Process for the detection of heterogeneities and/or for the determination of petrophysical characteristics of geological strata of a medium to be explored

Process for the detection of heterogeneities of a medium. It is defined in that logs of the attenuation of the shear waves and STONELEY waves are produced, the variations of each attenuation log in relation to a mean value are determined, and then the variations are compared with one another in such a manner as to determine the nature of the heterogeneity or of the petrophysical characteristic which has produced these variations. Application in particular in the area of oil and more particularly in the exploration of a medium. FIG. 9.

The present invention relates to a process for the detection of 
heterogeneities and/or for the determination of petrophysical 
characteristics of geological strata or formations of a medium to be 
explored. 
The knowledge of the structure of a compact formation in which a deposit or 
reservoir is situated is progressively improved in order to define better 
the conditions of production of the deposit and the reserves of the 
deposit, as well as the level of recovery of the hydrocarbons, which are 
likely to be trapped in the compact formation. 
The determination of a certain number of characteristics is essential for 
the study and the analysis of the structure. Among these characteristics, 
it is possible to mention saturation, porosity, permeability and formation 
fracturing. 
The saturation and the porosity of a formation are very accessible by usual 
techniques which are not always easy to carry out. Nevertheless, they seem 
to be well-controlled and, in any event, they give satisfactory results. 
On the other hand, the permeability and the formation fracturing are more 
difficult to determine in situ, since they may be closely linked. This 
originates from the fact that the permeability due to the presence of 
fractures in the formation of the medium to be explored is, in general, 
very high and greater than the matrix permeability, the brief definition 
of which is the capacity of a non-fractured porous stratum to allow the 
passage of a fluid. 
In the course of the exploitation of a deposit containing hydrocarbons, it 
is very important to determine whether a fractured rock or stratum is 
present, in such a manner as possibly to improve the flow of the fluids of 
the deposit through the said rock or stratum of the formation. In fact, 
the determination of fractures existing in any particular zone is a factor 
which plays a part in the decisions to be taken in the course of putting a 
well into production. Thus, priority is given to the perforation of the 
fractured zones containing hydrocarbons, or it is desired to know the 
stresses in situ which are developed in certain zones, in order to decide 
on possible fracturing by mechanical means or means of any other type, 
such as hydraulic. 
Consequently, numerous processes and corresponding tools have been 
developed and proposed, which are capable of providing a precise 
determination of the fracturing of a formation. 
Among the non-acoustic processes, it is possible to mention that known 
under the name MEST. In this process, the measurements are carried out by 
electrical or electromagnetic logging, by means of a tool which is lowered 
into the well. However, the measurements are small in number, by reason of 
the duration of immobilization of the well during the measurements, and of 
the relatively long time taken to acquire and to process the data 
collected by the sensors of the tool. Furthermore, this type of 
non-acoustic logging can concern only the first few centimeters of the 
formations surrounding the well, and above all cannot be utilized in cased 
wells, by reason of the presence of a metallic casing. 
Sonic logs also lack suitability for determining the presence of fractures 
in the formations, for the simple reason that they do not differentiate 
between a genuine fracture existing in the formation and, for example, a 
microfissure situated in the wall of the well. In fact, such logs detect 
any abnormality of the wall which has absorbed part of the energy emitted 
by the emitters of the measuring tool. Moreover, it should be noted that 
the presence of a cavity or local deformation in the wall of the well is 
analysed in the same manner as a microfissure or a fracture. Under these 
conditions, recourse should be had to other measurements in order to 
determine whether the abnormality detected is due to any particular 
modification of the structure of the formations surrounding the well. 
Finally, sonic logs must be produced in uncased wells, since the 
processing of the signals which is utilized in these logs is inappropriate 
for cased wells. 
Another sonic process consists in utilizing that to which it is 
conventional to refer as the "jump of cycles". A "jump of cycles" is a 
transit time of the compression wave measured between two pickings of two 
different waves, one of the pickings (sic) being offset by at least one 
period in relation to the other. 
The jumps of cycle which are observed on the recordings are due to 
variations of amplitude between the two waves which are picked, it being 
possible for these variations of amplitude to be attributed to fractures 
when these exist, but likewise to cavities, to a defective picking, early 
or late, of the arrival of the compression wave, to noise, or to 
modifications of the lithological structure between transducers. It 
follows that this technique of the jump of cycle does not exhibit an 
absolute character of repeatability. In any event, it is not suitable 
either for cased wells. 
Finally, a set of interpretation techniques has been proposed, which is 
known under the name DETFRA which, in principle, is intended for the 
determination of fractures. By using a tool referred to as "ARRAY SONIC", 
the amplitude of the compression waves P and of the shear waves S is 
measured and calculated. In the presence of fractures, the wave P arriving 
at an inclined fracture gives rise to a shear wave, which will be called 
Sp for the sake of greater convenience. Consequently, the receivers 
receive the customary shear waves S plus those Sp generated by the 
compression wave P on the inclined fractures. The amplitude of the waves 
S+Sp which are received on the receivers is thus greater than that of the 
waves S alone when there are no fractures. As regards the amplitude of the 
wave P, this is smaller in the presence of fractures. A measurement is 
then made of the variation of the ratio of the amplitude P to the 
amplitude of S+Sp. When the ratio decreases, it is inferred from this that 
there are fractures. 
Nevertheless, this new process is limited in its applications. In effect, 
in order that a wave Sp should be created by a wave P, it is necessary 
that the fracture generating such waves Sp should exhibit a large slope. 
In the case of a sub-horizontal fracture, for example, there is no 
creating of a wave Sp, and the measurements then lead to a non-existence 
or absence of a fracture, although in reality there is one. In a zone of 
multiple fractures, there is no longer a single reflecting plane, as is 
the case for a single fracture, but a plurality of reflecting planes with 
various orientations. Under these conditions, it will be difficult, if not 
impossible, to determine the variation of the ratio of the amplitudes 
measured previously. Furthermore, it is known that other phenomena, such 
as inclined thin beds, fluid, etc., may lead to the generation of the 
waves Sp from the waves P, without there being fractures in the zone under 
consideration. Finally, the ratio of the amplitudes of the waves P/Sp+S 
may vary for reasons other than fractures. This is, in particular, the 
case in the event of a lithological change or a change of fluid content in 
the zone under consideration. Thus, it is difficult to accept that such a 
process is really capable of discrimination. 
A traditional technique for logging which permits determination of the 
physical characteristics of the formations surrounding and situated in 
proximity, in the order of one meter, to a drilled well consists in 
creating, by means of one or more emitter transducers, an acoustic energy 
which propagates in all or a part of the said formations before reaching 
one or more receiver transducers which supply signals which are recorded 
on a recording medium situated, in general, at the surface of the medium 
to be explored. The recorded signals are then processed in such a manner, 
on the one hand, as to be able to separate, in particular, the compression 
waves or P waves from the shear waves or S waves, and, on the other hand, 
as to calculate the differing average acoustic velocities of the said 
waves in the formations. 
A high degree of effort has been devoted to the improvement of the tools 
used in such logging and the processing of the recorded signals. 
In French application No. 2,431,710, there are described a tool and a 
process for acoustic logging which are commercially designated EVA, and 
which provide original solutions to the problems posed by the traditional 
tools and processes. 
French application No. 2,568,020 relates to a process for the processing of 
the recorded signals, which consists in grouping elementary 
intercorrelation functions in at least one family, in which the 
measurement spaces are included within a single predetermined reference 
space, in transforming the time variables of the said functions in order 
to bring the size of the measurement space to that of reference, and then 
in summing the transformed functions. 
These processes offer the considerable advantage of being able to separate 
all the waves received, including the STONELEY waves, in particular by the 
technique of correlation. Nevertheless, the specialists in acoustic 
logging were interested only in the compression waves P and shear waves S, 
the importance of which was fully known in the formulation of the logs of 
velocities which permitted an improved knowledge of certain petrophysical 
characteristics of the strata traversed by the well. 
Now, it became evident that the STONELEY waves or the pseudo-RAYLEIGH waves 
could be studied and could contribute to an improved knowledge of the 
formations traversed by a well. The work undertaken by Messrs. TOKOZ of 
the MIT and MATHIEU of ELF AQUITAINE led to the conclusion that the 
STONELEY wave is affected in the presence of very widely open fractures, 
the STONELEY wave being, in fact, very highly attenuated by the said 
fractures. The physical mechanism which links these concepts is based on a 
transfer of energy in the form of a passage or flow of fluid within 
permeable formations (SEG. Atlanta 1984). 
By pursuing its research on the various waves propagating in a medium, the 
Applicant has found a novel process for the detection of heterogeneities 
present in the formations surrounding a drilled well. 
A subject of the present invention is thus a process for the detection of 
the said heterogeneities and/or for the determination of the petrophysical 
characteristics of the said formations which permits, in particular, the 
detection of the presence or absence of fractures. The process may also be 
implemented for the detection of permeable zones including or not 
including fractures. 
The tests which have been carried out have shown that even small fractures 
could be detected, this taking place in permeable zones, whereas the 
previous techniques had failed. 
The process according to the invention is of the type consisting in 
selecting the recordings obtained from at least three transducers, one of 
which is an emitter and another is a receiver, so as to gather pairs of 
recordings with a common emitter or receiver, and it is defined in that it 
consists, moreover, in determining for previously selected zones of 
interest the mean amplitudes of the waves received as a function of the 
depth; in producing, from the said mean amplitudes, logs of attenuation of 
the said waves; in selecting from among the attenuation logs those 
relating to the shear wave and to the wave referred to as the STONELY 
wave; in determining for each zone the mean value of each one of the said 
selected attenuation logs; in registering in the said zone and on the said 
selected attenuation logs the attenuation peaks exceeding a predetermined 
threshold value; and in comparing, for each zone, at least the peaks of 
the attenuation log of the shear wave in relation to the corresponding 
mean value of the part of the attenuation log of the STONELEY wave, in 
such a manner as to determine the significant relative variations of the 
attenuations, the said variations being representative of the 
heterogeneities or the petrophysical characteristics of the geological 
stratum situated around the said zone.

The process according to the invention was implemented by means of the 
logging tool called "EVA", which comprises sixteen transducers, four of 
which E.sub.1 to E.sub.4 are emitters, the other twelve transducers being 
receivers R.sub.1 to R.sub.12. The tool is lowered into a well 1 drilled 
vertically in a medium to be explored 2, by means of a cable 3 wound 
around a pulley 4 and the movement of which is controlled from control 
means 5. A recorder 6, disposed at the surface 7 of the medium, records 
the signals received on the receivers R.sub.1 to R.sub.12 and routed by 
the cable 3. The emitters E.sub.1 to E.sub.4 are separated by an interval 
which is constant and equal, for example, to 0.25 m. The receivers R.sub.1 
to R.sub.12 are separated from one another by a constant interval equal, 
for example, to 1 m. The interval between the last emitter E.sub.4 and the 
first receiver R.sub.1 is, for example, equal to 1 meter. 
Each emission of an acoustic wave by one of the emitters E.sub.1 to E.sub.4 
is received on each one of the receivers R.sub.1 to R.sub.12, and is then 
recorded on the recorder 6. Such a sequence of emission and of reception 
or recording is well known and produces what is known as a conventional 
sequence of 48 recordings or traces with an interval between the traces, 
for example, of 16 mm. It is this which is represented diagrammatically in 
FIG. 2. Starting from the conventional sequence, a composite sequence is 
constructed by intercalate regularly between the recordings of the 
sequence a trace corresponding to a given emitter-receiver pair' which is 
selected as a function of the criteria relating to the objectives of the 
measurement (in particular, a study of the quality of the cementation, 
detection of incline occurrences, study of the invaded zone). In the 
example of FIG. 3, there is intercalate between the trace R.sub.1 E.sub.1 
produced by a wave emitted by the emitter E.sub.1 and received on the 
receiver R.sub.1 on the one hand, and the trace R.sub.1 E.sub.2 produced 
by a wave emitted by the emitter E.sub.2 and received on the receiver 
R.sub.1, another trace No. 1 R.sub.4 E.sub.2 produced by a wave emitted by 
the emitter E.sub.2 and received on the receiver R.sub.4. The trace 
R.sub.4 E.sub.2 is intercalate in each sub-sequence constituted by the 
four emitters E.sub.1 to E.sub.4 and each one of the receivers R.sub.1 to 
R.sub.12. In this example, the trace R.sub.4 E.sub.2 is repeated every 
four traces, the interval between two intercalate traces being equal to 64 
mm (4.times.16 mm). As a result of this, a sequence such as that 
represented in FIG. 3 corresponds to a displacement of 768 mm of the tool 
in the well. 
The choice of the intercalate trace R.sub.4 E.sub.2 is motivated by the 
study of the quality of the cementation. In general, this criterion 
involves the intercalation of traces, the acoustic path of which is short. 
In the case where incline occurrences are of interest, an average acoustic 
path would be chosen, for example, corresponding to one of the emitters 
which is associated with one of the receivers R.sub.7 to R.sub.9. For the 
study of the invaded zone, a long acoustic path would be chosen, 
corresponding, for example, to an emitter E.sub.2 -receiver R.sub.11 pair. 
It is also possible to construct a composite sequence of the type 
represented in FIG. 4 by intercalate four different traces. For example, 
the trace No. 1 is intercalated between the conventional traces R.sub.1 
E.sub.1 and R.sub.1 E.sub.2 ; a trace No. 2 is intercalated between the 
conventional traces R.sub.1 E.sub.2 and R.sub.1 E.sub.3, and so on. This 
produces a sequence of 96 traces, with an interval between traces of 8 mm 
for a total length of displacement of the tool in the well which is in all 
cases equal to 768 mm. 
The set of recorded traces is represented, in part, in FIGS. 5 to 8, which 
constitute that to which it is appropriate to refer as a time-depth 
section formed of equidistant traces, FIG. 5 being the part of the section 
for a depth within the range, for example, between 570 and 640 m, while 
FIGS. 6 to 8 represent the parts of the section for depths within the 
range between 630 and 715 m, 700 and 780 m, and 770 and 850 m, 
respectively, the ordinates corresponding to times expressed in 
milliseconds. 
By referring to FIGS. 5 to 8, it can be observed that the section is 
strongly perturbed or highly chaotic between 550 and 593 m at the 
location, in particular, of the shear waves S and of the STONELEY waves ST 
which arrive at times greater than the times of arrival of the shear waves 
S which, themselves, arrive after the compression waves P. 
If this zone is compared with that in the range between 612 and 652 m, it 
is seen that the appearance is very different. 
By referring to FIG. 6, it is found that between 700 and 713 m the section 
is substantially homogeneous, without any significant change as regards 
the characteristics of the waves. However, it appears that an 
"interference phenomenon" is localized around the time of arrival of the S 
wave for the values 652 and 685 m. 
The conventional logs have established that there certainly was a fractured 
zone between 586 and 606 m, and that there was clearly something at the 
values 675 m, 687 m, 706 m, 784 m and 790 m, without definiteness being 
ascribed thereto. 
A confirmation and/or a removal of doubt or a detection which would 
otherwise be impossible to obtain by other means concerning the presence 
or absence of fractures may be effected by virtue of the present 
invention. 
Initially, a determination is made of a slowness (inverse of the velocity) 
of the shear waves S and of the STONELEY waves ST by utilizing all the 
traces with a common receiver, in such a manner as to carry out what is 
referred to as a BHC, which is an operation of compensation of the 
variations of diameter of the well or of an obliquity of the measurement 
tool. 
Once the time of arrival of each wave has been determined, the amplitude of 
each of the said waves is calculated, the said amplitude being obtained by 
means of a window around the time of arrival and by calculation of the 
mean amplitude in the said window. Likewise, a determination is made, by 
the same process, of the attenuation and the period for the P, S and 
STONELEY waves or those of any other type capable of being used. 
Assuming that the amplitude of a wave is given by the formula: 
##EQU1## 
in which 
x represents the acoustic path on which the measurement is made; 
a represents the intrinsic attenuation; 
n represents the geometric attenuation; 
b represents a coupling factor. 
For the iso-offset section represented, use has been made of a band-pass 
filter at most equal to 1-25 kHz for the determination of the amplitude, 
the period and the attenuation of the P and S waves, and a band-pass 
filter of 10-20 kHz for the velocity of these waves. So far as concerns 
the ST waves, band-pass filtering of 1-7 kHz was used. 
Subsequently, attenuation logs are produced, of which only those relating 
to the S and ST waves and for the depth within the range between 575 and 
850 m are represented in FIG. 9. In this figure, the attenuation log of 
the S wave is represented in broken lines and the attenuation log of the 
ST wave is in solid lines, the attenuation log of the S wave being 
inverted by 180.degree. in such a manner as to have peaks of attenuation 
of S and of ST in opposite directions and thus easier to localize and to 
pick out. 
From the two attenuation logs of S and ST, a determination is made, for 
each one of them, of the mean value in each zone. In fact, the mean value 
can change from one zone to the other. It is easy to see that the mean 
value between 575 and 625 m is different from that within the range 
between 625 and 670 m. Once this mean value, for each zone, has been 
determined, a threshold is chosen, for example one and a half times the 
mean value, which is utilized for the localization of the attenuation 
peaks on the two logs. 
By referring to the zone within the range between 575 and 612 m, a 
measurement is made of the variation of the attenuation of the STONELEY 
attenuation log which is small, the ST.sub.1 peaks which it is possible to 
pick out are not significant as they are not very far above the mean value 
for the zone under consideration. In any event, the ratio of the ST peaks 
to the corresponding mean value is less than 1.5. 
In the same zone, within the range between 575 and 612 m, the S.sub.1 
attenuation peaks of the shear log are, on the other hand, very 
significant; the ratio of these S.sub.1 peaks to the mean value of the 
attenuation is greater than 1.5. 
In the zone under consideration, between 575 and 612 m, there is a double 
condition, namely a relative stability of the attenuation log of the 
STONELEY wave, as well as significant S.sub.1 attenuation peaks of the 
attenuation log of the shear wave S. Such a double condition determines 
the presence of fractures, the depths of which in the drilled well are 
defined by the significant S.sub.1 peaks. 
In the zone within the range between 612 and 650 m, the attenuation logs of 
the STONELEY wave and of the shear wave indicate that there are 
practically no attenuation peaks or that, in any event, they are not 
significant. Under these conditions, no fracture is detected. 
By proceeding with the measurements indicated previously, it is possible to 
detect the presence of fractures R.sub.1 to F.sub.4 at the depth of 652 m, 
in the zones between 675 and 686 m, between 713 and 728 m and at the depth 
of 758 m, respectively. 
In the zone within the range between 642 and 662 m, the same measurements 
are made as previously, and it is determined that the ST.sub.2 peaks of 
the STONELEY attenuation log are significant, while the attenuation of the 
shear wave is substantially constant. This double condition leads to the 
conclusion that there are practically no fractures except at the depths of 
652 and 675 m, and that a porous and/or permeable stratum is present. 
Thus, on each occasion when a measurement is made of an attenuation peak 
of the STONELEY wave which corresponds to a relative stability of the 
attenuation of the shear wave, the conclusion is drawn that a porous 
and/or permeable stratum is present. 
Finally, by referring to the zone within the range between 800 and 850 m, 
the measurements of the attenuation peaks of the STONELEY and shear waves 
permit the deduction that they are significant and that they are directed 
in opposite directions in FIG. 9, by reason of the inversion made of the 
attenuation log of the STONELEY wave. Once more, a double condition is 
deduced from this--significant ST.sub.3 and S.sub.3 peaks--which permits 
the assertion that there are no fractures, that the corresponding 
geological stratum is neither porous nor permeable, but that it is 
probably a clay stratum having a high degree of agrillaceousness. 
In the above text, reference has been made to various heterogeneities 
and/or petrophysical characteristics of the geological strata surrounding 
the drilled well, which may be vertical as shown in FIG. 1, but likewise a 
well which is deflected to a slight extent or to a great extent, and even 
a horizontal well. In the case of a zone including fractures having a high 
degree of opening, of the conduit or drain type, the shear and STONELEY 
attenuation logs then exhibit significant peaks which are substantially 
opposite one another, similar to the S.sub.3 and ST.sub.3 peaks shown in 
FIG. 9, which permit the detection of an appreciable argillaceousness in 
the zone of the well under consideration. In order to eliminate the doubt 
between argillaceousness and fracture of a high degree of opening, 
reference is made to other measurements such as thoe relating to the 
acoustic velocities of the various waves which are present. In fact, the 
measurement of the acoustic velocities of the waves which have been 
propagated in the geological stratum situated between 800 and 850 m 
permits the determination, with precision, of whether or not an 
argillaceousness stratum is present. In the case where this measurement of 
acoustic velocities leads to the conclusion that the said geological 
stratum does not exhibit a high degree of argillaceousness, the 
significant peaks which are measured on the parts of the attenuation logs 
corresponding to the said geological stratum then permit the assertion, 
without ambiguity, that fractures having a high degree of opening are 
situated in the said geological stratum. 
The tests which have been carried out have demonstrated the great benefit 
of the present invention for the detection and/or the very precise 
location of certain heterogeneities exhibited by the geological strata 
surrounding a well. Thus, in certain drillings, it has been possible to 
detect the presence of fractures having a low degree of opening, which 
fractures could not have been detected by the traditional techniques.