Method for determining gas saturation of tight reservoir

This application claims priority to Chinese Patent Application No. 201910104816.1, filed on Feb. 1, 2019, which is hereby incorporated by reference in its entirety.

The present invention relates to a method for determining the gas saturation of a tight reservoir, which belongs to the technical field of oil and gas determination.

The natural gas content of underground rocks is usually calculated using a volume method, which requires four key parameters: gas saturation, porosity, rock density, and formation factor. Among them, the gas saturation varies widely. At present, the gas saturation of a tight reservoirs is measured by natural gas injection in a lab. Natural gas is injected into the rock with saturated water while maintaining a certain filling pressure. When it is observed that the water no longer flows out of the outlet, the rock core is taken out, followed by weighing the reduced mass of the rock core before and after the experiment to calculate the injected gas volume, and its ratio to the pore volume is the gas saturation. By changing different injection pressures, the gas saturation under different filling pressure conditions can be obtained.

Tight reservoirs have low porosity, are easily affected by experimental conditions, and have increased errors. Specifically, this method has the following disadvantages: 1. during the experiment, the liquid measurement is not accurate, the tight reservoir itself has low porosity, and the water content of the rock is small, which is easily affected; if the processes of sample loading, sampling and weighing take a long time, the water in the rock is volatile, resulting in a low water content; the adsorption of water on the surface of the rock tends to increase the water content; 2. the end condition of the experiment is to observe no more water flowing out of the outlet, the water existing in the pipeline cannot be observed, and the filled water cannot be seen in the pipeline, leading to the end of the experiment in advance, and a smaller gas saturation; 3. to determine the gas saturation at multiple pressure points, one need to take a sample to determine the gas saturation at each pressure point, then saturate water again and determine the saturation corresponding to the next pressure point; the sample is repeated for a long time and the process is tedious; 4. repetitive filling of rocks and saturating water can easily change the pore structure of rocks, such as resulting in micro-fractures; 5. repetitive filling of wet rocks and saturating water can easily change the wettability of rocks, for example, if becoming from hydrophilic to hydrophobic, it is easy to form dominant channels, resulting in incomplete gas filling; 6. it is difficult to fully achieve the temperature and pressure conditions in the experiment, the fluid pressure is less than 70 MPa and the temperature is less than 200° C.; 7. only a single value for total gas saturation can be determined.

In order to solve the above technical problems, an object of the present invention is to provide a method for determining the gas saturation of a tight reservoir, which can overcome the disadvantages of the methods in the prior art, such as complexity, inaccurate results, difficulty in meeting underground high temperature and high-pressure geological conditions, etc.

In order to achieve the above object, the present invention provides a method for determining the gas saturation of a tight reservoir, comprising:

a free water saturation calculation step: determining the pore size distribution of the tight reservoir rock sample, and calculating the free water saturation of the tight reservoir rock sample from the pore size distribution;

a water-membrane water saturation calculation step: calculating the water-membrane water saturation of the tight reservoir rock sample;

a corner water saturation calculation step: calculating the corner water saturation of the tight reservoir rock sample;

a gas saturation calculation step: calculating the gas saturation of the tight reservoir rock sample according to the following equation:

S g =100− S w

wherein S w is the water saturation in %; the water saturation is the sum of the free water saturation, the water-membrane water saturation and the corner water saturation;

S g is the gas saturation in %.

The determination method provided in the present invention is based on assuming that the reservoir rock contains only two fluids, formation water and natural gas, and the gas saturation is directly affected by the rock water saturation; if the reservoir water saturation is obtained, the gas saturation can be obtained; the size and stacking relationship of the rock-forming mineral particles determine the geometry and spatial distribution of the fluid storage space; the formation water exists in rocks in three main forms: (1) free water: pore water that cannot be expelled by natural gas, occupying small pore throats that cannot be filled, and its saturation is affected by natural gas filling pressure and pore throat capillary force; (2) water-membrane water: adsorbed on the pore wall in the form of a thin membrane, its saturation is related to the surface area of the pore wall and the thickness of the water-membrane; (3) corner water: existing in the corners that are difficult to displace among the particles, and its saturation is affected by the size and the contact relationship of the rock particles.

In the above method, the pore size distribution of the tight reservoir rock sample can be determined by mercury porosimetry, nuclear magnetic resonance method, etc., but it is not limited to the above two methods. The basic principle of rock pore diameter measurement using the mercury porosimetry is that mercury does not wet the rock surface and is a non-wetting phase, and the air in the rock pores is a wetting phase. The process of mercury injection into rock pores is to displace the wetting phase with a non-wetting phase. Due to the effect of the capillary pressure at the pore throat, mercury cannot enter the pores when the pressure is low. Only when the injection pressure exceeds the capillary pressure, mercury can enter the pore throat of the rock sample. The injection pressure at this time is the capillary pressure. The capillary radius corresponding to the capillary pressure is equivalent to the pore throat radius, and the volume of mercury entering the pores is the pore volume. The injection pressure is increased such that mercury enters the pores controlled by a smaller throat. In this manner, by continuously increasing the injection pressure, the relationship between the injection pressure, that is, the capillary pressure, and the pore volume can be obtained. The gas saturation of the wetting phase can be calculated from the volume of mercury injected and the pore volume of the tight reservoir rock sample, and then the curve of the pore size distribution of different pore throat radius ranges and the capillary pressure can be obtained. In the free water saturation calculation step, the data of the pore size distribution includes data of pore spaces corresponding to different pore throat radii.

In the above method, preferably, in the free water saturation calculation step, the data of pore spaces corresponding to different pore throat radii includes the pore throat radii corresponding to the pore spaces with different radii and the percentages of pore volumes occupied by the pore spaces with different radii.

In the above method, preferably, in the free water saturation calculation step, the pore size distribution of the tight reservoir rock sample is determined by the mercury porosimetry as follows:

injecting mercury into rock pores to displace air in rock pores;

when the injection pressure exceeds the capillary pressure, mercury entering the pore throat of the rock sample, and at this time, the injection pressure being the capillary pressure, the capillary radius corresponding to the pressure is equivalent to the pore throat radius, and the volume of mercury entering the pores being the pore volume;

increasing the injection pressure such that mercury enters the pores controlled by a smaller throat, to obtain the pore throat radius and the pore volume corresponding to the smaller throat; and repeatedly increasing the injection pressure to obtain the pore throat radii and the pore volumes corresponding to different throats.

In the above method, since the surface tension of the fluid system in the laboratory is different from that of the oil reservoir system, it is necessary to convert the laboratory's capillary pressure data into the oil reservoir capillary pressure. Assuming that the “J” function is a parameter of rock characteristics and does not change from the laboratory to the gas reservoir, the mercury porosimetry curve can be converted into the relationship between the water saturation and the filling pressure (e.g., FIG. 3 ), according to the actual gas reservoir conditions. In the free water saturation calculation step, the free water saturation of the tight reservoir rock sample is calculated from the pore size distribution as follows:

(1) obtaining the relationship between the injection pressure and the pore volume from the pore throat radii and the pore volumes corresponding to different throats in the tight reservoir rock sample;

wherein P mercury-air is the capillary pressure in 10 6 Pa; σ mercury-air is the fluid interfacial tension between mercury and air in mN/m; θ mercury-air is the wet contact angle between mercury and air in °; r is the capillary radius in 10 −9 m; in the mercury porosimetry, σ mercury-air =480 mN/M, θ mercury-air =140°;

(2) calculating the gas saturation from the volume of mercury injected and the pore volume of the tight reservoir rock sample, and then obtaining the curve of the pore size distribution of different pore throat radius ranges and the capillary pressure:

σ gas-water is the interfacial tension in mN/m; preferably, the gas-water interfacial tension G gas-water =20 mN/m;

θ gas-water is the gas-water contact angle; preferably, θ is 0, and cos θ=1;

P filling is the gas-water filling pressure in MPa;

r is the capillary radius in 10 −9 m;

(3) converting the curve of the pore size distribution of different pore throat radius ranges and the capillary pressure into the relationship curve of free water saturation and capillary pressure; obtaining the free water saturation from the filling pressure, that is, calculating the corresponding pore throat radius from the filling pressure, and summing the pore volumes larger than this radius, the percentage of this sum to the total pore volumes being the free water saturation.

In the above method, preferably, the water-membrane water saturation calculation step is performed as follows:

(1) it is inappropriate for the irreducible water saturation method to regard the thickness of the water-membrane in the reservoir as a constant value, and the thickness of the water-membrane in the reservoir cannot be the lower limit of the available pore throat radius in the reservoir (see FIG. 1 ). Using Dlvo theory, the calculation equation of the water-membrane thickness is given, that is, calculating the water-membrane thicknesses in the pore spaces corresponding to different pore throat radii according to the following equation:

wherein, a is a constant; the more hydrophilic the reservoir is, the greater is its value, which is preferably 1.18×10 −7 ; h is the water-membrane thickness in μm; σ is the fluid interfacial tension in mN/m; r is the pore throat radius in 10 −9 m;

(2) as seen from the above, a layer of water-membrane will leave after uniform oil and water displacement in a single pipe; the water-membrane is an integral part of the irreducible water; the thickness of the water-membrane will become thinner as the surface tension increases, and will gradually increase as the pore size becomes larger; the volume occupied by the water-membrane is compared with the capillary volume to calculate the irreducible water saturation of the water-membrane, that is, calculating the water-membrane saturation according to the following equation:

wherein S wmi is water-membrane water saturation of the pore space corresponding to the i-th pore throat radius;

r is the pore throat radius which corresponds to the pore space corresponding to the i-th pore throat radius;

h is the water-membrane thickness which corresponds to the pore space corresponding to the i-th pore throat radius.

In the above method, the corner water saturation of the reservoir is affected by the shape of the pores. In ideal cases, such as regular arrangement of spherical particles, triangular arrangement of spherical particles, regular triangle pores, square pores, regular hexagonal pores, the water saturation values can be calculated. For specific reservoirs, a simpler method can be used for the calculation of corner water saturation by observing the pore structure of the reservoir and directly determining the corner water saturation, or by counting the proportions of different types of pore structure, and determining the corner water saturation by weighted average. Preferably, the corner water saturation calculation step is performed by one of:

(1) observing the pore structure of the reservoir and directly determining the corner water saturation;

(2) counting the proportions of corner water corresponding to different types of pore structure, and determining the corner water saturation by weighted average;

(3) the pressure of the water-membrane on the surface of the particles is unstable, and it will gather towards the corner. In the particle model, most of the irreducible water is distributed in the corners between the particles (see FIG. 2 ), forming corner water. The water saturation equation of the particle model is as follows, and the corner water saturation can be calculated according to the following equation:

wherein, A2 is the circular cross-sectional area of the air-water interface;

A3 is the cross-sectional area of the corner water;

r is the arc radius of air-water interface, that is, the pore throat radius,

r = 2 ⁢ σ p filling

in 10 −9 m;

R is the particle radius of the tight reservoir rock sample in 10 −9 m;

S wyi is water saturation of the pore space corresponding to the i-th pore throat radius;

P filling is the filling pressure in MPa;

σ is the fluid interfacial tension in mN/m.

In the above method, preferably, the gas saturation calculation step is performed as follows:

(1) calculating the water saturation according to the following equation:

wherein S w is the water saturation;

λ i is the percentage of pore volume occupied by the pore space corresponding to the i-th pore throat radius;

S wyi is the corner water saturation in the pore space corresponding to the i-th pore throat radius;

S wmi is the water-membrane water saturation in the pore space corresponding to the i-th pore throat radius;

S wz is the free water saturation, that is, the proportion of pore volume occupied by free water pore space;

(2) calculating the gas saturation of the tight reservoir rock sample according to the following equation:

S g =100− S w

wherein S w is water saturation in %; the water saturation is the sum of the free water saturation, the water-membrane water saturation and the corner water saturation;

S g is the gas saturation in %.

According to a specific embodiment of the present invention, preferably, the method provided in the present invention comprises the following steps:

a free water saturation calculation step:

determining the pore size distribution of the tight reservoir rock sample by mercury porosimetry; when the injection pressure exceeds the capillary pressure, mercury entering the pore throat of the rock sample, and at this time, the injection pressure being the capillary pressure, the capillary radius corresponding to the capillary pressure being equivalent to the pore throat radius, and the volume of mercury entering the pores being the pore volume;

increasing the injection pressure such that mercury enters the pores controlled by a smaller throat, to obtain the corresponding pore throat radius and pore volume;

continuously increasing the injection pressure to obtain the relationship between the injection pressure, that is, the capillary pressure, and the pore volume;

calculating the gas saturation from the volume of mercury injected and the pore volume of the tight reservoir rock sample, and then obtaining the curve of the pore size distribution of different pore throat radius ranges and the capillary pressure;

converting the pore size distribution data of the tight reservoir rock sample into the capillary pressure of oil reservoir; and converting the mercury porosimetry curve into the relationship of water saturation according to the displacement pressure difference, in consideration of the interfacial tension and the contact angle under the conditions of actual gas reservoir temperature and pressure:

converting the curve of pore size distribution data and capillary pressure of the tight reservoir rock sample into the curve of free water saturation and capillary pressure, to obtain the filling pressure, so as to obtain the free water saturation; that is, calculating the corresponding pore throat radius from the filling pressure, and summing the pore volumes larger than this radius, the percentage of this sum to the total pore volumes being the free water saturation.

σ gas-water is the interfacial tension in mN/m; preferably, the gas-water interfacial tension σ gas-water =20 mN/m;

θ gas-water is the gas-water contact angle; preferably, θ is 0, and cos θ=1;

P filling is the filling pressure in MPa;

r is the capillary radius in 10 −9 m;

a water-membrane water saturation calculation step:

calculating the water-membrane thicknesses in the pore spaces corresponding to different pore throat radii according to the following equation:

wherein a is a constant; the more hydrophilic the reservoir is, the greater is its value, which is preferably 1.18×10 −7 ; h is the water-membrane thickness in μm; σ is the fluid interfacial tension in mN/m; r is the pore throat radius in 10 −9 m;

calculating the water-membrane saturation according to the following equation:

wherein S wmi is the water-membrane water saturation of the pore space corresponding to the i-th pore throat radius;

r is the pore throat radius which corresponds to the pore space corresponding to the i-th pore throat radius;

h is the water-membrane thickness which corresponds to the pore space corresponding to the i-th pore throat radius;

a corner water saturation calculation step:

calculating the corner water saturation according to the following equation:

wherein r is the arc radius of air-water interface, that is, the pore throat radius,

r = 2 ⁢ σ p filling

in 10 −9 m;

R is the particle radius of the tight reservoir rock sample in 10 −9 m;

S wyi is the water saturation of the pore space corresponding to the i-th pore throat radius;

P filling is the filling pressure in MPa;

σ is the fluid interfacial tension in mN/m;

a gas saturation calculation step:

calculating the water saturation according to the following equation:

wherein S w is the water saturation;

λ i is the percentage of pore volume occupied by the pore space corresponding to the i-th pore throat radius;

S wyi is the corner water saturation in the pore space corresponding to the i-th pore throat radius;

S wmi is the water-membrane water saturation in the pore space corresponding to the i-th pore throat radius;

S wz is the free water saturation, that is, the proportion of pore volume occupied by the free water pore space;

then calculating the gas saturation of the tight reservoir rock sample according to the following equation:

S g =100− S w

wherein S w is the water saturation in %; the water saturation is the sum of the free water saturation, the water-membrane water saturation and the corner water saturation; S g is the gas saturation in %.

The method for determining gas saturation of a tight reservoir provided in the present invention has the following beneficial effects:

1. using model calculations to avoid errors in the determination results of the gas saturation caused by water volatilization, surface adsorption, and observation of water flow during experiments;

2. avoiding the effect of repeated rock filling on the pore structure and wettability;

3. directly obtaining the gas saturation at multiple pressure points, which is simple and fast;

4. meeting the conditions of underground temperature and pressure, where the fluid pressure can be greater than 70 MPa, and the temperature can be greater than 200° C.;

5. separately obtaining the saturation and proportion of three kinds of water: free water, water-membrane and corner water;

6. not only determining the single value of total gas saturation, but also giving the gas saturation distribution at different pore sizes.

As compared with the existing method, the determination method of the present invention has shorter test time, requires less sample, has no requirements on the shape of the sample (either core plunger or rock cuttings), is safe and environmentally friendly, can use no mercury (e.g., Examples 2 and 3), and saves test gas. By subdividing the irreducible water saturation of the prior art into water-membrane water saturation and corner water saturation, the method of the present invention can overcome the disadvantages of the methods in prior art, such as complexity, inaccurate results caused by insufficient filling time, difficulty in meeting underground high temperature and high pressure geological conditions, etc, while it avoids errors in determination results of the gas saturation caused by water volatilization, surface adsorption, and observation of water flow during experiments.

FIG. 1 is a schematic diagram of water-membrane water saturation calculation.

FIG. 2 is a schematic diagram of corner water calculation in a particle model.

FIG. 3 is a relationship diagram between the filling pressure and the gas saturation in Example 1.

FIG. 4 is a relationship diagram between the filled pore throat radius and the gas saturation in Example 1.

FIG. 5 is a relationship diagram between the filling pressure and the gas saturation in Example 2.

FIG. 6 is a relationship diagram between the filled pore throat radius and the gas saturation in Example 2.

FIG. 7 is a relationship diagram between the filling pressure and the gas saturation in Example 3.

FIG. 8 is a relationship diagram between the filled pore throat radius and the gas saturation in Example 3.

In order to have a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the technical solutions of the present invention will now be described below in detail, but cannot be understood as limiting the implementable scope of the present invention.