WO2014154242A1

WO2014154242A1 - Determining a condition of a subsurface reservoir

Info

Publication number: WO2014154242A1
Application number: PCT/EP2013/056293
Authority: WO
Inventors: Per Atle OLSEN
Original assignee: Statoil Petroleum As
Priority date: 2013-03-25
Filing date: 2013-03-25
Publication date: 2014-10-02

Abstract

The reservoir may have a region extending between an upper portion and a lower portion and having at least one geological formation containing first fluid at said upper portion and second fluid at said lower portion. A resistivity of the formation and a resistivity gradient of the formation may be provided at a first height within said region. A geological model may be arranged to predict a property of the formation at a second height within said region in dependence upon a model parameter, for example an exponent. The resistivity and resistivity gradient may be used to determine the parameter, which may be used to determine said condition of the reservoir.

Description

Determining a condition of a subsurface reservoir

Technical field The present invention relates to methods and apparatus for determining a condition of a reservoir. In particular embodiments, measurements of resistivity of the reservoir formation are obtained and used to estimate a profile with respect to height within the reservoir to determine the state of drainage of the reservoir. Background

It is of interest to obtain an understanding of a subsurface reservoir in order to help recover fluid from the reservoir. For example, it may be useful to determine a condition of the reservoir to determine the ability to recover fluid from the reservoir over time or to determine the extent of fluid depletion after previous recovery operations, or to determine where to place a well to recover such fluid.

In order to provide an understanding of the reservoir, formation evaluation techniques may be employed. Formation resistivity evaluation represents one such technique. Resistivity is sensitive to the fluid content in the geological formation of the reservoir, and can therefore be useful to determine the extent to which hydrocarbons or water/brine is present in the formation. Accordingly, the resistivity may provide an indication of fluid distribution in the formation. In order to position a well in the right place in the reservoir for recovery of hydrocarbons, resistivity images of the surroundings are often acquired (see for example Zhang Z., C. Gonguet, V. Rajani and R. Roeterdink, 2008, Directional LWD resistivity tools and their business impacts, SPWLA 49th Annual Logging Symposium, (paper FFFFJ). Measurements using directional resistivity tools may help detect the resistivity contrasts and the distance to such contrasts in the surroundings. If a relatively conductive layer is found below the well-bore while drilling a horizontal well, this may indicate water filled reservoir rock that should be avoided by the production well. A technique for directional resistivity measurements is described in the patent publication US6181 138. A technique for directional resistivity measurement while drilling is described in the patent publication US7057392. Current procedures for interpreting resistivity data assume a simplified 1 D distribution of resistivity above and below the wellbore with constant resistivity in each layer. For example, the measured resistivity may be assumed to apply uniformly in a 1 D layer above and below the measurement point. If a horizontal production well is positioned close to the oil-water contact, the assumption of constant resistivity above the oil-water-contact may be inaccurate, such that images of the resistivity and fluid distribution around the well may also be inaccurate. Inferences based on such data, such as determinations of reservoir condition, may therefore also be inaccurate in such situations.

Summary of the invention

According to a first aspect of the invention there is provided a method of determining a condition of a reservoir having at least one geological formation containing an amount of fluid that varies in dependence upon height within the reservoir, the method comprising the steps of:

a. providing data comprising either or both of (i) a first resistivity and a resistivity gradient of the formation, and (ii) second and third resistivities of the formation at different respective heights;

b. providing at least one reference formation property response, or parameter thereof, which response is dependent upon the amount of fluid and height within the reservoir; and

c. comparing the provided data, or a component based on the data, with the formation property response to determine the condition of the reservoir.

The reference formation property response may comprise any one or more of an imbibition response, a primary drainage response, and a secondary drainage response. The determined condition may be a state of drainage of the reservoir. The reference formation property response may be obtained from either or both of: earlier- performed formation property measurements (e.g. resistivity or the like) at the reservoir or theoretical data, or other prior knowledge. The reference formation property response may comprise either or both of resistivity or fluid saturation. The reference formation property response may comprise a curve or profile of resistivity or fluid saturation in dependence upon height within the reservoir.

The method may further comprise estimating a resistivity or fluid saturation of the formation using the data to provide said component based on the data. The method may further comprise comparing the estimated resistivity or fluid saturation with the at least one reference formation property response to determine the condition of the reservoir.

According to a second aspect of the invention, there is provided a method of determining a condition of a reservoir having at least one geological formation containing an amount of fluid that varies in dependence upon height within the reservoir, the method comprising the steps of:

b. using the data to estimate at least one formation property which is dependent upon the amount of fluid and height within the reservoir; and

c. determining the condition of the reservoir based on the estimated formation property.

Step b may be performed to estimate the formation property at a plurality of heights within the reservoir. The formation property may comprise either or both of resistivity or fluid saturation, e.g. with respect to height. Step b may be performed to estimate a curve or profile of the formation property in dependence upon height within the reservoir. Step b may comprise performing an inversion of the data to estimate the formation property. Step c may comprise determining the condition based on the estimated curve or profile, for example by analysing characteristics of the curve or profile, e.g. shape, curvature or the like.

Step b may further comprise using the data to determine at least one parameter of a model, for example the Brooks & Corey parameterised model. The method may include using either or both of the parameter and the model to estimate the formation property. The parameter may comprise any one or more of the following in any combination :

i. a saturation exponent;

ii. a pore size distribution index;

iii. irreducible water saturation ; or

iv. entry pressure.

The model may be or may be based upon,

R, (H) = Ro (S_wirr + (1 -SwirrX Δρ g H /Ρ_β)^~λ ) ⁿ wherein

Rt is the resistivity of the reservoir partially filled with hydrocarbon fluid;

H is a height above free water level;

R₀ is the resistivity of water filled reservoir;

g is gravity;

Δρ is the difference in density between said water and hydrocarbon fluid;

P_e is entry pressure;

S_wirr is irreducible water saturation;

A is a pore size distribution index; and

n is a saturation exponent.

The estimated formation property may be compared with a reference formation property or reference formation property response as set out in relation to any of the other aspects.

According to a third aspect of the invention, there is provided a method of determining a condition of a reservoir having at least one geological formation containing an amount of fluid that varies in dependence upon height within the reservoir, the method comprising the steps of:

a. providing data comprising either (i) a first resistivity and a resistivity gradient of the formation, or (ii) second and third resistivities of the formation at different respective heights; b. using the data to estimate at least one parameter for, or associated with, a formation property response which is dependent upon the amount of fluid and height within the reservoir; and

c. determining the condition of the reservoir based on the estimated parameter.

The parameter may comprise any one or more of the following in any combination: i. a saturation exponent;

ii. a pore size distribution index;

iii. irreducible water saturation; or

iv. entry pressure.

The parameter may be a parameter of a parameterised model. The model may be or may be based on,

R, (H) = R₀ (S_wirr + (1 -S_wirr)( Δρ g H /Ρ_θ)^"λ ) ⁿ wherein

Rt is the resistivity of the reservoir partially filled with hydrocarbon fluid;

H is a height above free water level;

R₀ is the resistivity of water filled reservoir;

g is gravity;

Δρ is the difference in density between said water and hydrocarbon fluid;

P_e is entry pressure;

S_wirr is irreducible water saturation;

A is a pore size distribution index; and

n is a saturation exponent.

The method may further comprise using either or both of the parameter and the model to estimate the formation property response. The estimated parameter or formation property response may be compared with a reference formation property response or parameter thereof. The reference formation property response may have further features as set out in relation to any of the above aspects. The reference formation property response may comprise either or both of resistivity or fluid saturation. The formation property response may comprise a curve or profile of resistivity or fluid saturation in dependence upon height within the reservoir. The condition of the reservoir may be a state of drainage of the reservoir. Determining the condition of the reservoir in any of the above aspects may comprise determining whether the reservoir is in the state of one of primary drainage, imbibition, and secondary drainage. In any of the above aspects, said fluid may comprise any one or more of:

i. hydrocarbon gas;

ii. oil; or

iii. water. In any of the above aspects, the data may be obtained by performing measurements from the formation. The steps of providing data may be performed by energising the formation and measuring or detecting a response to said energisation. The methods may further comprise drilling a borehole in said region. Step a may be performed during the process of said drilling.

According to a fourth aspect of the invention, there is provided apparatus configured to perform a method of any of the above aspects.

According to a fifth aspect of the invention, there is provided a computer program for use in performing the method of any of the first to third aspects, the program being arranged to determine the condition of the reservoir using the data.

According to sixth aspect of the invention, there is provided a computer device arranged to execute the program of the fifth aspect, to determine said condition of the reservoir.

According to a seventh aspect of the invention, there is provided a computer readable medium containing the program of the fifth aspect. Each aspect may include further features as set out in the claims or in the present description or the drawings in any combination. Features may be combined between any of the different aspects. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

The invention can provide an improved determination of properties or the condition of the reservoir by taking into account formation property dependence as a function of height within a reservoir, in particular properties that depend on the resistivity distribution in proximity to an oil water contact. The invention provides numerous further advantages as will be apparent from the description, drawings and claims.

Drawings and description There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, in which:

Figure 1 is a graph of capillary pressure versus water saturation for two rocks with different air entry pressure and irreducible water saturation fitted with Brooks & Corey (B&C) parameters;

Figure 2a is a graph of height above free water level (FWL) versus resistivity for the rocks of Figure 1 assuming an Archie exponent of n=2; Figure 2b is a graph of height above free water level (FWL) versus resistivity for the rocks in Figure 1 with a potential imbibition cycle included.

Figure 3 is a representation showing principles of a method for determining a condition of a reservoir according to an embodiment of the invention;

Figure 4 is a diagram of steps of a method for determining a condition of a subsurface reservoir according to a further embodiment of the invention; and

Figure 5 is a representation of a computer device for use in performing the method of Figures 3 and 4. Reservoirs in static equilibrium have an increasing oil contents with distance from the oil-water-contact, which is characterized by the capillary pressure curve as shown in Figure 1 . The most common parameterization of the capillary pressure curve for water wet reservoirs is the Brooks & Corey (1966) equation (see Brooks, R.H.. Corey, A. T.. 1966, Properties of porous media affecting fluid flow, J. Irrig. Drain. Div. 6, 61):

P_c = P_e S_wn ^{( a)} (Equation 1 ) This equation indicates that the capillary pressure (P_c) equals the air entry pressure (P_e) times normalized water saturation (S_wn) to an exponent. The parameter λ is called the pore size distribution index and is related to the pore size distribution of the rock in a drainage experiment. Normalized water saturation is related to irreducible water as S_wn = (S_w-S_wirr)/(1 -S_wirr) (Equation 2) in which S_w is the water saturation and S_wirr is the irreducible water saturation.

In static reservoirs, equilibrium between the capillary forces and gravity occur, as:

Pc = Δρ g H (Equation 3)

This indicates that capillary pressure equals the buoyancy. Δρ is density difference between oil and water, g is the gravitational constant and H is distance from the free water level. Therefore, the water saturation will vary with distance from the free water level as:

S_wn = (Pc/Pe) ^λ = (H/He) ^"λ (Equation 4) Here the entry height H_e is introduced and is related to the entry pressure as H_e = Ρ_β/Δρ g. The result is that the water saturation depends on the height above the free water level with an exponential function (Η^"λ).

In reservoir sand the most common resistivity equation is the Archie (1942) formula (see Archie. G.E.. 1942, The electrical resistivity log as an aid in determining some reservoir characteristics, AIME, Petroleum Tech. 5, 1-8). It relates formation resistivity (R_t) partially saturated with oil to resistivity saturated with water (R₀) and the water saturation (S_w) as: R, = R₀ S_w ⁿ (Equation 5)

The ratio of R,/R₀ is called the resistivity index and n is called the saturation exponent. This means that also resistivity has a transition profile from the oil-water contact as shown in Equation 6, as follows (and exemplified in Figures 2a and 2b):

R, (H) = R₀ (S_wirr + (1 -S_wirr)(H/H_e)-^¾ ) ⁿ (Equation 6)

Taking the derivative of Equation 6 the gradient of the resistivity will be: δ (R_t)/6 (H) = ηλ R, H^" (1 -S_wirr) (Η/Η_θ) ^λ /(S_wirr + (1 -S_wirr) (Η/Η_θ) ^λ) (Equation 7)

The shape of the resistivity profile is indicative of the drainage status of the reservoir. Different shapes for different reservoir states are shown in Figure 2B. Measurements of the resistivity and the resistivity gradient at a particular height above the OWC can indicate if the reservoir is in a primary drainage or imbibition state, for example by comparing the resistivity and the gradient with the profiles of Figure 2B (using the profiles as reference responses). In principle, measurements of the resistivity and the resistivity gradient at two or more heights can allow the Archie and Brooks & Corey parameters n, λ, S_wirr and P_e to be resolved or estimated. To estimate one or more of these parameters, the Brooks & Corey parameterised model as described above may be used (by combining and evaluating equations 6 and 7). The resistivity or fluid saturation profile may be estimated from the resistivity and gradient measurements and the Brooks & Corey parameters. Comparing such estimates with such parameters, profiles or the like obtained before reservoir production can reveal changes in reservoir drainage state. Figure 2B shows the potential difference in resistivity and resistivity gradient of the two rock types at two different reservoir drainage states.

While drilling with an angle to the oil-water-contact, predicted resistivity and resistivity gradient at different elevations can be compared with the actual observed resistivity profile. For example crossing the oil-water-contact, in a reservoir in drainage state, the resistivity at the oil-water contact (P_c = P_e) is Ro. While drilling upwards, the resistivity prediction at an small elevation ΔΗ is: R_t1 ~ R₀ ( 1 + ηλ(1 -8ννί_ΓΓ) ΔΗ/Η_β), where H_e is the entry height corresponding to the entry pressure (P_e = Δρ g H_e). In general, the predicted resistivity and resistivity gradients could be calculated with Equation 6 and 7 and the Archie and Brooks & Corey parameters adjusted to fit the observed resistivity and resistivity gradients.

In cases where the irreducible water saturation is small and can be neglected equation 7 can be simplified to δ R/δ H ~ ηλ R, H^" , i.e. Rt₂ = Rti ( 1 + ηλ ΔΗ/Η_!), meaning that the combination ηλ can be estimated either from these expressions from measurements of resistivity and resistivity gradient at one elevation or by measuring resistivity at two elevations. This may then be sufficient to determine the condition of the reservoir. For example, a certain value of the parameter may be compared with that of a reference imbibition or drainage curve.

With reference to Figure 3, an embodiment of the method is described in further detail. On the left hand side of Figure 3, denoted A, a reservoir 1 has a transition region 4 extending between an upper portion 2 and a lower portion 3. The transition region 4 comprises a geological formation in which reservoir fluid is contained therein. The formation contains fluid comprising oil and water, with an oil-water contact present in the transition region at the lower end 3. The free water level is found below the oil- water contact in water wet reservoirs. The reservoir fluid contains increasingly more oil and less water with height within the region, i.e. moving from the lower portion 3 to the upper portion 2. Accordingly, the saturation of water reduces until the irreducible saturation is reached, whilst the oil component increases. As a result, the resistivity of the formation (i.e. the formation rock and the reservoir fluid within the rock) also increases with height, as explained further above.

A borehole 5 is drilled into the region 4 using drilling equipment 6 provided in a bore hole assembly. The drilling equipment includes a resistivity tool 7 which is used for acquiring a resistivity of the formation in proximity to the tool, in this example at height H 1 . In practice, the tool 7 may include a transmitter used to energise the formation, for example to generate an electrical current in the formation or transmit an electromagnetic field into the formation. The tool may further be adapted to measure a response from the energised formation e.g., measuring a component of said field or the current using a detector positioned a distance away from the transmitter. The measured response may be directly indicative of the resistivity or may otherwise be used to determine the resistivity. The drilling equipment may further include a directional resistivity tool 8, for acquiring a resistivity gradient of the formation in proximity to the tool, for example at height H1 . This tool may include a transmitter and a detector in the same way as the resistivity tool 7, except being adapted to measure a response that is indicative of a gradient or otherwise useable to determine the resistivity gradient.

In one variant, a combined tool may be used to acquire the resistivity and resistivity gradient, for example using the same transmitter and detector.

It will be appreciated that in other variants, a resistivity and/or resistivity gradient at a certain height may be pre-obtained from an earlier process, e.g. provided from historical measurements or a dataset.

Referring to the right hand side of Figure 3, denoted B, the resistivity and gradient may map to a second resistivity at H2 according to a set of Archie and Brooks & Corey parameters, indicating a first reservoir condition. Under different conditions, different resistivity and gradient values may be obtained at the borehole at H1 , and a second parameter set may be obtained, mapping to a similar resistivity at H2. This may be the case for an imbibition and drainage state, which can be distinguishable based on resistivity and gradient measurement or parameters derived therefrom.

In the general case, initial estimates of resistivity and resistivity gradient measured at first height H1 may be compared with estimates from Equation 6 and 7 by using initial values of n, λ, S_wirr and P_e obtained before drilling. Subsequently measured resistivity and resistivity gradients at two or more different elevations may be solved for the needed parameters.

Turning to Figure 4, a further example of how the method may be performed is indicated by the following steps: S1 : Identifying the free water level (FWL) and the transition region in the reservoir can be obtained either from landing the producer well close the oil-water-contact, from a pilot well before drilling a production well or from an offset well drilled at an earlier time. This may be a preliminary step for determining where to apply the model and where to drill.

S2: Providing the resistivity of the formation and the resistivity gradient of the formation. These properties are provided at a location in the transition region and may involve drilling a borehole and making measurements using drilling equipment during the drilling process.

S3: Inverting the resistivity and resistivity gradient to obtain the n, λ, S_wirr and P_e parameters. S4 & S5: Evaluating the model via Equation 6 at multiple heights within the region using the obtained parameters and producing a curve or profile of a formation property, e.g. the saturation of water or resistivity, as a function of height.

S6: Using the curve to determine a state of drainage of the reservoir. This may be done for example by comparing the estimated curve from the data with reference data.

It can be noted from the above that the determination of the condition of the reservoir can be obtained using the data in various ways. In a basic example, a simple resistivity and a resistivity gradient measurement can be considered on its own with reference to pre-provided reference curves, for example those in Figure 2b, and curve which fits the data identified. From the relevant curve, the condition of the reservoir can be inferred. In other embodiments, a parameter such as the combined exponent (ηλ) or other parameter may be calculated from the data, and compared to a corresponding exponent or parameter associated with a reference response such as the reference curves as in Figure 2b or other data e.g reference database or the like. In other embodiments still, the curve of saturation or resistivity may be calculated as a function of height within the reservoir region from the measured data and/or combined exponent using a parameterised model such as described above. The calculated curve could then be analysed to indicate the state of reservoir drainage. The calculated curve can also be compared with a reference curve or reference data. It should be appreciated that a number of measurements may be obtained using the drilling equipment as the borehole is drilled and used for providing resistivity and resistivity gradient data at several locations along the borehole. The resistivity tool may provide a measurement focused at different depths of investigation. This may provide resistivity data at different lateral positions and different heights. The resistivity and resistivity gradient at each such position may be used to determine the parameters for that position. For a common lateral position, two calculations for an exponent may be for example averaged, to produce a more robust prediction of the resistivity according to the model at other (non-measurement) heights.

It may also be noted that although the description above refers mainly to the prediction of resistivity, other properties of the formation may be predicted using the model parameter, such as the saturation of the water or hydrocarbon (e.g. oil) component, by solving Equation 5.

In Figure 5, there is shown a computer device 10 as may typically be used in performing the method. The computer device 10 comprises an In/Out device 12 which may be used for reading in the resistivity and resistivity gradient data, for example at the first height H1 . Further, the computer device has a microprocessor 13 and memory 14. The microprocessor is arranged to perform operations according to instructions provided in a computer program. The computer program may be stored in the memory 14. The computer program in this case may comprise instructions to determine the parameters based on the data in accordance with Equations 8 and 9, and may further comprise instructions to evaluate the model to predict a property of the formation, e.g. a saturation or resistivity using the parameters. The microprocessor is used for processing the data read in from the In/Out device and to execute the program using those data. The input data and results output from execution of the program may be stored in the memory. The results may be displayed on a display 15.

It will be appreciated that the computer device 10 may take the form of an individual unit, or alternatively may be a distributed arrangement where all or some of the components 12-14 are provided separately, for example in separate locations. It may also be noted that communication between the individual devices may take place wirelessly over a communication network. The computer program may be provided on a removable storage medium such as an optical disk, memory stick or the like. The storage medium may be connected to a computer device (for example remotely and wirelessly) when required so that the processor may execute the program stored on the disk.

The invention provides a number of advantages. For example, by determining a resistivity or saturation profile according to the model described above, a more useful resistivity image may be obtained for the reservoir region because it takes better account of the gradual change in oil and water contents near the oil-water contact. The simplified assumption of constant resistivity near the oil-water-contact in prior art techniques in many cases is not realistic. The transition profile can be obtained and reservoir condition determined simply from a measurement of resistivity and resistivity gradient in the transition region, for example while drilling. This is convenient. In addition, by obtaining the parameters which control the shape of the transition profile, it can reveal if the reservoir is in a primary drainage state or in an imbibition or secondary drainage state which is of great importance for reservoir monitoring. This will be important information for monitoring reservoir properties with for example 4D seismic and can act as constraints to time-lapse interpretation.

It can be noted that the term height is used to mean vertical position, level, or elevation, relative to some reference level.

Various modifications and improvements may be made without departing from the scope of the invention herein described.

Claims

CLAIMS:

1 . A method of determining a condition of a reservoir having at least one geological formation containing an amount of fluid that varies in dependence upon height within the reservoir, the method comprising the steps of:

a. providing data comprising either (i) a first resistivity and a resistivity gradient of the formation, or (ii) second and third resistivities of the formation at different respective heights;

b. providing at least one reference formation property response, or parameter thereof, which is dependent upon the amount of fluid and height within the reservoir; and

2. A method as claimed in claim 1 , wherein the reference formation property response comprises any one or more of an imbibition response, a primary drainage response, and a secondary drainage response.

3. A method as claimed in claim 1 or 2, wherein the determined condition is a state of drainage of the reservoir.

4. A method as claimed in any preceding claim, wherein the reference formation property response is obtained from either or both of: earlier-performed formation property measurements at the reservoir or theoretical data.

5. A method as claimed in any preceding claim, wherein the reference formation property response comprises either or both of resistivity or fluid saturation.

6. A method as claimed in any preceding claim, wherein the reference formation property response comprises a curve or profile of resistivity or fluid saturation in dependence upon height within the reservoir.

7. A method as claimed in any preceding claim, which further comprises estimating a resistivity or fluid saturation of the formation using the data to provide said component based on the data, and comparing the estimated resistivity or fluid saturation with the at least one reference formation property response to determine the condition of the reservoir.

8. A method of determining a condition of a reservoir having at least one geological formation containing an amount of fluid that varies in dependence upon height within the reservoir, the method comprising the steps of:

9. A method as claimed in claim 8, where step b is performed to estimate the formation property at a plurality of heights within the reservoir.

10. A method as claimed in claim 8 or claim 9, wherein the formation property comprises either or both of resistivity or fluid saturation.

1 1 . A method as claimed in any of claims 8 to 10, wherein step b is performed to estimate a curve or profile of the formation property in dependence upon height within the reservoir.

12. A method as claimed in any of claims 8 to 1 1 , wherein step b comprises performing an inversion of the data to estimate the formation property.

13. A method as claimed in any of claims 8 to 12, wherein step b further comprises using the data to determine a parameter of a model, and using either or both of the parameter and the model to estimate the formation property.

14. A method as claimed in claim 13, wherein the parameter comprises any one or more of the following in any combination:

i. a saturation exponent;

ii. a pore size distribution index; iii. irreducible water saturation ; or

iv. entry pressure.

1 5. A method as claimed in claim 1 3 or 14, wherein the model is or is based on,

R, (H) = Ro (S_wirr + (1 -SwirrX Δρ g H /Ρ_β)^~λ ) ⁿ wherein

Rt is the resistivity of the reservoir partially filled with hydrocarbon fluid;

H is a height above free water level;

R₀ is the resistivity of water filled reservoir;

g is gravity;

Δρ is the difference in density between said water and hydrocarbon fluid;

P_e is entry pressure;

S_wirr is irreducible water saturation;

A is a pore size distribution index; and

n is a saturation exponent.

16. A method of determining a condition of a reservoir having at least one geological formation containing an amount of fluid that varies in dependence upon height within the reservoir, the method comprising the steps of:

b. using the data to estimate at least one parameter for a formation property response which is dependent upon the amount of fluid and height within the reservoir; and

c. determining the condition of the reservoir based on the estimated parameter.

17. A method as claimed in claim 16, wherein the parameter comprises any one or more of the following in any combination:

i. a saturation exponent;

ii. a pore size distribution index;

iii. irreducible water saturation ; or iv. entry pressure.

1 8. A method as claimed in claim 1 6 or claim 1 7, wherein the parameter is a parameter of a model, and the model is or is based on,

R, (H) = Ro (S_wirr + (1 -SwirrX Δρ g H /Ρ_β)^~λ ) ⁿ wherein

Rt is the resistivity of the reservoir partially filled with hydrocarbon fluid;

H is a height above free water level;

R₀ is the resistivity of water filled reservoir;

g is gravity;

Δρ is the difference in density between said water and hydrocarbon fluid;

P_e is entry pressure;

S_wirr is irreducible water saturation;

A is a pore size distribution index; and

n is a saturation exponent.

19. A method as claimed in any of claims 16 to 1 8, which further comprises using either or both of the parameter and the model to estimate the formation property response.

20. A method as claimed in any of claims 1 6 to 19, wherein the reference formation property response comprises either or both of resistivity or fluid saturation.

21 . A method as claimed in any of claims 1 6 to 20, wherein the formation property response comprises a curve or profile of resistivity or fluid saturation in dependence upon height within the reservoir.

22. A method as claimed in any preceding claim, wherein said condition of the reservoir is a state of drainage of the reservoir.

23. A method as claimed in any preceding claim, wherein determining the condition of the reservoir in step c comprises determining whether the reservoir is in the state of one of primary drainage, imbibition, and secondary drainage.

24. A method as claimed in any preceding claim, wherein said fluid comprises any one or more of:

i. hydrocarbon gas;

ii. oil; or

iii. water.

25. A method as claimed in any preceding claim, wherein the data are obtained by performing measurements from the formation.

26. A method as claimed in any preceding claim wherein the step of providing data is performed by energising the formation and measuring or detecting a response to said energisation.

27. A method as claimed in any preceding claim, which further comprises drilling a borehole in said region, and wherein step a is performed during the process of said drilling.

28. Apparatus configured to perform a method as claimed in any preceding claim.

29. A computer program for use in performing the method of any of claims 1 to 27, the program being arranged to determine the condition of the reservoir using the data.

30. A computer device arranged to execute the program of claim 29, to determine said condition of the reservoir.

31 . A computer readable medium containing the program of claim 29.