CN1074833C - Method for detecting rock cation-exchange capacity - Google Patents

Abstract

The present invention relates to a new chemical method for measuring the cation exchange capacity (CEC) of rocks. A sodium ion exchange method is adopted to measure the CEC value (CEC) Na<+> of a rock sample. The quantitative correlation of the CEC value measured through the sodium ion exchange method and the CEC value (CEC) <NH<+>4> measured by a general ammonium ion exchange method is set up. The quantitative relational expression is used as a standard, so other rock samples to be measured only need convenient measurement of (CEC) Na <+>. Then the (CEC) Na <+> value is converted into the (CEC) <NH4<+>> value, and can be used as the numerical value of the actual cation exchange capacity. The method is suitable for measuring various clay minerals and the CEC values of rocks in various oil gas storage layers. The method has the characteristics of high measurement accuracy, good repeatability, time saving, low cost, etc.

Description

Method for measuring cation exchange capacity of rock

The invention relates to a novel method for determining the Cation Exchange Capacity (CEC) of rock. The method is suitable for measuring CEC values of various clay minerals and various oil and gas reservoir rocks. The invention adopts the chemical flame photometric method to determine the CEC value of the rock sample, establishes quantitative correlation with the CEC value determined by the classical chemical ammonium acetate leaching method, and is suitable for determining the CEC value of various clay minerals and various oil and gas reservoir rocks.

Common clay minerals in the formation rock comprise kaolinite, montmorillonite, illite, chlorite and the like, the clay minerals have ion exchange surfaces with different activity degrees, the ion exchange surfaces have great influence on the physical and chemical properties of the oil and gas reservoir rock, and the measurement of the cation exchange capacity of the formation rock provides important quantitative parameters for rock resistivity, natural potential, natural gamma and neutron logging and the like. The most common cation for which the exchange reaction can be carried out is Ca²⁺、Mg²⁺、K⁺、Na⁺And NH₄ ⁺The ions, typically expressed in milliequivalents of exchangeable cations (meq) per 100 grams of dry rock. CEC is determined by various methods, and at present, three methods are mainly adopted at home and abroad: A. conductivity measurements under various fluid mineralization conditions (Mortland and mellor); B. thin film potential method (Thomas); C. ammonium acetate leaching chemistries (Campos) and (Hilchie). Many mineralization methods do not require breaking rock samples, the measurements are relatively consistent, and the samples may be closest to reservoir conditions, but many have the disadvantage of being time consuming and requiring a special resistivity measuring device, and therefore expensive. The CEC value measured by the thin film potential measuring method is more accurate for rock samples with low effective clay content, but the CEC measured value is poorer in accuracy and unreliable for rock samples with high montmorillonite content, and the experimental method is troublesome. The ammonium acetate leaching chemical method is easy to realize, simple in instrument and equipment, good in analysis accuracy and repeatability, but multiple in measurement steps, time-consuming in analysis, large in analysis workload and low in efficiency, and one sample can be analyzed generally within three days.

GB2235488A discloses a method for measuring the Cation Exchange Capacity (CEC) of rock, which is a monolithic geological core (core material) that cannot be damaged at all during the measurement, and therefore requires analysis and measurement under non-aqueous conditions, and requires a specific experimental apparatus and a non-water-soluble organic compound solvent as a exchanging liquid, and the measurement process is complicated, long in cycle, and low in precision.

CN1062210A discloses a spectrophotometric method for measuring the cation exchange capacity of clay, which adopts NH used for a long time in the prior art₄ ⁺Exchange method, only with K⁺Exchange of NH₄ ⁺Determination of NH₄ ⁺Instead of the conventional and customary absorption of NH by hydrochloric acid₄The method comprises the following steps of performing two-step exchange, namely, taking an ammonium chloride solution as an exchange solution, exchanging cations for ammonium ions, taking a potassium chloride solution as the exchange solution to obtain a solution to be detected, containing the ammonium ions, measuring the content of the ammonium ions in the solution by a spectrophotometry method, and calculating the cation exchange capacity. The precision of the method is 0.5-1.0%, the accuracy of the determination is generally 5-10%, the determination speed is generally 3-4 days, and the problems of complex determination process, long period and low precision exist.

The invention aims to provide a novel chemical method for measuring the cation exchange capacity of rock, which has the advantages of simple instrument and equipment and measuring method, high analysis accuracy and good repeatability, simplifies the operation steps, takes less labor and time and is suitable for measuring CEC values of various clay minerals and various oil and gas reservoir rocks.

The invention is designed based on the basic principle of measuring rock-like cation exchange capacity, cations on clay minerals for balancing charge are bound on the surface of a crystal layer by coulomb force, when the clay minerals are in a hydroelectric medium, due to the action of water molecules adsorbed on the clay surface and water molecules hydrated around the cations, the balancing cations and the clay surface have a certain distance, the centers of the cations are positioned on a plane called OHP (outer Helmholtz plane) away from the clay Xd, and the coulomb force applied to the cations adsorbed on the clay surface is as follows:

F＝KQZ/Xd²

wherein K is a constant, and is related to the types of clay and medium, temperature, etc., Q is the average charge of the clay wafer, Z is the charge of the cation, and Xd is the closest distance between the cation and the clay wafer. Therefore, cations adsorbed on the same clay wafer are more easily adsorbed or exchanged on the clay surface as the amount of charge is larger andthe hydration radius is smaller (Xd is smaller) and the coulomb attraction force is larger. The hydration radii of some common cations are listed in table 1.

TABLE 1 hydration radius of certain cations Li⁺Na⁺K⁺Rb⁺Cs⁺

Mg²⁺Ca²⁺Ba²⁺Hydrated radius (. crclbar.) 3.823.583.313.293.293.314.284.124.04

For monovalent and divalent cations, the order of preference for adsorption or exchange on the crystal surface is:

Cs⁺，Rb⁺＞K⁺，＞Na⁺＞Li⁺

Ba²⁺＞Ca²⁺＞Mg²⁺

the invention is realized by the following steps: the first step in the determination of CEC values in rock samples is the use of Na⁺Cation (Mg) exchangeable on the surface of clay²⁺，Ca²⁺，K⁺，Na⁺And H⁺Etc.) are exchanged to become sody soil; in the second exchange step, the Na earth is replaced by Mg²⁺Exchanging Na on the first exchange⁺All the Na ions are replaced into an aqueous medium, and then the Na content in the second exchange liquid is measured by a flame photometer⁺The CEC value of the rock sample is calculated, and the principle of the method is represented by the following chemical formula:

the first step of exchange:

R-[Mg²⁺，Ca²⁺，K⁺，Na⁺，H⁺]+NaAC (PH＝5～7)

→R-Na⁺+Mg(AC)₂，Ca(AC)₂KAC, NaAC, HAC + NaAC (excess)

And a second step of exchange:

R-Na⁺+Mg(AC)₂(PH＝7)

→R-Mg²⁺+NaAC+Mg(AC)₂(surplus)

Wherein R represents rock (containing clay), NaAC and Mg (AC)₂Sodium acetate and magnesium acetate, respectively. The invention adopts NaAC and Mg (AC)₂Two-step exchange, Na is firstly⁺Total exchange of exchangeable cations on the surface of the rock sample, followed by Mg²⁺Adsorbing all Na on the surface of the rock sample⁺Then, the Na content was measured by flame photometer⁺And (4) calculating the CEC value. The invention respectively uses ammonium acetate method and flame photometry to measure corresponding rock samples of different typesAndand establishing a quantitative relation between the two, and obtaining a linear relation between CEC values determined by an ammonium ion exchange method and a sodium ion exchange method through a large amount of data: ${\left(\mathrm{CEC}\right)}_{{\mathrm{NH}}_4^{+}}=K{\left(\mathrm{CEC}\right)}_{N{a}^{+}}+b$

or ${\left(\mathrm{CEC}\right)}_{{\mathrm{Na}}^{+}}=[{\left(\mathrm{CEC}\right)}_{{\mathrm{NH}}_4^{+}}-b]/K$

Wherein the constant K (1.0-10.0) is the slope of the straight line, and the constant b (0.1-2.5) is the intercept of the straight line. The quantitative linear relation or linear relation graph is used as a standard, and only other rock samples to be measured need to be conveniently measured

Then convert to

And then the cation exchange capacity can be used as an actual cation exchange capacity value.

The CEC measuring method provided by the invention is sequentially carried out by the following steps:

1. block diagram of the assay:

sample pretreatment → decarbonation → first step ion exchange (with sodium acetate) → second step ion exchange (with magnesium acetate) → measurement of sodium ion content in the sample solution after exchange (with flame photometry) → calculation→ correction to

(standard linear equations or straight lines obtained with the present invention).

2. The determination step comprises:

(1) pretreatment of the sample: the pretreatment comprises oil washing, crushing and drying. Pre-crushing the rock sample into small pieces with the square of 1cm, performing oil washing distillation in a fat extractor by using extract containing 70% of toluene, 15% of acetone and 15% of ethanol (volume percentage), replacing solution containing 50% of ethanol and 50% of acetone after oil is cleaned, distilling again in the fat extractor, and washing out the extract residual in the rock sample. Crushing and screening the sample subjected to oil washing to 40-60 meshes, and drying the sample at 105-120 ℃ for 2-4 hours;

(2) removing carbonate in the rock sample: some rock samples contain carbonate and need to be removed in advance, and the method comprises the steps of accurately weighing 4-5 g of the pretreated 40-60-mesh rock samples, adding 50-60 ml of 1N HCl, heating to 50-60 ℃, removing the carbonate in the rock samples, washing the samples to be neutral by using distilled water, and removing the excessive hydrochloric acid.

(3) First step ion exchange: putting 150-200 ml (PH 5-7) of 1-2N NaAC solution into an erlenmeyer flask with a sample, carrying out exchange reaction for 2-4 hours in a constant-temperature oscillating water bath at 60-70 ℃, repeatedly carrying out exchange reaction for 2-3 times by using fresh NaAC exchange solution until the exchange reaction is completely carried out, and then washing residual Na on the surface of the rock sample by using distilled water⁺The cleaned rock sample surface is totally Na absorbed by exchange⁺；

(4) The second step of ion exchange: putting 150-200 ml of 1-2N alkaline earth metal acetate solution (PH 7) selected from one of Sr, Ca and Mg into a conical flask with a sample, carrying out exchange reaction for 2-4 hours in a constant-temperature shaking water bath at 60-70 ℃, repeatedly carrying out exchange reaction for 2-3 times by using fresh exchange solution until the exchange reaction is complete, and repeatedly filtering and washing after each exchange and collecting all filtrate and washing solution;

(5) and (3) measuring the content of sodium ions in the exchange test solution: in the second step of ion exchange, Mg is used²⁺Adsorbing Na on the surface of the rock sample⁺All exchange reaction is carried out, and Na content after each exchange reaction is collected⁺The Na content in the exchanged sample solution is measured by a flame photometer⁺Content (to the nearest 0.05 ppm);

(6) calculation of Na⁺CEC value of the exchange method is calculated by the following formula: ${\left(\mathrm{CEC}\right)}_{{\mathrm{Na}}^{+}}=\left[{\mathrm{Na}}^{+}\right]V/10\mathrm{WM}---\mathrm{meq}/100g$

in the formula: v is the total volume (ml) of the total exchange fluid collected in the second exchange stage, [ Na]⁺]For Na in the whole exchange liquid collected in the second exchange⁺Concentration (ppm), M is the atomic weight of sodium ions (22.99), W is the weight (g) of the dry rock sample;

(7) by

Is corrected to

；

Due to Na⁺Has a switching capacity less than

Therefore, it isIs less thanBuilt by the inventionAnd

linear relationship therebetween, by

Is corrected to

To do so by

The cation exchange capacity obtained by the exchange is the actual cation exchange capacity value of the rock sample.

About using

The method for measuring CEC by exchange is a common chemical method at present, and is reported at home and abroad since the 80 s. About

Methods for exchanging CEC assays can be found in reference to relevant data (e.g., j.c. campos and d.w. hilchie, SPWLA Symposium, 1980).

The block diagram of the measurement procedure is as follows:

sample pretreatment (Washing oil, crushing, drying → decarbonation → first step ion exchange (with NH)₄AC, repeated exchange 3-4 times) → washing to remove residues on the sample

Distilling off ammonia gas (adding solid MgO) in water in a sample of → rockExcess hydrochloric acid absorbs NH₄OH → titrate excess hydrochloric acid with standard NaOH solution → Calculation

${\left(\mathrm{CEC}\right)}_{{\mathrm{NH}}_4^{+}}=(N_{\mathrm{HCl}}{V}_{\mathrm{HCl}}-N_{\mathrm{NaOH}}{V}_{\mathrm{NaOH}})*100/W$

In the formula:

N_HClis hydrochloric acid absorption liquid equivalent concentration (N);

V_HClis the volume (ml) of hydrochloric acid absorption solution;

N_NaOHis the standard concentration (N) of sodium hydroxide solution;

V_NaOHvolume of sodium hydroxide (ml) consumed at titration;

w is the weight of the dry rock sample;the unit of (1) is meq/100g.

The method of the invention is completely different from the currently and commonly used ammonium acetate chemical exchange method, and the basic steps of the ammonium acetate exchange method are as follows:

1、R-[Mg²⁺，Ca²⁺，K⁺，Na⁺，H⁺]+NH₄AC(PH＝7)→R-+Mg(AC)₂，Ca(AC)₂，KAC，NaAC，HAC+NH₄AC (excess)

3. And titrating the excessive hydrochloric acid in the absorption liquid by using a standard NaOH solution, and calculating the equivalent number of consumed hydrochloric acid to obtain the CEC value of the rock sample.

The steps of the exchange method are cumbersome and include exchange, distillation, absorption and titration operations. With Na⁺The exchange can omit distillation, absorption and titration operations, but because ofHas a hydration radius of less than Na⁺Radius of hydration of (1) thereforeHas a stronger exchange capacity than Na⁺By using Na⁺Obtained by exchangeIs less than using

Obtained by exchange method (CEC)

Need to be aligned withBy using

Comparing and establishingThe quantitative relationship between the two was corrected.

The method for measuring the cation exchange capacity of the rock has the following advantages: 1. the method is simple, a special measuring instrument and a device are not needed, only common chemical reagents and chemical instruments are adopted, the measuring steps are mainly two-step ion exchange, and the distillation and chemical analysis steps are omitted compared with the common ammonium acetate exchange method, so that the time required by measurement can be shortened, the analysis of one sample can be completed only by 3-4 days when the CEC value is measured by the common chemical method, the sodium ion exchange method can complete the analysis of one sample in only 1-2 days, and a group of samples can be conveniently subjected to parallel tests simultaneously, so that the test efficiency is improved. 2. The method has the advantages that the determination accuracy is high, the repeatability is good, the accuracy of generally determining the CEC value is 5-10%, the CEC value of the pure clay minerals of montmorillonite, illite and kaolinite is respectively 89.5, 5.83 and 4.32meq/100g, and the CEC values are all in the range of the CEC standard values of the three pure minerals. The method provided by the invention is used for repeatedly measuring dozens of rock samples, and the relative error of the parallel test of the same sample is within the range of 0.1-3% (most of the relative errors are 1.0-1.5%), which is much lower than the relative error (5-8%) of the CEC value measured by other methods, and the repeatability is much higher. 3. The method is suitable for measuring CEC values of various clay minerals and various oil and gas reservoir rocks, and the reliability of the CEC measured value is still higher for rock samples with high montmorillonite content. 4. The determination method is simple, low in cost and good in feasibility.

Example 1:

(1) pretreatment of the sample: taking montmorillonite, illite and kaolinite (the purity is more than 97%) as tested rock samples, crushing the rock samples to obtain 40-60 meshes, drying the samples at 120 ℃ for 3 hours, and accurately weighing 3-5 g of each sample (each tested rock sample is called 2-3 parallel test samples);

(2) first step ion exchange: the method comprises the steps of placing 200ml (PH is 5) of 1N sodium acetate (NaAC) solution into a conical flask containing a sample, carrying out exchange reaction in a constant-temperature oscillating water bath at 65-70 ℃ for 3 hours (4-5 samples can be placed in the constant-temperature oscillating water bath at the same time), separating and removing the exchange solution through a high-speed centrifugal separator (4000-5000 r/min), washing the sample with distilled water, replacing 200ml of fresh 1N AC exchange solution, and repeating the first exchange, separation and washing until the exchange reaction is completely carried out (generally, three times of exchange is needed). Residual Na on the cleaned surface of rock sample⁺The surface of the obtained rock sample is totally Na absorbed by exchange⁺；

(3) The second step of ion exchange: with 1N magnesium acetate [ Mg (AC)₂]200ml of the solution (pH 7) is placed in an erlenmeyer flask containing the sample after the first step of exchange and is put into a constant temperature oscillating water bath at 65-70 DEG CThe reaction mixture was subjected to a column exchange reaction for 3 hours, separated by a high-speed centrifugal separator and washed with distilled water (purification of distilled water must be strictly controlled to prevent Na⁺Present), the whole separation and washing liquid was collected and reused with fresh 1N Mg (AC)₂150ml of exchange solution is subjected to exchange, separation and washing by repeating the first condition until the exchange reaction is completely carried out (generally, exchange is carried out for three times), and the separation solution and the washing solution after each exchange are completely collected and fully and uniformly mixed;

(4) and (4) measuring the content of sodium ions in the uniformly mixed test solution, and measuring by using a flame photometer to obtain the content of sodium ions of 0.05 ppm.

(5) CEC values were calculated for each sample by sodium ion exchange method according to the following formula: ${\left(\mathrm{CEC}\right)}_{{\mathrm{Na}}^{+}}=[\mathrm{Na}+]V/10\mathrm{WM}---\mathrm{meq}/100g$ wherein V is the total volume (ml) of the total exchange solution collected in the second exchange step and [ Na]⁺]The concentration (ppm) of sodium ions in all the exchange liquid collected in the second step of exchange, M is the atomic weight (22.99) of the sodium ions, and W is the weight (g) of the dry rock sample;

(6) from Na⁺Correction of the CEC values determined by the exchange method to

CEC value determined by the exchange method:

built by the invention

And (CEC) Na⁺Linear relation (or straight line) between them to obtain

Is the cation exchange capacity of the actual rock sampleNumerical values.

For the inventionAnd Na⁺Two exchange methods measure more than 20 rock samples of various types, and establish

And

the linear relationship between: ${\left(\mathrm{CEC}\right)}_{{\mathrm{NH}}_4^{+}}=K{\left(\mathrm{CEC}\right)}_{{\mathrm{Na}}^{+}}+b$

or ${\left(\mathrm{CEC}\right)}_{{\mathrm{Na}}^{+}}=[{\left(\mathrm{CEC}\right)}_{{\mathrm{NH}}_4^{+}}-b]/K$

Wherein the constant K (2.8-3.0) is the slope of the straight line, and the constant b (0.8-1.0) is the intercept of the straight line. The cation exchange capacities of the montmorillonite, the illite and the kaolinite are respectively 89.5, 5.83 and 4.32meq/100g measured by the steps, and the relative error of the parallel test is 0.1-0.7%.

Example 2:

(1) pretreatment of the sample: taking oil reservoir rock samples of the victory oil field and the Liaohe oil field as an example, the rock samples contain various clay minerals such as montmorillonite, kaolinite and the like, and generally contain oil and carbonate, so the pretreatment of the samples comprises oil washing, crushing and drying. The rock sample is pre-crushed into 1cm square pieces, and the remaining extract is washed away by distillation in an extractor with extract containing 70% toluene, 15% acetone and 15% ethanol (all by volume percentage) in a fat extractor. And crushing the samples after oil washing, taking the samples of 40-60 meshes, drying the samples at 120 ℃ for 3 hours, and accurately weighing 3-5 g of each sample.

(2) Removing carbonate in the rock sample: adding 50-60 ml of 1N hydrochloric acid into the rock sample, heating to 50-60 ℃, removing carbonate in the rock sample, washing the sample with distilled water to be neutral, and removing the excessive hydrochloric acid;

(3) first step ion exchange: as with the step (2) of the example 1, only the separation of the rock sample after the exchange reaction and the exchange liquid is changed into suction filtration by a sand core funnel, and because the rock sample of the oil storage layer is easy to keep the original granularity after being oscillated in the exchange liquid and is easy to separate, a high-speed centrifugal separator is not needed;

(4) the second step of ion exchange: as with the step (3) of example 1, only the separation of the rock sample after the exchange reaction and the exchange solution was performed by suction filtration with a sand core funnel;

(5) and (3) measuring the content of sodium ions in the mixed test solution: same as in step (4) of example 1;

(6) the CEC value of the sodium ion exchange method was calculated for each sample: same as in step (5) of example 1;

(7) from Na⁺Correction of the CEC values determined by the exchange method to

CEC value determined by the exchange method: same as in step (6) of example 1.

The cation exchange capacities of a certain rock sample of the victory oil field and a certain rock sample of the Liaohe oil field are measured to be 5.113 and 11.530meq/100g respectively through the steps, and the relative error of a parallel test is 0.5-1.0%.

Claims (1)

1. A method for measuring the cation exchange capacity of rock sequentially comprises the following steps:

(1) pretreatment of the sample: the method comprises the steps of washing oil, crushing, drying and weighing, wherein sample washing oil is carried out in a fat extractor, and after washing oil is distilled, residual washing oil liquid is washed by 50% ethanol and 50% acetone;

(2) reacting 1-3N hydrochloric acid with the rock sample to remove carbonate in the rock sample;

(3) first step ion exchange: adopting sodium ions to completely exchange exchangeable cations on the surface of the clay, and performing exchange reaction on 150-200 ml of 1-2N sodium acetate (NaAC) solution with pH of 5-7 and a rock sample in a shaking water bath at 60-70 ℃ for 2-4 hours, wherein the exchange reaction is repeated for 2-3 times by using fresh NaAC exchange liquid each time until the exchange reaction is completely performed;

(4) the second step of ion exchange: sodium ion Na exchanged in the first step by alkaline earth metal ion⁺Completely replacing the raw materials into an aqueous medium, performing exchange reaction on the raw materials with 1-2N alkaline earth metal acetate solution selected from one of strontium (Sr), calcium (Ca) and magnesium (Mg) and 150-200 ml of rock sample in a shaking water bath at 60-70 ℃ for 2-4 hours, repeatedly performing the exchange reaction for 2-3 times by using fresh exchange solution each time until the exchange reaction is completed, and filtering, washing and collecting all filtrate and washing liquid;

(5) measuring the sodium ion content in the second exchange test solution by using a flame photometer with the accuracy of 0.05 ppm;

(6) calculating the Cation Exchange Capacity (CEC) of the rock sample obtained by sodium ion exchange_Na+A value;

(7) composed of (CEC)_Na+And (CEC)_NH4+The linear relationship between:

or (CEC)_Na+＝[(CEC)_NH4+-b]/K

Wherein the slope of the constant K line is 1.0 to 10.0, the intercept of the constant b line is 0.1 to 2.5, and the measured rock sample is measured by the sodium method (CEC)_Na+Then corrected to ammonium method determined (CEC)_NH4+And taking the cation exchange capacity as the cation exchange capacity value of the actual rock sample.