Apparatus and method for determination of bound water in subsurface formations

(74) Agent

Siv.ing. Rolf Larsen, Bryn & Aarflot A / S, Oslo.

(56) Cited publications

Norwegian (NO) application no. 780578 available

Norwegian (NO) Patent No. 144233

The present invention relates to the investigation of subsurface formations, and more particularly to a method and apparatus for determining bound water in formations surrounding a borehole.

Modern borehole logging technology has reached a point where a number of subsurface parameters, such as porosity and lithology, can often be determined with reasonable accuracy. However, a reliable technique for determining the permeability of formations (i.e., a measure by which lightness fluid can flow through a pore system) is not known. Resistance gradients have been used to estimate the order of permeability of formations, but this technique has been found useful only in certain types of formations. It has been suggested that a measure of the amount of free fluid in shale formations would be a good permeability indicator. A known technique for measuring free fluid is the nuclear magnetic resonance apparatus, but achievable signal-to-noise ratios tend to be a limiting factor for the performance of this apparatus.

U.S. Patent 4,063,151 to the same proprietor of the present patent discloses a technique for determining the amount of bound water in formations surrounding a borehole by measuring the dielectric constant of the formations at two different microwave frequencies. The difference between measurements at the two frequencies is used to determine the amount of bound water in the formations. In another U.S. patent 4,077,003, which also has the same holder, determinations of dielectric loss factors are made at two different frequencies and used to obtain information on the amount of bound water in the formations.

It is an object of the present invention to determine the water filling porosity in formations surrounding a borehole, without the use of measurements at two frequencies.

In connection with this invention, it has been discovered that the water filling porosity (amount of bound water) in formations surrounding a borehole can be determined from a measurement of the attenuation of electromagnetic microwave energy, at a frequency within a predetermined range propagated through the formations. Especially over a frequency range of about 0.9 GHz i

to about 1.3 GHz, and preferably at about 1.1 GHz, the water filling porosity is substantially proportional to the square of the measured attenuation constant of the electromagnetic microwave energy passing through the formations being investigated. Said relationship has been found to apply mainly even in the presence of a significant amount of unbound (free) water, usually sludge filtrate (preferably of relatively fresh sludge) which has replaced moving fluids washed away from the invaded zone in the formations.

The present invention is directed to a method and apparatus for determining bound-water filling porosity in formations surrounding a borehole. As used herein, bound-water filling porosity is meant as the fraction of a volume unit of formation (matrix plus fluid) occupied by bound water, with bound water being the portion of the total water absorbed or bound, or is held by the matrix. Slate usually contains bound water, and terms such as shale water are also sometimes used to denote bound water. In accordance with an embodiment of the invention, a first size discharge means is provided which is a measure of the attenuation of electromagnetic microwave energy passing through the formations being examined. This first magnitude may, for example, be the damping constant determined for the electromagnetic microwave energy propagated through the formations. Means are provided for producing a second size which is substantially proportional to the square of the first size. The second size indicates the formation water filling porosity of bound water.

More specifically, the invention is based on a method for determining bound-water filling porosity in formations surrounding a borehole, at which a first signal is derived which is a measure of the attenuation of electromagnetic microwave energy propagated through the formations. The new and distinctive feature of the method of the invention consists in generating a second signal which is substantially proportional to the square of the first signal, for registration as an indication of the formation bottleneck porosity of the formations. The invention also includes an apparatus as further specified in the claims.

Further features and advantages of the invention will become apparent from the following detailed description in connection with the accompanying drawings.

Figure 1 is a diagram, partly in block form, of an apparatus embodying an embodiment of the invention.

Figure 2 illustrates in simplified form the nature of the propagation of a lateral electromagnetic microwave in the formations.

Figure 3 is a block diagram of the amplitude comparator of Figure 1.

Figure 4 is a block diagram of the calculator of Figure 1. Consider a planar electromagnetic wave propagating in a loss medium. The propagation constant, V, for the wave is usually represented as

Τ '= ω V / U 8 \ / 1 + j (1) where uy is the angular frequency of the wave, is the permeability of the medium, ε is the dielectric constant of the medium and C is the conductivity of the medium. It is common to express the real and imaginary parts of the propagation constant as β and a, respectively, so that

T- β + yes (2) where β is a phase constant and a is the damping constant of the wave. (Note that the propagation constant is used in the well-known wave equation in the form e^J , so that the real part of the propagation constant becomes the imaginary part of the exponent, and vice versa. This explains why the imaginary part of the propagation constant is associated with loss.) Squaring the equations (1) and (2) and calculating the real and imaginary parts gives β - a = (3) and 2αβ = yuCit¹ (4)

In the above-mentioned US patents and in US patent no.

No. 3,944,910 discloses techniques for determining the phase and damping constants of the electromagnetic microwave energy propagating through subsurface formations. The particular phase and / or attenuation information is then utilized, for example using Equations (3) and (4), to determine the properties of the formations, such as their dielectric constant or their porosity. As noted above, measurements taken at two different frequencies can also be used to determine the amount of bound water in the formations. In the present invention, the bound water filling porosity of the formations is designated as being substantially proportional to the square of the measured attenuation constant of the electromagnetic microwave energy propagated through the formations. In equation form, this is expressed by =^K“² <⁵>

The free fluid index (FFI) is the fraction of the formation volume unit occupied by free (unbound) fluid, and it is ^{FFI =} * t tb ⁽⁶⁾ where is the total porosity of the formation.

In Figure 1 there is shown a representative embodiment of an apparatus according to the invention for examining subsurface formations 31 penetrated by a borehole 32. The borehole 32 is usually filled with a drilling fluid or mud containing finely divided solid particles in solution. The survey apparatus or logging device 30 hangs in the borehole 32 of an armored cable 33, the length of which mainly determines the relative depth of the device 30. The cable length is determined by appropriate means on the surface, such as a drum and winch mechanism (not shown).

The logging device 30 comprises an elongated cylindrical carrier 34, the inner portion of which has a fluid-tight housing containing the downhole electronics. Mounted on the support member 34 are a pair of curved springs 35 and 36. The spring 35 is mounted on a block member 37 which, among other things, contains a transmitter antenna T and vertically separated receiver antennas and R₂. Mounted on the spring 36 is a second block means 38 which may be a passive block which facilitates smooth vertical movement of the device 30 through the borehole. However, if desired, the block 38 may contain electrodes or similar auxiliary means for examining the surrounding formations. Electronic signals indicating the information provided by the logging device are transmitted through cable 32 to a calculator 85 and a printer 95 located on the ground surface. The particular means shown in Figure 1 for keeping the antennas in contact with the borehole wall are illustrative and it will be appreciated that other suitable means for this purpose, such as hydraulic means, may be used.

Figure 2 shows in simplified form the propagation of the electromagnetic wave to be measured with the apparatus of Figure 1. (For a more detailed description of the wave propagation and further description of logging devices of the type used here and known as an electromagnetic propagator or EPT, see to Figure 2, the block 37 is shown positioned against the side of borehole 32, which is filled with drilling mud as mentioned above. Generally, the fluid pressure in formations penetrated by a borehole is less than the hydrostatic pressure of the mud column in the borehole, so that mud and sludge filtrate flow somewhat into the formations. The formations tend to filter out the small particles so that a mud cake is formed on the walls of the borehole. In Figure 2, the block 37 is in contact with a mud cake 40 which is shown with excessive thickness for the sake of clarity.

The transmitting antenna T emits electromagnetic microwave energy in the formations as shown by arrow A. A resultant surface wave propagating in the formation is represented by arrow B and its extension, arrow C. The surface wave continuously throws energy back to the more loss-filled medium (the mud cake), and those parts of the energy that are ejected at the positions of the receivers and R₂is represented by arrows D and E. If the path lengths represented by arrows D and E are assumed to be substantially equal, it is seen that the difference in path length between the energy received at R 2 (via the path ABD) and the energy received at R₂ (via the road ABCE) is the distance represented by the arrow C, i.e. the distance between the receivers. Accordingly, a differential receiver arrangement permits examination of the portion of the formation approximately opposite to the line between R₂· Generally, but not necessarily, the examined formation will be the leached or invaded zone surrounding the borehole mud cake and containing fluids from the mud filtered through the mud cake. EPT-type devices used herein are particularly effective for investigating the invaded zone in a borehole drilled with slurry based on relatively fresh water.

Referring again to Figure 1, the electronics in the member 34 are shown adjacent to the borehole to facilitate illustration. A solid state oscillator 45 generates output energy in the microwave's microwave range. The microwave range is here defined as the range of frequencies between about 300 MHz and 300 GHz. Oscillator 45 operates at the appropriate frequency of 1.1 GHz, ie 1.1 x 10 periods per second. second. The output of the oscillator 45 is connected through an insulator 46 to the transmitter antenna T. Microwave energy is transmitted into the surrounding formations and propagates through the formations in the manner described above. The energy arriving at the receiving antennas and being respectively connected to the input terminals of mixing stages 47 and 48. The signals arriving from R 1 and R 2 are out of phase with each other to a degree which depends on the phase constant β and has an amplitude ratio which depends on the damping constant a. Secondary input terminals on the mixing stages are supplied with microwave energy at a frequency different from the transmitter frequency with a relatively low frequency usually in the radio frequency range. In the embodiment shown, a solid oscillator 49 supplies the mixing stages 47 and 48 with microwave energy at a frequency of 1,1001 GHz, or 100 kHz above the transmitter frequency. Therefore, the output signals 47A and 48A from the mixing stages 47 and 48 contain the 100 kHz differential frequency. In accordance with well-known principles, signals 47A and 48A contain the phase and amplitude ratios of the signals from and R 2, but the task of phase detection (usually performed in this type of logging devices but not necessary for the present invention) is much easier at the lower level. frequency of the mixed signals. To ensure that the differential frequency between the outputs of the two oscillators 45 and 49 remains 100 kHz, the oscillator outputs are sampled and fed to a mixing stage 50. The output of the mixing stage is received by a frequency stabilization circuit 51 which detects deviations from the 100 kHz standard and produces a correction signal 51 which controls oscillator 49 in the usual manner with a phase-locked loop.

The signals 47A and 48A are usually fed to a phase detection circuit (not necessary here and not shown) and to an amplitude comparator 54. The output of the amplitude comparator 54 is a signal level proportional to the attenuation constant a. A suitable circuit 54 to obtain a signal level which is proportion with a is shown in Figure 3. The signals 47A and 48A are respectively fed to logarithmic amplifiers 55 and 56 whose outputs are fed to the differential amplifier 57. The output of the differential amplifier 57 is a signal level proportional to a. This can be illustrated by representing the amplitude of the wave energy received at as Ae, where A is an amplitude constant and z is the distance separating T and R 2. It follows that the amplitude of the wave energy received at R₂ is Ae^-a^^{z + L}^, where L is the distance between receivers R1 and r₂“The relationship between the wave amplitudes at the two receivers is therefore _Ae-a (z + D _ - ^e

The logarithm of the relation between the wave amplitudes is therefore proportional to a. It will be understood that the circuit 54 in Figure 3 performs the same mathematical result by taking the difference between the logarithms of the wave amplitudes.

The output representing ot is transmitted to the surface over a conductor 54a which actually passes through the reinforced cable 33. The signal can typically be a DC voltage level which is magnified by amplification prior to transmission to the surface. At the ground surface, the signal on conductor 54a is fed to a calculating unit 85 which calculates the bound water filling porosity, δ, for the formations according to the ratio (5). A signal representative of the total porosity island_t may also be the input to the calculator 85 which may then determine the free fluid index (FFI) of the formations in accordance with Equation (6). The calculated sizes are recorded by a printer 95 which is conventionally driven as a function of borehole depth by mechanical coupling to a rotating wheel 96. Wheel 96 is connected to cable 33 and rotates in synchronism therewith to move as a function of borehole depth. Thus, the bound water filling porosity and / or free water index in the formations is recorded as a function of the borehole depth of the printer 95.

Figure 4 is a block diagram of the calculator 85 receiving the signal on conductor 54a indicating the measured value of a. The signal representative of a is coupled to a squared circuit 91, the output of which is representative of a. This signal is then fed to one input of a multiplication circuit 92, the other input of which receives an adjustable input of a value denoted K. The output of the multiplication circuit 92 is thus a signal of a value Ka and representative of the bound water filling porosity, φ, of the formations according to equation (5). The output of the multiplication circuit 92 (conductor 85A) is connected to the printer 95 and also to the negative input terminal of a differential amplifier 93. The positive input terminal of amplifier 93 receives a signal representative of the total porosity of the formations being investigated, φφ. This last signal can be determined, for example, from other logging information, such as from neutron / density logging information. The output of differential amplifier 93 (conductor 85B) also connected to printer 95 is representative of the free fluid index of the formations, in accordance with Equation (6). As used herein, the bound water filling porosity and the free fluid index are defined as fractions of the total volume unit of the formation. In this sense, these terms are interchangeable with similar terms expressing the amount, volume or fraction of bound or free fluid in the formations.

The invention has been described with reference to a particular embodiment, but variations within the scope of the invention may be made by those skilled in the art. For example, while the circuits have been described for analog signals representing the desired sizes, it will be appreciated that a digital universal computer can be easily programmed to perform the techniques described. It should also be noted that the advantageous principles of known borehole compensation techniques and / or redundant treatment channels, such as described in U.S. Patent No. 3,849,721, may be utilized, if desired, in connection with the present invention. It will further be understood that the measured values, if desired, can be corrected for mud cake effects, scattering or temperature variations as known in the art. Although the disclosed embodiment further describes various sizes derived directly from a logging device, these can be sized.

late is alternatively derived from storage media or communicated from a log site.

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