The present invention relates to a process for the fluidification of a high-viscosity oil directly inside the reservoir.

More specifically, the present invention relates to a process for the fluidification of a high-viscosity oil directly inside the reservoir with the use of microwaves/radiofrequencies (RF) .

Even more specifically, the present invention relates to a process for the fluidification of a high- viscosity oil directly inside the reservoir, with the use of microwaves/ RF, suitable for both on-shore and off-shore production fields.

As is known, oil wells which produce high-viscosity oil, for example oil with a viscosity higher than 100 cps, generally ranging from 300 to 10 ⁶ cps, have the necessity of fluidifying the oil directly in situ to favour its flow towards the well and allow the extraction pumps to pump the oil to the surface with a flow-rate which makes the exploitation of the reservoir convenient. Reservoirs with these characteristics are, for example, heavy oil reservoirs or tar fields.

The fluidification of heavy/viscous crude oils or non-conventional oils, such as tars, oil sands, oil shales, etc., directly in the reservoir is, however, an extremely difficult operation.

For the extraction of viscous oils, it is known to use advanced techniques, such as CSS (Cyclic Steam Simulation) or SAGD (Steam-Assisted Gravity Drainage) , which are based on injections of hot steam into the reservoir to fluidify the oil and favour its movement towards the well and therefore its emission onto the surface. Techniques of this type are also defined as "thermal stimulation" of the reservoir. Other known alternative techniques envisage the injection of hot C0 ₂ or combustion in situ.

For effecting the above techniques, however, either large water and energy consumptions are required, for the production and injection of vapour and/or C0 ₂ , or a combustion must be effected at the well bottom which implies considerable risks. Furthermore, the implementation of these techniques can be problematical in off-shore environments where the availability of space is not excessive.

An alternative to the known thermal stimulation systems is represented by the use of microwaves whose use is described, for example, in the patent USA 3,104,711. The use of microwaves generated in situ in the reservoir causes a targeted heating of the crude oil, which facilitates its movement and consequently its extraction.

Applications of this type are also described in patent USA 5,082,054 which indicates how to apply electromagnetic radiations having a suitable frequency to favour the extraction of high-viscosity oils from reservoirs essentially by means of selective heating induced by microwaves .

Other references in literature which describe the use of microwaves for heating high-viscosity oils in situ are patent application USA 2007/131591 and patent USA 4,419, 214.

The use of microwaves for thermally stimulating crude oil directly in the reservoir is therefore a known process but the actual application in a production well has proved to be not particularly effective due to the limited capacity of the heating effect .

The Applicant has now found, as better illustrated in the enclosed claims, a new way for heating a viscous oil directly in the reservoir with the use of microwaves or radiofrequencies (RF) . This method allows the heat produced by the microwaves or radiofrequencies to be diffused in depth in the reservoir providing a heating effect of the crude oil and consequently its fluidification, higher than that of the known methods.

An object of the present invention therefore relates to a process for the fluidification of a high- viscosity oil directly inside a reservoir which comprises :

a. producing a well by the drilling of an oil field until the oil reservoir is reached;

b. injecting into the reservoir, substantially at the bottom of the well, finely subdivided particles of ceramic material having a high thermal conductivity, dispersed in a fluid with a variable viscosity, by means of an induced fracturing operation;

c. installing, substantially in correspondence with the fractured area of the reservoir, a microwave/RF emission antenna connected by means of a waveguide or cable, to a magnetron; and

d. fluidifying the viscous oil distributed in the reservoir by heating it with the heat produced by the microwaves/RF .

According to the present invention, the oil production well can be a vertical or tilted, simple or with one or more branchings which radiate into the reservoir. As is known, the walls of the well, proceeding towards the reservoir, are reinforced with a cylindrical steel structure, called casing, which adheres and is constrained to the walls with cement/mortar or concrete, generally consisting of a mixture of Portland cement and inert aggregates such as siliceous sand and possibly gravel. When the drilling phase of the well is terminated, with the possible formation of one or more branchings, the operative phases are initiated for completing the safety set-up phase of the well and activating the oil production.

When the well enters into production, the oil is recovered by means of a specific tubing, known as production tubing. This is a steel tubing which is inserted into the well and also into each branching, until it reaches the level of the reservoir. At the well bottom (also in correspondence with each branching) , the tubing is fixed by means of a combined hydraulic and mechanical sealing system, called packer, which forces the oil to rise to the surface from inside the tubing without touching the walls of the casing.

According to the present invention, the terminal part of the casing, downstream of the packer, is made with a cylindrical structure of plastic material, as the organic polymeric materials used as plastic material are much more transparent to microwaves/RF than steel. The dimension of the casing section or segment made of plastic material is substantially determined by the dimension of the reservoir and size of the well. The segment (substantially circular section) generally has a length ranging from 2 to 20 metres and a wall thickness ranging from 1 to 5 cm.

The plastic material can be a thermoplastic polymer or a thermosetting resin. In the former case, a polymer such as polypropylene, polyvinylchloride , polyesters (for example PET, PBT) , thermotropic polyesters, "engineering polymers", polyamides etc., can be used, and in the latter case thermosetting polyester resins or epoxy resins can be used. In both cases, the materials can be reinforced with mineral fillers, such as talc or calcium carbonate, and/or with fibrous material such as glass fibres, carbon fibres, aramidic fibres, continuous or short.

In order to favour the transparency to microwaves, the terminal section of the casing made of plastic material is fixed to the walls of the well with mortar wherein the inert part essentially consists of alumina (A1 ₂ 0 ₃ ) , which is more transparent to microwaves/RF than silica-based sand.

The part of casing overlying the section made of plastic material is made of steel. The steel part of casing directly above the plastic casing and below the packer in turn comprises the production section, which can be associated with a screen for allowing the oil to flow from the reservoir into the well without producing possible sandy materials, and then rise to the surface through the tubing.

The second step (b) of the process, object of the present invention, envisages injecting into the reservoir, substantially at the bottom of the well, finely subdivided particles of ceramic material having a high thermal conductivity, dispersed in a fluid with a variable viscosity, by means of an induced fracturing operation. The induced fracturing operation is, as a general principle, an operation which is applied in the oil industry, for allowing the oil to flow more easily towards the well. According to the process, object of the present invention, on the other hand, the induced fracturing operation allows the heat generated by the emission of microwaves/RF to be transferred in depth into the reservoir, by means of fractures which propagate in all directions, and involving, in the heating and fluidification phase, greater volumes of high-viscosity oil with respect to those treated with the methods of the known art .

Induced fracturing is a known technique which envisages subjecting a fluid having a variable viscosity to high pressure at the well bottom so that this can penetrate into the reservoir by means of a fracture in the geological formation in which the reservoir itself is situated. The fluid, which under shear stress has a viscous rheological behaviour, entrains a finely subdivided solid material known as "proppant" which is a preservation agent of fractures. At the end of the fracturing phase, the suspension (viscous fluid + proppant) present in the fractures passes from the shaken state, associated with the shear action of the fracture-creating system, to a rest state during which the viscous fluid tends to have a Newtonian fluid rheological behaviour. As it becomes less viscous, the fluid is slowly absorbed by the reservoir and flow from the fractures diluting in the formation. The proppant, on the other hand, remains in the fractures consolidating them.

Information and details on induced fracturing techniques, on fluids with a variable viscosity and proppants, are available in scientific literature, for example in J.M. McGowen et al . "Fluid Selection for Fracturing High-Permeability Formations", SPE 26559, 1993, 451-466 or in M.B. Smith et al . "High- Permeability Fracturing: The Evolution of a Technology", JPT, July 1996, 628-633.

According to the present invention, the fluid with a variable viscosity generally consists of a base selected from water, oil, alcohols, etc., in which a substance is dispersed/dissolved, capable of providing the desired rheological fluid behaviour suitable for producing an induced fracture at the well bottom. Examples of these substances are guar rubber or materials of the type CMHPG (Carboxy Methyl Hydroxy Propyl Guar) or CMHEC (Carboxy Methyl Hydroxy Ethyl Cellulose) .

The finely subdivided ceramic solid, which is dispersed in the fluid having a variable viscosity, as a traditional proppant, preferably consists of silicon carbide, SiC, with a particle-size ranging from 1 to 20 μπι, preferably from 1 to 10 μπι. This material is preferred as it allows an optimum diffusion of the heat generated by the microwaves/RF . The concentration of finely subdivided ceramic material, for example SiC, in the suspension varies from 20 to 60% by weight (calculated with respect to the total of fluid + ceramic material), preferably from 30 to 50%. If required, conventional suspending agents (surfactants) can be used, to favour, for example, the re-dispersion of the solid and prevent its accumulation, in the case of sedimentation of the suspension following prolonged storage .

The choice of silicon carbide is substantially determined by the fact that it is a material with extremely good mechanical properties, among which a high hardness and a high elastic modulus, it has a low density, a low thermal expansion coefficient, an excellent thermal shock resistance and is substantially inert to the chemical products generally present in the reservoir, for example H ₂ S, mercaptans, nitrogenated compounds, C0 ₂ , etc. In particular, however, the preferred characteristics of silicon carbide are its good absorbing capacity of microwaves/RF and a high thermal conductivity, which enables the heat generated by the microwaves/RF to be transferred in depth in the reservoir.

According to a preferred embodiment of the present invention, the ceramic material, i.e. the silicon carbide, is mixed with stainless steel microbeads, in a quantity ranging from 2 to 10% by weight (calculated with respect to the total of ceramic material + steel microbeads) . As the latter are not transparent to microwaves, they reflect them in all directions improving the diffusion of heat in the fractures and from these towards the reservoir, thus favouring the fluidification of the oil contained therein.

Thanks to the viscosity of the fluid injected at the well bottom, fractures can be obtained, which are distributed in depth in the geological formation in which the reservoir is present. The fractures can be distributed and developed at 360°, in all directions, with lengths which can be greater than 20 metres, generally ranging from 20 to 50 meters, preferably from 25 to 30 metres.

The part of the well involved in the induced fracturing operation is preferably the terminal part of the casing, which is made of plastic material. In order to allow the induced fracturing generation fluid to penetrate and fracture the formation, the casing made of plastic is previously perforated so that the viscous suspension can flow through the corresponding holes. At the end of the fracturing phase, in order to prevent the crude oil fluidified by the heat generated by the microwaves/RF from entering these holes, at the production regime, the plastic casing is internally covered with a continuous elastic lamina, again made of thermoplastic material, which blocks the holes as it elastically pushes against the internal cylindrical walls. Nothing can prevent, however, the part of the well involved in the induced fracturing operation from being that which corresponds to the production area immediately overlying the plastic casing. If the formation of the reservoir is not consolidated, screens (steel casings with a mesh structure) can be added in front of the production area to prevent the entrainment of sand, or other material of the formation, which could block the well.

At the end of the induced fracturing phase, the well can enter into production. In order to favour the flow from the reservoir to the well, the viscous oil is heated with the microwaves/RF . In particular, a microwave/RF antenna is lowered to the well bottom, in the bed of the casing made of plastic material, which is capable of irradiating radiofrequencies ranging from 0.1 to 1 GHz or microwaves with frequencies ranging from 1 GHz to 300 GHz and an overall wavelength ranging from 0.001 to 3 metres. The antenna is connected, by- cable or waveguide, to a magnetron having a power ranging from 1 to 5 MW, situated on the surface or inside the well in a specific position.

In the operative phase, the antenna continuously emits microwave beams/RF which are diffused in the formation and reservoir at 360° in all directions. In this overall diffusion phase, the microwaves encounter both the reservoir and the fractures filled with ceramic material, in particular silicon carbide (SiC) and possibly stainless steel microbeads. The electromagnetic waves are absorbed by the SiC and homogeneously reflected by the stainless steel beads in all directions and transformed into heat which is further diffused in the reservoir. In this way, the fractures become further heat sources which penetrate the reservoir allowing greater volumes of viscous oil to be fluidified with respect to those fluidified with the methods of the known art .

The process for the fluidification of a high- viscosity oil directly inside a reservoir, object of the present invention, can be better understood with reference to the drawing of the enclosed Figure which represents an illustrative and non- limiting embodiment.

In the Figure, the numerical reference (1) indicates the reservoir in which a heavy crude oil is contained. The reference number (2) indicates the steel casing whose terminal part, reference number (3) indicates the screen or "production section or area" .

Below the production section there is the perforated casing made of plastic material, reference number (4) , whereby the induced fracturing operation takes place. The reference number (5) indicates the fractures filled with the thermally conductive ceramic material, for example with Sic, whereas the reference numbers (6) and (7) respectively indicate the antenna for microwaves/RF and the cable or waveguide which connect the antenna to the magnetron, not illustrated in the Figure .

The dashed and compact arrows respectively indicate the microwaves/RF emitted by the antenna, reference number (8) , and the flow of heated and fluidified oil, reference number (9) , which moves from the reservoir towards the production area.

The functioning of the process, object of the present invention, is evident from the above 11000427

description and scheme of the enclosed figure. Under regime conditions, the antenna (6) transmits beams of microwaves or radiofrequencies (8) which are diffused in the reservoir (1) and cause its heating. During this irradiation phase, moreover, the waves also encounter and heat the SiC contained in the fractures (5) . Thanks to its thermal properties, including its thermal conductivity, the SiC can be easily heated and allows the heat to be transferred in depth inside the reservoir. The hot oil which is consequently more fluid (9) can be easily transferred towards the production area (3) to rise into the casing (2) towards the pumping system (not illustrated in the Figure) which will take it to the surface by means of the tubing.