0042] The resistivity of water (R.sub.w) and tortuosity factor (a), which are functions of rock depositional environment, mineralogy, organic matter type, and maturity may vary in nature within the same area. A stochastic approach employing Monte Carlo 340 simulations can be utilised to take into account the resultant TOC uncertainty. The input values of Rw, and a, in this case, will be fed randomly in the form of normal, or other suitable distributions.

[0043] The technical solution is only one embodiment of the present invention, to those skilled in the art, the present invention discloses a basic principle of the method and applications, very easy to make various types of modifications or variations, the method is not limited to the specific embodiments of the present invention described above, and therefore the manner described above are only preferred and is not in a limiting sense.

REFERENCES CITED

[0044]

TABLE-US-00001 PATENT DOCUMENTS WO 63725A1 October 2000 Michael John Wiltshire CN 104564037A April 2015 Shi Qiang, Chen   i. Peng, Xiao Yufeng, Zeng Qingcai,  ii. Liu Fengxin and Zhiyu Wang iii. Shuyin WO 136448A1 July 2018 Shubham Mishra

PUBLICATIONS

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