Field of the Invention

The present invention relates to a process to prepare encapsulated expansive mineral and said expansive mineral encapsulated by a wax. The present invention also relates to the use of said encapsulated minerals to exert high pressures by volumetric expansion of the minerals upon hydration.

Background of the Invention

It is known in the art to use expansive mineral to fracture rock. The volumetric expansion relies on hydration as a result the volume of the mineral increases dramatically. The minerals are transported and placed in an aqueous slurry. Aqueous media, such as brine, are used as carrier fluids. However, hydration and expansion of the minerals start too soon before being delivered at their final destination.

US2014/0144635 discloses the use of a cement slurry comprising an expandable cementitious material and a breakable gel fluid. The expandable cementitious material will be suspended by a gel formulation, which gel will induce the sedimentation upon chemical breaking of the gel. Said chemical breaking will be temperature

activated. Although the placement with the gel

formulation will lead to faster penetration into the fracture and delayed sedimentation, complicated and costly chemistry is needed for gelling and breaking of the gel used for expandable material placement .

US20080108524 relates to a water absorbing

composition comprising a particle having a core of a water-swelling material and a coating substantially surrounding the core that temporarily prevents contact of water with the water-swelling material. The water- swelling material is at least one of a clay and a superabsorbing material .

The water-swelling material used in US20080108524 to treat a formation penetrated by a wellbore may however decrease in size again due to the use of the

superabsorbent material that has been surface cross- linked .

It is an object of the invention to provide a process which allows the minerals to be transported and placed in an aqueous slurry delaying the hydration until delivered at their final destination.

It is a further object of the present invention to provide a carrier containing the expansive minerals which may control the volume expansion of the minerals in such a way that the expansion is not dependent on the

solubility of the carrier.

Another object of the present invention is to provide a carrier containing the expansive minerals which allows permanent hydration of the expansive minerals.

Summary of the invention

From a first aspect, above and other objects may be achieved according to the present invention by providing a process to prepare expansive mineral encapsulated by a wax, the process comprises the steps of:

(a) addition of a wax to a heated expansive mineral to obtain a blend with a temperature above the congealing point of the wax;

(b) mixing the blend of step (a) until a homogeneous fluid mixture with well dispersed minerals is obtained;

(c) forming a particulate matter of the fluid mixture of step (b) by a forming technique to obtain an expansive mineral encapsulated by a wax. It has been found according to the present invention that the encapsulation process allows the minerals to be transported and placed in an aqueous slurry without hydration to occur immediately; i.e., the minerals will only start to expand when delivered at their final destination .

From a second aspect, the invention embraces a expansive mineral encapsulated by a wax. An advantage of the present invention is, is that at temperatures above the melting point of the wax, the reactive surface of the mineral will be exposed and expansion can occur when in contact with water.

In this way, the start of hydration and subsequent volume expansion of the mineral can be controlled.

From a third aspect, the invention resides in use of expansive minerals encapsulated by a wax to exert high pressures by volumetric expansion of the minerals upon hydration. The advantage of using the encapsulated minerals according to the present invention to hydraulic (re-) subterrean frack formations is that the use of water and/or horse power for pumps may be reduced, the expanded minerals might also acts as proppant and existing wells can be re-fracked without re-applying excessive water pressure.

From a third aspect, the invention embraces a the use of Fischer-Tropsch derived wax to encapsulate an expansive mineral.

An advantage of the use of Fischer-Tropsch derived wax is that the Fischer-Tropsch derived wax has very low levels of aromatics, sulphur, napthenics and impurities. A further advantage is that the Fischer-Tropsch derived waxes have high melting temperatures.

Detailed description of the invention In step (a) according to the process of the present invention, a wax is added to a heated expansive mineral to obtain a blend with a temperature above the congealing point of the wax.

Preferably, the expansive mineral is calcium oxide

(CaO) , magnesium oxide (MgO) , or aluminium oxide (A1 ₂ 0 ₃ ) . The present invention is not limited to the use of expansive minerals as listed above. Typically minerals to be used according to the present invention can be found in the abstract of M. Huynh and D. Laefer, titled

"Expansive cements and Soundless Chemical demolition agents: State of technology review", published for the lithe Conference on Science and Technology, 21-23 October 2009 in Ho Chi Minh City Vietnam.

The expansive mineral is preferably heated at a temperature in a range of from 50 to 100°C, more

preferably from 70 to 100°C, most preferably in range of from 70 to 90°C. The person skilled in the art will understand that the temperature used is dependent on the wax to be used. In addition, the blend of step (a) comprises a wax in the range of from 5 to 30 wt . % based on the total amount of the blend.

In another embodiment of the process according to the present invention the expansive mineral is heated together is the wax to a temperature above the congealing point of the wax.

In step (b) according to the process of the present invention the blend of step (a) is mixed until a

homogeneous fluid mixture with well dispersed minerals is obtained. In other words, in step (b) according to the process of the present invention the blend of step (a) is mixed to obtain a homogeneous fluid mixture with well dispersed minerals. Preferably, the amount of wax is such that a homogeneous fluid mixture with well dispersed minerals is obtained. In addition, an extra amount of wax may be added to step (b) in order to obtain the

homogenous fluid mixture.

Suitably, the wax added in step (a) is a natural wax, such as beeswax, a petroleum derived wax or a synthetic derived wax.

Suitable natural waxes are for example disclosed in the "International Journal for Applied Science, 4-2011, Natural waxes-Properties, Compositions and Applications,

E. Endlein, K Peleikis; Natural Waxes-Properties,

Compositions and Applications".

Preferably, the wax added in step (a) is a paraffin wax. Paraffin wax may be obtained by various processes. US 2,692,835 discloses a method for deriving paraffin wax from crude oil. Also, paraffin wax may be obtained using the so called Fischer-Tropsch process. An example of such process is disclosed in WO 2002/102941, EP 1 498 469, and WO 2004/009739. In addition, the wax is preferably a Fischer-Tropsch derived wax. Also, the Fischer-Tropsch derived wax has a congealing point of at least 30°C. The Fischer-Tropsch derived wax has preferably a congealing point of at most 130°C.

In step (c) of the process according to the present invention a particulate matter of the fluid mixture of step (b) is formed by a forming technique to obtain an expansive mineral encapsulated by a wax. Typically, depending on the forming technique the fluid is first cooled before being formed into a particulate matter. Also, the particulate form of the particulate matter is preferably a pellet, flake, sphere, granule or prill. Formation techniques and particulate forms formed by these formation techniques are known by the skilled person in the art and are similar to sulphur forming and can for example be found in the presentation of J.

d'Aquin, titled "Airborne Sulfur Dust: Composition and Control" published for Mespon 2015, 20 October 2015 in Abu Dabi .

In a further aspect, the present invention provides an expansive mineral encapsulated by a wax.

Preferably, the expansive mineral in the encapsulated expansive mineral is CaO. Also, the wax encapsulating the expansive mineral is preferably Fischer-Tropsch derived wax. More preferably, the Fischer-Tropsch derived wax has a congealing point of at least 30°C, more preferably the Fischer-Tropsch derived wax has a congealing point of 50°C. In addition, the amount of Fischer-Tropsch derived wax in the encapsulated expansive mineral is in a range between 5 to 30 wt . % .

In another aspect, the present invention provides the use of encapsulated expansive minerals to exert high pressures by volumetric expansion of the minerals upon hydration.

Preferably, the high pressures are exerted to hydraulic subterrean (re-) frack formations or to seal openings or cavities .

Upon hydration of minerals, such as CaO, MgO and A1 ₂ 0 ₃ , the hydroxides (Ca(OH) ₂ , Mg(OH) ₂ and Al (OH) ₃ ) of these minerals are formed. The volumetric expansion of these minerals to their hydroxides is in a range of from 200 to 250%. The pressures exerted by this volume expansion are large enough to break high strength concrete as shown in "Magazine of Congrete Research,

2010, 62, NoO, Month 1-10, D. Laefer et.al, titled:

Expansive fracture agent behaviour for concrete cracking. Typically, expansive minerals are used to fracture formations, such as sand, rock, and clay. The layer deep underground is warmer than the surface of the ground. The wax according to the present invention may melt going deeper underground resulting in the hydration of the expansive mineral.

The process for hydraulic (re-) frack of formations is a known process .

In another embodiment of the present invention the process to hydraulic frack a rock using the encapsulated expansive minerals encapsulated by a wax comprising the following steps:

(a) addition of a wax to a heated expansive mineral to obtain a blend with a temperature above the congealing point of the wax;

(b) mixing of the blend of step (a) until a homogenous fluid mixture with well dispersed minerals is obtained;

(c) forming a particulate matter of the fluid mixture of step (b) by a forming technique to obtain an expansive mineral encapsulated wax;

(d) Filling of a hole drilled into a rock with the expansive mineral encapsulated by a wax of step (c) ;

(e) Adding water to the hole comprising the expansive

mineral encapsulated by a wax;

(f) Mixing and compacting the expansive mineral encapsulated by a wax in the hole of the rock;

(g) Repeating step (d) and (e) until the complete hole is filled.

(h) heat the rock to the melting temperature of the wax or wait in order to hydrate and expand the expansive mineral encapsulated by a wax.

In step (a) of the process to hydraulic frack a rock a wax is added to a heated expansive mineral to obtain a blend with a temperature above the congealing point of a wax. In another embodiment of the process according to the present invention in step (a) the expansive mineral is heated together is the wax to a temperature above the congealing point of the wax.

In step (b) of the process to hydraulic frack a rock the blend of step (a) is mixed until a homogenous fluid mixture with well dispersed minerals is obtained.

Preferably, the amount of wax is such that a homogeneous fluid mixture with well dispersed minerals is obtained. In addition, an extra amount of wax may be added to step (b) in order to obtain the homogenous fluid mixture.

In step (c) of the process to hydraulic frack a rock the fluid mixture of step (b) is formed to a particulate matter by a forming technique to obtain an expansive mineral encapsulated by a wax.

Typically, depending on the forming technique the fluid is first cooled before being formed into a particulate matter. Also, the particulate form of the particulate matter is preferably a pellet, flake, sphere, granule or pill. Formation techniques and particulate forms formed by these formation techniques are known by the skilled person in the art and are similar to sulphur forming and can for example be found in the presentation of J.

d'Aquin, titled "Airborne Sulfur Dust: Composition and Control" published for Mespon 2015, 20 October 2015 in Abu Dabi .

In a further aspect, the present invention provides the use of Fischer-Tropsch derived wax to encapsulate an expansive mineral.

The advantage of using the encapsulated expansive minerals encapsulated by the Fischer-Tropsch derived wax to hydraulic (re-) frack a rock is that the minerals will only expand when the melting temperature of the Fischer- Tropsch derived wax is reached and not earlier. In this way, early and uncontrolled hydration and expansion is avoided .

The present invention is described below with reference to the following Examples, which are not intended to limit the scope of the present invention in any way.

Examples

Fracturing of a concrete block by encapsulated expansive minerals

Dexpan®, which is a commercial expansive mortar (>80% CaO) and obtained from the company Dexpan®was heated with Fischer-Tropsch derived wax with a congealing point of 50°C to a temperature of 90°C. The mixture was stirred with a spatula for 2 minutes followed by cooling the mixture to room temperature yielding a powder.

The powder was mixed with water and placed in a hole of an autoclaved cellular concrete (Block: 60mm x 60mm x 100 mm, hole: 12 mm diameter x 70 mm) . The mixture was compacted into the hole with a cotton swab. The block was kept in a drip tray (approx. 3 cm water) to keep moist. The block was kept at different conditions, which conditions can be found in Table 1. In Table 1 and Figure 1 the results of the different conditions on the

autoclaved cellular concrete are shown.

Table 1

Example 2

Sealing with a Fischer-Tropsch wax having a congealing point of 70°C (SX70)

36 g of Calcium Oxide and 4 g of SX 70 was transferred to a 250 ml tin can. The can was heated to 120°C to allow that wax to melt. After melting, the Calcium Oxide was thoroughly mixed with the molten SX70 wax, and a

hydrophobic powder was obtained

First a small piece of foam was placed in a copper pipe (15x113 mm) to act as a stopper. The pipe was then partially filled with 6.4g of CaO + SX70 (90/10) mixed water. The mixture was compressed with a metal rod and left to cure for 4 hours, sealing the pipe. The pipe was then filled with water to see if the seal holds. After 18 hours, no water went through the seal.

Discussion

The results of Table 1 show that the Fischer-Tropsch derived wax inhibits hydration and volume expansion at room temperature. Furthermore, at 86°C in the oven hydration and expansion will occur.

These observations indicate that expansion switches on at temperatures above the melting point of the wax. Using the expansive minerals as a seal e.g. the example 2 of the pipe above it was successfully demonstrated that a water seal could be established upon expansion of the mineral. The seal remained intact for at least the whole duration of the experiment (18 h) .