WO2000031209A1

WO2000031209A1 - Precipitation of inorganic salts in porous media

Info

Publication number: WO2000031209A1
Application number: PCT/NO1999/000341
Authority: WO
Inventors: Alkiviades C. Payatakes; Peter Koutsoukos; Terje Östvold; Peter Read
Original assignee: ICE/HT-FORTH
Current assignee: ICE/HT-FORTH
Priority date: 1998-11-26
Filing date: 1999-11-12
Publication date: 2000-06-02
Anticipated expiration: 2001-05-26
Also published as: NO985505D0; EP1153102A1; NO985505L; NO313203B1; AU1417400A

Abstract

Present invention concerns a method for precipitation of inorganic salts in porous media, with controlled consolidation and permeability loss. Inorganic salts are precipitated within the pore structure and inter granular spaces of granular sand stones such as unconsolidated hydrocarbon reservoir strata or similar, or within the pore structure of water permeable natural rocks. A first solution of a first soluble salt and a second solution of a second soluble salt are mixed at the desired place for precipitation, and the method comprises use of a spacer solution in order to control the pH and reaction rate. The first solution and the second solution are injected alternating with the spacer solution between and preferably last, and a pumping device is connected to the desired place, which device creates a reciprocating flow pulsation in that the device works both in forward and backward direction.

Description

Precipitation of inorganic salts in porous media.

This invention relates to a method for precipitation of inorganic salts in porous media with controlled consolidation and permeability loss, in accordance with the preamble of Claim 1.

Background

Sand production from a soft or poorly consolidated formation is a persistent problem which causes severe damage to the well-casing pipes, valves, pumps etc., and, most important, reduces oil production levels or may even lead to the collapse of the local reservoir. Moreover, removal of the sand from very viscous oil at the later stages in production operation is a costly and time-consuming procedure, which requires additional production stages and equipment or chemical additives.

A number of techniques have been proposed for controlling sand production. These techniques include in situ chemical consolidation of the formation adjacent to the well, which is unconsolidated, with the use of different agents such as various types of resinous material (e.g. furan epoxy and phenolic resins, sodium silicate based aluminum oxide, cementing material and silicon halide). Other techniques include placing tubular screens in the well bores and gravel packs between the sand formations and the screen. Gravel pack and/or hard resins fill the space, which is in contact with the formation and the screen to prevent the flow of sand.

Despite the fact that these techniques have proved to be successful to some extent, each of them has its own set of problems and limitations. For instance, it is not certain that every perforation becomes tightly packed with gravel, and such failure may lead to sand flowing into the production well. On the other hand, epoxy resins besides their high costs, may lead to the undesirable effect of reducing the well productivity because of the significant reduction of the formation permeability around the well bore. Furthermore, the additional hardware and procedures required by this method, and the associated costs, are additional disadvantages with high cost penalty. On the other hand, water permeation in natural rocks may be a considerable problem for the construction and use of underground facilities. During construction water leaks may be plugged with different types of organic chemicals or fine grained and rapidly consolidating cements. The problem, however, often is the very fine pore structure found in natural rocks. The penetration of the above-mentioned compounds into the water permeable pores may therefore be difficult, and the consequence is continued water penetration.

Objects To increase oil production from a reservoir, which consists of weakly consolidated sandstone ("sandy" reservoirs), the grains of reservoir rock must be interconnected and consolidated to prevent sand production. In this way the permeation of both water and oil may be reduced. By performing a controlled precipitation of inorganic salts in the porous structure of the near well bore area, sufficient consolidation may be achieved with acceptable permeability loss. The main object of the invention is to provide a method with the following advantages:

- oil production should be increased following the consolidation treatment by increasing sand free draw down pressure,

- the well treatment consists of bullheading¹ with proper solutions compatible with the formation in the near well bore area using the techniques described below, and

- in cases of accidental well damage the process can be completely reversed by a traditional acid wash.

Another objective of the invention is to stop water leaks in underground constructions. Water leaks in connection with tunnels, dams and other types of underground constructions may sometimes be a significant problem. To stop such leaks a maximum precipitation of inorganic salts in the pore structure of the natural rock is needed. The main advantages with this method are:

- the very good wettability and the permeation of the aqueous solutions in the natural rock since no solids are present in the solutions to be injected, - the compatibility of the aqueous solutions with the natural surroundings, and

- bullheading the proper aqueous solutions into the rock structure using the techniques described below can perform the rock treatment.

Invention The objectives are fulfilled with a method according to the characteristic part of claim 1. Further features of the invention are defined in the accompanying dependent claims.

¹ Bullheading is pumping two miscible liquids with such rate as to minimise the mixing of one into other at the junction between them in the tubing. Present invention provides a method, which can be used, for two different applications; the re-mediation of unconsolidated hydrocarbon reservoirs and the effective plugging of water permeable natural rocks, through the controlled precipitation of properly selected inorganic salt(s). A method according to the present invention makes it possible to achieve adequate consolidation of initially unconsolidated or poorly consolidated hydrocarbon reservoirs to a sufficient depth around the production well (e.g. 0J0 to 0,50 m), without excessive loss of permeability. The invention can also be applied for the effective plugging of underground constructions, in cases where water leaking is a significant problem. The use of inorganic salts to assist rock consolidation and or plugging of water leaks combines several advantages. Inexpensive materials are used, compatible with the natural environment, and the desirable consolidation or rock porosity plugging is obtained in an environmentally friendly way. The consolidation of hydrocarbon reservoirs producing sand, which causes serious problems to the oil and hydrocarbon production, avoids losses in production volume and other operational costs. Another major advantage of the proposed method is that the delivery of aqueous solutions into the reservoir around the well can be performed with relatively minor modifications of the hardware in the well. This is opposed to other methods (such as the injection of epoxy resins, etc.), that require complex equipment and procedures. Finally, a considerable advantage of the proposed method is that it is readily and fully reversible, because the inorganic salts that are used to consolidate the granular stratum are soluble in simple acids. Thus errors can be corrected and fine-tuning can be achieved.

During rock porosity plugging the very fine pores are not accessible with traditional injection methods. The permeation of aqueous solutions, however, is good, and water under pressure should reach the finest pores in the water permeable rock, especially in rocks, which are water wet, and water leaks can be closed.

The use of inorganic salts for the two objectives may be different, but similar placement techniques are used for the aqueous solutions to be injected with the active precipitating minerals.

The invention will in the following be described with reference to the accompanying figures, where:

Fig. 1 a shows a cylindrical device, mounted in an oil field well or in a bore hole of a water leaking rock, injecting the solutions,

Fig. lb shows a device in three parts, where each part ejects one solution, Fig. lc shows a system of perforated tubes for the injection of the solutions,

Fig. 2 shows a number of perforated tubes injecting the solutions in an oil field reservoir or a bore hole of a water leaking rock,

Fig. 3a shows a device of tubes for injection of solutions, where the tubes are not inserted into the formation,

Fig. 3b shows a cross section of fig. 3a,

Fig. 4 shows an alternative to fig. 3 with only three tubes for injection of solutions,

Fig. 5 shows the placement of the 3 solutions using the Crossover technology

Fig. 6 shows the concentration profiles at the end of the injection procedure, Fig. 7 shows the concentration profiles at the end of the mixing,

Fig. 8 and 9 show injection procedures used in the examples,

Fig. 10 shows a consolidated sand pack, consolidated with the procedure according to the invention,

Fig. 11 shows a picture of two grains connected by OCP crystals, and Fig. 12 shows the structure of OCP crystals.

Two aqueous solutions with different compositions of inorganic materials compatible with the natural environment are mixed in situ. Proper selection of the inorganic materials, the manner of their transport, mixing, and reaction leads in all cases to the precipitation of sparingly oil and water soluble salts. This precipitation again leads to the objects of the invention.

Consolidation of granular strata with the desired loss of permeability is achieved as follows: the crystals of the inorganic salts are small compared to the pores and precipitate and grow at grain boundaries or on the grain surfaces forming uniform coatings. Thus, the grains connect and interlock with each other forming a consolidated material.

Drastic reduction of permeability of natural rocks and their eventual plugging is achieved through the precipitation and growth of large crystals that tend to fill and plug the pores or the inter-granular spacing. Both the nature and moφhological characteristics of the substrates affects the crystals forming. Best results are obtained by introducing the reactants in a series of predetermined, variable pulses using an appropriate pumping device. The degree of loss of permeability of the porous formation, and the degree of consolidation can be controlled by the type and by the amount of inorganic salts being deployed in the permeable region of the system. The two soluble salts are accordingly introduced in the sand bed in successive layers. To avoid reaction of the salts near the boundary layers, and to control pH and reaction rate, a spacer solution is placed between the salt solution layers. Then, a gentle flow pulsation is applied. Pulsation increases the rate of mixing within the porous medium through the phenomenon of dispersion in porous media. For a typical experimental condition the pulsation time is 0.5-5 min, preferably 2 min, the amplitude is 0,5-5 cm, preferably 2 cm, the frequency is 0J-5 Hz, preferably 1Hz, and the relaxation time is 5-15 min preferably 8 min. It was found that the effective diffusivities are 10³ to 10⁴ times larger than the respective molecular diffusivities. Perfect mixing ensures complete reaction of the soluble salts to produce insoluble crystallites. Pulsation also increases the rate of salt crystallite deposition.

The method of injecting the solutions in the sand bed or the rock is very important and determines the degree of success or failure of the consolidation or plugging processes. The proposed process is designed in such a way as to utilize the inorganic salts fully, and to reduce as much as possible the duration of the process. Figures 1-5 show alternative methods for the in-situ injection of the chemical solutions.

After the first injection-process and the relaxtion time, there are in most cases, several consecutive injection-processes. The near well bore formation is then filled with the non- reactive solution mixture from the previous injection. This solution may either be pushed further into the formation by the next injection, or allowed to return to the injection-site. In the beginning of the consolidation-process, it will be most advantageous to push the solultions futher into the formation, as their return probably will wash out sand from the still loosely consolidated formation. As the sand gets consolidated, it will be most advantageous to release the pressure after det relaxtion step, so that the non-reactive solution mixture may return to the injection-site to be removed.

Placement of the solution and in-situ mixing

Figure la shows one of the proposed in-situ consolidation procedures. It shows a cylindrical device with diameter approximately equal to the diameter of the well or the bore-hole in the rock. The vessel has been immersed into the well as far as the lower end of the poorly consolidated granular stratum, or to the position of the water leak in the bore hole. Elastomer (or equivalent function) seals are placed between the device and the walls in order to prevent undesirable leakage. The chemical solutions (the first solution A, the spacer solution N, and the second solution B) are injected with the aid of a centrifugal pump, successively, from the top of the device. The injected volume of each solution is much larger than the volume of the device. Thus, the solutions move radially and they are distributed into the porous formation. After the termination of the injection procedure, the device works by alternatively pressurizing and depressurizing the system (pulsation) in order to achieve mixing. This pulsation lasts for a sufficiently long time to ensure that the mixing has been successfully completed (typically 2-3 minutes). The relaxation step follows after the mixing of the solutions. A typical relaxation time is 8 minutes and during this period the chemicals are mixed, react and the insoluble salt precipitates. The entire procedure is repeated until the in-depth consolidation of the walls which are in contact with the outer surface of the device, is completed. The device then moves upward to a higher position, and the entire procedure is repeated. The device continues moving upwards until the consolidation of the poorly consolidated stratum is completed or until the water leaks are stopped.

Figures lb and lc show similar equipment that can be used instead of the device of Figure la. Figure lb shows a device in three parts that can be used for the successive injection of the solutions instead of the cylindrical device shown in Fig. la. The procedure in this case is slightly different from the one described before. Firstly, we select the area to be treated. Each part ejects only one of the solutions needed for the formation of the insoluble salt. The part of the device containing solution B is in contact with the surface and with the aid of a pumping device the predetermined volume of this solution is injected. Next, the device moves downward in order to allow for the second part, to contact the wall at the same area and the spacer solution (a very diluted aqueous KOH solution, 10³ M) is injected and distributed radially into the formation. The same step is repeated for a third time and the third solution (A) is injected. After the third injection, we apply pulsation for better mixing and the procedure is repeated until the satisfactory result for the selected area. Then, the same procedure is repeated in a different part of the surface,

- until the consolidation of the entire poorly consolidated stratum,

- until no water is produced from the bore-hole.

Figure lc shows a similar device in which a system of perforated tubes that can be used as alternative equipment for the injection of the solutions instead of the cylindrical device shown in Fig. la. The tubes, which are perforated and allow the injection of the chemical solutions, replace the device in the case described above.

The idea illustrated in Figures 2 is quite different in comparison with case 1. A number of capillary tubes are inserted into the formation. The tubes are perforated and solutions can be diffused to the unconsolidated formation. All the tubes are connected and end up in a cylindrical tube, which supplies the solutions. In each horizontal level there are at least two uniformly distributed implanted tubes. Thus, the same quantities of the solutions pass through each tube and eventually the entire adjacent porous formation is consolidated. The number radial distribution of the tubes and, consequently, the radial distribution of the chemical solutions ensure the uniform consolidation of the formation. This radial distribution of tubes is repeated every ~30 cm of the well depth until the total length of the poorly unconsolidated stratum is covered. From each tube the first A, the second B and the spacer N solutions may be either passed alternating or only one at a time. Another option of this idea is to inject only one solution into the porous formation in every horizontal level. With only two perforated tubes in each horizontal level, the solutions are injected from one tube while the other is used to apply suction. The solution is thus forced to move circularly into the porous formation. After the injection of the first solution, the injection and the flow of the other two solutions in curvilinear flow lines follow. When the injection procedure is over, the pulsation facilitates solution mixing in situ (within the porous formation).

Figures 3 a-b and 4 present a different approach for the in-situ injection: A system of tubes is inserted into the well, and the tube ends touch the well walls. The length of each tube extends out to the well and their ends touch the walls of the well. The different solution may be injected into the formation the same way as described for figure 2, above. Figures 4 shows a system with three tubes only. Each tube ejects only one solution. The tubes move upwards and downwards to consolidate the desired area as in case 1. As soon as the selected area is consolidated, the three-tube system moves to another area and the procedure is repeated until sufficient consolidation of the entire formation is achieved. At the end of this method the tubes are removed from the well.

The device that is used in the Crossover Gravel-Pack technique (Figure 5) can be used for the alternating injection of the chemical solutions into the formation, as it is proposed in the present invention. Solution A may be first injected with the aid of a pumping device through the central tube. The major part of solution A is injected into the formation through the screen annulus. The rest of the volume of solution A returns to its reservoir (or to the surface) through the wall annulus following the flow path that is shown in Figure 5. Then solution KOH is injected through the central tube. A part of it intrudes into the formation through the screen annulus and the rest of it removes the remains of solution A from the bulk space between the device and the screen. In the same way, solution B is inserted into the formation and a part of it removes the remains of solution KOH and so on. The procedure is repeated a number of times until the desired volume for consolidation is filled with the chemical solutions. The chemical solutions are mixed with the same pressure pulsation technique that has been mentioned above. One further approach is to use a small bore tubing, such as coiled tubing typically 1.25 inches in diameter, inserted inside the well to conduct the sequenced slugs of solutions A, spacer solution and B into the formation to be consolidated. The interval to be treated may be isolated by placement of a retrievable packer at the base of the interval and an expandable packer carried on the small-bore tubing. Alternatively a straddle packer can be used.

An amount of solution A is pumped into the small-bore tubing followed by a volume of spacer solution followed by an amount of solution B. The volume of the spacer solution is such as to prevent premature mixing of solutions A and B within the insert tubing. This sequence is repeated such as to develop a train of sequenced slugs of the solutions passing down the insert tubing and out into the selected and isolated formation interval. As the solutions disperse radially from the well bore they become thinned and once placed, can be pulsated to allow the precipitation reaction to take place. The process may be repeated as many times as needed in order to achieve the desired rock strength.

The placement of the aqueous solutions for plugging of water leaks in underground constructions (rocks) can be made using the same techniques as described in the above figures. The essence of the injection method is either to

- inject the aqueous solutions to be mixed in separate neighboring bore-holes in the rock above and along the ceiling of e.g. a leaking tunnel in such a way that the solutions meet in the pores of the rock with the resulting mineral precipitation and pore-plugging, or - inject the two aqueous solutions, A + B, in the same bore-hole with the intermediate neutral solution, C, to separate the solutions A and B from being mixed before they reach the rock pores. Then a pulsating pressure is applied to mix the aqueous solutions A and B inside the pore structure of the rock, utilizing the phenomenon of axial and lateral dispersion.

Insoluble Salt Precipitation

Several salts have been tested in the laboratory. Some salts have been proven to consolidate sand beds effectively with very low permeability reduction and are suitable for oil well application. These salts are therefore proper for the main object. Other salts can diminish the permeability of the underground formation so much that water transport is eliminated, and are therefore more suitable to civil engineering application. The chemical equation for both cases can be written in the following general form:

MX₂ (aq.)+ B₂Y (aq.) = MY(s) + 2BX (aq.)

Salt, Salt₂

The crystals of the precipitated sparingly soluble MY salt deposit uniformly around the grain surfaces linking them without reducing the permeability significantly. Depending on the nature, shape and size of the crystals, the inter-granular spacings and pores may be sealed and water transport may be completely blocked off. A number of substances, compatible with their natural surroundings, can be used in order to form crystals with the desired end result. For example,

M= Ca²\ Mg²⁺, Ba²⁺ and Si^2*

X= Cr, F, Br \ r and NO₃- B= K⁺, Na⁺, Rb⁺ and Cs⁺

Y= SO₄ ²\ PO₄ ^3" and CO₃ ^2"

Rb and Cs are substances that are not recommended because of their toxicity.

Precipitation of all the modifications of calcium carbonate polymorphs, calcium sulfate and calcium phosphate has been tested extensively in the laboratory. All of the above salts resulted in consolidated sand packs with large variations in both degrees of consolidation and porous medium permeability losses. For the stabilization of the desirable crystals, an optional fixing step is used. This step includes the injection of an aqueous solution of an organo-phosphorous compound in the sand bed after the termination of the salt precipitation process. This solution acts as an inhibitor for the hydrolysis of the precipitated salt into other crystals forms that may lead to undesirable results (poor consolidation), caused by changes of the moφhology of the deposits.

The spacer solution is a solution of any ions, which do not react with the ions in the first or second solution, to form insoluble crystals. Examples of such solutions are KOH, NaOH, or NH₃ as most salts of Na⁺, K⁺ and NH₄ ⁺ are soluble in water. The major problem that faces all the proposed techniques for consolidation of the unconsolidated formation (either the gravel pack technique, or the treatment with epoxy resins, or other chemical substances) is the placement of the consolidating material at the right place. In most cases the area that needs remediation is very far from the surface (hundreds of meters or even kilometers) In some cases therefore we suggest the use of the x-pipe technology (PCT WO96/04502), which is a technology that allows the three solutions to be readily introduced near the well bore formation. This technology offers a smart way of controlled fluid delivery in oil wells, and can be used as described above for the puφoses of the present consolidation invention. Technologies similar to the X-pipe technology can also be used for the delivery of consolidation solutions as discussed above.

Example: Precipitation of Octocalcium phosphate (OCP) and sand bed consolidation

Calcium carbonate, calcium sulfate and calcium phosphate are some of the insoluble salts, which have been tested extensively in the laboratory. All of the above salts resulted in strongly consolidated sand packs with acceptable reduction of the porous medium permeability value (up to 40-50 %). It should be noted here that each of these salts could precipitate in different forms. These forms may differ both in chemical composition and in their moφhology. For instance, depending on experimental conditions (pH, ionic strength, additives, etc.), four crystal forms of calcium phosphate can be precipitated (Koutsoukos and Nancollas, 1987), namely,

Hydroxyapatite, HAP, Ca₅(PO)₃OH Dicalcium phosphate dihydrate, DCPD, CaHPO₄ 2H₂O Tricalcium phosphate, β-TCP, Ca₃(PO₄)₂ Octacalcium phosphate, OCP, Ca₈H₂(PO₄)₆.5H₂O

The last phase of the calcium phosphate, OCP, has been found to be the most appropriate for the specific process. OCP crystals have a platelike shape, are small in size (1-3 μm) and precipitate nearly uniformly around the grains, figure 10 shows a picture of two grains connected by OCP-crystals, and figure 11 shows the structure of OCP-crystals. These characteristics cause strong consolidation with a relatively small reduction of the permeability. Of course, it may be possible to identify other types of crystals with comparable properties that would be suitable for the same puφose.

OCP can be produced from the reaction of soluble calcium salts (e.g. CaCl₂) and alkali phosphates (e.g. KH₂PO₄, K₂HPO₄ or a mixture of them): CaCl₂ K₂HPO₄ or KH₂PO₄ (A) (B)

Ca_gH₂(PO₄)₆* 5H₂O (OCP, octacalcium phosphate)

OCP crystals are formed in solutions in which the molar ratio of the total calcium, Ca, over total phosphate, P, is in the range 0,5-1,5, preferably 1,33 and the pH in the range of 6,0-7,0 (Cheng, 1987). For pH values larger than 7,0 the HAP precipitates, whereas for pH values smaller than 6,0 the phase DCPD is stabilised (Koutsoukos et al, 1980). Even for pH values between 6,0 to 7,0, magnesium chloride hexahydrate (MgCl₂.H₂O) or other stabilising chemical agents should be added in the solution to stabilise OCP crystals and avoid their transformation to the more stable HAP (Cheng, 1987). The ionic strength of the solution is adjusted by adding a spacer solution, such as an alkali nitrate, e.g. (KNO₃). As the deposit of the precipitated OCP crystals increases in size, the sand grains are strongly interconnected without the elimination of the porous medium permeability.

There are many parameters affecting the mixing and precipitation process. These include the thickness of the successive aqueous solution layers, the concentration of the salts in the layers, the flow pulsation characteristics (amplitude, frequency, duration/ number of pulses), and the total duration of the process. A preliminary model of the solution injection procedure and mixing has been developed. This model can predict the concentration profile of the two salts at the end of the injection procedure.

Experiments for precipitation of OCP were performed at 25 and 70°C. Procedures with 2, 4 or 8 plugs of solutions have been tested and were found effective at 25°C. Figure 8 shows the injection procedures for 6 plugs. At 70°C, the best results were obtained with only 1 plug each of the two active solutions as shown in Figure 9.

The conditions for the experiment at 25 °C were: injection time for each plug, t₀ = 4 min., flowrate, Q=l ml/min, number of plugs = 6. The above model is used for the calculation of the soluble salt concentration profiles after the solution injection, and the number of pulses needed to achieve good mixing as a function of the amplitude and frequency of pulsation. Figures 6 and 7 show the concentration profile of the two salts before and avfter the mixing process, respectively.

The mixing was performed with an amplitude of the pulsation piston motion s = 1cm, frequency of f=0J5 Hz, the number of pulses were 20, and the duration of the pulsation were 80 sec. The estimated values have been tested experimentally.

Table I summarises the data and the results of successful experiments for the precipitation of OCP and sand-bed consolidation at 25°C . The beds were initially filled with silicate grains with mean grain size d^ 0,71 mm. After packing the porosity was measured and was found, to be ε₀= 0,36, while the initial permeability was 137 Darcy. The void space of the column was filled with 3 plugs KH₂PO₄ solution and 3 plugs CaCl₂ solution and 12 plugs of 10^"3 M KOH solution. (Volume of each plug, V_plug= 4,45 ml). After the perfect mixing, with the appropriate pulsator, the CaCl₂ concentration was 20 mM, whereas the KH₂PO₄ concentration was 15 mM (for the formation of OCP the molar ratio of the total calcium Ca_t over total phosphate P_t must be 1,33), due to dilution with the aqueous solution of KOH. Figures 8 show schematically the different injection procedures used.

Table II summarises the data and the results of the experiment at 70°C . The bed was filled with silicate grains with varying grain size. The porosity was measured to ε₀= 0.40, while the initial permeability was 27 Darcy. The void space was filled with 1 plug of 360 mM K₂ HPO₄ solution and 1 plug of 180 mM CaCl₂ solution and 7 plugs of 10^"3 M KOH solution. (Volume of each plug, V_plug=10 ml). After the perfect mixing, the CaCl₂ and K₂HPO₄ concentrations were reduced due to dilution with the aqueous solution of KOH (Ca_j= 20 mM and P_t= 40 mM. Figure 9 shows schematically the injection procedure.

As it may be seen, the two solutions are injected alternately and between them there are plugs of aqueous solution of 10^~3 M KOH. The aqueous solution of KOH controls the pH and the reaction rate. This method of sequential injection of the soluble salts with plugs of KOH solution in between prevents the initiation of the reaction and the plugging at the top layers of the sand-bed, allowing for in depth penetration of the reactants. Thanks to this procedure the reaction takes place only after the mixing using the pulsation technique, and during the relaxation step, t_r = 8 min. The duration of the relaxation step was determined by independent batch experiments in glass vessels where the two solutions are mixed gently, and the reaction is monitored by the change of the pH value.

The procedure according to figure 9, with only one layer of solution one, and one layer of solution two, had several advantages. The large amount of KOH containing solution injected after the second solutio resulted in that most ions from the first solution were removed from the upper layer when the second solution was injceted. IN the same way were most of the ions from the second solution removed when the first solution was injected again. Thus, the precipitation became easier to control, and took place at the furthermost end of the area to be consolidated. Less consolidation occurred near the incjection point. By adjusting the injection procedure in this way, controlled consolidation could easily be achieved over the whole volume.

Figure 10 shows an example of consolidated sand pack of 29 cm after 36 hours using experimental conditions shown in Table I. The depth of consolidation was up to 29 cm and the reduction of the permeability up to 70 %. Samples were taken from the top, the middle and the bottom of the sand-bed and checked for the presence of OCP. The OCP was identified as the main mineral deposit by several techniques such as x-ray Diffraction, Fourier Transform Infra Red Spectroscopy, Raman Spectroscopy and by Scanning Electron Microscopy.

Figure 11 shows bridging of two sand grains with OCP crystals, whereas Figure 12 shows the structure of OCP crystals (both photographs were taken using a scanning electron microscopy).

Table I: EXPERIMENTAL DATA AND RESULTS FOR CONSOLIDATION AT 25°C

Sand bed geometrical characteristics

Mean grain size: 0.71 mm

Porosity: 0J6

Initial Permeability: 137 Darcy

Diameter of the bed: 3 cm

Length of the bed: 31.5 cm

Data for the sand consolidation

Flow rate: 21 ml/min

Number of plugs: 6

Volume of each plug: 4.45 ml

Pulsation time: 1 min

Amplitude 2 cm

Relaxation time: 10 min

Data for the OCP formation Table I: EXPERIMENTAL DATA AND RESULTS FOR CONSOLIDATION AT 25°C

Ca,: 20 mM

P,: 15 mM

Aquatic phase: distilled water + KOH

Temperature: 25 °C

Initial pH: 6.8

Ionic strength: 0.1

Results

Duration of the experiments: 36 hr

Precipitated salt: OCP

Identification techniques: XRD, SEM, FTIR, RAMAN

Final permeability: 42 Darcy

Length of consolidation: 29 m

Table II: EXPERIMENTAL DATA AND RESULTS FOR CONSOLIDATION AT

70°C.

Sand bed geometrical characteristics

Grain size: 0J25<dg<0J 80 mm

Porosity: 0.4

Initial Permeability: 27 Darcy

Diameter of the bed: 2.8 cm

Length of the bed: 29.5 cm

Data for the sand consolidation

Flow rate: 18 ml/min

Number of plugs: 2

Volume of each plug: 10 ml

Pulsation time: 3 min

Amplitude 2 cm

Relaxation time: 8 min

Data for the OCP formation Table II: EXPERIMENTAL DATA AND RESULTS FOR CONSOLIDATION AT

70°C.

Ca,: 20 mM

P_t(K₂HPO₄): 40 mM

Aquatic phase: distilled water + KOH

Temperature: 70 °C

Initial pH: 8.5

Ionic strength: 0.1

Results

Duration of the experiments: 32 hr

Precipitated salt: OCP/ DPCD

Identification techniques: XRD, SEM

Final permeability: 5.5 Darcy

Length of consolidation: 25 cm

Claims

1. A method for precipitation of inorganic salts in porous media, with controlled consolidation and permeability loss, by which inorganic salts are precipitated within the pore structure and inter granular spaces of granular sand stones such as unconsolidated hydrocarbon reservoir strata or similar, or within the pore structure of water permeable natural rocks, wherein a first solution of a first soluble salt and a second solution of a second soluble salt are mixed at the desired place for precipitation, and the method comprises use of a spacer solution in order to control the pH and reaction rate, characterized - in that the first solution and the second solution are injected alternating with the spacer solution between and preferably last, and

- in that a pumping device is connected to the desired place, which device creates a reciprocating flow pulsation in that the device works both in forward and backward direction.

2. A method according to claim 1, characterized in that subsequent to the flow pulsating, there is a relaxation time, preferably 8 min.

3. A method according to claim 2, characterized in that the method according to claim 2 is repeated until the desired area is consolidated, and subsequent to the last relaxtion time, an organo-phosphorus compound solution is injected.

4. A method according to anyone of the previous claims, characterized in that the first solution is an aqueous solution of calcium salt, preferably CaCl₂, 5 - 30 mM.

5. A method according to anyone of the previous claims, characterized in that the second solution is an aqueous solution of a phosphate, preferably KH₂PO₄ or K₂HPO₄.

6. A method according to anyone of the previous claims, characterized in that the spacer solution is an aqueous solution, preferably of KOH, 0,001 -

0,1 M.

7. A method according to anyone of the previous claims, characterized in that the ionic strength of the solutions is adjusted approximately to 0J M if distilled water is used, 0,4 M if fresh water is used, and 0,62 if sea water is used.

8. A method according to anyone of the previous claims, characterized in that the number of layers of the first and second solution are 1-8, preferably 2.

9. A method according to anyone of the previous claims, characterized in that the precipitate is a calcium sparingly soluble salt, preferably Octacalcium Phosphate, Ca₈H₂(PO₄)₆ * 5 H₂O, OCP.

10. A method according to anyone of the previous claims, characterized in that the preferred characteristics of the flow pulsation are:

- pulsation time in the range 0.5-5 min, preferably 2 min, - amplitude in the range 0,5 - 5 cm, preferably 2 cm,

- frequency in the range 0J-5 Hz, preferably 1Hz,

- relaxation time in the range 5-15 min, preferably 8 min.

11. A method according to anyone of the previous claims, characterized in that a salt containing Mg²⁺ is added to anyone of the solutions.

12. A method according to anyone of the previous claims, characterized in that it is carried out with a pH about 6,0 - 7,0, preferably 6,5.

13. Use of a method according to any of the claims 1-12, in consolidation of hydrocarbon reservoirs in order to prevent sand production, without significant permeability loss.

14. Use of a method according to any of the claims 1-12, in stopping water leaks in underground constructions.