EP1506388A1 - Method for evaluating capillary pressure curve of an underground deposit rocks based on rock cuttings measurements - Google Patents

Abstract

The invention concerns a method for evaluating the capillary pressure curve of an underground deposit rocks based on measurements of rock cuttings or fragments such as drill cuttings derived therefrom, over the entire saturation interval of said rocks, in reduced time and at less cost based on said measurements. The method essentially consists in: measuring the permeability k of the rock cuttings, measuring the capillary pressure curve Pc based on the saturation of said fragments initially saturated with a fluid (for example brine) by subjecting them to centrifuging, and parameterizing a capillary pressure curve Pc satisfying empirical relationships depending on adjustable parameters, which are constrained to be adjusted to an asymptotic portion of the capillary curve measured by centrifuging, and to the value of permeability measured on the drill cuttings, so as to obtain the complete capillary pressure curve. The invention is applicable to evaluation of a hydrocarbon deposit for example.

Description

METHOD FOR EVALUATING THE CAPILLARY PRESSURE CURVE OF ROCKS OF A SUBTERRANEAN DEPOSIT FROM MEASUREMENTS OF ROCK DEBRIS

The present invention relates to a method for evaluating a capillary pressure curve of the rocks of an underground deposit from measurements on rock debris taken therefrom.

State of the art

Laboratory measurements on cores or drilling debris

The measurement of petrophysical parameters such as permeability, porosity and capillary properties on rock fragments raised during the drilling of a well through an underground formation, constitutes an interesting opportunity for operating companies to obtain quickly a first petrophysical characterization of producing zones crossed by the well.

By patent FR 2 809 821 of the applicant, a system is known for evaluating physical parameters such as their absolute permeability of porous rocks in an area of an underground deposit, from rocky debris brought up in the mud of a borehole . In an enclosure where the debris is immersed in a viscous fluid, this same fluid is injected under increasing pressure over time, up to a defined pressure threshold, so as to compress the gas trapped in the pores of the rock. This injection phase is followed by a relaxation phase with stopping the injection. The evolution of the pressure during the injection process having been modeled from initial values chosen for the physical parameters of the debris, a computer iteratively adjusts them to make the modeled pressure curve coincide as well as possible with the pressure curve actually measured

By patent application FR 02/0023 of the applicant, another method is known for evaluating physical parameters such as the absolute permeability and the porosity of rocks in an area of an underground deposit, also from drilling debris. An enclosure containing the rock fragments and filled with a viscous fluid is placed in communication with a reservoir of this same fluid under a pressure defined so as to compress the gas trapped in the pores of the rock. The duration of application of this pressure depending on whether it is short or longer makes it possible to measure either the variation of the pressure in the enclosure or the variation of the volume actually absorbed by the rock fragments. Then, the evolution of the pressure or the volume in the enclosure is modeled, starting from initial values chosen for the physical parameters of the fragments, and the values of the physical parameters of the rock fragments are adjusted iteratively so that the evolution modeled adjusts as best as possible with the measured change in the physical parameter in the enclosure.

In the field of petrophysical characterization, capillary pressure is also a very important datum for operators because it conditions:

• the initial distribution of fluids in the reservoir from the aquifer zone (known as WOC for "Water Oil Contact" by those skilled in the art) to the upper part of the reservoir (transition zone). Depending on the capillary pressure curve associated with a reservoir rock and the nature of the fluids in place, this transition zone can extend over a few meters or a few tens of meters, which has a significant impact on the determination of accumulations in place.

• the inlet pressure of a rock which is particularly important for cover rocks. For example, for a gas storage tank, the inlet pressure of the cover rocks directly conditions the permissible overpressure in the storage levels without having any leaks.

With current techniques, the capillary pressure curve is obtained by means of laboratory measurements on deposit cores. These methods are expensive both because of the coring operations and because of the measurement operations on the cores and the results are often not available until several months after drilling. Approaches to quickly obtain the capillary pressure curve

However, there are alternative methods described in the literature for evaluating the capillary pressure curve quickly, either during drilling or with a slight delay.

The most commonly used approach consists in using the mercury porosimetry technique to measure the capillary pressure curve Pc air / mercury directly from cuttings or "cuttings". However, the curve obtained differs widely from the reference curve obtained on carrot with high saturation in wetting fluid. On the other hand, this approach is based on the use of mercury which is extremely polluting and progressively prohibited by legislation in many countries, which poses a major problem for the application of this technique in the near future.

Another known method uses nuclear magnetic resonance (NMR) to quickly estimate the capillary pressure curve from log data measured in the well soon after drilling. It is implemented in particular in the following publications:

- Bowers, M., A. et al. : "Prediction of permeability from capillary pressure curves derived with NMR", September 17, 1998;

- Marshall, D., et al. : "Method for correlating NMR relaxometry and mercury injection data", SCA No. Society of Core Analyste International Symposium 1995;

- Volokitin, Y., W. J. et al. : "A practical approach to obtain lst drainage capillary pressure curves from NMR core and Log data", SCA n ° Society of Core Analysts International Symposium 1999.

The NMR relaxation signal is first converted in terms of pore size distribution and then in terms of threshold size distribution, which makes it possible to calculate a pseudo capillary pressure curve. This approach has been tested on several samples of known Pc curve. The results show that obtaining good agreement with the reference curves requires a rigorous calibration phase to be carried out on a case-by-case basis depending on the nature of the rocks studied. This calibration phase is necessary due to the uncertainty in: • NMR signal conversion - pore size distribution which depends on the value of the surface relaxivity which varies according to the rocks; and

• the conversion pore size distribution - distribution of threshold sizes which depends on the nature of the rock and the diagenesis process.

This approach is therefore not recommended in a predictive context of exploration. In any case, it would not be applicable to drilling debris.

Image analysis has also been the subject of work aimed at obtaining a Pc curve. The porous medium is previously prepared in the form of a thin section photographed by scanning electron microscopy called SEM for "Scanning Electron Microscopy". The image obtained is then analyzed so as to determine parameters representative of the proportion and the shape of the voids relative to the rock. In particular, it is possible to determine a threshold size distribution for reconstructing a pseudo capillary pressure curve Pc. The main limitation of this method is the two-dimensional (2D) nature of the thin section while capillary pressure is by definition a three-dimensional (3D) property. On the other hand, this technique requires fairly heavy conditioning which is not compatible with obtaining a slightly delayed result. Image analysis could possibly be applicable on rock debris but would require careful calibration to acquire good predictability.

To conclude, note that the centrifugation technique is sometimes applied on site to drilling debris, but this is in order to extract as much drilling fluid as possible from the rock to minimize polluting releases to the environment, but also to limit costs by recycling the recovered drilling fluid. To our knowledge, no centrifugation measurement on drilling debris has been envisaged in order to determine capillary properties.

The method according to the invention

The method according to the invention makes it possible to determine the capillary pressure curve of rocks of an underground deposit from measurements on debris or fragments of rock which are taken there (such as drilling debris), over the entire interval. saturation of these rocks, this in a reduced time and at lower cost from these measurements. It involves : - a measurement of the permeability k of rock debris;

a measurement of the capillary pressure curve Pc as a function of the saturation of the rock debris initially saturated with a fluid by subjecting them to centrifugation; and

- the parametrization of a capillary pressure curve Pc satisfying empirical relationships depending on adjustable parameters, which one is forced to adjust to an asymptotic part of the capillary curve measured by centrifugation, and to the value of the permeability k measured on drilling debris, so as to obtain the entire capillary pressure curve.

The parameterization of the curve is advantageously carried out by selecting by default a set of said parameters allowing a setting on the asymptotic part of the capillary pressure Pc at low saturations, and by a modification step by step near the parameters so that the estimation of the permeability given by one of the empirical relations used, is best adjusted with the permeability measurements k carried out on rock debris and with this asymptotic part.

The measurement of the permeability k of the rock debris is carried out for example from measurements of the pressure variations inside a container filled with a fluid containing the rock debris after it has been placed in communication for a defined time with a reservoir of this same fluid under pressure, and from the volume actually absorbed by the drilling debris, and from a modeling of the evolution of the pressure or the volume in the container, from initial values chosen for the physical parameters rock debris, which is adjusted iteratively so that the modeled evolution of the pressure is best adjusted with the measured evolution of the physical parameters of the rock debris.

The method is advantageous in particular in that it provides the capillary pressure of the rocks on the basis of simple drilling debris more readily available and less expensive to obtain. The results are also obtained much more quickly than with carrots. 1. Summary presentation of the figures

The characteristics and advantages of the method according to the invention will appear more clearly on reading the description below of a non-limiting example of implementation, with reference to the appended drawings where:

- Fig.l shows the evolution of the NMR signal during different centrifugation stages performed on drilling debris;

fig.2a shows the curves of Pc obtained on the same rock (B7) from debris (cuttings) and a carrot;

fig. 2b shows the comparison between the reference Pc curve measured on core with the reconstructed Pc curve over the entire saturation interval from the centrifugation measurements and the permeability measurement;

fig.3a and 3b show results comparable to those shown in Fig.2a and Fig.2b respectively for another rock GDV1;

- Fig.4a and 4b show results comparable to those shown in Fig.2a and Fig.2b respectively for another rock Rotl;

fig.5a and 5b show results comparable to those shown in Fig.2a and Fig.2b respectively for another St Max rock;

Fig.6 shows the comparison between the reference permeabilities and the permeabilities measured on drilling debris as part of the method described in filed in the aforementioned patent application FR 02/0023; and

- Fig.7 schematically shows the flowchart for implementing the method.

Detailed description of the method

The method for rapid evaluation of a capillary pressure curve Pc from cuttings or rock fragments according to the invention is illustrated in FIG. 7. The method is based on two experimental measurement phases followed by a parameterization phase by reference with known curves, to reconstruct the curve of Pc over the entire saturation interval. During one of the measurement phases, pressure data are acquired capillary Pc by centrifuging the rock fragments previously saturated with water. The other experimental phase makes it possible to calculate the permeability value k of the rock using the method described in patent application FR 02/0023 already cited. The reconstruction of the curve Pc over the entire saturation interval is carried out using a parameterized form. The parameters of the curve are determined so that the curve agrees with the capillary pressure data Pc obtained experimentally and with the estimated value of permeability from the curve of Pc obtained according to a known method such as that of Thomeer or that of Swanson Kamath which will be recalled later, with the value measured on the rock fragments. Several application cases are proposed which show the very good agreement obtained with reference curves without any particular prior calibration procedure.

I) Measurement of Pc by centrifugation from rock fragments

For the implementation of the method, it can use standard centrifugation means or more sophisticated means with automatic monitoring of the volumes of fluid produced such as those described for example in patents EP 603040

(US 5,463,894), FR 2,763,690, FR 2,772,477 (US 6,185,985) or FR 2,798,734 of the applicant.

The rock fragments brought up during drilling are cleaned beforehand with solvents in a Soxhlet type device, then dried and saturated in brine 30 g / 1. The rock fragments are then drained in a damp cloth so as to remove the water trapped between the different rock fragments d and then introduced into a cell or bucket fixed at the end of a rotating arm. The water expelled by centrifugation out of the rock debris passes through a grid and is collected at the base of the bucket. The acquisition of experimental data is done in the same way as in the context of centrifugation on a carrot. For a centrifugation stage (given rotation speed), we measure the evolution of the water production until no more significant variations are observed, then we increase the rotation speed to start a new stage .

As shown in fig. 1, a gradual decrease in the NMR signal is observed and a shift towards the low relaxation times T2 which reflect a desaturation of the porous medium with the increase in the speed of rotation. This type of measurement proves that there is indeed a capillary contact between the rock debris which makes it possible to measure the Pc by centrifugation from drilling debris.

The volume of water produced during the experiment is converted into saturation data from the volume of water initially contained in the rock fragments. The latter is determined by weighing (difference in weight of the rock fragments before and after saturation) or directly by NMR measurement.

FIGS. 2a to 5a show the result of experiments carried out using fragments of model rock, of size 1 to 2 mm, manufactured in the laboratory from rocks of known properties for which there is a curve of Pc measured so classic by centrifuging a carrot. It can be checked that a good correspondence with the reference curve is obtained at the asymptotic part (low saturation in wetting fluid). On the other hand, there is a significant difference for the higher saturations in wetting fluid. We therefore obtain results equivalent to those obtained in the framework of porosimetry measurements using mercury on rock fragments and this without risk of pollution.

The capillary pressure curve Pc measured is however only representative on the asymptotic part. A reconstruction procedure is therefore necessary to evaluate the behavior of the curve over the entire saturation interval.

II) Measurement of the permeability K of rock fragments

To measure the permeability of the rock fragments, the method described in the aforementioned patent application 02/02242 is applied. To this end, the rock fragments are immersed in a confinement enclosure containing a viscous fluid. The enclosure is then placed in communication with a container containing this same fluid under pressure, so as to compress the gas trapped in the pores of the rock. According to a first mode of implementation, this communication can be very brief and followed after a waiting time, a measurement of the evolution of the pressure in the enclosure. According to another mode of implementation, the communication is long enough so that one can observe and measure the variation in the volume actually absorbed by the rock fragments. Then, the evolution of the pressure or the volume in the enclosure is modeled, starting from initial values chosen for the physical parameters of the rock fragments, and the values of the physical parameters of the rock fragments are adjusted iteratively so that the modeled evolution is best adjusted with the measured evolution of the physical parameter in the enclosure.

This procedure gives excellent results. The permeability values k of the rock fragments are entirely in accordance with the reference measurements obtained from cores.

III) Reconstruction of the total capillary pressure curve

In this third step, we synthesize the previous measurements which allowed us to construct the asymptotic part of capillary pressure Pc and the measurements of permeability k, using empirical relationships, which are reputed to model well the physical parameters of rocks. Through the following publications:

**** Thomeer, J. H. M.: "Introduction of a pore geometrical factor defined by the capillary pressure curve", Trans AIME, vol, March, pp 73-77, 1960; and

"Thomeer, J. H. M.:" Air permeability as a function of three pore network parameters ", Trans AIME, vol, April, pp 809-814, 1983.

procedures for evaluating permeability from a capillary pressure curve are known. The capillary pressure curve Pc is modeled as

• G is a shape parameter to take into account the curvature of the capillary pressure curve (related to the shape of the pore size distribution);

• Pd, the displacement pressure extrapolated to S _HÊ equal to zero; and

• V _b _ = _, the percentage of volume occupied by mercury at the end of the experiment at infinite capillary pressure (equal to φxSπg), the three parameters of the model being linked to the permeability by the following expression

In the following publication:

Swanson, B. F.: "A simple correlation between permeability and mercury capillary pressures", JPT, vol, December, pp 2498-2504, 1981,

It is also proposed to correlate with the permeability value of the maximum value of the ratio (N / _P.) Of mercury porosimetry curve. This particular point generally corresponds to the change in regime which occurs at the end of the percolati on regimen, just before the significant increase in capillary pressure. From a fluid flow capacity point of view in the porous medium, this point is particularly important because it represents the pore size for which the entire porous network is connected and which therefore controls the flow. The most general correlation provided by the author is given by the expression (Swanson 1981).

The following publication:

Kamath, J.: "Evaluation of accuracy of estimating air permeability from mercury injection data", SPE Formation evaluation, vol 7, 4, pp 304-310, 1992,

also relates to the comparative evaluation of the empirical correlation or physical model approaches to determine the value of the permeability from a mercury porosimetry curve. The best agreement is obtained with a new correlation based on the characteristic length of Swanson 1981 already cited:

Jt = 347 x Z} ^ if k> l mD L _max being defined as follows

With:

• λ: exponent translating the curvature of the capillary pressure curve (related to the shape of the pore size distribution).

• P _e : the displacement pressure extrapolated to S _ϋ g equal to zero.

• S _r : residual saturation occupied by the wetting fluid (%).

To configure the empirical capillary pressure curve Pc from the approaches of Thomeer, Swanson or Kamath, we constrain it to fit the asymptotic part obtained by centrifugation during the first step of the method To constrain the whole of the curve, we also uses the value of the permeability k measured on drilling debris during the second step of the method, which is compared with what the empirical relationships give. The parameters of the capillary pressure Pc are then modified until both the measured asymptotic behavior and the estimation of the permeability are honored, which makes it possible to constrain the capillary pressure curve over the entire saturation interval. sw.

The inversion process starts with a set of default parameters which allow the asymptotic behavior of the capillary pressure Pc to be calibrated at low water saturations. These parameters are then modified step by step (mainly the inlet pressure, noted Pe or Pd, and the form factor, noted λ or G) so that the estimate of the permeability given by one of the preceding relationships is in good agreement with the permeability measurement carried out on drilling debris while maintaining good agreement with the Pc measurements at low water saturations.

1.1. Results obtained

FIGS. 2b, 3b and 4b and 5b show the comparison between the curve of Pc reconstituted according to the previous procedure and the curve of Pc of reference obtained on a carrot. It can be seen that, whatever the example considered, the reconstruction method makes it possible to obtain a relevant evolution of the Pc over the entire saturation interval and in particular at high water saturations, whatever the permeability of the rock. .

Claims

1) Method for determining the capillary pressure curve of rocks of an underground deposit from measurements on rock debris taken there, comprising a measurement of the permeability K of rock debris, characterized in that it includes: a measurement of the capillary pressure curve Pc as a function of saturation by subjecting rock debris initially saturated with a fluid to centrifugation; and the parametrization of a capillary pressure curve Pc satisfying empirical relationships dependent on adjustable parameters, which one is forced to adjust to an asymptotic part of the capillary curve measured by centrifugation, and to the value of the permeability k measured on drilling debris, so as to obtain the entire capillary pressure curve. 2) Method according to claim 1, characterized in that the configuration includes the default selection of a set of said parameters allowing a calibration on the asymptotic part of the capillary pressure Pc at low saturations, and a modification from step to step of parameters so that the estimation of the permeability given by one of the empirical relations used is best adjusted with the permeability measurements carried out on rock debris and with the said asymptotic part. 3) Method according to claim 1 or 2, characterized in that one measures the permeability k of rock debris from measurements of pressure variations inside a container filled with a fluid containing the debris of rock after its communication for a defined time with a reservoir of the same fluid under pressure and from the volume actually absorbed by the drilling debris, and from a modeling of the evolution of the pressure or the volume in the container , from initial values chosen for the physical parameters of the rock debris, which are adjusted iteratively so that the modeled evolution of the pressure is best adjusted with the measured evolution of the physical parameters of the rock debris.