BACKGROUND OF THE INVENTION

The present invention relates to the hydro-fracturing of subsoil and especially to methods for evaluating if the quality of the fracturing of the subsoil. The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.

When hydro-fracturing operations are performed in an oil/gas wellbore, the result may be quite unpredictable and the quality of the fracturing may be uncertain.

To evaluate the presence of formation operating intervals, the document

SU672333 proposes registering the temperature distribution of the fluid along wellbore during injection into the wellbore (fracturing phase) and re-registering the temperature during the production of fluid (production phase). At a temperature anomaly in the absorption zone during fluid injection and the same zone during producing, an operating interval may be identified. The disadvantage of this method is that it is impossible to identify formation fracture (only formation operating intervals) as temperature anomaly appears in the formation both with fractures and without fracture.

To evaluate vertical motion of a fluid in the wellbore, " Vasilevskiy V.N,, Petrov A.I. "Wells evaluation operator". Moscow: Nedra, 1933 _; pp.195-199" proposes to simultaneously record pressure variation curves, which define the fluid density, and temperature. Then calculate the change in temperature due to adiabatic compression and expansion. The disadvantage of this method is a significant cost and complexity in implementing it in the wells, since the method is associated with multiple tool displacements while measuring temperature and pressure along the wellbore. Another drawback of this method is practical impossibility of use in horizontal wells. To evaluate flow rate at each layer during production at steady state based, " V.M. Zaporozhets "Geophysical methods of wells evaluation. Manual for Geophysisists", - M.: Hedpa, 1983, p.201~203" propose detecting temperature anomalies introduced by each producing interval. Temperature anomalies are recorded by moving thermometer lowered into the well on the logging cable or wire. The disadvantage of this method is that it is impossible to identify fractures in low-rate wells.

To evaluate hydrodynamic of horizontal wells, the document RU2243372 propose to install autonomous devices along the borehole depth. The devices are installed in containers and perform registration of pressure and temperature in the range of inflow. The disadvantage of this method is that it is required to achieve a steady-state mode, which significantly increases the duration of the test in low permeability formations. The method is not enough informative for fracture diagnosis.

There is thus a need for a method for accurately evaluating the fractures in a wellbore and / or provide a quality measurement of said fractures. SUMMARY OF THE INVENTION

The invention relates to a method for evaluating fractures of a wellbore, wherein the method comprises:

- installing at least one temperature sensor in a zone of the wellbore;

- monitoring a temperature for a plurality of time thanks to at least one temperature sensor;

- modifying on purpose a pressure in the wellbore;

- determining a quality indicator based on at least:

- a direction of a variation of the monitored temperature after the modification of the pressure ; - a comparison of the monitored temperature after the modification of the pressure with at least two reference curves.

Therefore, it is possible to determine the presence or the absence of fractures in a wellbore based on the determined quality indicator. The quality indicator may be, for instance, a Boolean, a string such as "presence", "absence", "high concentration of fractures", "low concentration of fractures", etc. or a numeric value which value may correspond to a concentration level/quality of the fractures (e.g. 0 means no fracture, 1 means "100% of fractures" or "high quality of fractures" and intermediate values between 0 and 1 corresponds to intermediate quality/concentration of fractures).

The quality indicator takes into account the variation of temperature in a zone of the wellbore:

- either the direction of said variation (which may be simply read and may provide a qualitative measure of the quality of the fractures);

- or the comparison with reference curves (which may thus provide a quantitative measure of the quality of the fractures).

It has been found that the variations of temperature just after an abrupt modification of pressure (e.g. 10 or 100 times quicker that a low-pass pressure variation that may occur during a production period) may provide valuable information on the quality of the fracture in the wellbore.

Therefore, the aim of the invention is to use this information to provide to the engineers a quality measurement of the fractures in the wellbore and enables a cost effective exploitation of said wellbore.

Advantageously, modifying on purpose a pressure in the wellbore may be performed thanks to a jet pump or any other methods.

In a particular embodiment, the quality indicator may be determined based on a derivative of the variation of the temperature.

Said derivative may be the derivative for a time after the pressure modification (e.g. 10 min after the pressure variation) or a mean derivative for a plurality of points (in time) taken after the pressure modification (e.g. 10 points taken every 3 minutes after the pressure modification).

For instance, if the derivative is negative, the determined quality indicator may correspond to the presence of fractures, and if the derivative is positive, the determined quality indicator may correspond to the absence of fracture.

Thus the quality indicator may be for instance "good quality of fractures" or "poor quality of fractures".

In addition, the at least two reference curves may correspond respectively to a reference curve representing a reference variation of temperature in a zone with no fracture and to a reference curve representing a reference variation of temperature in a zone with fractures only.

These reference curves may be ideal curves (i.e. mathematically computed curves) or experimental curves determined based on experimentations.

In a possible embodiment, determining a quality indicator may comprise:

- determining a value Q that minimize a difference between a function of the monitored temperature and Q. C _refl (t) + (1 - Q). C _ref2 {t) with C _refl (t) and C _ref2 {t) the two at least reference curves. The determined quality factor may thus be function of said determined value Q.

The difference between said functions (i.e. the function of the monitored temperature (function of t, the time) and the function Q. C _refl (t) + (1 - Q). C _ref2 {t)) may be the sum of the absolute difference of each value of said functions for a plurality of times divided by the number of used time in the plurality of time.

The minimized difference may be a least square difference. The difference between said functions (i.e. the function of the monitored temperature (function of t, the time) and the function Q. C _refl (t) + (1 - Q). C _ref2 {t)) may be the sum of the square difference of each value of said functions for a plurality of times divided by the number of used time in the plurality of time.

The function of the monitored temperature T may be where t is a time variable.

Indeed, this function has been identified as providing an accurate discrimination for determining the quality of fractures.

A second aspect relates to a computer program product comprising a computer readable medium, having thereon a computer program comprising program instructions. The computer program is loadable into a data-processing unit and adapted to cause the data-processing unit to carry out the method described above when the computer program is run by the data-processing unit.

Other features and advantages of the method and apparatus disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings.

In addition, an aspect of the invention may relate to a method for determining fracture surface in a predetermined zone of a wellbore, wherein the method comprises:

- installing at least one temperature sensor in a zone of the wellbore;

- monitoring a temperature for a plurality of time thanks to at least one temperature sensor;

- modifying on purpose a pressure in the wellbore;

- determining a minimum of temperature in the monitored temperature after the modification of the pressure;

- receiving a permeability value of a reservoir in which the wellbore is dug and a production flow value of the wellbore;

- determining a surface value of fractures of the wellbore for the predetermined zone based on the received permeability value, the received production flow value, the determined minimum of temperature and based on a nomogram adapted for said predetermined zone.

The zone of the wellbore may be a section of the wellbore.

This method enables a proper determination of the parameters of fractures in wellbore.

The nomogram may be adapted to the size of the size/section of the wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements and in which:

- Figure 1 is a side view of subsoil with a wellbore;

- Figure 2a and 2b are flow charts describing possible embodiments of the present invention; - Figure 3 illustrates two extreme cases of variations of after abrupt

pressure change;

- Figure 4 illustrates temperature variations T in a wellbore;

- Figure 5 is a possible embodiment for a device that enables the present invention; - Figure 6 is a representation of the variation of the temperature after an abrupt change in pressure for three wellbore with various permeability parameters;

- Figure 7 is an example of nomogram used to determine parameters of the fractures

DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 is a side view of subsoil (101 ) with a wellbore (102). It is assumed that wellbore have fractures (103) (natural or made by fracturing the subsoil).

As a matter of fact, two types of flows exist in the wellbore:

- a radial flow (104) related the flow that enter in the wellbore by its wall;

- a linear flow (105) related to the flow that enter in the wellbore by the fracture.

In traditional approach, it is not possible to distinguish radial and linear flows associated with the fractures.

Figure 2a are flow charts describing two possible embodiments of the present invention. Part of these flow charts can represent steps of an example of a computer program which may be executed by a computer or a device according to Figure 5.

The following description refers to the Figure 2a.

Prior to the other steps, it is possible to install (step 201 ) monitoring tool(s) such as temperature and pressure sensors in wellbore: namely, temperature sensors may be placed in/near inflow zone(s) and both temperature and pressure sensors may be placed in zone with no inflow (normally bottom hole of the wellbore).

It is noted that:

- the pressure sensor is optional in various embodiments;

- it is possible to have only one temperature sensor in various embodiment (this sensor will monitor the local zone, close to the sensor).

Once the sensors are installed, it is possible to create an abrupt change in pressure in the wellbore (step 202} and to perform simultaneous recording (step 203} of temperature and pressure for some sensors (e.g. all sensors). The recording can last several hours. The recording can be performed in a continuous manner or for a plurality of times (e.g. every minute, a record is created). This change of pressure may be created by the use of a pump, such as a jet pump, which must be able to reduce bottom-hole pressure (preferably abruptly) to initiate production from the well or increase production from the well. Alternatively, shutting down the production of the well also can be used for the estimations.

Then, once the recording is ended (100h after abrupt change in pressure, for instance), it is possible to create a representation of the variation of temperature (step 204, optional). This representation may be done by creating a graph, for instance:

- with the variation of temperature T in ordinate and the time in abscissa ; or

- with the variation of— in ordinate and the time in abscissa.

a ln(t) For instance, the Figure 3 illustrates two extreme/ideal cases:

- a curve 301 representing the variation of for an inflow zone of the

wellbore where there are "only" linear flows ;

- a curve 302 representing the variation of for an inflow zone of the

wellbore where there are "only" radial flows. In this figure, t=0 corresponds to the moment where the abrupt pressure change is performed.

Actually, a "real" case scenario (expressed also with _j j^ to be comparable with the reference curves) is a in-between scenario between these two extreme cases and therefore, a "real" curve is close to a linear combination of these two curves 301 and 302.

It should be understood that curves representing the variation of T (or any other variation of a function of T and/or t) may be directly derived from these curves 301 and 302: this is a simple implementation choice and other functions (i.e. other than —— " ) may be chosen.

a ln(t) ' ^J

In regard of the two curves 301 and 302, it is useful to note that:

- the derivative of T is negative just after the abrupt change (i.e. t=0) if monitored zone has many fractures or if the quality of the fractures is good (i.e. linear flow) and therefore the temperature is expected to go down just after the change. Additional criteria for linear flow registration (indicator of good quality of the fracture) may be the linear dependence of the dynamics of temperature some time after the drop of the temperature (curve 301 );

- the derivative of T is positive just after the abrupt change (i.e. t=0) if monitored zone has no fractures or if the quality of the fractures is poor (i.e. radial flow) and therefore the temperature is expected to go up just after the change. Additional criteria for radial flow registration (indicator of bad quality of the fracture) may be the constant temperature some time after raise of the temperature (curve 302);

It is possible to determine (step 205) what the "qualitative" variation of the temperature (or any other function of the temperature T) just after the abrupt change in pressure is.

For instance, Figure 4 illustrates temperature variations in a "real" wellbore. In this Figure 4, it is possible to determine that the temperature 402 is going up just after the change of pressure 403 and therefore, the monitored zone is expected to have no fracture or the quality of the fractures is expected to be poor. It is also possible to determine that the temperature 401 is going down just after the change of pressure 403 and therefore, the monitored zone is expected to have various fractures or the quality of the fractures is expected to be good.

Step 205 only provides a qualitative assessment: if the variation of the temperature is important (respectively positive or negative), it is possible to assess that the monitored zone is close to one extreme case scenario described respectively by curve 301 or 302. If the variation is weak, it is possible to assess that the monitored zone is an intermediate case scenario with an average quality of the fractures.

This qualitative assessment Qi (208) may be provided to a user (e.g. a string). It is also possible to determine a "quantitative" measure of the fracture quality (step 206). This may be done by comparing the obtained curve in step 204 with the two curves 301 and 302 (or similar, i.e. reference curves) which may be stored in a database 209. The quality factor Q ₂ corresponding to this "quantitative" measure may be for instance the value which minimizes the least square difference between Creaii and Q2- ^C refi t + (1 - Q2) . C _ref2 (t) with C _real (t) the real curve computed in step 204, and C _refl (t) and C _ref2 {t) the two reference curves (e.g. the curves 301 and 302).

This quantitative assessment Q ₂ (207) may be provided to a user.

Once the temperature values and pressure values are recorded (step 203), it is also possible to compute the following coefficients for each temperature sensor in a zone i (step 21 0): i - _AP where ΔΡ represents the pressure variation in the wellbore and represents the temperature variation in the zone I monitored by the temperature sensor in said zone. The variations may be computed between any two different times (i.e. At). The values of 7 ; (21 1 ) can then be output.

In this specific embodiment, at least one pressure and at least one temperature sensors may be installed at a reference point in the well to determine n] _re f . The reference point is the location in the well where there is no any fluids movements/fluxes. Normally (for most current field cases) the well bottom has to be chosen as reference point in the well, hence _bo ttom is used as ?7 _re . In order to determine whether there is an influx from the zone/area i or not, one have to compare 7 ; and T] _re . If η _ί and η _τβί are different then an influx from the i-th zone is proved. If η _ί and ?7 _re are equal, then there is no inflow from the i-th zone.

Figure 2b is a flow chart that illustrates an alternative embodiment of Figure 2a. It is noted that if the blocks have the same reference number between Figure 2a and Figure 2b, it means that the description corresponding to said block are substantially the same (otherwise indicated).

In this embodiment, the value of η _ί (21 1 ) can be used to determine / estimate the flow (i.e. the presence of influx as indicated above). If it is determined that there is no influx (test 212, output KO), the process may be exited (step 213). If it is determined that there is an influx (test 212, output OK), step 204 may be executed and the two quality indicators Qi (208) and Q2 (207) may be determined as presented above.

Figure 5 is a possible embodiment for a device that enables the present invention. In this embodiment, the device 500 comprise a computer, this computer comprising a memory 505 to store program instructions loadable into a circuit and adapted to cause circuit 504 to carry out the steps of the present invention when the program instructions are run by the circuit 504.

The memory 505 may also store data and useful information for carrying the steps of the present invention as described above.

The circuit 504 may be for instance:

- a processor or a processing unit adapted to interpret instructions in a computer language, the processor or the processing unit may comprise, may be associated with or be attached to a memory comprising the instructions, or

- the association of a processor / processing unit and a memory, the processor or the processing unit adapted to interpret instructions in a computer language, the memory comprising said instructions, or - an electronic card wherein the steps of the invention are described within silicon, or

- a programmable electronic chip such as a FPGA chip (for « Field- Programmable Gate Array >>). This computer comprises an input interface 503 for the reception of temperature/pressure input according to the invention and an output interface 506 for providing the quality parameter.

To ease the interaction with the computer, a screen 501 and a keyboard 502 may be provided and connected to the computer circuit 504.

Figure 6 is a representation of the variation of the temperature after an abrupt change in pressure for three wellbore with various permeability parameters.

It is assumed that:

- flow for all wellbores are identical (Q=120m ³ /day); - the permeability parameter for the curve 601 is k _res =30mD;

- the permeability parameter for the curve 602 is k _res =60mD;

- the permeability parameter for the curve 603 is k _res =90mD.

On Figure 6, it is possible to see that the minimum of temperature is not reached after the same period for all wellbores. This minimum of temperature is then (at least) function of the permeability parameter.

Thus, it is possible to determine, for a real scenario, the time needed to reach the minimum temperature value (noted t _m in)-

Once this t _m in is determined, parameters of the fractures may be determined thanks to nomograms (which may be determined experimentally of mathematically by the person skilled in the art).

Figure 7 is an example of nomogram used to determine the parameters of the fractures (the surface of the fractures, S). Knowing the minimum temperature value (noted t _m in and determined as detailed in Figure 6), the permeability of the reservoir k _res and the production rate Q, it is possible to determine the surface of the fracture in a determined zone (i.e. the determined zone is a zone for adjusting the nomogram, e.g. a cylinder with a given radius and height).

Assuming that k _res =10mD and 0=120m ³ / day, the curve 701 is selected in the nomogram of Figure 7.

Once the curve 701 selected and assuming that t _m in = 2h, the point 702 may be selected on the curve 701 (it corresponds to the point with ordinate 2h). Once the point 702 selected, the abscissa is read and may provide the surface value of fractures in the determined zone (i.e. about 17m ² in this example).

Expressions such as "comprise", "include", "incorporate", "contain", "is" and "have" are to be construed in a non-exclusive manner when interpreting the description and its associated claims, namely construed to allow for other items or components which are not explicitly defined also to be present. Reference to the singular is also to be construed in be a reference to the plural and vice versa.

A person skilled in the art will readily appreciate that various parameters disclosed in the description may be modified and that various embodiments disclosed may be combined without departing from the scope of the invention.