GB2092783A

GB2092783A - Apparatus and Method for Determining Characteristics of Subsurface Formations

Info

Publication number: GB2092783A
Application number: GB8111490A
Authority: GB
Original assignee: Schlumberger NV
Current assignee: Schlumberger NV
Priority date: 1977-10-07
Filing date: 1978-10-09
Publication date: 1982-08-18
Also published as: MY8500187A; IE781980L; NO783345L; BR7806647A; GB2092784A; AU4017878A; FR2405487B1; NL7810079A; DE2843871A1; IE831875L; AU529348B2; IE48034B1; GB2092783B; TR20684A; GB2092784B; MY8500171A; OA08260A; IE48033B1; CA1120543A; IE48035B1

Abstract

A method of determining a free water parameter of formations surrounding a borehole comprises determining a parameter, eg attenuation, of the water in a region of the formations where substantially all of the water present is free water, eg. a clean sand region. The water content of the formations at a particular depth level of interest is also determined. First and second signals representative of the parameter and the water content thus determined are then combined to determine the free water parameter at the particular depth level of interest.

Description

1 GB 2 092 783 A 1

SPECIFICATION

Apparatus and Method for Determining Characteristics of Subsurface Formations This invention relates to an apparatus and method for investigating subsurface formations and, more particularly, to an apparatus and method for determining a parameter of the formation water in formations surrounding a borehole.

The amount of oil or gas contained in a unit volume of a subsurface reservoir is a product of its 70 porosity and its hydrocarbon saturation. The total porosity of a formation, designated 0, is the fraction of the formation unit volume occupied by 15 pore spaces. Hydrocarbon saturation, designated Sh, is the fraction of the pore volume filled with 75 hydrocarbons, In addition to the porosity and hydrocarbon saturation, two other factors are necessary to determine whether a reservoir has commercial potential; viz. the volume of the reservoir and its producibility. In evaluating producibility, it is important to know how easily fluid can flow through the pore system. This depends upon the manner in which the pores are interconnected and is a property known as permeability.

To determine the amount of producible hydrocarbons in a formation, it is useful to obtain a measure of the bulk volume fraction of hydrocarbons displaced in invasion of the drilling mud during the drilling operation. During drilling, 85 the mud in the borehole is usually conditioned so that the hydrostatic pressure of the mud column is greater than the pore pressure of the formations. The differential pressure forces mud filtrate into the permeable formations. Very close to the borehole, virtually all of the formation water and some of the formation hydrocarbons, if present, are flushed away by the mud filtrate. This region is known as the "flushed zone". The bulk volume fraction of hydrocarbons displaced by invasion in the flushed zone is an indication of the amount of "movable" hydrocarbons in the particular portion of the formations. This bulk volume fraction of the hydrocarbons displaced by invasion can be expressed as Ot (Sh-Shr), where S., is the residual hydrocarbon saturation in the flushed zone (i.e., the saturation of hydrocarbons which were not flushed away by the mud filtrate and generally considered as immovable), The saturation of the mud filtrate, designated as S.0, can be represented as Sxo(1 -Shr) (1) for the bulk volume fraction of oil displaced by invasion, 6t(Sh-Shd' can be expressed as Ot(Sh-Shr)0t(Sxo-Sw) (3) Generally, relatively accurate determinations of Ot can be obtained using known logging techniques, so accurate determinations of S. and S are 0 W highly useful, inter alia, for determining the bulk volume fraction of hydrocarbons displaced by in vasion and, therefore, the fraction of producible hydrocarbons for particular formations sur rounding the borehole.

Classical prior art techniques exist for determining water saturation and/or related parameters. It has been established that the resistivity of a clean formation (i.e., one containing no appreciable amount of clay), fully saturated with water, is proportional to the resistivity of the water. The constant of proportionality, designated F, is called the formation factor. Thus we have

RO FR, where R. is the resistivity of the formation 100% saturated with water of resistivity Rw. Formation factor is a function of porosity, and can be expressed as a 0ni t where a and m are generally taken to be 1 and 2, respectively. Using these values, the true resistivity, designated Rt, of a clean formation containing hydrocarbons is expressed as Rw JR,:S n o2 W t (4) (5) (6) where n, the saturation exponent, is generally taken to be 2. Using the classic equation set forth, one conventional prior art technique computes a value, designated R 7 which is a computed "wet" resistivity value and assumes that the formation is fully saturated with water; i.e., Sw=1. From relationship (6), it can be seen that Rw Rr=z0 02 t In this computation, Ot may be obtained from The saturation of hydrocarbons in the uninvaded 100 logging information, for example from neutron 55 formations, designated Sh, can be expressed as Sh=(1 -SW) (2) where Sw is the water saturation of the formations; i.e., the fraction of the pore spaces filled with water. From the equations (1) and (2), it can 60 be seen that the previously set forth expression (7) and/or density log readings, and Rw may be obtained from local knowledge of connate water resistivity or, for example, from a clean waterbearing section of a resistivity log. The computed value of R. is compared with a measured value of resistivity, designated Rt, obtained, for example, from a deep investigation resistivity or induction log. In clean zones having no hydrocarbons RO will 2 GB 2 092 783 A 2 track Rt, but when R.' is less than Rt, there is an indication of the presence of hydrocarbons. Thus, by overlaying the computed wet resistivity (R.) and the measured resistivity (RJ, potential hydrocarbon bearing zones can be identified. From equations (6) and (7), it is seen that another way of using this information is to obtain a computed value of apparent water saturation, designated S ',from the relationship W R.' R 1 S 1= - "" FRO.

(8) Substantial deviations of S ' from unity also W indicate potential hydrocarbon bearing zones.

The described types of techniques are effective in relatively clean formations, but in shaly formations the shales contribute to the conductivity, and the usual resistivity relationships, as set forth, do not apply. Accordingly, and for example, the previously described overlay or R' and Rt can lead to P incorrect conclusions in a shaly section of the formations, and the overlay in these sections (as well as the determination of water saturation taken therefrom) is generally, of necessity, ignored. In addition to the results being less useful than they might be, this consequence can tend to 90 diminish the credibility of the entire computed log comparison and is a disadvantage when attempting to commercl - ally exploit the resultant information. Accurate determination of Sw can also be difficult in shaly sections. Of course, these are just limited examples of how shallness can interfere with measurement interpretation, but similar problems with shaliness arise in other situations, such as when invaded zone characteristics (like S..) are to be determined or when interpreting readings from thermal decay time logs in cased boreholes.

A number of techniques, of varying complexity, are in existence for aiding in the interpretation of results obtained in shaly formations. The manner in which shallness affects a log reading depends on the proportion of shale and its physical properties. It may also depend upon the way the shale is distributed in the formations. It is generally believed that the shaly material is 110 distributed in shaly sands in three possible ways; i.e., "laminar shale" where the shale exists in the form of laminae between which are layers of sand, "structural shale" where the shale exists as grains or nodules in the formation matrix, and "dispersed shale" where the shaly material is dispersed throughout the sand partially filling the intergranular interstices. Shaly-sand evaluations are typically made by assuming a particular type of shale distribution model and incorporating into 120 the model information which indicates the volume of shale or the like. For example, in a laminated sand-shale simplified model, an equation of the form of equation (6) is set forth, but includes a second term which is a function of the bulk- 125 volume fraction of shale in the laminae. The same is true for another known model wherein a term is developed which depends upon the volume fraction of shale as determined from a total clay indicator. In a dispersed shale simplified model, values are developed for an "intermatrix porosity" which includes all the space occupied by fluids and dispersed shale and another value is developed representing the fraction of that porosity occupied by the shale. Still another approach relates the conductivity contribution of the shale to its cation exchange capacity, this capacity being determined, inter alia, from the volume of clay.

The described prior art techniques, which require either a determination of the volume of shale or clay, or similar information, have been satisfactory in some applications. However, in addition to the difficulty of accurately obtaining information concerning the volume and composition of shale or clay and its conductivity, a further problem with prior art simplified models is that various forms of shale may occur simultaneously in the same formation. Reliable techniques, some of which use extensive statistical treatment of data, do exist and generally yield good results, but tend to be relatively complex and may require either powerful computing equipment and/or substantial processing time.

It is one object of the present invention to provide a solution to the indicated prior art problems and to set forth techniques which are effective even in shaly formations, but which are not unduly complex or difficult to implement.

Applicant has discovered that determination of a "composite" parameter of the formation water in formations surrounding a borehole, for example the composite conductivity of the formation water, allows a relatively accurate determination of formation characteristics, such as water saturation, the determined values being meaningful even in shaly regions of the formations. In contrast to past approaches which attempted to determine the volume of shale or clay present in the formations and then introduce appropriate factors which often involve substantial guesswork, applicants' technique determines a composite water parameter, for example a composite water conductivity, which represents the conductivity of the bulk water in the formations, including both free water and bound water. Bound water trapped in shales is accounted for in this determination, so unlike prior techniques, the shales can be considered as having a porosity. Having determined at each depth level, the composite water conductivity, water saturation can be directly obtained using relatively straightforward relationships which do not require estimates of the volume of shale in the formations. Shale effects are accounted for in the present invention by the different conductivities (or other parameter such as capture crosssections) of the formation water constituents (free and bound) which make up the total water. As used herein, "free water" is generally intended to 3 GB 2 092 783 A 3 mean water that is reasonably free to be moved under normal reservoir dynamics, whereas "bound water" is generally intended to mean water that is not reasonably free to be moved under normal reservoir dynamics.

According to one aspect of the invention, there 55 is provided a method of determining a free water parameter of formations surrounding a borehole, the method comprising the steps of:

deriving a plurality of measurement signals each representative of a respective characteristic of the formation surrounding said borehole; deriving from said measurement signals a first signal representative of said parameter in at least one region of said formations in which substantially all of the water present is free water, and deriving a second signal representative of water content in said formations; and determining said free water parameter from said first and second signals.

The invention also provides computing apparatus programmed or otherwise arranged to implement the method of the preceding paragraph.

1 he composite water conductivity, designated o-w'c is expressed by the following relationship:

Swb CrwcUwf±(0'wb-Uwf (9) S W where o-,,,f is the conductivity of the free water in the formations, u is the conductivity of the bound water in the formations Sw is the water saturation of the formations (which equals OW ot and SM is the saturation of the bound water in the formations (which equals The expression (9) apportions the composite water conductivity as between the conductivity of 90 the free water (the above-indicated first quantity) and the conductivity of a difference term which expresses the difference between the conductivities of the bound water and the free water (the above-indicated fourth quantity). Mathematical manipulation shows that another form of expression (9) is (YWI.

(Sw-Swb) SW SWb O'M + -Uwb (10) SW In this form, the composite water conductivity can be viewed as the sum of a first term, which represents the fraction of free water times the conductivity of the free water, plus a second term which represents the fraction of bound water times the conductivity of the bound water. As implied above, the fraction of free water, SjSw, (which is the unity complement of the bound water fraction-since the total water volume consists of the free water volume plus the bound water volum) could alternately be used in expressions (9) or (10). For example, the form of expression (10) would then be SWf (sw-swf) 01 rw, UW + - Orwb (1 Oa) S W SW which can be seen to be equivalent to 0 0) since S,,=Swf+Swb, Accordingly, when the term "fraction of bound water", or the like, is used in this context, it will be understood that its complement (the fraction of free water) could alternatively be employed in appropriate form.

Another composite parameter of the formation water is the composite water capture cross section, designated Y-,',c. As is known in the art, capture cross section is a measure of the fraction of thermal neutrons absorbed per unit time, and is typically measured using a thermal neutron decay time C'NIDT") logging device of the type described, for example, in U.S. Patent No. RE 28,477. The composite water capture cross section E ' is expressed herein as WC, Swb E,', =Y. (11) WC wf±-y-wb -1wd SW which is similar to expression (9), but where Zw, is the capture cross section of the free water in the formations and Y-M is the capture cross section of the bound water in the formations.

Relationships similar to (9) or (10) can be set forth in terms of a generalized parameter, "P", and utilized to obtain a free, a bound, or a composite water parameter, depending on what information is desired and what information is measurable or derivable. In particular, if it is desired to obtain a parameter of the free water, one can set forth the following generalized relationship which is similar in form to relationship (9) above swb P 1 Pwl:=pw'f±(pwb- Wd (9a) SW where Pwc is a composite water parameter, Pwb'S a bound water parameter, and P 'f is the free W water parameter to be determined. As will be described hereinafter, the free water parameter to be determined is in the form of a variable ow awf, defined as the signal attenuation attributable to formations when assuming that substantially all of the water therein is free water. Means are provided for deriving a function representative of the parameter (attenuation in this case) in at least one region of the formations (typically a 4 GB 2 092 783 A 4 capture cross section that can be compared to measured values of capture cross section. 60 Fig. 10 is a block diagram of a circuit useful in obtaining values of the invaded zone water saturation. Fig. 11 is a block diagram of a circuit useful in obtaining an alternate value of bound water saturation.

Fig. 12 is a block diagram of a circuit useful in obtaining a signal representative of the bound water fraction.

Fig. 13 is a frequency cross-p.lot useful in obtaining subsurface characteristic values.

Fig. 14 is a block diagram of a circuit for obtaining signals representative of free water attenuation, bulk free water attenuation, and EMP derived conductivity.

Referring to Fig. 1, there is shown a representative embodiment of an apparatus in accordance with the present invention for investigating subsurface formations 31 traversed by a borehole 32. The borehole 32 is typically filled with drilling fluid or mud wFich contains finely divided solids in suspension. The investigating apparatus or logging device 40 is suspended in the borehole 32 on an armored cable 33, the length of which substantially determines the relative depth of the device 40: The cable length is controlled by suitable means at the surface such as a drum and winch mechanism (not shown). Circuitry 5 1, shown at the surface, although portions thereof may typically be downhole, represents the overall processing circuitry for the various logging units of apparatus 40.

- The investigating apparatus 40 includes a suitable resistivitydetermining device such as an induction logging device 41. As is known in the art, formation resistivity or conductivity is indicated by the induction log readings, the measured conductivity being designated as ut. The downhole investigating apparatus also includes a sidewall epithermal neutron exploring device 42 having a source and detector mounted on a skid 42A. A device of this type is disclosed, for example, in U.S. Patent No. 2,769,918. Each count registered in the epithermal neutron detector is received by a processing circuit in the overall circuitry 51 which includes a function former that operates in well known manner to produce a signal ON which represents the formation porosity as determined by the neutron logging device. The investigating apparatus 40 further includes a formation density exploring device 43 for producing well logging measurements which can be utilized to calculate the bulk density of the adjoining formations, in known manner. In this regard, a skid 43A houses a source and two detectors (not shown) spaced different differences from the source. This arrangement of source and detectors produces signals that correspond to the bulk density of the earth formations as is described, for example, in the U.S. Patent No. 3, 321,625. The circuitry 51 includes conventional circuits which convert the clean sand region) in which substantially all of the water present is free water. Means are also provided for deriving a quantity representative of water content in the formations surrounding a particular depth location in the borehole. This quantity may be tp,, the travel time of microwave electromagnetic energy in the formations, which is dependent on water content. The free water parameter (in the form of the variable 0,, cvwf at the particular depth level is then determined from the derived function and the water content representative quantity. Measurements of attenuation and travel time are typically obtained using an "EMP" microwave electromagnetic propagation logging device.

In terms of the attenuation, a, the relationship 19a) can be expressed as Swb awcawf±(tlwb-awf) (9b) S..

where a,,b is the bound water counterpart of avf, 80 and aw,: is a -composite- attenuation for the actual formation water.

As will be described further hereinbelow, the "apportionment" of attenuation, as betweeen the free and bound water which is indicated by expression (9b) leads to a technique for determining the fraction of bound water, S,,b/Sw once the values of a, awf and aWb have been established. In particular, SwJS,, can be determined from Swb awc-tlwf (90 Vwf SW Cwb-L which follows directly from relationship (9b).

Further features and advantages of the invention-will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

Fig. 1 is a.simplified block diagram of an apparatus in accordance with the invention.

Fig. 2 is a block diagram of the computing module 60 of Fig. 1.

Fig. 3 is a block diagram of the computing 105 module 70 of Fig. 1.

Fig. 4 is a block diagram of the computing module 80 of Fig. 1.

Fig. 5 is a frequency cross-plot useful in obtaining subsurface characteristic values that can be utilized in the apparatus of Figure 1.

Fig. 6 is a log of values, including computed values, versus depth.

Fig. 7 is a block diagram of circuitry useful in obtaining a signal representative of apparent 115 composite capture cross section of subsurface formations.

Fig. 8 is a block diagram of a circuit useful in obtaining values of apparent water capture cross section of subsurface formations.

Fig. 9 is a block diagram of a circuit useful in obtaining signals representative cif a "weV GB 2 092 783 A 5 signals derived from the short and long spacing detectors to a computed bulk density. If desired, a caliper signal may also be applied in determining bulk density, as is known in the art. The resulting bulk density is applied to porosity computing circuitry within the block 51 which computes the porosity, as derived from the bulk density, in well known f3shion. The derived porosity is designated as 0, The investigating apparatus includes a still further device 44 which is a gamma ray logging device for measuring the natural radioactivity of the formations. The device 44, as known in the art, may typically include a detector, for example a scintillation counter, which measures the gamma radiation originating in the formations adjacent the detector. An output of circuitry 51 is a signal designated "GR" which represents the gamma ray log reading. Further devices may be provided, as required in accordance with variations of the invention as described hereinbelow. For example, a device 45 is available for obtaining measurement of the spontaneous potential ("SP") of the formations. This device may be of the type disclosed in U.S. Patent No. 3, 453,530, this patent also disclosing deep and shallow resistivity devices. Also, an electromagnetic propagation tool ("EMP-) 46 is available, and includes a pad member 46A that has transmitting and receiving antennas therein. Microwave electromagnetic energy is transmitted through the formations (typically the invaded zone) and formation characteristics are determined by measuring the attenuation and/or phase (or velocity) of received microwave energy. This type of logging tool is described in U.S. Patent No. 3,944,910. 100 Measurements indicative of attenuation, designated a, and of travel time (which depends on velocity), designated tpl, are available from this tooL Also, in the copencling U.S. Patent application Ser. Nos. 806,983 and 788,393, 105 there are disclosed techniques for obtaining an "EMP"-derived conductivity measurement, designated UEM, and for obtaining a measurement of bound water filled porosity, designated 0,,b Signals representative of these measurement 110 values are illustrated as being available outputs of circuitry 5 1. An NDT (Neutron Detection Tool) device 47, for example of the type disclosed in U.S. Patent No. RE 28,477, is also available and results in an output capture cross section value, Y_ 115 - from processing circuitry 51.

To keep the investigating apparatus 40 centered in the borehole, extendable wall engag,ng members 42B, 43B and 46B may be provided opposite the members 42A, 43A and 46A. For centering the upper portion of the investigating apparatus, centralizers 49 may also be provided. As noted, a borehole caliper can be combined with the arms which extend the skids and supply a signal representative of borehole diameter to the circuitry 51.

While all of the measurements to be used in practising the invention are shown, for ease of explanation in this illustrative embodiment, as being derived from a single exploring device, it will be understood that these measurements could typically be derived from a plurality of exploring devices which are passed through the borehole at different times.. In such case, the data from each run can be stored, such as on magnetic tape, for subsequent processing consistent with the principles of the invention. Also, the data may be derived from a remote location, such as by transmission therefrom.

One or more of the signal outputs of block 51 are illustrated in Fig. 1 as being available to computing modules 60, 70, 80 and 510. In the embodiment of Fig. 1, the computing module 60 generates a signal representative of an apparent composite water conductivity, designated or I WC01 consistent with the relationship (9). The computing module 70 is responsive to the signal representative of a'., and to the signal from WC block 51 (in particular a porosity-indicative signal), to generate a "wet" conductivity signal, orO. The computing module 80 generates a computed value of water apparent saturation, S,,, in accordance with a relationship to be set forth. The computing module 510 is utilized in the generation of free and bound water attenuation values and a signal representative of the bound water fraction. These signals, along with some or all of the outputs of circuitry 51, are recorded as a function of depth on recorder 90.

Referring to Figs. 2 and 3, there are shown embodiments of the computing modules, 60 and of Fig. 1. Initially, structural components of the modules will be described. The source of various signals, along with further rationale of the configurations, will then be set forth. A pair of difference circuits 601 and 602 are provided. The positive input terminal of circuit 601 receives the signal GR, i.e., a signal representative of the output of the gamma ray logging device 44. The positive input terminal of circuit 602 receives a signal designated GRyvbl which is a signal level representative of a gamma ray log level for the bound water of the formations being investigated.

The negative input terminals of both difference circuits 601 and 602 receive a signal level designated GR,,f, which is a gamma ray log level for the free water in the formations being investigated. The outputs of circuit 601 and 602, which are respectively GR-GR,,f and GRwb-GRwf, are coupled to a ratio circuit 603 which produces a signal proportional to the ratio of the output of circuit 601 divided by the output of circuit 602.

The output of ratio circuit 603 is a signal representative of Swbl i.e., the saturation of the bound water of the formations in accordance with the relationship G R-G Rwf SM GRwb-GIRwf (12) The output of ratio circuit 603 is coupled via limiter 604 to one input to a multiplier circuit 605. The other input to multiplier circuit 605 is the output of a difference circuit 606. The circuit 6 GB 2 092 783 A 6 606 receives at its positive input terminal a signal level representative of owbl i.e. the conductivity of the bound water in the formations being investigated. The negative input terminal of difference circuit 606 receives a signal level representative of u,,,f, i. e. the conductivity of the free water of the formations. This latter signal is also one input to a summing circuit 607 whose other input is the output of multiplier circuit 605.

The output of summing circuit 607 is a signal representative of the apparent composite water conductivity of the formations being investigated, i.e.

UwcoC'wf+Swb(o'wb-(Twf) (13) This expression is seen to be the same as the expression (9) above for composite water conductivity, uwc, except that Sw is assumed to be 1, which means that the result is an "apparent" composite water conductivity.

In Fig. 3 there is shown an implementation of the computing module 70 of Fig. 1 which is utilized 80 to generate a signal representative of u., i.e. the computed---wet-conductivity of the investigated formations. The circuitry 5 1 (Fig. 1) includes a porosity computing circuit 511 which is responsive to the signals representative of ON and 85 0.. The circuit 511 uses this information, in well known manner, to produce a signal generally known as OND that incorporates information from both the neutron and the density log readings to obtain an indication of formation total porosity, designated Ot. Techniques for obtaining OND are well known in the art, and a suitable neutron density porosity computing circuit is disclosed, for example, in the U.S. Patent No. 3,590,228 of Burke. It will be understood however, that any suitable alternate technique for obtaining Ot can be employed, including, for example, techniques that use other logging information, such as from a sonic log. The output of circuit 511 is coupled to a squaring circuit 701 whose output is accordingly 100 proportional to 02 t. This signal is, in turn, coupled to one inputterminal of a multiplier circuit 702, the other input to which is u 'c., i.e. the apparent W composite water conductivity as determined by computing module 60 (Fig. 1, Fig. 2). Accordingly, 105 the output of multiplier circuit 702 (which is also the output of computing module 70-Fig. 1), is a signal proportional to U... multiplied by 02, and is -thus indicative of the computed "wet" conductivity of the formations, u., in accordance 110 with a relationship analogous to (7) above; viz.:

U01=U 1 02 WC0 t (14) The manner in which the inputs to computing 115 module 60 can be developed will now be described. In particular, one preferred technique for obtaining values of S owb and uwf is as follows: Log values of u, GR and Ot are initially obtained over a depth range of interest. Using the 120 measured conductivity, ut (which is preferably obtained from a deep resistivity measurement), one can compute, at each depth level over the range of interest, a value designated u,,, as U- U 1 wa 02 t (15) This is similar in form to relationship (7) above, and it is seen that Owa is a simple computed apparent water conductivity [not to be confused with the apparent composite water conductivity, o-w'e developed in accordance with relationship (1 31; that is, it is the computed value of water conductivity that would be expected in order for the obtained conductivity measurement (a,) to result from the obtained total porosity measurement, assuming that the total porosity is water-filled (viz. assuming that Sw=l). Stated another way, a formation of porosity Ot which is filled with water of conductivity a' would Wa (according to the basic Archie relationship) result in the measured formation conductivity o,, If desired, a computing circuit of the type employed in Fig. 3 (which uses another form of the relationship to develop u' from a,',, 0.) could be utilized to obtain a,,. in accordance with relationship (15) by substituting at as the conductivity input to multiplier 702. Having obtained u,,,',, at each depth level over the depth range of interest, the inverse of these values can now be utilized, in conjunction with gamma ray (GR) log readings taken over the same depth range, to generate a frequency cross-plot of the type illustrated in Fig. 5. Frequency cross-plots are commonly used in the well logging art (see, for example, Schlumberger "Log InterpretationVolume 11", 1974 Edition). At each depth level, the values of lluw' " and GR result in a point on the cross-plot. When all points have been plotted, the number of points which fall within each small elemental area (of a selected size) on the plot are summed and presented numerically. The resultant plot is as shown in Fig. 5, with the numbers thereon representative of the frequency of occurrence of points at each particular elemental area on the plot. In the illustrated example, the region designated by enclosure 501 contained the highest concentration of points (i.e. more than five points at each elemental area), so the frequencies of occurrence within this region are omitted for clarity of illustration. The position on the GR axis designated as GRf is indicated by the line of lowest gamma ray readings on the plot, as shown in dashed line. The position of the GR axis designated as GRyvb is indicated by the GR value at which increasing GR no longer results in increasing values of 1 lor '.. This means that a W GRwb essentially all the water in the formations is bound (typically by whatever shaliness is present). Any further shaliness or increases in the volume of clay would mean an increase in GR, but would not increase the bound water fraction since essentially all water present was indicated as bound at the GR,b line. The fraction of bound water is then determined by interpolation 7 GB 2 092 783 A 7 between the reference lines GRwf and GR that is, as GR-GRwf S""b=GRM-GRwf (16) The line on the l lu 1, axis at which l lu 1. no W W longer varies substantially with GR (beyond GRwb) is indicative of 1/0'wbI since, as previously noted, at this point on the plot essentially all of the formation water is bound. Accordingly Uwb'S derived from the dashed line labelled with this designation. Applicant has found that OrM 'S substantially a constant and has a value of about 7mhos/m at 751C. It is not, however, considered a universal constant and may vary somewhat in different regions. In any event, it is determinable from e.g. the cross-plot of Fig. 5. The value of the free water conductivity uwf can be obtained, for example, from the free water dashed line on the Fig. 5 plot. Alternatively, as is known in the art, uwf can be obtained from a clean sand section of a 75 resistivity log or from local knowledge. It will be understood that alternate techniques can be utilized to obtain at least some of the values considered herein.

With values of GRwf, GR owf, and uwb having been established for the depth range of interest, corresponding signal levels can be input to the computing module 60 (Fig. 2). Now, log values of GR (as a function of depth can be input to module 60 and a,,' c . can be output and recorded (if desired) on a dynamic basis. At the same time, the computing module 70 (Fig. 3) generates u.' as an output to recorder 90. This signal can now be overlayed with u, to great advantage in identifying potential hydrocarbon bearing zones.

Fig. 6 illustrates the nature of the signals which can be recorded by the recorder 90 in the embodiment of Fig. 1. The vertical axis represents depth. The middle track shows the inverses of or.

(dashed line) and u, (solid line); i.e., the computed "wet" resistivity and the measured deep resistivity respectively. The regions of divergence of these curves, for example the regions designated 2 and 3, indicate that the measured deep resistivity is substantially greater than the 45 computed---wet-resistivity (or, conversely, that the measured deep conductivity is substantially less than the computed "wet" conductivity), thereby indicating that they are potential hydrocarbon bearing zones. The left hand track 50 illustrates the output of a spontaneous potential (SP) log over the same depth range. Relatively stable value of the SP, for example in the regions designated 4 and 5 are at the shale baseline and characteristic of shaly regions. It is seen that the 55 resistivity curves general iy track each other even 100 in the shaly zones, as should be the case for water-bearing shale regions. This continuous tracking of the measured and derived resistivity signals is an important advantage of the present 60 invention since comparable prior art techniques 105 are generally unreliable in shaly regions, as discussed in the Background section hereof.

The determination of a computed value of water saturation, Sw, will now be considered.

Relation (9) above indicated that the composite water conductivity, uw, is expressed as:

GFWC(Twf+ Swb (0wb-o'wf) (9) SW From equation (6) we can write Grt=uWS 202 W t (17) where u,,, is the (unknown) actual conductivity of the formation water. Substituting the expression for composite water conductivity (uwc) for u,, in (17) gives:

S202 oiuwc W t SM =02S 2 t W1UVVf±(CrWil-UJ1 SW 2S2 W Or f+02 WS t W ts Jo'wb_UM) (18) The apparent water conductivity (as described in conjunction with Fig. 5) is equal to Orj02. Substituting into (18) gives t U la=S 2 which can be rewritten as:

[U IS2+ 1 M W Swb(Uvvb-o'wf)JSW-UWao (20) This quadratic equation can be solved for Sw to obtain:

Orj12+ 4Gwfu 'a-SWb(UWb-o'wf) VSwb(o'wb- W SW= 2uwf (21) From relationship (2 1) it is seen that a value of water saturation, obtained using the composite (free and bound) water technique of the pre"nt invention, can provide meaningful information even in shaly regions, since the effects of the shales in binding a portion of the formation waters is accounted for in the relationship. Accordingly, the prior art technique of estimating an appropriate "cementation" exponent for shaly formations is obviated.

Fig. 4 illustrates an implementation of the computing module 80 utilized to generate a signal representative of computed water saturation, designated Sw, in accordance with relationship (21). The signal representative of ---true-or measured resistivity, ut (Fig. 1), is one input to a ratio circuit 811. The other input to ratio circuit is the output of a squaring circuit 812 whose input is a signal representative of Ot. Accordingly, the output of ratio circuit 811 is proportional to utlogt, which equals the apparent formation conductivity, u',,. This signal is, in turn, W 8 GB 2 092 783 A 8 coupled as one input to a multiplier circuit 805 whose other input is a signal representative of u,,. The output of multiplier 805 is coupled, with a weighting factor of 4, to one input of a summing circuit 804. The signal uwf is also coupled to the negative input terminal of a difference circuit 801, the positive input terminal of which receives a signal representative of owb. The output of difference circuit 801 is one input to a multiplier 802. The other input to multiplier 802 is a signal representative of SM, which may be derived, for example, from the output of the limiter 604 of Fig.

2. Accordingly, the output of multiplier 802 is a 65 signal representative of Swb(o'wb-ovvf). This signal is coupled to a squaring circuit 803 and to the negative input terminal of a difference circuit 807.

The output of squaring circuit 803 is coupled to the other input terminal of summing circuit 804 whose output is, in turn, coupled to a square root 70 circuit 806. The output of the square root circuit 806 is coupled to the positive input terminal of difference circuit 807. The output of difference circuit 807 is coupled to one input of a ratio circuit 808, the other input of which receives the 75 signal representative of u., this signal being afforded a weighting factor of 2. The output of ratio circuit 808 is the desired signal representative of Sw, in accordance with relationship (2 1). The right track of Fig. 6 80 illustrates the recorded values of the computed water saturation, S,,.

The determination of a composite conductivity and determination of water saturation, in accordance with the principles of the invention, applies equally well in the invaded zone of the formations. In the relationships (9) and (18) for example, the quantity of u,,,f would be replaced by a., (i.e. the conductivity of the invading mud filtrate) and the water saturation S,, would be replaced by the invaded zone saturation S... The EMP logging device referred to above measures characteristics of the invaded zone. In the abovereferenced U.S. Patent Application Serial No. 788,393, a technique is disclosed for measuring OWb using an EMP logging device. This technique can be utilized as an alternate herein for obtaining Swh from SwbO,J0t' In another abovereferenced U.S. Patent Application Serial No. 806,983 it is disclosed that conductivity as measured using an EMP device, and designated OrEMP, is related to the conductivity of the formation water, u, as a linear function of waterfilled porosity, 0,,, Le.:

UEMPow Ow (22) Since S,,=Ow/ot and S,,, relationship (22) can be expressed as:

UEMPOt SW GW Substituting the expression (9) composite water conductivity for a,, into (23) gives:

S,,b UEMPOt Sw [0'wf±(Uwb-Uwf)l SW =Ot SW UM+Ot SWJUM-(rwd (24) Substituting anf for o-wf and S', for Sw and solving for S'. yields X X O,EMP/Ot-Swb(t7wb-o-mf) S[ X0 Umf (25) Referring to Fig. 10, there is shown a block diagram of a computing module 80' suitable for obtaining a signal which represents the computed invaded zone water saturation, S'O, in accordance X with relationship (25). A ratio circuit 111 receives as one input a signal representative of oEMp, and as its other input a signal representative of Ot. The signal UEMP may be derived from the EMP device 46 (Fig. 1) by using processing circuitry 51 as disclosed, for example, in the above referenced copending U.S. Patent Application Serial No. 806,983. Another ratio circuit 112 receives as one input a signal representative of Owb, and as its other input the signal representative of total porosity, Ot. As noted just above O.M can be derived from the measurements taken with an EMP logging device and, in this example, is utilized, in conjunction with ot, to obtain S,,,, (the output of ratio circuit 112). It will be understood, however, that Swb can be obtained using alternate techniques, such as those described herein. A difference circuit 113 receives as its input the signals representative of UM (which may be obtained as indicated above and is typically, although not necessarily, about 7mhos/m at 750C) and o-,,,f. The output of ratio circuit 112 and difference circuit 113 are coupled to a multiplier circuit 114 whose output is therefore Swb(Uvvb-Ujlif). The output of ratio circuit 111 and multiplier circuit 114 are coupled to still another difference circuit 115. The output of difference circuit 115 is therefore seen to represent the numerator in expression (25). This output, and the signal representative of umf, are the inputs to another ratio circuit 116, whose output is seen to be representative of S.'., in accordance with expression (25). This signal can be recorded, in the manner of the illustration in Fig. 5.

The spontaneous potential measurements from SP device 45 (Fig. 1) can also be used, for example, as an alternate technique for obtaining values of Swb, The SP measurement can be expressed as SP=K log,, S,, U' WC Scr r 0 Mf (26) where K is a constant dependent upon absolute (23) 110 temperature and o-'f isa composite conductivity m for the invaded zone mud filtrate, similar in form to u'c as expressed by relationship (9). Using W relationship (9) as a basis, we have:

9 GB 2 092 783 A 9 Sw U ',:=S,, Uwf+Swb((yvvb-t7wf (27) W and Sxo CTmfSxo (Tmf+Swb(OFwb-Ormf) (28) 50 Substituting (27) and (28) into (26) and rearranging gives SW CFM Ulb SX0 1 OSP/K-- Sb Umf Umf 1TWIF 0-1 OSPIK) + 1 OSPIK - - Umf In a water-bearing region of the formation where S,,.=Sw relationship (29) reduces to:

1 0 SM 1 l+p C'wbl /GrMf(l -1 OSP/K) where v (30) 1 oSPIK-orwornf Therefore, the relationship (30) can be utilized (taking SP from a waterbearing region) as an alternate technique for obtaining Swb Fig. 11 illustrates circuitry that can be utilized to obtain a signal representative of Swb in accordance with relationship (30). The combination of ratio circuit 12 1, antilog circuit 122, difference circuit 124 and multiplier 126 are used to obtain the numerator, while ratio circuit 123, antilog circuit 75 122, and difference circuit 125 are used to obtain the denominator of v. The ratio circuit 127 then yields v and summing circuit 128 and inverter 129 are used to obtain a signal representative of Swb' In the previously described embodiments, the determined composite parameter of the formations has been the composite conductivity (or resistivity). Another composite parameter which can be determined is the composite capture cross section, as obtained using an NDT log plus inputs corresponding to those indicated above. As is well known, the NDT is particularly useful in cased holes where resistivity logs cannot be used. In such case, the relationship (11) as set forth above is:

SWb 1: ±(Ewb-y-) (11) W Wif M SW An apparent composite capture cross section, designated Y_ ', can be obtained in the same WCO manner that u'c W. was developed above, and by using the computing module 60' illustrated in Fig. 7. In Fig. 7, the multiplier 705, difference circuit 706, and summing circuit 707 operate in the same fashion as the corresponding units 605, and SWb can be obtained by cross-piotting YE against GR in the manner described in conjunction with Fig. 5. The only difference is that instead of using relationship (15) to obtain a computed apparent water conductivity, an apparent water capture cross section, 2: ', to be W plotted against GR, is obtained from the known relationship E-YME1 E", -+Y-ma W, (31) ot (29) 55 where Y-ma is the matrix capture cross section for the particular lithology encountered. The circuitry of Fig. 8, including difference circuit 881, ratio circuit 882 and summing circuit 883, can be employed to obtain Zvva in accordance with relationship (31). After plotting -pw'a against GR, Ewf and lwb can be determined, for example, as indicated in conjunction with Fig. 5, Swb can be obtained using the arrangement of circuits 601, 602, 603, 604 of Fig. 2, as described in conjunction therewith. Having determined YE WC01 one can now compute a---wet-capture cross section (analagous to u,, obtained using relationship (14) (above) from:

7-0=01 YW1.0+0 (32) The circuitry of Fig. 9, including difference circuit 901, multipliers 902 and 903, and summing circuit 904, can be utilized to generate a signal representative of E.'. This signal can then be overlayed with the measured log value, YE, in the manner illustrated in the central track of Fig. 5, to reveal potential hydrocarbon bearing zones.

A further composite parameter which can be expressed by the generalized relationship (9a) is attenuation, a, i.e. the relative attenuation (typically corrected for temperature and spreading loss) measured by the microwave electromagnetic propagation tool ("EMP"-46 of Fig. 1). The relationship for this parameter is set forth above (9b), and will be considered momentarily. First, and consistent with the teachings of US Patent No. 4, 092,583, consider that the measured attenuation of the bulk formation (designated a) can be expressed as a=Ow aw.+(' -0Jam (33) where aw, is the attenuation attributable to the formation water (i.e., its composite water, in accordance with the teachings hereof) and a,,, is the attenuation attributable to the formation matrix. Since am is very small compared to aw, one can write a=Ow awc (34) This relationship expresses that the bulk formation attenuation is volumetrically -adjusted by a factor ow to take account of the fact that loss 606 and 607 of Fig. 2. Suitable values of YEf, lwb 100 is essentially occurring in that fraction of the GB 2 092 783 A 10 bulk formation occupied by the water. Returning, now, to relationship (9b), we have Swb awcawf±(0wb-tywf) (9b) SW where a,,,f is the attenuation attributable to the free water (i.e. the attenuation which one would measure with the---EMP-logging device in a theoretical environment consisting exclusively of the formation free water), tlvvb is the attenuation attributable to the bound water (i.e. the attenuation which one would measure with the ---EMP-logging device in a theoretical environment consisting exclusively of the formation bound water), and a,,,c is the attenuation attributable to the composite water (i.e. the attenuation which one would measure with a---EMP-logging device in a theoretical environment consisting exclusively of the actual formation water).

Solving relationship (9b) for the bound water fraction, SJS,, yields the relationship (9c) first set forth above:

S,,b awc-awf S,, Cwb-Cwf (9c) In the form of the present invention, a,,,f and a.b (or these parameters multiplied by water filled porosity, Ow, to obtain "bulk" variables Oawf and Otlvb) are determined using attenuation and travel time (or velocity) measurement taken with an electromagnetic propagation logging device such as "EMP- 46 of Fig. 1. The conductivity (generally of the formation invaded zone) obtained using the---EMP-device, designated uEmp, can be expressed as a t P1 O'EMP=_ K where K is a constant, tP, is the measured travel time through the formations, and a is the bulk attenuation determined from the measured attenuation corrected for spreading loss and temperature, where a=ow a,,c (relationship (34) above). While the relationship (35) for conductivity is expected to hold substantially independent of the salinity of the formation water, it has been observed that frequency uamp exceeds the conductivity measured from other tools. An explanation for the observed differences in conductivity is that not all of the losses represented by the bulk attenuation measurement a are due to the conductivity or salinity of the formation water. Extraordinary losses are believed to occur in the presence of bound water, these losses being more dielectric than conductive in nature. Applicant has discovered that treating bound water losses separate from the ordinary expected free water losses resolves the problem (35) 95 and produces more realistic values of UEMP In accordance with a feature of the invention, and as will be described, an attenuation representative variable is determined that is, inter alia, more appropriate for use in obtaining UEMP In the example below, this attenuation representative variable is the free water variable 0,,, crwf. The determined variable is also useful in conjunction with other techniques where attenuation is utilized as an input or a correction.

Referring to Fig. 12, there is shown implementation of the computing module 510 of Fig. 1 which is utilized to generate a signal representative of the bound water fraction, S,JS,, A pair of difference circuits 501 and 502 are provided. The positive input terminal of circuit 501 receives a signal representative of the quantity awe and the negative input terminal of circuit 501 receives a signal representative of the quantity awf. The positive input terminal of dircuit 502 receives a signal representative of the quantity awb, and the negative input terminal of circuit 502 receives the signal representative of the quantity cwf. The outputs of difference circuits 501 and 502 are respectively coupled to a ratio circuit 503 which produces a signal proportional to the ratio of the output of circuit 501 divided by the output of circuit 502. The output of ratio circuit 503 is accordingly a signal representative of the bound water fraction, SwJS, in accordance with relationship (9c). In actuality, and as will be clarified shortly, the inputs to computing module 5 10 may each have a common multiplier, Ow.

The manner in which the inputs to computing module 510 can be developed will now be described. In particular one preferred technique for deriving values of awf and awb (or, of related bulk attenuation variables ow cr,,,f and ow a, ,b) is as follows: Log values of a (attenuation) and tP, (travel time) are initially obtained over a range of depth levels of interest (e.g., using EMP device 46 of Fig. 1 -these outputs being indicated as being available from processing circuitry 51). The obtained values of a and tP, are cross plotted, as shown in the frequency cross plot of Fig. 13. The values of a may first be corrected for temperature and for spreading loss. The cross plot of Fig. 13 can be initially understood by recognizing that higher porosity generally results in higher values of both attenuations and travel time (at least, when that porosity contains water). This is because the water is much lossier than the rock matrix (thus: greater attenuation) and the velocity of the electromagnetic energy through water is lower than through the matrix (thus: greater travel time). Accordingly, increasing values of tP, and a on the cross plot generally correspond to increasing values of porosity. It can be noted that a could alternatively be cross-plotted against other nonconductivity related measurements reflecting total porosity,Ot, such as OND, previously described.

The point designated tP, on the tp, axis represents the travel time through the formation 11 GB 2 092 783 A 11 matrix. Two trend lines, designated as the "free water trend line" and the -bound water trend line" are constructed by starting at the point tpm and drawing lines through the approximate bottom and top edges of the main cluster of points on the cross plot. These trend lines can be understood in the following terms; In those portions of the formations containing substantially only free water, both tP, and a will increase with porosity, with the increase in travel time being dependent upon the volume of water and the increase in attenuation being dependent upon both the volume of water and its conductivity. Accordingly, the slope of the free water trend line will depend upon the conductivity 80 or lossiness associated with the free water. The same will generally be true of those portions of the formations in which substantially all of the water is bound water. However, in this case, attenuation will be a function of not only the volume of water and its conductivity, but also of the generally higher losses, included dipolar losses, associated with the bound water.

Accordingly, the bound water trend line usually has substantially greater slope than the free water trend line. It will be understood that these trends representing the relationships between attenuations and travel time in a substantially free water region (such as a clean sand) and a bound 90 water region (such as a shale) could be determined initially from logs taken in such formation regions. Also, it will be uderstood that these relationships are determinable functions which need not necessarily be linear, but are illustrated as being linear in the graph of Fig. 13.

Having established free water and bound water trend lines (or functions), one can now, at each depth level of interest, obtain a free water attenuation quantity representative of the attenuation attributable to the formations (surrounding the depth level of interest) as if substantially all of the water in the formations was free water. Similarly, one can derive a bound water attenuation quantity representative of the 105 attenuation attributable to said formations (surrounding the depth level of interest) as if substantially all of the water in the formations was bound water. Using these quantities, in conjunction with the measured attenuation at the depth level of interest, one can then determine the bound water fraction in the formations surrounding the particular depth level. With reference to Fig. 13, consider the illustrated individual point (a, tp,) and the vertical line drawn therethrough. At the particular measured value of t P11 the intersection with free water trend line indicates the attenuation value that one would have measured if the water in the pore snaces of this particular formation contained exclusively free water (i.e., 0, awf) whereas the intersection with the bound water trend line indicates the attenuation that would have been measured if the pore spaces of this formation contained exclusively bound water (i.e., Ow awb)' In 120 actuality, the measured attenuation (a=ow aw,) is an attenuation which has a value between these two extreme values, and the total water in the pore spaces can be considered as a composite water having attenuation awc. Accordingly, it is seen that relationship (9c) and the output of computing module 510 represents a linear apportionment between the two extreme values and yields the bound water fraction, Swb/Sw' (Note that the multiplier ow before each term will be cancelled in the output of computing module 510 if 0,, awc, Ow awf and OW awb are used as the input quantiti es.) In addition to the use of ow a,,f and Ow awb in obtaining the bound water fraction, the bulk formation attenuation if all the water was free water (i.e., 0,, aJ is useful, as first noted above, in determining ormp, since attenuation due to whatever bound water is present will not then result in an unduly high value of GrEmp, In particular, CrEMP can be determined from Ow awf tpi UEMP=_ K (36) which is a modified form of relationship (35) wherein the bulk free water attenuations (Ow awf) is substituted for the bulk composite water attenuation (Ow a,,,c which is the equivalent of the measured a in accordance with (34) above).

An alternative technique for obtaining the bulk free water attenuations, Ow am, is to use the apparatus of Fig. 14. A ratio circuit 431 receives at its inputs signals representative of a and Ow, both as determined from measurements taken with an EMP device 46 (Fig. 1) in a clean nonhydrocarbon-bearing region of the formations in which substantially all of the water present is free water. (The signal representative of Ow may be obtained, for example, using the technique of U.S. Patent No. 4,092,583). The ratio CVIOW, in this region, will be representative of awf in accordance with relationships (34) and (9b), where Swb=o for this case.In particular Swb aOw awcovv awf+O%-&-(Cwb-awf) (37) SW cv=O,, a,,f (when Swb=0) (38) so that etwf=crlo,,, when Swb=0. Having obtained the parameter crwf for the formations, the variable Ow awf (i.e., the bulk free water attenuation) can now be determined at a particular depth level of interest by multiplying the output of ratio circuit 431 by a signal representative of 0, at that depth level; this being implemented by multiplier circuit 432. A further multiplier circuit 433 can then be employed to obtain a signal representative of or,,p in accordance with relationship (36). It will be understood that analagous circuitry could be used to obtain a corresponding bound water parameter, awbI from information in a shaly region, and then the bulk bound water attenuation 12 GB 2 092 783 A 12 at specific depth levels of interest could be obtained using a multiplier circuit to produce a signal representative of Ow awb, The signals representative of Ow cr,,f and 6EMP can also be recorded, if desired, by recorder 90 of Fig. 1.

It can be noted, in the context of obtaining either the bound water or free water related values, that non-linear interpolation can be employed, if desired (e.g., in Fig. 13). Further, since tp, may be effected by residual hydrocarbons left in the formation near the borehole, the indicated attenuation corresponding to free or bound water conditions may be slightly inaccurate. However, since both tp, and a will decrease due to hydrocarbon effects, there is some compensation in the indicated bound or free 80 water saturations. When awf or 0,, a,,f is determined, the hydrocarbon effects will lower corresponding tp, values and will produce slightly lower awf values and hence, when applied in conductivity measurements, lower ump values. Use of a 0, measurement (relatively independent of hydrocarbon effects) in place of tp,, in the technique illustrated in Fig. 13, may be advisable in some instances.

The invention has been described with reference to particular embodiments, but variations of these embodiments will occur to those skilled in the art. For example, while circuitry has been described for generating analog signals representative of the desired quantities, it - will be understood that a general purpose digital computer could readily be programmed to implement the techniques as set forth herein.

Also, while conductivity values have been utilized for purposes of illustration, it will be recognized that the inverses of values utilized herein could be employed in conjunction with the inverse of conductivity; i.e., resistivity. Attention is directed to our co-pending United Kingdom Patent Application No. 39868/78, from which the present application has been divided and to our co-pending United Kingdom Patent Application 105 No.8 1, which was also divided from Application No. 39869/78.

Claims

1. A method of determining a free water 110 parameter of formations surrounding a borehole, the method comprising the steps of:

deriving a plurality of measurement signals each representative of a respective characteristic of the formations surrounding said borehole; 115 deriving from said measurement signals a first signal representative of said parameter in at least one region of said formations in which substantially all of the water present is free water, and deriving a second signal representative of water content in said formations; and determining said free water parameter from said first and second signals.

2. The method as defined by claim 1, wherein said second signal is derived from at least one 125 measurement in the formations surrounding a particular depth location in said borehole.

3. The method as defined by claim 2, wherein said free water parameter is the attenuation of electromagnetic energy attributable to the formations surrounding said particular depth location if substantially all of the water in said surrounding formations was free water, and wherein said first signal is derived from attenuation measurements.

4. The method as defined by any one of claims 1 to 3, wherein said second signal is derived from at least one travel time measurement.

5. The method as defined by claim 3, wherein said attenuation measurements are measurements taken with a microwave electro magnetic propagation logging device.

6. The method as defined by claim 4, wherein said travel time measurements are measurements taken with a microwave electromagnetic propagation logging device.

7. Computing apparatus programmed or otherwise arranged to implement the method of any one of claims 1 to 6.

New Claims or Amendments to Claims Filed on 18 Feb 1982. Superseded Claims 1 and 7.

New or Amended Claims I and 7-14:- 1. A method of determining a parameter of the free water in formations surrounding a borehole, said parameter being useful in the exploration for hydrocarbons, the method comprising the steps of:

deriving a plurality of measurement signals each representative of a respective characteristic of the formations surrounding said borehole, respective ones of said characteristics being related to said parameter in at least one region of said formation in which substantially all of the water present is free water, and to the water content in said formations; deriving from said measurement signals a first signal representative of said parameter in at least one region of said formations in which substantially all of the water present in free water, and a second signal representative of water content in said formations; and combining said first a - nd second signals in accordance with a function selected to produce an output signal representative of said free water parameter.

7. The method of any one of the preceding claims, further comprising the step of moving along the borehole an investigating device which incorporates measuring means for effecting measurements dependent upon said characteristics, in order to derive said plurality of measurement signals.

8. Apparatus for automatically determining a parameter of the free water in formations surrounding a borehole, said parameter being useful in the exploration for hydrocarbons, the apparatus comprising:

means arranged to produce a plurality of measurement signals each representative of a 13 GB 2 092 783 A 13 respective characteristic of the formations 25 surrounding said borehole, respective ones of said characteristics being related to said parameter in at least one region of said formations in which substantially all of the water present is free water, and to the water content in said formations; 30 means arranged to derive from said measurement signals a first signal representative of said parameter in at least one region of said formations in which substantially all of the water present is free water, and a second signal 35 representative of water content in said formations; and means arranged to combine said first and second signals in accordance with a function selected to produce an output signal representative of said free water parameter.

9. Apparatus as defined by claim 8, wherein said second signal is derived from at least one measurement in the formations surrounding a particular depth location in said borehole.

10. Apparatus as defined by claim 9, wherein said free water parameter is the attenuation of electromagnetic energy attributable to the formations surrounding said particular depth location if substantially all the water in said surrounding formations were free water, and wherein said first signal is derived from attenuation measurements.

11. Apparatus as defined by any one of claims 8 to 10, wherein said second signal is derived from at least one travel time measurement.

12. Apparatus as defined by claim 10, wherein said attenuation measurements are measurements taken with a microwave electromagnetic propagation logging device.

13. Apparatus as defined by claim 11, wherein said travel time measurements are measurements taken with a microwave electromagnetic propagation logging device.

14. Apparatus as defined in any one of claims 8 to 13, wherein said measurement signal producing means comprises an investigating device adapted to be moved along the borehole and incorporating measuring means arranged to effect measurements dependent upon said characteristics in order to produce said measurement signals.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa7 1982. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.