Methods of determining cation exchange sites in rock core samples

Abstract

A method for determining properties of different cation exchange sites in a rock core sample may include providing a rock core sample that is in either a preserved state or a non-preserved state, wherein a preserved form of the rock core sample includes a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites, a plurality of cation exchange sites occupied by a crude oil, and one or more fluids occupying pore spaces in the rock core sample; subjecting the rock core sample to a plurality of coreflooding steps, the plurality of coreflooding step displacing the plurality of indigenous exchangeable cations and the one or more fluids in at least two separate coreflooding steps to render the rock core sample clean of indigenous exchangeable cations; and determining an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites.

Claims

1. A method for determining properties of different cation exchange sites in a rock core sample in a reservoir, the method comprising: providing a rock core sample, wherein the rock core sample comprises a preserved form of the rock core sample including a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites, a plurality of cation exchange sites occupied by a crude oil, and one or more fluids occupying pore spaces in the rock core sample, wherein the preserved form of the rock sample is in the original reservoir condition; subjecting the rock core sample to a plurality of coreflooding steps, the plurality of coreflooding step displacing the plurality of indigenous exchangeable cations and the one or more fluids in at least two separate coreflooding steps to render the rock core sample clean of indigenous exchangeable cations; and determining an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites.

2. The method of claim 1, wherein the provided rock core sample is in the original reservoir condition.

3. The method of claim 1, further comprising: determining a pore volume of the rock core sample.

4. The method of claim 1, wherein the amount of indigenous exchangeable cations is quantified from an extract of an injection fluid upon completion of extraction by an analytical method.

5. The method of claim 4, wherein the injection fluid is an ammonium acetate solution.

6. The method of claim 1, wherein the subjecting the rock core sample to a plurality of coreflooding steps comprises: displacing the crude oil in the rock core sample with a formation brine until oil ceases production; displacing an excess of cations present in a plurality of interstitial pore spaces of the rock core sample by using a first organic solvent; and displacing the plurality of indigenous exchangeable cations from the cation exchange sites of the rock core sample with a first injection fluid until completion of extraction.

7. The method of claim 6, wherein the amount of indigenous exchangeable cations is quantified from an extract of the first injection fluid upon completion of extraction by an analytical method.

8. The method of claim 6, wherein the first organic solvent is ethanol.

9. The method of claim 1, further comprising: displacing all native components out of the pore space of the rock core sample; subjecting a non-preserved form of the rock core sample clean of native components to a coreflooding steps to inject formation brine into the rock core sample; injecting a reservoir crude oil into the rock core sample until reaching irreducible water saturation and equilibrium between formation brine, the reservoir crude oil, and the cation exchange sites, such that rock core sample includes a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites, a plurality of cation exchange sites occupied by a crude oil, and one or more fluids occupying pore spaces in the rock core sample.

10. The method of claim 9, wherein displacing all native components out of the pore space of the rock core sample comprises: alternately injecting a second organic solvent and a third organic solvent, wherein the third organic solvent is the last injected to render the pore space of the rock core sample clean of native components.

11. The method of claim 10, wherein the second organic solvent is toluene and the third organic solvent is methanol.

12. A method for determining an amount of indigenous exchangeable cations adsorbed onto cation exchange sites in a rock core sample in a reservoir, at a preserved state of the rock core sample, wherein the rock sample is in the original reservoir condition, the method comprising: providing a rock core sample that includes at least a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites and a plurality of cation exchange sites occupied by a crude oil; displacing the crude oil in the rock core sample with a formation brine until oil ceases production; displacing an excess of cations present in a plurality of interstitial pore spaces of the rock core sample by using a first organic solvent; displacing the plurality of indigenous exchangeable cations from the cation exchange sites of the rock core sample with an injection fluid until completion of extraction; and calculating an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites.

13. The method of claim 12, wherein the amount of indigenous exchangeable cations is quantified from extracts of the injection fluid upon completion of extraction by analytical methods.

14. The method of claim 12, further comprising: determining a pore volume of the rock core sample.

15. A method for determining an amount of indigenous exchangeable cations adsorbed onto cation exchange sites in a rock core sample in a reservoir, at a non-preserved state of the rock core sample, wherein the rock sample is not in the original reservoir condition, the method comprising: providing a rock core sample in a non-preserved state; displacing all native components out of the pore space of the rock core sample by alternately injecting a first organic solvent and a second organic solvent, wherein the second organic solvent is the last injected; displacing the second organic solvent with a formation brine to adsorb a plurality of exchangeable cations onto the different cation exchange sites of the rock core sample; injecting a reservoir crude oil into the rock core sample until reaching irreducible water saturation and equilibrium between formation brine, the reservoir crude oil, and the cation exchange sites, such that rock core sample includes a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites, a plurality of cation exchange sites occupied by a crude oil, and one or more fluids occupying pore spaces in the rock core sample; displacing the reservoir crude oil in the rock core sample with formation brine until oil ceases production; displacing an excess of cations present in a plurality of interstitial pore spaces of the rock core sample by using a third organic solvent; displacing the plurality of indigenous cations adsorbed onto the cation exchange sites of the rock core sample with a second injection fluid until completion of extraction; and calculating an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites.

16. The method of claim 15, wherein the amount of indigenous exchangeable cations is quantified from extracts of the injection fluid upon completion of extraction by analytical methods.

17. The method of claim 15, further comprising: determining a pore volume of the rock core sample.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a flow chart according to one or more embodiments of the present disclosure.

(2) FIGS. 2-4 show schematics of a rock core sample during sequential coreflooding operations in accordance with one or more embodiments of the present disclosure.

(3) FIG. 5 shows a flow chart according to one or more embodiments of the present disclosure.

(4) FIGS. 6-10 show schematics of a rock core sample during sequential coreflooding operations in accordance with one or more embodiments of the present disclosure.

DETAILED DESCRIPTION

(5) In one aspect, embodiments disclosed herein relate to methods of rock sample analysis to provide determinations concerning different cation exchange sites present in the rock core samples. Clay minerals in a reservoir or rock sample have negatively charged sites (X.sup.−) on their surfaces which adsorb and hold cations (e.g., Ca.sup.2+, Mg.sup.2+, Na.sup.+, and K.sup.+) by electrostatic force. In particular, the present methods are directed to methodologies that allow for determinations concerning the contents of different cation exchange sites in a rock core sample, specifically the amount of each indigenous cation adsorbed onto the exchange sites, which may also be referred to as reservoir representative exchangeable cations.

(6) Conventional methods of rock core analysis do not provide for distinctions based on different cation exchange sites, i.e., differentiating between Ca.sup.2+, Mg.sup.2+, Na.sup.+, and K.sup.+, for example. However, in accordance with embodiments of the present disclosure, a rock core sample in a preserved state or a non-preserved state may be subjected to a series of coreflood steps to provide such differentiation by considering the indigenous exchangeable cations of Na.sup.+, K.sup.+, Ca.sup.2+ and Mg.sup.2+ adsorbed onto cation exchange sites ([NaX].sub.e, [KX].sub.e, [CaX.sub.2].sub.e and [MgX.sub.2].sub.e). This may advantageously allow for enhanced oil recovery operations to be designed based on such different cation exchange sites and the properties thereof to result in greater efficacy in EOR operations. As used herein, the indigenous exchangeable cations adsorbed onto the cation exchange sites in the native state in the reservoir. Thus, the present methods may determine the different exchange sites, whether or not the rock core sample was preserved. However, as discussed below, the methodology varies depending on whether or not the rock core sample is preserved or not. As used herein, when the rock is in a preserved state, it, and specifically the cation exchange sites, is in the original reservoir condition, whereas a non-preserved state is not in the original reservoir condition.

(7) As described herein, the present methodology uses coreflooding to sequentially displace native components out of the rock core sample and inject (and displace) replacement fluids therethrough during the analysis. In one or more embodiments, the present methods separately displace excess components such as fluids (including excess cations) from the pore spaces, then indigenous cations from exchange sites (by replacing the indigenous cations with replacement cations) in order to quantify the different indigenous cations. In embodiments involving a rock core sample that is in a non-preserved state, the pore space of the rock core sample is entirely cleaned of all native components. Formation brine and then reservoir crude oil may be injected into the rock sample until reaching irreducible water saturation and equilibrium to replicate the rock sample in a native state. Then, the present method may separately displace excess components such as fluids (including excess cations) from the pore spaces, and then indigenous cations from exchange sites (by replacing the indigenous cations with replacement cations), in order to quantify the different indigenous exchangeable cations.

(8) Such a coreflooding system may include a coreholder, a pumping system, an effluent collection system, a measurement system, as well as temperature and pressure control so that coreflooding experiments may be conducted at conditions mimicking reservoir conditions. Such systems are commercially available. Coreflooding may be utilized on rock types having a permeability of at least 0.1 millidarcy.

(9) Preserved State

(10) Referring now to FIG. 1, a flow chart according to one or more embodiments is shown. As shown, stage 10 may include providing a rock core sample that includes a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites, a plurality of cation exchange sites occupied by a crude oil, and one or more fluids occupying pore spaces in the rock core sample. Stage 20 may include subjecting the rock core sample to a plurality of coreflooding steps, the plurality of coreflooding step displacing the plurality of indigenous exchangeable cations and the one or more fluids in at least two separate coreflooding steps to render the rock core sample void of indigenous cations. Stage 30 may include determining an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites.

(11) FIGS. 2-4 illustrate schematics of a rock core sample during progressive stages of the present coreflooding operations occurring in a rock core sample at a preserved state. Initially, the rock core in the preserved state (i.e., in original reservoir conditions, including preserving the cation exchange sites) may be coreflooded with formation brine to displace crude oil from the rock core sample. Formation brine may be injected until oil ceases production from the rock core sample. Referring to FIG. 2, FIG. 2 shows a schematic of a rock core sample 60 following the initial coreflooding with a formation brine. As shown in FIG. 2, rock particles 65 have exchange sites 70 present on the surface thereof. The exchange sites 70 are shown to have some cations 100 and some oil 105 adsorbed thereto. The cations 100 adsorbed onto the exchange sites 70 are referred to as the indigenous exchangeable cations. Otherwise, following the initial coreflooding with a formation brine 110, the pore spaces between the rock particles 65 are occupied by formation brine 110, including an excess of cations 115 (not adsorbed to exchange sites 70) therein. It is also envisioned that some quantity of residual oil 120 may also be present in the pore spaces between rock particles 65.

(12) Following the displacement illustrated in FIG. 2, the rock core sample 60 may be coreflooded with an organic solvent, such as but not limited to 95% ethanol, to displace the excess cations 115 (shown in FIG. 2) from the rock core sample 60. To complete the displacement of excess cations 115, a large volume of organic solvent may be used, for example, ranging from an estimated 50 to 80 pore volumes. For the purpose of estimating the volume of organic solvent or other fluid that may be used for the coreflooding, the pore volume may be estimated by measuring the length and diameter and assuming a porosity of 30% for the rock core sample. The effect of such displacement is shown in the schematic illustrated in FIG. 3. As shown in FIG. 3, while the exchange sites 70 still have the cations 100 and oil 105 adsorbed thereto, the pore space between the rock particles 65 is now occupied by organic solvent 125.

(13) Following the displacement of excess cations, the effect of which is illustrated in FIG. 3, the rock core sample 60 is conducted with an injection fluid to displace the cations 100 (i.e., indigenous exchangeable cations) adsorbed on the exchange sites 70 out of the rock sample 60, the effect of which is illustrated in the schematic shown in FIG. 4. As shown in FIG. 4, following such displacement, in addition to displacing cations (100 in FIG. 3), the organic solvent (125 in FIG. 3) is also displaced from the rock sample 60 such that the pore space between the rock particles 65 is occupied by injection fluid 130 in FIG. 4. Injection fluid 130 may include a replacement cation 135, such as NH.sub.4.sup.+ adsorbed onto the exchange sites 70. Additionally, injection fluid 130 may also include excess replacement cations 140 that are not adsorbed onto the exchange sites 70, but which are present in the injection fluid 130. To ensure complete displacement of indigenous exchangeable cations 100, about 50-80 pore volumes of injection fluid 130 may be injected into rock sample 60. In one or more embodiments, the injection fluid 130 may be an ammonium acetate solution, having a concentration ranging from 0.5 to 2.0 M and a pH ranging from 7 to 8.5. It is also envisioned that other injection fluids such as a hexaaminecobalt (III) solution may be used.

(14) From the extract collected from the coreflooding with the injection fluid, the amount/concentration of the indigenous exchangeable cations (those cations 100 that were originally adsorbed to exchange sites 70, e.g., Na.sup.+, K.sup.+, Ca.sup.2+ and Mg.sup.2+) in the injection fluid extract may be determined by analytical methods, such as but not limited to ion chromatography (IC) specifically cation chromatography, atomic spectroscopic methods such as atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), atomic emission spectrometry (ICP-AES), and optical emission spectrometry (ICP-OES), as well as capillary electrophoresis (CE). In one or more embodiments, the amount of indigenous exchangeable cations may be considered as a mole equivalent per liter of pore volume and represented as [NaX].sub.e, [KX].sub.e, [CaX.sub.2].sub.e, and [MgX.sub.2].sub.e.

(15) In one or more embodiments, the pore volume of the rock core sample 60 may be determined by NMR. Preferably, this determination may be performed as the rock core sample 60 is in a state illustrated in FIG. 3, which may allow for fewer complexities in the NMR determination. The pore volume may be used to quantify the amount of cations relative to the pore volume of the rock core sample 60. However, a NMR analysis performed at another time may account for the presence of more than one fluid, such as a brine and oil.

(16) Non-Preserved State

(17) Referring now to FIG. 5, a flow chart according to one or more embodiments is shown. As indicated from reference to stages having the same reference number as used in FIG. 1, the methodology presented in FIG. 5 diverges from that in FIG. 1 in the initial stages, but then upon restoring the rock core sample to as near native form as possible, the final stages of the present embodiment share common stages as discussed in FIG. 1. As shown, initially, stage 40 may include providing a rock core sample in a non-preserved state. While the rock core sample is provided in a non-preserved state, a preserved form of the rock core sample includes a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites, a plurality of cation exchange sites occupied by a crude oil, and one or more fluids occupying pore spaces in the rock core sample. Stage 50 may include displacing all native components out of the pore space of the rock core sample. Stage 60 may include subjecting the rock core sample clean of native components to a coreflooding steps to inject formation brine into the rock core sample. Stage 70 may include injecting a reservoir crude oil into the rock core sample until reaching irreducible water saturation and equilibrium between formation brine, the reservoir crude oil, and the cation exchange sites, such that rock core sample includes a plurality of indigenous exchangeable cations adsorbed onto the cation exchange sites, a plurality of cation exchange sites occupied by a crude oil, and one or more fluids occupying pore spaces in the rock core sample. Following stage 70, the exchange sites in the rock core sample may be equivalent to native form. Thus, the method may follow a similar set of stages discussed with respective to the embodiments using a rock core sample in a preserved state. Next, stage 20 may include subjecting the rock core sample to a plurality of coreflooding steps, the plurality of coreflooding step displacing the plurality of indigenous exchangeable cations, the crude oil, and the one or more fluids in at least two separate coreflooding steps to render the rock core sample void of the indigenous cations. Stage 30 may include determining an amount of indigenous exchangeable cations adsorbed onto the cation exchange sites.

(18) FIGS. 6-10 illustrate schematics of a rock core sample 80 during progressive stages of the present coreflooding operations occurring in a rock core sample at a non-preserved state. Initially, the rock core sample 80 in the non-preserved state (i.e., not at original reservoir conditions) has a plurality of rock particles 85. The rock particles have exchange sites 90 present on the surface thereof. While there may be exchangeable cations 95 adsorbed to the exchange sites, because the rock core sample 80 is a non-preserved stated, the exchangeable cations 90 may not be used in determining the properties of the exchange sites. Thus, the rock core sample is coreflooded with an alternating sequence of a plurality of organic solvents. Such organic solvents may include at least one solvent that may be effective to remove any residual oil present in the rock core sample 80, including oil adsorbed to the exchange sites 90 as well as residual oil present in the pore spaces between rock particles. The effect of such coreflooding is shown in FIG. 6. Additionally, the organic solvents may also include at least one solvent that is effective to remove water and salts from the pore space of the rock core sample 80. In one or more embodiments, one solvent may be toluene and the other may be methanol. It is envisioned that the solvent miscible in water (e.g., methanol) may be the last organic solvent injected into the rock core sample 80 (such that the water miscible organic solvent can be completely displaced out of the rock core sample by formation brine as descried in the following paragraph). As shown in FIG. 6, the rock core sample 80, after alternating injection of organic solvents, may have the exchangeable cations 95 adsorbed to the exchange sites 90, with the pore space being occupied entirely by the water-miscible organic solvent 145.

(19) Following the alternating solvent injection, the effect of which is illustrated in FIG. 6, a formation brine may be coreflooded into the rock core sample 80 to displace the water-miscible organic solvent 145 from the rock sample core 80. As shown in FIG. 7, following such displacement, formation brine 150 may be present in the pore spaces between rock particles 85. Formation brine 150 is injected in sufficient volume for complete displacement of the water-miscible organic solvent (145 in FIG. 6). Cations in formation brine exchange with exchangeable cations 95 such that exchangeable cations 155 are adsorbed to all exchange sites 90 present on the surface of rock particles 85. Excess cations 160 in formation brine 150 are also present in the pore spaces between rock particles 85. In order to have complete displacement and equilibrium of the exchange sites 90, a large volume of formation brine 150 may be used, for example, ranging from an estimated 50 to 80 pore volumes. For the purpose of estimating the volume of formation brine or other fluid that may be used for the coreflooding, the pore volume may be estimated by measuring the length and diameter and assuming a porosity of 30% for the rock core sample.

(20) In one or more embodiments, the pore volume of the rock core sample 80 may be determined by NMR. Preferably, this determination may be performed as the rock core sample 80 is in a state illustrated in FIG. 7, as the rock core sample 80 contains a single fluid type therein, which may allow for fewer complexities in the NMR determination. The pore volume may be used to quantify the amount of cations relative to the pore volume of the rock core sample 80. However, a NMR analysis performed at another time may account for the presence of more than one fluid, such as a brine and oil.

(21) Having determined the pore volume, the rock core sample 80 may be coreflooded with a reservoir crude oil until reaching irreducible water saturation and equilibrium between formation brine, crude oil, and cation exchange sites 90. In particular, a large volume of reservoir crude oil may be used, such as about 50-80 pore volumes. Reaching such equilibrium, the rock core sample 80, while originally in a non-preserved state, has now been brought into an estimated native state based on the injection of formation brine and reservoir crude oil until reaching irreducible water saturation and equilibrium at the exchange sites 90.

(22) After being brought to an estimated native state, the rock core sample 80 may be coreflooded with formation brine to displace crude oil from the rock core sample. Formation brine may be injected until oil ceases production from the rock core sample. Referring to FIG. 8, FIG. 8 shows a schematic of a rock core sample 80 following the coreflooding with formation brine to displace crude oil. As shown in FIG. 8, rock particles 85 have exchange sites 90 present on the surface thereof. The exchange sites 90 are shown to have some cations 100 and some oil 105 adsorbed thereto. The cations 100 adsorbed onto the exchange sites 90 are referred to as the indigenous exchangeable cations. Otherwise, following the coreflooding with a formation brine 110, the pore spaces between the rock particles 85 are occupied by formation brine 110, including an excess of cations 115 (not adsorbed to exchange sites 90) therein. It is also envisioned that some quantity of residual oil 120 may also be present in the pore spaces between rock particles 85.

(23) Following the displacement illustrated in FIG. 8, the rock core sample 80 may be coreflooded with an organic solvent, such as but not limited to 95% ethanol, to displace the excess cations 115 (shown in FIG. 8) from the pore space of the rock core sample 80. To complete the displacement of excess cations 115, a large volume of organic solvent may be used, for example, ranging from an estimated 50 to 80 pore volumes. The effect of such displacement is shown in the schematic illustrated in FIG. 9. As shown in FIG. 9, while the exchange sites 90 still have the cations 100 and oil 105 adsorbed thereto, the pore space between the rock particles 85 is now occupied by organic solvent 125.

(24) Following the displacement of excess cations, the effect of which is illustrated in FIG. 9, the rock core sample 80 is conducted with an injection fluid to displace the cations 100 (i.e., indigenous exchangeable cations) adsorbed on the exchange sites 90 out of the rock sample 80, the effect of which is illustrated in the schematic shown in FIG. 10. As shown in FIG. 10, following such displacement, in addition to displacing cations (100 in FIG. 9), the organic solvent (115 in FIG. 9) is also displaced from the rock sample 80 such that the pore space between the rock particles 85 is occupied by injection fluid 130 in FIG. 10. Injection fluid 130 may include a replacement cation 135, such as NH.sub.4.sup.+, adsorbed onto the exchange sites 90. Additionally, injection fluid 130 may also include excess replacement cations 140 that are not adsorbed onto the exchange sites 90, but which are present in the injection fluid 130. To ensure complete displacement of indigenous exchangeable cations 100, about 50-80 pore volumes of injection fluid 130 may be injected into rock sample 80. In one or more embodiments, the injection fluid 130 may be an ammonium acetate solution, having a concentration ranging from 0.5 to 2.0M and a pH ranging from 7 to 8.5. It is also envisioned that another injection fluid such as hexaaminecobalt (III) chloride solution may be used.

(25) From the extract collected from the coreflooding with the injection fluid, the amount/concentration of the indigenous exchangeable cations (those cations 100 that were adsorbed to exchange sites 90, e.g., Na.sup.+, K.sup.+, Ca.sup.2+ and Mg.sup.2+ after the rock core sample 80 was brought to an estimated native state) in the injection fluid extract may be determined by analytical methods, such as but not limited to ion chromatography (IC) specifically cation chromatography, atomic spectroscopic methods such as atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), atomic emission spectrometry (ICP-AES), and optical emission spectrometry (ICP-OES), as well as capillary electrophoresis (CE). In one or more embodiments, the amount of indigenous exchangeable cations may be considered as a mole equivalent per liter of pore volume and represented as [NaX].sub.e, [KX].sub.e, [CaX.sub.2].sub.e, and [MgX.sub.2].sub.e.

(26) As shown, irrespective of the state of the rock core sample, the present methods may provide for determinations concerning cation exchange sites in a rock sample, in a manner that differentiates between different cation exchange sites, such as Na.sup.+, K.sup.+, Ca.sup.2+, and Mg.sup.2+. Specifically, the amount of exchangeable cations which are representative of the reservoir, specifically [NaX].sub.e, [KX].sub.e, [CaX.sub.2].sub.e, and [MgX.sub.2].sub.e, may be determined. Using such determinations, an enhanced oil recovery operation may be better designed, for example, in terms of the compositional components included in an EOR injection fluid, whether in water flooding, or chemical flooding such as surfactant flooding, polymer flooding, alkaline/surfactant/polymer flooding, or reservoir preflushes for the chemical flooding processes, or the like.

(27) Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.