NO340688B1 - tracing Substance - Google Patents

Description

Tracer substance

The area of technology

The present disclosure relates to tracer-containing materials for use in flow monitoring of underground reservoirs. Specifically, the present invention relates to materials which provide a controlled and/or sustained release of a tracer material, in particular a tracer material for gas in an underground reservoir.

The background of the invention

Tracer technology is widely used in the exploration and extraction of oil and gas, and both radioactive and non-radioactive tracers that can be measured at low concentrations are used. Tracer can be injected as pulses in well-to-well studies to measure flow paths and velocities.

Tracer sources can also be placed in oil or gas production wells to monitor inflow from the surrounding formation. For

inflow monitoring studies, a tracer may be encapsulated in a polymer that is placed along the outer surface of the production tubing at various locations before the well is completed, as described in US 6,645,769. The tracer can be attached to or encapsulated in polymers or in different types of particles, and the release of the tracer can be made depending on the type of fluid passing (oil or water), chemical properties of the fluid (for example pH, or salinity) or for example of the temperature.

Hydraulic fracturing is often used to stimulate gas or oil production from oil reservoirs, and is often used in connection with hydrocarbon production from tight formations. Hydraulic fractures radiate outward from the wellbore (typically from several hundred to a few meters) and expand the surface area from which hydrocarbons can be efficiently produced. This process requires that "slurry" (fluid which may or may not contain proppants) is pumped at high pressure into a defined section of the well. As the slurry is pumped into the formation, a pressure difference between the well bore and the reservoir pressure is generated. Eventually, the pressure difference will reach a point where it will exceed the natural stresses in the formation, generating a tensile fracture that breaks the rock apart and creates a fracture. The plugging agents (usually ceramic or sand particles) from the slurry will prevent the fracture surfaces from closing against each other, leaving a permeable path for the reservoir fluids to flow into the wellbore. If the treatment has been designed correctly, this will lead to an increase in production. The fracturing slurry may be accompanied by particles, including tracers, the influx of which is to provide information about the fractured formation, as described in WO2010/140033. Oilfield chemicals, for example tracer, are distributed in the form of "particles" - preferably as a result of solidification of an emulsion - within a relatively large carrier matrix particle through which the tracer can diffuse.

WO2010/140033 deals with tracers for rapid efflux for 5-10 days or less, and does not take into account the harsh mechanical conditions to which these particles are subjected. Furthermore, the tracer particles can easily be broken up into smaller particles, or be limited to very small particles. The largest median size quoted is 800 micrometers. As the tracer diffuses through the entire surface of the particle, it will be difficult to achieve a controlled release of tracers or flow measurements over a long period, as the tracer will be released quickly and after a short time decrease to very low concentrations. In addition, as there is no control given on the size of the particles and tracer release will be critically dependent on the surface area and diffusion distances, the tracer concentration will be far from reflective of the flow for given physical properties of the environment, composition of production fluid and temperature in particular.

WO 2007/063325 deals with a particle for use in a well, which deals with cavities for storage of treatment agent and/or flow of well fluids (so that there is less hydraulic resistance). WO 2007/063325 is further focused on showing the least possible resistance, and is not suitable for a slow and controlled release of chemicals.

WO 2013/078031 deals with porous particles with adsorbed/absorbed tracer. The particles may be covered with an outer coating, but no word suggests that the coating should not be completely covering. As the surface available for adsorption is limited to the outer layer of the particle, the amount of tracer per particle is limited, and its release occurs over a shorter period. If the particle shows a coating, release will also occur quickly as soon as the same coating is destroyed or dissolved.

WO 2012/012224, which is similar to WO 2013/078031, deals with porous particles with adsorbed/absorbed tracer. The particles may be covered with one coating, but the text does not suggest that the coating should not be completely covered. D3 thus shows the same limitations as D2.

WO 01/94744 deals with delayed release of tracer substance. It shows particles with an island of tracer inside a matrix of material, so that the release is more progressive. The particles may have a coating, but there is no indication that the coating should not be completely covering. We therefore lack the additional control parameter of a partial and robust coating.

US2006/0052251 deals with the delayed release of "markers" by means of a coating. US2006/0052251 thus focuses on release delay characteristics with different coatings, which are permeable and/or crushable. No partial and robust coverage is shown.

In light of the above, it would obviously be advantageous to provide a tracer-releasing material that can provide tracer-releasing over a longer period of time. It would be a further advantage to provide a tracer release material with a predictable release rate and/or change in release rate over time. It would also be advantageous to provide a tracer release material that is physically robust enough to maintain its release properties when delivered to an underground reservoir. This applies in particular to tracer release materials that can be used at the locations of hydraulic fracturing, for example when injecting with plugging materials. It would be a further advantage if the tracer-releasing materials allow practical and reproducible production methods.

Brief description of the invention

The inventors have now determined that by providing a tracer-releasing material such as the hair's robust outer layer of low permeability, combined with a more permeable inner matrix and a controlled degree of encapsulation of the matrix by the outer layer, a tracer-releasing material can be provided that addresses one or more of the problems stated above, and provides the possibility of application in a variety of configurations.

In a first aspect, as shown in claim 1, the invention therefore provides a gas or oil tracer releasing material in the form of particles comprising: a) an outer coating; b) a diffusion matrix within said outer coating, said matrix comprising at least one polymeric material; c) at least one porous material encapsulated within said matrix;

d) at least one tracer that is inside the porous material,

where the outer coating is partial and comprises at least one robust material

Typically, the tracer releasing material can be in the form of a tube with at least one open end. In such cases, the tube will be formed from the robust material and will contain the matrix, porous material and tracer components.

The tracer releasing materials according to all aspects and embodiments of the invention will provide release of a tracer material over a longer period of time. This tracer material can be detected at low levels in the fluid produced from an underground reservoir, and can thus provide information about the movement of fluids within the underground reservoir.

In a further aspect, as shown in claim 12, the invention thus provides a method for assessing the flow of fluid(s) in an underground reservoir, where the method comprises the application of a first tracer releasing material as described here, and which comprises a first tracer to at least one position within said reservoir and detection of the presence, absence, concentration and/or rate of change of the concentration of tracer in at least one fluid produced from at least one producing area at said reservoir. Such detections can be made under normal operating conditions, or, in an alternative embodiment, can be made under conditions chosen to clarify certain properties of the reservoir and/or flow of fluids through it. For example, the particles according to the present invention can be used in estimation methods for multizone flow ("multizone flow estimation") based on tracer transient analysis ("tracer transient analysis").

In such a method, as shown in claim 14, particles comprising different tracers are positioned in different production zones. After a well is closed, or at least choked, production is resumed and the various tracers are analyzed downstream, for example on the platform deck. They will then usually provide a peak of concentration (corresponding to the build-up of tracer released during a limited flow), and the analysis of the time interval between the peaks of the respective tracers will provide information with respect to flow produced at the level of each zone marked with a tracer substance (and possibly also from other zones). Depending on the characteristics of the tracer enrichment design (release constant with time, release proportional to flow), additional information can also be obtained from the amount of tracer flowing up. In such a method, it is usually the different tracer concentrations and the rate of change of the concentrations that will provide information about relative flow rates.

The methods according to the invention can be carried out using one or more tracer materials. In such an embodiment, each tracer material will typically contain different and identifiable tracers. Thus, for example, the above-mentioned method can additionally comprise the application of a second tracer material comprising a second tracer to a second position inside the reservoir, for detecting the presence, absence, concentration and/or the rate of change of the concentration of the said first tracer and the said second tracer, characterized in that the second tracer is different from the said first tracer. This may provide a further advantage in determining, or helping to determine, regions of production in said reservoirs.

In more general terms, the methods may employ a plurality (eg, 2 to 30, preferably 2 to 10) of tracer materials as described herein. In general, each tracer material will comprise a different tracer from the other materials used in the same reservoir. In one such aspect, the invention provides a method for assessing the flow of at least one fluid in an underground reservoir, the method comprising applying a plurality of tracer release materials as set forth in claim 1, each comprising a different tracer at a plurality of positions within the said reservoir and detection of the presence, absence, concentration and/or the rate of change of the concentration of each of the said tracers in at least one fluid produced from at least one producing area of the said reservoir, and relating detected presence, absence, concentration and/or the rate of change of the concentration of each of said tracers and the position of application of each tracer-releasing material to a stream of fluid within said reservoir. In a similar embodiment, different types of tracer can be placed in a single fracturing step. This can provide information about the hydraulic fracture, such as its effective dimensions, for example the degree of fractures with different

dimensions, and on fluid movement and drainage patterns at the fracturing locations.

The materials of the present invention are highly effective in analyzing and monitoring the flow of at least one fluid in an underground reservoir. The robust nature and/or controlled release properties provided by the materials of the invention allow for highly advantageous applications both for individual materials over long periods of time, and for multiple materials (usually comprising different tracers) distributed in one or more locations.

In a further aspect, as shown in claim 15, the invention thus provides the use of at least one tracer-releasing material as described herein, to monitor the flow of at least one fluid in an underground reservoir. Preferred embodiments of such an application include use in monitoring the relative production of hydrocarbons from a plurality of areas at different layers in an underground reservoir. An example of this would be to monitor the relative production of gas from different areas in a shale gas reservoir.

The materials according to the present invention are typically formed under controlled conditions to ensure effective encapsulation and release of the tracer in the underground reservoir environment. In a further aspect, as shown in claim 20, the present invention therefore provides a method for forming a material, as described herein comprising: i) encapsulating at least one gas or oil tracer within particles of at least one porous material thereby providing an encapsulated tracer; ii) mixing said encapsulated tracer with at least one diffusion matrix medium, wherein to provide a tracer matrix medium; iii) introducing said tracer matrix medium into at least one cavity within at least one particle coating to provide a coated tracer matrix medium;

iv) curing the matrix medium component of said

tracer matrix medium to thereby provide at least one tracer-releasing particle or a tracer-releasing particle assembly.

v) optionally: dividing said tracer-releasing particle assembly to thereby provide a majority of tracer-releasing particles.

In general, the methods according to the invention will be carried out at a temperature below the glass transition temperature for the hardened matrix component. The methods can also be carried out at or below the boiling point of the tracer. The hardening of the matrix medium will typically be by non-thermal methods. Such methods include exposure to electromagnetic radiation, such as visible or preferably UV light. Where the manufacturing process involves elevated temperatures, such as for curing the matrix material, this will generally be a temperature below the boiling point of the tracer under the conditions within the particle. Therefore, in one embodiment, the temperature of the process may be above the boiling point of the tracer at atmospheric pressure, but below the boiling point of the tracer at the pressure of the process. Such a process can be carried out at, for example, 1.2 to 10 bar pressure.

The particles according to the present invention can be designed as a single size and/or can by optional step v) generate a single size and/or shape of the particle. Alternatively, if more than one size and/or shape of particles is formed by the manufacturing method according to the invention then the particles can be sorted to generate a desired size and possibly the size distribution of the particles (as described herein). The sizes and/or the size distribution can be regulated by choosing the appropriate size of the particle coating(s), by dividing a particle assembly into optional step v) and/or by sorting the particles. The division method comprising optional step v) has the advantage of allowing strict control of particle size and the ability to reconfigure particle size by regulating the division process. Sorting of particles into desired sizes and/or the size distribution constitutes optional additional step vi).

Detailed description of the invention

The tracer release materials according to the present invention comprise three main components. First, an outer layer of robust material with at least one defined opening that is indissoluble under field conditions. The primary purpose of this robust material is to provide mechanical strength and/or to define a tracer release surface (surface area) by providing a partial encapsulation. Second, a matrix component (which may be a polymer) that fills the volume within the outer robust layer and serves to limit and/or control the rate of release of the tracer from the open capsule. The matrix also provides structure to the open portion of the encapsulation volume and provides the area where the tracer can diffuse out through and into the surrounding fluid. Third, within the matrix component are dispersed particles of a porous material (eg silica gel or any of the porous materials described herein) containing (in their porous volume) tracer.

In one aspect, the invention provides controlled release of at least one tracer. The tracer is released by diffusion through the matrix and out of the defined opening(s). The release rate is generally limited compared to non-encapsulated tracers. The release rate may be predictable and/or reproducible due to the defined capsule opening and diffusion pathway.

Tracer release materials according to the invention can be used when placed in the formation by hydraulic fracturing or in defined positions in an underground reservoir, for example in a gravel-packed section of the well annulus or on a well screen (such as a sand screen) or inlet control device (ICD) completion location. Many suitable areas for tracer utilization are widely known in the oil industry, and can use the compositions according to the present invention.

Compounds and compounds

The compositions of the present invention (which are applicable to all aspects of the invention) are "controlled release", in that the tracer included in the compositions is not released into the fluid immediately, but rather is released over a period of time. This release may be exclusively or primarily dependent on the period of contact between the tracer-releasing material and the relevant fluid in the formation (for example oil or gas). Alternatively, the release may be dependent on flow and/or conditions in the reservoir. In general, 50% of the tracer can be released over a period of from 2 days to 180 days. That is, the release of a tracer can have a half-life (preferably under the conditions of the underground reservoir) of 2-180 days. If the half-life is 180 days, a quarter of the initial release rate will be maintained after one year under stable reservoir conditions. The tracer is typically detectable at concentrations down to 0.lppt and, depending on the amount of tracer that is initially placed in a fracture and the total fluid flow in which the tracer is distributed in the well, can be effectively detected in many half-lives for release. Thus, tracer release materials of the invention may be effective in providing a detectable marker for at least one year, preferably at least 3 years, after introduction into the reservoir environment for preferably 5 to 120 days, or 10 to 60 days (eg 5 to 60 days). At constant temperature and pressure, the diffusion release rate may be predictable, or substantially predictable, and may depend on a small number of factors, such as the matrix composition, the tracer boiling point, the dimensions of the particles, and the surface area of the orifice. This allows prediction of the expected release rate under constant conditions, and thus so that changes in these conditions (for example changes in temperature or flow) are to be detected, where the observed concentrations deviate from what is expected. The release can therefore be modeled and/or compared against one or more experimentally derived standards in order to be able to predict the speed of the release at varying times. From this information, predictions can be made regarding the concentration of tracer expected in produced fluids under different conditions, and/or the conditions in the reservoir can be derived by fitting the model release rate to the observed concentration of the tracer in the produced fluid. Changes in the tracer concentration in the produced fluid overtime can help confirm that the conditions in the reservoir are stable (for example, if the drop in concentration follows the predicted level for stable conditions), or can indicate a change in flow or temperature (for example, if the change deviates from what is expected for stable conditions). In embodiments as described herein, one tracer or a plurality of tracers may be used to indicate absolute and/or relative conditions at locations within a reservoir.

Although it is potentially beneficial to derive quantitative data from the tracer concentration(s) in the produced fluid(s), this is not essential for deriving useful information from tracer availability and detection. The tracer measurements can thus be qualitative (for example presence or absence at detectable levels), semi-quantitative (for example concentration above or below a threshold value), or quantitative either as an absolute concentration or a relative concentration of a tracer in relation to at least one other.

Tracer supply material

The controlled release particles of the present invention can be used in the release of many "oil field chemicals". An important embodiment of the present invention is the use of tracer material such as this "oilfield chemical", and it is tracer material that is the core of that invention and related claims, and the invention relates to the use of tracer material.

An important aspect of the present invention relates to a "tracer releasing material". As used herein, this is a composite material that serves to encapsulate a tracer in a petroleum reservoir, and to release it over a period of time as described herein. The first component of the composite material is a "robust material" which serves to form at least one outer layer, partially encapsulating the other components, and providing physical and release properties.

The "robust" material used in the present invention will typically be in the form of a tube (for example a hollow cylindrical or other hollow prismatic shaped tube). The outer layer of robust material serves to define the size of the particles of the tracer-releasing material, and in the final material, will thus have the size and distribution indicated herein for the particles of the tracer-releasing material. In one embodiment of the invention, tracer releasing material can be formed by introducing the remaining components into a tube of robust material, followed by curing, and splitting (cutting). In such an embodiment, the robust material which initially has one or two dimensions corresponding to that of the final product, but will be longer in at least one dimension before being filled, cured and divided.

Where a tube of robust material is used in the present invention, this will typically be of an external diameter of 0.1 to 50 mm, preferably 0.2 to 10 mm and most preferably 0.5 to 5 mm. The internal diameter will obviously be smaller than the external diameter, and can be 0.005 to 45 mm, preferably 0.01 to 9.5 mm, most preferably between 0.025 to 4.5 mm. The internal diameter will usually be between 10% and 90% of the external diameter.

The robust material for use in the present invention provides at least part of the mechanical strength of the tracer-releasing material, and thus should be physically robust. For example, the robust material when cut to the dimensions of tracer-bearing particles may exhibit crush resistance such that under closed state stresses of at least 100 psi, (eg at least 500 or at least 1000 psi), preferably at least 5000 psi, and more preferably at least 10,000 psi (eg at least 12,000 psi, up to 500,000 psi). The robust material may additionally or alternatively have a compressive strength at yield strength (according to ASTM D695) of at least 5,000 psi, preferably at least 8,000 psi, more preferably at least 10,000 psi (eg, at least 50,000 psi, such as 5,000 to 200,000 psi). Furthermore, the robust material can withstand temperatures of at least 140 °C, preferably at least 160 °C, more preferably at least 180 °C or 200 °C (for example up to 250 or 300 °C). A typical robust material may have a heat deflection temperature at 264psi (according to ASTM648) of at least 140°C, preferably at least 160°C and most preferably at least 200°C. Certain advantageous robust materials may have a heat deflection temperature of 264psi (according to ASTM648) of at least 220°C, preferably at least 250°C.

Typical materials suitable for use as the robust material of the particles of the present invention include polymers and metals. Suitable polymers include cross-linked and thermosetting polymers, especially those having the physical properties described herein. Some preferred examples of suitable polymers include polyphenylsulfone (radel R), polyetheretherketone (PEEK), polyetherimide, and polyketone.

In one embodiment, the robust material is transparent in at least one region of the electromagnetic spectrum, preferably transparent or partially transparent (such that it absorbs no more than 70% of the incident radiation) to visible and/or UV radiation, most preferably UV. This has the advantage of allowing hardening of the matrix material upon exposure to radiation, such as UV irradiation (see below).

An alternative material suitable for use as the robust material of the particles according to the present invention is metal. Metallic materials are advantageous in having high strength and stiffness, but are not transparent to electromagnetic radiation of frequencies applicable to curing the matrix material. Suitable metals include steel, stainless steel and other iron alloys, aluminum and aluminum alloys, nickel and nickel alloys, copper and titanium. A further advantage of metal is that it has a relatively high density and can thus be used to increase the overall density of the tracer releasing material. This may allow the density of the tracer proppant material to be more closely matched to the proppant particles, for example in embodiments where the tracer proppant material is injected with the proppant slurry during a hydraulic fracturing process.

In a further embodiment of the invention, the robust material can comprise more than one layer, for example two, three or four layers. In such an embodiment with several layers of robust material, the layers can be of different materials, for example a two-layer material with a metal layer on the outside, with a polymer lining. Such external multi-layer materials have advantages compared to single-layer coatings. For example, a fused metal oxide material, such as fused silicon dioxide, can be used as a layer of robust material for its excellent barrier properties and low reactivity. However, such a layer would require an outer coating of another material, such as a polymer, to provide sufficient toughness. Similarly, a robust metal material can be coated with a polymer layer to reduce the risk of damage to the pumping equipment and to improve the filling of cracks when used in combination with a proppant in hydraulic fracturing methods. Furthermore, if the outer polymer coating provides greater diffusion of tracer, the polymer coating can then encapsulate both the robust metal material and the opening(s) through which the tracer diffuses, to help maintain the integrity of the matrix. The reverse layer arrangement having a polymer layer coated within a robust metal material may serve to allow control of the inner diameter of the robust layer and may also aid in controlling the rate of release.

The outer layer(s) of robust material (such as those layers outside the matrix material) additionally serve to define at least one release area of the tracer release particles of the present invention. This is reinforced by the robust material, which has a significantly lower permeability to the tracer than the permeability of the matrix material. In this way, the tracer material is transported out of the particle primarily by contact between the matrix material and the reservoir environment in the areas where the outer layer of robust material is not present. In one embodiment, the permeability of the outer robust material to the tracer is no more than 10% (eg, 0.001 to 10%) of the permeability of the matrix material to the tracer. This can preferably be no more than 5%, and more preferably no more than 1% of the permeability of the matrix material for the tracer. In those cases with a robust metal material, or in the case of a robust outer material with several layers comprising at least one layer of metal, the permeability of the robust material to the tracer may be close to zero.

Where the robust material is a polymer, it will be desirable for the glass transition point of this polymer to be higher than the temperature in the reservoir environment. This will help ensure that the permeability of the robust material to the tracer remains low. Thus, in one embodiment, the robust material will be a polymer with a glass transition temperature (GTT) above 60°C, for example above 100°C, preferably above 140°C (for example above 160°C, for example 160 °C to 250 °C or even up to 300 °C) and possibly above 180 °C or 200 °C. In a further embodiment, the robust material has a glass transition temperature higher than that of the matrix component. The GTT of the robust material can thus be greater than that of the matrix component. Thus, the GTT of the robust material may be at least 50°C (eg 50 to 150°C) higher than that of the matrix component. This is preferably at least 70 °C higher, and more preferably at least 80 °C higher. Obviously, where the robust material is a metal, this will not have a corresponding glass transition temperature, but will have a melting point higher than the GTT of the matrix material.

In one embodiment, the robust material is not soluble in oil or water. In a further embodiment, the robust material is not soluble in any fluid commonly encountered in an underground reservoir under the conditions of such a reservoir.

In the tracer releasing particles of the present invention, a matrix material is held within the outer casing of robust material. The matrix material is typically a polymer and may be a cross-linked polymer. The matrix material serves to enclose the porous material within the outer robust material, and helps to control the rate of release of tracer to the reservoir environment.

In order to achieve a relatively high permeability from the matrix material to the tracer, it is preferred that the matrix material has a glass transition point below the temperature in the reservoir. It may also be beneficial for the matrix material to have a GTT above normal handling temperatures (eg above room temperature) to limit the permeability of the matrix to the tracer prior to introduction into the reservoir. The matrix material may therefore have a glass transition point of 30 to 200°C (for example 30°C to 180°C), preferably 50 to 145°C, more preferably 60 to 135°C.

Matrix materials will typically be polymer, especially cross-linked polymer. Some suitable polymers include chemically and/or ultraviolet (UV) cured polymers. Thermally cured polymers are generally less preferred, since the application of heat to cure the polymer matrix can cause significant release of tracer components from the tracer release material. Specific examples of suitable matrix polymers include polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyvinyl alcohol (PVA), polystyrene, polymethyl methacrylate, acrylonitrile-butadiene-styrene (ABS), polytetrafluoroethylene (PTFE), polycarbonate, epoxy resin (chemical and UV cured) . Most preferred matrix materials include UV curing epoxy resins such as UV-15-DC-80 and UV-15TK.

The matrix material will typically be formed or is formable from at least one matrix precursor material. This allows the matrix medium(s) to be introduced into at least one void in the robust material and then cured to form the matrix material. It is preferred that the matrix is formed or is formable from matrix medium which has a viscosity of 1 to 150,000 cP at 25 °C. This is preferably 300 to 100,000 cP and preferably 5,000 to 80,000 cP at 25°C. The most suitable matrix medium will be chemically curable or preferably by exposure to electromagnetic radiation, such as visible and/or UV light.

In one embodiment, the matrix according to the present invention can be formed of a polymer emulsion containing particles of porous material, itself containing a tracer as described herein. Such an emulsion can then be hardened inside a shell of robust material as described herein.

The tracer-releasing materials according to the present invention comprise a particulate porous material encapsulated in the matrix material. Suitable porous materials will typically have a pore size of from 2 to 50 nm, preferably 3 to 8 nm. The porous material may also have a specific pore volume of 0.2 to 0.95 cm 3 /g, preferably 0.4 to 0.8, more preferably 0.55 to 0.75 cm 3 /g. The porous material can further have a specific surface area of 100 to 1000 m2/g, preferably 150-800 m2/g, more preferably 200-700 m2/g. Suitable particle sizes for the porous material will be in the range of 10 to 1000 mesh, preferably 30-650 mesh. The particles of porous material may be of uniform or mono-modal size distribution, or alternatively they may be of bi-modal or multi-modal size distribution. A bimodal or multimodal particle size distribution may have the advantage of allowing more porous material to be incorporated without unduly increasing the viscosity of the matrix medium.

Metal oxides and/or metal silicates form a preferred group of materials which are suitable for forming the porous particulate material for use in tracer substance-releasing materials according to the present invention. Particularly suitable metal oxides include oxides of silicon, aluminium, titanium, tin or magnesium, as well as mixed oxides thereof. Silicates of any of these (except for silicon) are also suitable, especially magnesium silicate (florisil). Preferred examples of porous materials include silicon dioxide, aluminum oxide, florisil and celite.

The porous particulate material may also be (or may comprise) boron carbide. As boron has a high probability of accepting neutrons, neutron activation logging can be used to determine which fractures have received tracer material part i clay according to the invention and correspondingly which have also received normal proppant during the hydraulic fracturing process. In this method, a logging tool with a neutron source that emits is placed in the fractured well, as the tool moves along the well path, the surroundings are irradiated with neutrons. The logging tool may also contain a neutron detector or a gamma ray detector. As the boron carbide inside the tracer release particles is irradiated, there is a high probability of accepting neutrons and thus a reduction in the neutron signal on the neutron detector will be observed, alternatively after boron has accepted a neutron it will release gamma radiation at a known wavelength, this can be detected by gamma radiation detector included in the logging tool. Both methods can be used to determine which fractures have accepted tracer release particles containing boron carbide and thus which fractures have accepted normal proppants.

The amount of porous material incorporated in the tracer-releasing material according to the invention will depend on the viscosity of the matrix medium, the size of the particles and similar factors. Typically, an amount of 2 to 50 percent by weight of porous material will be encapsulated in the matrix material, preferably 5 to 20 percent by weight, specifically 7 to 15 percent by weight.

The porous material in and for use in tracer release materials according to the invention is capable of containing at least one tracer within the pores thereof. In one embodiment, the porous material contains, or is capable of containing, at least one tracer to a level of at least 20% (eg, 10 to 70%) of the combined weight of tracer and porous material. This is preferably 30% or 40%, and more preferably at least 50% tracer by weight of tracer and porous material.

The porous material according to the invention can "encapsulate" tracer material in any suitable form, and by any method. The encapsulation may be, for example, by physical encapsulation, by absorption into the bulk of the porous material, by adsorption on the porous surface of the porous material or in some other way.

The tracer releasing materials of the present invention require a tracer component for release. Such a tracer can be an oil tracer or a gas tracer (preferably a gas tracer), and can be detectable down to a very low level. Tracers detectable at a level down to at least 1x10-9 vol/vol (eg down to 1x10-9 to 1x10-13 by volume) would be preferred, more preferably detectable to 1x10-12 and most preferably detectable down to 1x10- 13. Suitable tracers can be those that are detected by means of radioactive detection (for example scintillation), by photometric methods such as color or fluorescence, but will most often be detectable by means of chromatographic methods, for example gas chromatography mass spectrometry (GCMS). Tracer materials will also typically be stable above the conditions in the reservoir, and in particular be stable towards water and/or hydrocarbons at elevated temperatures, temperatures such as up to 160 °C, preferably up to 200 °C and more preferably to temperatures above 200 °C, such as up to 220 °C, 250 °C or 300 °C.

Suitable classes of tracer include halogenated compounds, especially per-halogenated compounds such as per-fluorinated compounds, including per-fluorinated hydrocarbons. Partially fluorinated and per-deuterated aromatic compounds, partially fluorinated alkyl chains and ethers are also suitable.

In one embodiment, the tracer is a gas tracer. A suitable gas tracer may have a boiling point lower than the temperature of the reservoir (eg the gas reservoir). A suitable boiling point may be below 200 °C (for example 20 to 200 °C), preferably below 180 °C, more preferably below 140 °C (for example below 120 °C).

Where the reservoir is operated at elevated pressures, the boiling point of the tracer may be chosen to be lower than the temperature in the reservoir at pressures such as in the reservoir. In one embodiment, the boiling point of the tracer at pressures such as those in the reservoir may thus be lower than the temperature of the reservoir. The tracer can therefore be chosen according to the conditions of use that are appropriate.

Some examples of halogenated tracer compounds are shown below with their boiling points. Each of these tracers forms a highly preferred indicator for use in all aspects of the present invention.

In a further embodiment, the tracer material can be an oil tracer. Suitable oil tracers may have a boiling point above the reservoir temperature. Thus, for example, the tracer can be an oil tracer and have a boiling point above 140 °C, preferably above 160 °C, more preferably above 200 °C.

In one embodiment, a majority of the tracer-releasing materials according to the present invention can be used where at least one such tracer-releasing material comprises a gas tracer, and at least one such tracer-releasing material comprises an oil tracer. Such an embodiment allows for simultaneous monitoring of the flow of both fluids (oil and gas) and allows a relative increase in the production of one fluid in relation to the other to be detected. Otherwise, this embodiment can be carried out according to any of the methods and applications described here, and the use of any of the materials described here.

The tracer-releasing material according to the present invention is in the form of composite particles which have components as described herein. The outer capsule of robust material provides the general shape and size of the particles, and defines at least one opening through which the matrix material is in contact with the surrounding environment

(usually the reservoir environment when in use). Each particle may have one or more apertures (eg two, three or four apertures). Most often, each particle will have two openings. In one embodiment, the robust material is a polymer and the particles each have two openings. In an alternative embodiment, the robust material is a metal and the particles each have one or two openings. Typically, the ratio of the combined surface area of all the openings to the volume of the matrix encapsulated by robust material will be in the range of 0.0001 to 4 m_<1>, preferably 0.001 to 4 mm"<1>, more preferably 0.01 to 2 mm<1>.

Typical sizes for tracer releasing particles of the present invention will be in the range of 0.1 to 50 mm, preferably 0.2 to 10 mm and most preferably 0.5 to 5 mm. The internal diameter will obviously be smaller than the external diameter, and may be, 0.005 to 45 mm, preferably 0.01 to 9.5 mm, preferably 0.025 to 4.5 mm. Typically, the particles may be made from a tube of robust material and may be generally prismatic (especially cylindrical) in overall shape. Any form of particles can be used in the present invention and preferred forms will depend on the method of manufacture and the specific use. Spheres, open tubes and tubes with a closed end are among the preferred options. Various properties of the tracer-releasing materials (eg frequency of tracer release, duration, physical strength) can be regulated by varying the ratio of the cross-section of such a prism to its length. For example, a typical ratio of the prism mean diameter to the length may be from 2:1 to 1:20, more preferably 1:1 to 1:10 and more preferably from 1:2 to 1:8.

Where the tracer-releasing particles according to the invention are for use, or suitable for use, in hydraulic fracturing methods where it is preferred that the particles are of a comparable size to the propellants used, so that the propellant pack permeability is not significantly affected. In such an embodiment, the particles will generally not be larger than 2 mm in their smallest dimension, more preferably no more than 1.2 mm and most preferably no more than 1 mm in their smallest dimension. Preferably, the particles will conform to this dimensioning in at least two dimensions.

Applications and methods suitable for the particles according to the present invention include applications at any suitable location in a reservoir, where a fluid flow will, or can be expected, and there may be a desire to monitor the fluid flow. Examples of processes and places where the particles according to the present invention can be used include: High Rate Water-Pack: injection of tracer particles together with gravel under high pressure to achieve that the pressure pack will enable monitoring of the execution and success of the completion of the gravel pack.

Hydraulic fracturing without proppants: even when neither gravel nor proppants are injected, a limited amount of tracer particles that do not function as proppants can be useful for monitoring the results of the hydraulic fracturing, especially when such fracturing is carried out at several zones in the well.

Acid fracturing: provided that the particle coating and the matrix are designed to resist acid, and the particle is produced with small sizes (for example, less than 1 mm), tracer particles entering wormholes and pores will help to assess the result of acid fracturing. This is of particular interest in multi-zone operation. In acid fracturing embodiments, at least the outer robust material and/or coating material thereof should be stable to the acidic conditions of acid fracturing. When a coating material is used, and where this is acid-resistant, the coating can extend over the openings in the robust material in order to protect the matrix. In such a case, the coating should be permeable to the tracer with a corresponding degree of permeability to the matrix.

Inlet control devices: tracer particles fixed in the inlet zone by inlet control devices (ICD) will help to monitor the flow on the ICD, and thus different tracers will inform about relative production for several zones. Tracer particles can be trapped at the level of screens, for example, contained in materials positioned so that fluid flowing in the ICD carries leaked tracer. A plurality of particles according to the invention, each of which contains a different tracer, can be used in such a method (for example with different ICDs). This may allow the generation of a tracer inflow profile and thus to identify relative flows on each ICD.

Flow and sediment monitoring: an alternative is to introduce tracer particles into sand screens, prepacked or freestanding, and monitor inflow profiles and their development overtime to detect scaling. Tracer particles according to the invention may be useful in long screens, probably at several intervals, either prepackaged or freestanding to identify inflow profiles and their development overtime (for example, due to deposition and/or plugging).

Cementing/clamping job control: by injecting the particles into the zone to be isolated before the cementing or clamping job, work integrity control can be achieved by monitoring tracer release. The particles according to the invention can be used to validate remedial cementing jobs, for example to show isolation of zones or completion intervals or to prove integrity. For example, when shutting down or abandoning a well, a branch or in a completion interval/zone and/or when carrying out a completion repair. The particles according to the invention can be used over a long period of time to prove the integrity of barriers or to detect failure.

Injector profile control «flood control» / gas breakthrough: The particles according to the invention can be used in control and monitoring when using gas injectors (pressure maintenance for oil reservoirs or CO2 flooding) and/or injectors for WAG (Water Alternate Gas) systems. With the help of the particles according to the invention, information can be obtained about the connection between the reservoir, flow properties and the gas breakthrough mechanisms. Particles with different tracers can be used at each injection location to help identify breakthroughs.

Tracking the effectiveness of Improved Oil Recovery (IOR) injection processes and Enhanced Oil Recovery (EOR) injection processes: by prior (or possibly concurrent with water injection) introduction of tracer particles into the reservoir, monitoring IOR injection processes such as WAG, polymer flooding and characterization of the reservoir be possible.

Each of the above applications obviously serves as a further aspect of the present invention and may be used in combination with any of the tracer particles described herein as appropriate.

Typically, the particles will be used in a mono-modal distribution of particle sizes. In a further embodiment, however, a bi-modal distribution of particle sizes can be used. This will be most useful when a mixture consists of two tracer release materials that have different tracer and different particle sizes. This allows information to be provided on the flow of at least one fluid at properties in the reservoir of different size. For example, a mixture of small particles (eg, less than 1.5 mm in smallest dimension), and large particles (eg, greater than 5 mm in smallest dimension) with a first and a second tracer, respectively, can provide information about the fluid flow within a network of smaller fractures from tracer 1 and information about the fluid flow through larger fractures in the well from tracer 2.

In one embodiment, the tracer-releasing particles according to the present invention can be used for monitoring or investigations in connection with hydraulic fracturing methods. In such an embodiment, tracer release materials can be introduced into the reservoir with the plug material. In one embodiment, it is therefore preferable that the tracer-releasing particles have a density similar to that of the plug particles so that they do not have a strong tendency to settle out of the slurry of plugging agent, nor to float to the top of it. In one embodiment, therefore, the tracer release particles have a total density in the range from 1 to 4 g/cm 3 . This is preferably 1.5 to 3.5 g/cm 3 , more preferably 1.8 to 3 g/cm 3 .

The methods for forming tracer-releasing materials according to the present invention comprise steps i) to v) as described herein.

It is preferred that the method of formation is carried out at a temperature below the glass transition temperature of the matrix material. It is also preferred that the method for the formation is carried out at a temperature below the boiling point of the tracer. Both of these requirements have the advantage of reducing the amount of tracer lost from the particles during handling and preparation, and maximizing the utility of the tracer-releasing particles. As such, the method should preferably be carried out at below 50°C, preferably below 35°C, and more preferably below 30°C.

With regard to the preferred limitations on the operating temperature of the process, it is preferred that curing step iv) is not a thermal curing step. Curing by electromagnetic radiation, especially by visible or ultraviolet light is preferred. Chemical curing is possible, but this should preferably not require heating to above the temperatures stated here and there should not be a large generation of heat during curing, so that the temperature remains as stated herein above.

Use of tracer release material for inflow monitoring along a production pipe or inside the formation

The tracer release materials according to the invention can be used individually. Alternatively, they can be used as a plurality of materials according to the invention, each with different tracers. These can be used in investigations of fluid (especially gas) inflow into reservoirs, such as hydrocarbon reservoir production wells. In suitable methods according to the invention, two or more tracer-releasing materials according to the invention are used.

In one embodiment, the tracer-releasing materials containing defined tracers are placed at specific locations, either in the reservoir or along the production pipe so that identification of zones where fluid inflow is possible (see Figure 10), or at the reservoir's completion interface. Each different composition is typically applied at a different known position within the reservoir, for example along the well, in the gravel pack or screens (for example, sand screens), at ICDs, behind or in isolation devices (cemented intervals) and produced fluid at one or more locations ( er) of the aforementioned reservoir is monitored for the presence of tracer substances. These locations can be different layers of the same reservoir, so that monitoring of layer separation or flow between the layers is possible. Alternatively or in combination, those locations may be different zones of the same layer. By analyzing the produced fluid at the wellhead, information about the flow through the different locations in the reservoir can be derived. This can be through the presence, absence, quantitative determination and/or variation of one or more tracers in the produced fluid.

In an alternative embodiment (which can be used in combination with other embodiments described herein), tracer release materials containing defined tracers are placed in the formation by hydraulic fracturing which enables the identification of fractures where fluid flows into the well.

The tracer releasing materials according to the present invention can be used on an underground reservoir by many known methods and at a number of locations. Some suitable areas for using the materials according to the present invention are shown in figures 5 to 9. These include placing the material in the gravel pack around a sand screen in a defined position, placing the tracer release materials in a contact chamber between the sand screen and the production pipe, or placing the material according to the invention in a contact chamber inside the production pipe through which material must flow as it passes from the inflow point in the production pipe. Other application methods, such as use in hydraulic fracturing, are described here.

The different methods and applications of the present invention can be used with different degrees of accuracy when assessing the detected tracers. In one embodiment, the tracer may simply be assessed for presence or absence at one or more threshold concentrations. Alternatively, one can

absolute concentration is measured to provide information about the flow of fluid(s)

in the region of the tracer-releasing material, or a relative concentration may be measured to allow assessment of the relative flow of fluid at two or more locations. Furthermore, since the rate of change of concentration can provide information about the stability or in other ways of the fluid flow in the area of the tracer-releasing material(s), this can also be subject to measurement.

Application of the invention to investigations of fractures

A preferred application of the tracer releasing materials according to the present invention is in investigations and/or monitoring of fractures in hydraulic fracturing operations. The present invention provides a method for making observations of fluid movements in fractures in oil or gas wells. A plurality of sets of tracer releasing materials containing various tracers are guided down the wellbore and allowed to enter fractures at a first location. The first set of tracer releasing materials containing a first set of tracer is gradually released by diffusion in the fluid present at the first location in the reservoir. A second set of tracer-releasing materials may be transported from the surface to a second location, and allowed to enter fractures at a second location. The number of sets of tracer-releasing materials may be greater than two (ie, three, four, five or more), thus allowing the tracer-releasing materials to indicate fractures at more than two locations. The tracers can be separated from each other, so that detection of the tracer in the produced fluid will allow the identification of the tracer-releasing materials, and the location where they were placed. In one embodiment, all the tracers used can be detected by the same detection method (eg GC or GCMS). Preferably, all the tracers can be detected simultaneously.

General advantages of using the present invention

The advantages of using tracer-releasing materials according to the invention compared to other techniques are primarily that the release of tracer can be spread over a longer period of time, be more constant and/or predictable throughout the lifetime of the tracer source, and that the release rate is easier to calculate as the capsules have a defined opening which is fairly constant over the lifetime of the capsule. In contrast to other applications used, where tracer particles with low physical strength or where they are designed to dissolve completely, the tracer releasing materials according to the invention can be used, which are able to withstand hard physical loads over long periods of time. Thus, the particles according to the invention are designed to withstand the forces of injection, either alone or as part of a fracturing process. Furthermore, they can withstand the sustained pressure and forces in the reservoir over a longer period (for example up to 3 years). The materials according to the invention are furthermore not weakened during the process by tracer diffusion, and thus they maintain their strength regardless of tracer diffusion.

Brief description of the te<g>nin<g>s

Figure 1 shows a schematic representation of tracer-releasing particles according to the present invention. Figure 2 shows a schematic representation of the apparatus used in the example below. Figure 3 shows a graph showing the calculated release concentration versus time (from 7 hours to 134 days) of 1,2-perfluoromethylcyclohexane. Figure 4 shows a graph showing the calculated release concentration versus time (83 to 134 days) of 1,2-perfluoromethylcyclohexane. Figure 5 shows the materials according to the present invention applied to the gravel pack that surrounds the sand screen at the inflow point. Figure 6 shows the materials according to the invention used in a tracer contact chamber (for example in the form of a cartridge) between the sand screen and the production pipe at the inflow point. Figure 7 shows the materials according to the invention used in a delay chamber between the sand screen and the production pipe at the inflow point. Figure 8 shows the material according to the invention used as a cavity contact chamber inside the production pipe at the inflow point. Figure 9 shows the material according to the invention used with the plug particles at a fracture in a production zone. Figure 10 shows the use of three tracer materials according to the present invention to monitor material flow at three inflow points along a production pipe.

The following numbers are used to indicate certain features of the various figures:

1 - robust material

2 - matrix

3 - porous material (with encapsulated tracer substance)

4 - shale or similar with low productivity and/or unstimulated layers

5 - production layer

6 - the gravel pack

7 - cartridge of tracer particles

8 - tracer particles in the delay chamber

9 - tracer particles in the cavity contact chamber within the production pipe 5.1 - production layer 1

5.2 - production layer 2

5.3 - production layer 3

Examples

Manufacture of the capsules

In an experiment carried out in the inventor's laboratory, open capsules for controlled release containing four gas tracers were produced, an example is shown in Figures 1 and 2. Briefly, silicate particles (merck 10181 40Å 35/70 mesh) containing 50% by weight tracer mixture was mixed into the resin at a concentration of 10% by weight. The liquid resin was transferred to a plastic syringe. The syringe was connected to a 20 cm long piece of polymer tubing and the mixture was injected into the tubing. After the resin had been cured either chemically (polymethyl methacrylate) or with the help of a UV lamp, the tube was cut into pieces of 3 or 5 mm length. The acrylic resin could only be filled into 1/8" tubing, and not into 1/16" tubing because the tubing tended to get clogged by the silicate particles. This problem can be solved by using other resins and/or different sizes or proportions of particles. UV curing resins with silicate can be filled into both 1/8" and 1/16" production tubing. Gas tracers, such as those listed in Table 1, can be added to silicate to provide various identifiable open encapsulations for controlled release. Other fluorinated or partially fluorinated carbon compounds may be added, as well as per-deuterated aromatic compounds. Partially fluorinated alkyl chains and ethers are also suitable.

Release rates, in a natural gas environment, from a selection of tracers from a variety of different open controlled release canisters (Table 2) were tested using a flow apparatus at 160 °C and 400 bar pressure. Figure 3 shows the calculated release concentration versus time (from 7 hours to 134 days) for 1,2-perfluoromethylcyclohexane from line 3 (UV-15DV-80 1/8 "<*>3 mm), and Figure 4 shows the calculated release concentration versus time (83 to 134 days) for 1,2-perfluoromethylcyclohexane from line 3 (UV-15DV-80 1/8 "<*>3 mm). The curves shown in Figures 3 and 4 are typical for each open capsule type, but the release type changes depending on capsule dimension and polymer matrix. Table 3 shows the amount of tracer that is produced from each open enclosure type in the first 134 days of tracer availability in the trial.

Claims (21)

1) A gas or oil tracer repellent material in the form of particles comprising: a) an outer coating; b) a diffusion matrix within said outer coating, said matrix comprising at least one polymeric material; c) at least one porous material encapsulated within said matrix; d) at least one tracer inside the porous material, characterized by the outer coating is partial and comprises at least one robust material. 2) Trace substance-releasing material as stated in claim 1, in which the said partial outer coating is in the form of a tube having at least one opening. 3) Tracer release material as claimed in any one of the preceding claims, wherein the density of said particles is in the range: 1 to 3 g/cm<3>. 4) Tracer release material as set forth in any one of the preceding claims, wherein the robust material has a glass transition temperature above 140°C. 5) Tracer release material as set forth in any one of the preceding claims, wherein the robust material is at least one selected from: polyphenylsulfone (Radel R), polyetheretherketone (PEEK), polyetherimide, polyketone, steel, stainless steel, aluminum, aluminum alloys, nickel, brass and titanium. 6) A tracer release material as claimed in any one of the preceding claims, wherein the matrix material has a glass transition temperature of from 50 to 145°C. 7) Tracer release material as set forth in any of the preceding claims, wherein the matrix material is at least one selected from: polyethylene terephthalate, polyvinyl chloride, polyvinyl alcohol, polystyrene, polymethyl methacrylate, acrylonitrile butadiene styrene, polytetrafluoroethylene, polycarbonate, chemically cured epoxy resins and UV curing epoxy resin. 8) A tracer release material as claimed in any one of the preceding claims, wherein the porous material comprises at least one material selected from silicon dioxide, aluminum oxide, boron carbide, florisil and celite. 9) A tracer release material as claimed in any one of the preceding claims, wherein the tracer is a fully or partially fluorinated compound, a fully deuterated aromatic compound, a partially fluorinated alkane or a partially fluorinated ether. 10) Tracer releasing material as specified in any of the preceding claims, characterized in that the tracer is at least one selected from perfluorodimethylcyclobutane, perfluoromethylcyclopentane, perfluoromethylcyclohexane, 1,2-perfluorodimethylcyclohexane, perfluoroethylcyclohexane, octafluorotoluene, 1,3,5-Perfluoro trimethylcyclohexane, n-perfluoropropylcyclohexane , perfluorodecalin, perfluoromethyldecalin, and pe rf I u o rope rhy d rof I u o re n. 11) Tracer release material as set forth in any one of the preceding claims, characterized in that the robust material is a polymer and defines two openings. 12) A method for assessing the flow of at least one fluid in at least one underground reservoir, wherein the method comprises applying a first tracer release material as set forth in any one of claims 1 to 11, and comprising a first tracer in at least one position inside the said reservoir(s) and detection of the presence, absence, concentration and/or rate of change of the concentration of tracer in at least one fluid produced from at least one production location at the said reservoir(s). 13) The method according to claim 12, further comprising applying a second tracer material as set forth in any of claims 1 to 11, and comprising a second tracer in a second position within said reservoir(s), and detection of presence, absence, concentration and/or rate of concentration of said first tracer and said second tracer, wherein said second tracer is different from said first tracer. 14) A method for assessing the flow of at least one fluid within at least one underground reservoir, wherein the method comprises applying a plurality of tracer-releasing materials according to any one of claims 1 to 11 each comprising a different tracer at one plurality of positions within the said reservoir(s), and detection of the presence, absence, concentration and/or rate of change of the concentration of each of the said tracers in at least one fluid produced from at least one production location at the said reservoir(s), and relating the detected presence, absence, concentration and/or rate of change of concentration of each of said tracers and the position of application of each tracer releasing material in a stream of fluids within said reservoir(s). 15) Use of at least one tracer propellant material as set forth in any one of claims 1 to 11 to monitor the flow of at least one fluid in at least one underground reservoir. 16) Use as stated in claim 15 for monitoring the relative production of hydrocarbons from a plurality of locations at different layers within a hydrocarbon reservoir. 17) Use as stated in claim 16, wherein at least one tracer is added to the hydrocarbon reservoir as part of a proppant slurry. 18) Use as stated in claim 16 or claim 17 for monitoring the relative production of gas from a plurality of locations at different layers in a gas reservoir. 19) Application as stated in claim 18, wherein the reservoir is a shale gas reservoir. 20) A method for forming a gas or oil tracer release material as set forth in any one of claims 1 to 11, comprising: i) encapsulating at least one tracer within particles of at least one porous material to thereby provide an encapsulated tracer; ii) mixing said encapsulated tracer with at least one diffusion matrix medium, wherein to provide a tracer matrix medium; iii) introducing said tracer matrix medium into at least one cavity within at least one particle coating to provide a coated tracer matrix medium; iv) curing the matrix medium component of said tracer matrix medium to thereby provide at least one tracer-releasing particle or a tracer-releasing particle assembly. v) optionally: dividing said tracer-releasing particle assembly to thereby provide a majority of tracer-releasing particles. 21) The method according to claim 20, in which each of the steps i) to v) is carried out at a temperature below the boiling point of the tracer and/or below the glass transition temperature of the matrix material.