Abstract: Downhole tools having at least one component made of a doped magnesium alloy solid solution that at least partially degrades in the presence of an electrolyte, wherein the doped magnesium alloy is selected from the group consisting of a doped MG magnesium alloy, a doped WE magnesium alloy, a doped AZ magnesium alloy, a doped ZK magnesium alloy, a doped AM magnesium alloy, and any combination thereof.

Abstract: A swellable article, including a matrix material and an exfoliatable graphene-based material disposed in the matrix material. The exfoliatable graphene-based material is operatively arranged to facilitate swelling of the swellable article upon exposure to a selected fluid by sorbing particles in the fluid. The swelling enables the swellable article to engage an adjacent structure. Methods of making and using a swellable article are also included.

Abstract: An expandable tool can be lowered in a contracted state into a subterranean well. The expandable tool includes a tool frame that is movable between a frame contracted position and a frame extended position, and a plurality of tool segments. The plurality of tool segments include a plurality of moveable tool segments and a plurality of stationary tool segments. An outer diameter of the expandable tool can be increased by moving each of the tool segments radially outward. The expandable tool can be rotated so that the plurality of moveable tool segments rotate relative to the tool frame and the plurality of stationary tool segments remain fixed relative to the tool frame. Applying axial force to the expandable tool will move moveable tool segments axially relative to the tool frame, and an axial length of the expandable tool is reduced until the expandable tool is in a final state.

Abstract: A well packer includes an expandable member carried on a mandrel, the mandrel including a groove formed on at least a portion of an outer radial surface of the mandrel; a back-up assembly carried on the mandrel adjacent the expandable member; and a fold back assembly carried on the mandrel adjacent the back-up assembly, the fold back assembly including a snap ring carried on the mandrel and adapted to move into the groove and adjust the expandable member into an expanded position in substantially sealing contact with a tubular.

Abstract: Downhole tools including a wellbore isolation device that provides a plurality of components including a mandrel, and a slip assembly comprising a plurality of slip elements including a slip wedge and a slip, wherein an element of the slip assembly is composed of a degradable metal material, and wherein the slip comprises a slip insert embedded therein, the slip insert having a compressive strength of greater than about 120,000 psi.

Abstract: A method of sealing a well comprises creating one or more openings in a tubing installed in a wellbore, in a location for a well seal; using a wireline to locate a stinger in that location; setting the stinger in that location; and forming a seal in the well in that location, by injecting sealant through said stinger.

Abstract: A formation of a borehole casing by application of material layers is achieved by means of kinetic sputtering. Specifically, the formation is achieved by additive kinetic sputtering of metallic, non-metallic and composite materials using acceleration and heating of material powder particles and subsequent plastic deformation of deformable powder fraction upon the impact onto the surface. The accelerated and heated particles of material in the form of powder impinge onto the surface of a borehole wall and/or on a mould and/or onto the surface of a previous layer of the casing in such manner that they form the layered composite casing on the inside wall of the borehole and/or on the mould, particularly in a liquid medium.

Abstract: Apparatus and methods are described for autonomously controlling fluid flow in a tubular in a wellbore. A fluid is flowed through an inlet passageway into a biasing mechanism. A fluid flow distribution is established across the biasing mechanism. The fluid flow distribution is altered in response to a change in the fluid characteristic over time. In response, fluid flow through a downstream sticky switch assembly is altered, thereby altering fluid flow patterns in a downstream vortex assembly. The method selects based on a fluid characteristic, such as viscosity, density, velocity, flow rate, etc. The biasing mechanism includes a semi-doughnut-shaped wall contour element formed along one side.

Abstract: A valve assembly can include a longitudinal flow passage, a first longitudinal section of the flow passage having less flow area than a second longitudinal section of the flow passage, and a valve that permits flow in only one direction through the flow passage, the valve including a closure member. In an open position, the closure member can be disposed between the second flow passage section and a chamber in fluid communication with the first flow passage section. A method can include connecting a valve assembly in a tubular string, the valve assembly including a valve that permits flow in only one direction through a longitudinal flow passage of the valve assembly, and flowing fluid in the one direction through the tubular string, thereby displacing a closure member of the valve to an open position and preventing displacement of the closure member toward a closed position.

Abstract: A valve assembly can include a longitudinal flow passage, a first longitudinal section of the flow passage having less flow area than a second longitudinal section of the flow passage, and a valve that permits flow in only one direction through the flow passage, the valve including a closure member. In an open position, the closure member can be disposed between the second flow passage section and a chamber in fluid communication with the first flow passage section. A method can include connecting a valve assembly in a tubular string, the valve assembly including a valve that permits flow in only one direction through a longitudinal flow passage of the valve assembly, and flowing fluid in the one direction through the tubular string, thereby displacing a closure member of the valve to an open position and preventing displacement of the closure member toward a closed position.

Abstract: Disclosed herein is a seat assembly for use in wellbore servicing systems, comprising a cylindrical baffle with an annular shaped seat with an upward facing seat for receiving an obturator, the seat defining a central passageway. Erosion resistance rings are placed inside of and in front the baffle to protect the baffle and seat from erosion cause by treatment fluids and solids passing through the servicing system.

Abstract: A wellbore tool that can withstand pressure tests without becoming hydraulically actuated. The wellbore tool includes a tubular housing including an inner bore; a tool mechanism responsive to fluid pressure; and a pressure isolator for the tool mechanism moveable between an active and an inactive position.

Abstract: A method and apparatus for controlling the flow of fluid in an injection well includes a main valve assembly having a valve and a retrievable nozzle selective lock assembly (RNSLA). The RNSLA is operable when positioned within the valve body to open the valve when fluid is pumped into the well and closes the valve when fluid flow is terminated. The RNSLA includes a replaceable orifice nozzle so that orifices of different dimensions may be used in conjunction with the valve assembly. In an alternate embodiment, the RNSLA includes a variable output nozzle assembly to maintain the valve in a protected open position without chattering over a wide range of flow rates.

Abstract: An apparatus to inject a fluid into a geological formation. The apparatus includes a central bore running axially through the apparatus; a normally-closed sleeve valve with a sliding sleeve, the sleeve valve configured to open at a sleeve activation pressure; an upstream packer disposed upstream from the sleeve valve; a downstream packer disposed downstream from the sleeve valve; and a normally-open bottom valve disposed downstream from the downstream packer, the bottom valve configured to block axial fluid flow at a first bore pressure. The upstream packer and the downstream packer are configured to set at a second bore pressure between the first bore pressure and the sleeve activation pressure.

Abstract: Downhole wireline cleaning tool (1) adapted to scrape and remove objects, such as scale decreasing an inner diameter of a casing in a wellbore, comprising a tool housing (4) having a first housing part (5) and a second housing part (6), a projectable arm (7) which is pivotably connected with the first housing part at a first end of the projectable arm, the projectable arm having a plurality of bits (9) in a second end, said projectable arm (7) being movable between a retracted position and a projected position in relation to the tool housing (4), an arm activation assembly (11) for moving the projectable arm between the retracted position and the projected position, and a rotatable shaft (12) for rotating the projectable arm (7), wherein the bits are arranged along an extension of the projectable arm, wherein the extension of the arm extends across the centre tool axis in at least the retracted position, so that the bits of the arm are arranged to scrape the entire internal cross-sectional area of the casing.

Abstract: Disclosed are wellbore flow control devices that allow on-site field adjustments to flow characteristics. One autonomous inflow control device (AICD) assembly includes a base pipe defining one or more flow ports and an interior, at least one AICD arranged on the base pipe and having at least one fluid inlet and an outlet in fluid communication with one of the one or more flow ports, and a plug configured to be arranged in at least one of the at least one fluid inlet and the outlet of the at least one AICD by a well operator on-site.

Abstract: An intake manifold provided as an integral one-piece manifold body. The manifold has a base plate with a series of spaced apart discharge ports and an elongate tapered pipe portion formed integrally with the base plate, having an inlet, and having inner tapered walls for directing flow through the manifold body from the inlet through the discharge ports of the base plate.

Abstract: A system and method of performing a fracture operation at a wellsite about a subterranean formation is disclosed. The method involves, obtaining wellsite measurements by placing a downhole tool in a wellbore and using the downhole tool to acquire measurements of the subterranean formation, simulating the obtained wellsite measurements to determine formation parameters comprising conductivity tensors based on a formation model of the measured subterranean formation, validating the formation model by comparing the obtained wellsite measurements with the simulated wellsite measurements, generating fracture parameters and triaxiality indicators based on the validated formation model, and fracturing the subterranean formation based on the generated fracture parameters and triaxiality indicators.

Abstract: Systems and methods for monitoring, calculating, and/or optimizing the consumption of fuel in operating motorized equipment in well site operations or other jobs are provided. In one embodiment the methods comprise: calculating a set of fuel consumption rates for one or more engines at a job site as a function of a speed of each engine, a hydraulic horsepower load to be provided by each engine, and an external parasitic load to be provided by each engine; identifying one or more operating speeds for the one or more engines during an operation at the job site based at least in part on the set of fuel consumption rates for the one or more engines; and operating the one or more engines at the one or more operating speeds during an operation at the job site.

Abstract: Techniques for managing a hydraulic fracturing operation include receiving a selection of a proxy model that represents a first principles model of a hydraulic fracturing operation, the proxy model including at least one property of a plurality of properties of the first principles model associated with the hydraulic fracturing operation; simulating the selected proxy model to generate a modeled output based on the property; and determining a value of a control setpoint for hydraulic fracturing operation equipment based on the modeled output.

Abstract: Methods, systems, and computer program products for developing and presenting wellbore pipe string related operation engineering parameter roadmaps in hydrocarbon reservoir modeling are disclosed. A computer-implemented method may include receiving a plurality of depth intervals, associating different computational model calculations with each depth interval, selecting input parameter values for each set of computational model calculations, generating computational model results for each set of computational model calculations associated with each interval to yield a graphical interval plot for the depth interval; and displaying a plurality of graphical interval plots together on a wellbore roadmap. A control list for the roadmap is correspondingly generated and dynamically linked to the wellbore roadmap so that individual entries in the control list are associated with individual interval plots.

Abstract: A system is configured to fracture a formation surrounding a subterranean wellbore. In one example, the system includes a tool string configured to be deployed in the wellbore, and an acoustic pulse generator connected to the tool string and configured to transmit acoustic pressure pulses into the formation. The acoustic pulse generator is configured to generate acoustic pressure pulses with a magnitude of at least 1000 pounds per square inch.

Abstract: The presented invention pertains to devices and techniques for containment of burning, as well as of non-burning, hydrocarbon energy resources blown out of a damaged wellhead or pipeline on the land or under water. The device can be monolithic or modular and assembled into a bell-shaped dome structure using a single or multi-layer flexible fire-proof material. Upper section of the dome has connected one or more flexible arms with attached ducts. The attached arms can be branched. The arms and their branches are used to divert fire flames and released products away from the affected area into a safe zone or a collection reservoir. The device can be deployed for containment and localization of the blown out resources from horizontal, vertical or inclined pipelines. Materials needed for fire suppression (e.g. gaseous or vaporous inert materials) or required for stopping and capping damaged area (e.g.

Abstract: A downhole pump flushing system has a pump body defining an interior cavity, an inlet end, an outlet end and at least one flushing inlet. A lifting member is positioned within the interior cavity of the pump body for facilitating lifting of wellbore fluid. An injection mandrel in circumferentially engaging relation to an exterior surface of the pump body is provided such that it encompasses the at least one flushing inlet. The injection mandrel has an injection port fluidly connected to a fluid outlet port. The fluid outlet port is in fluid communication with the at least one flushing inlet of the pump body. An injection line is connected to the injection port of the injection mandrel and to a fluid supply. An annular channel is provided between the pump body and the injection mandrel adjacent to the at least one flushing inlet such that fluid is guided into the at least one flushing inlet from the fluid outlet port of the injection mandrel.

Abstract: The disclosed device provides a high-accuracy plunger arrival detection system comprising a low-power magnetometer with high sensitivity and which is capable of sampling low or high intensity magnetic fields. The device processes gathered data from sensors, stores at least some processed data in memory, executes a trending algorithm which compares the magnetic field of the plunger to the ambient magnetic field or a predetermined set of initialization values, and generates an output which is relayed to a well controller. An output signal may be via hard wire, RF, wireless or other known means. In addition, the implementation of two sensing devices mounted in series and in spaced relation to each other, can provide for an actual plunger average velocity. An actual plunger average velocity, as opposed to approximate average velocity, can be used to better optimize well control and improve safety of the overall well production system.

Abstract: One illustrative artificial lift method includes deriving compressed natural gas (CNG) from liquefied natural gas (LNG) and injecting the CNG into the well as a lift gas that aids in conveying fluid from the well. An illustrative system embodiment includes an evaporator and a controller. The evaporator converts LNG into CNG, which the controller injects into a well to enter a lift conduit as a lift gas to aid in conveying fluid from the well. Further disclosed herein is the use of a virtual pipeline to supply LNG for such artificial lift systems and methods. It includes: liquefying natural gas to fill a transport trailer at an offsite facility; transporting the trailer to a site of a well; and coupling the trailer to surface equipment to supply LNG as needed for supplying gas lift in the well. Once emptied, the trailer may be returned to the offsite facility for refilling.

Abstract: A crossover valve assembly for insertion into production tubing, or integral with production tubing, includes an outer housing, an inner production tube, a pilot section responsive to external pressure to open an activation passage above a pre-determined pressure, a power section responsive to pressure in the activation passage to open an injection opening; and a crossover valve responsive to pressure in the injection opening to open a crossover port, allowing fluid communication from outside the outer housing to within the inner production tube.

Abstract: Disclosed herein is an experiment apparatus for estimating ground deformation during gas hydrate recovery. The experiment apparatus may include: a high-pressure cell having a space in which a sample containing gas hydrate is stored; a recovery member inserted into the sample so as to recover the gas hydrate contained in the sample to the outside; and a transparent region formed at one or more parts facing the space of the high-pressure cell, such that the sample stored in the space is observed from outside.

Abstract: A process useful in mixing reagents delivered to an injector well which reacts with bitumen-heavy petroleum products in the injector well. The reaction produces significant heat to the bitumen reservoir rock, the heat lowers the viscosity of the material, and permits the flowable material to be collected by a nearby producer well, usually beneath the injector well. The well is assisted with a vacuum system to remove the bitumen containing petroleum materials. This is a method of selectively initiating in situ chemical reaction causing the heating of a targeted subterranean hydrocarbon-bearing formation wherein a substance capable of reacting with hydrogen peroxide is introduced into the formation via an injector well. The resultant mixture of hydrogen peroxide and water is injected to cause an exothermic reaction that increases the temperature of the formation and resident bitumen resulting in a less viscous product with enhanced quality and increased market value.

Abstract: A system and method are provided for providing access to surfaces within a formation is provided, the method including: providing a wellbore from a first surface location to a second surface location; inserting into the wellbore a cylindrical cutting assembly connected to at least two wellbore tubulars, one of the wellbore tubular extending to each of the first surface location and the second surface location; and rotating the radial cylindrical cutting element.

Abstract: An embodiment of a method of stimulating an earth formation includes: disposing a stimulation device at a borehole in an earth formation, the earth formation having been stimulated by an initial stimulation operation; subsequent to the stimulation operation, performing a probe operation configured to cause movement of existing fractures in the formation; and measuring microseismic events occurring in the formation by one or more seismic receivers. The method further includes: identifying one or more target zones in the formation based on the measuring, the one or more target zones exhibiting a reduced micro seismicity relative to another zone in the formation; and designing a re-stimulation operation configured to stimulate the one or more target zones to increase hydrocarbon production from the formation.

Abstract: Systems for stimulation a formation include a plurality of variable frequency drives, each in communication with at least two high pressure pumps. A variable frequency drive actuates at least two electric motors associated with the high pressure pumps, such that the pumps pressurize fracturing fluid, proppant, or combinations thereof for flowing the fracturing fluid, proppant, or combinations thereof into the formation.

Abstract: Improving the flow of hydrocarbons from shale lateral wellbores in unconventional wellbores may be accomplished with various configurations of assisting lateral wellbores and/or kick-off wellbores from primary lateral wellbores and/or secondary or assisting lateral wellbores. By extending fracture networks from adjacent lateral wellbores and/or adjacent kick-off wellbores so that the fracture networks from different wellbores are in fluid communication with one another, the flow of various fluids between the adjacent wellbores provides another dimension of control over the wellbores and fracture networks used to recover hydrocarbons from the shale intervals of interest.

Abstract: This invention has three parts: Part 1 Two-phase extraction of contaminated water and vapor from extraction wells using drop tubes with vertical slots or orifices; Part 2, Separation of extracted water and vapor/air at the wellhead or in a common manifold installed in ground, Part 3 injection of the extracted water using injection well with a drop tube similar to drop tube in the extraction well without the orifices or slots that extend above and below the water table. This invention strips volatile organic compounds from water, creates a recirculation cell in subsurface, increases biodegradation of organic compounds within the recirculation zone. This inventions also facilitate performing enhanced in-situ bioremediation or chemical oxidation by adding oxidation chemicals or bioremediation substance into the injection stream.

Abstract: The subject matter of this specification can be embodied in, among other things, a method that includes providing an outer tubular member having a bore with an inner surface, applying a lubricant layer to at least a portion of the inner surface of the outer tubular member, positioning the outer tubular member in at least a portion of the wellbore, providing a drilling assembly including an inner member having an outer surface, applying a lubricant layer to at least a portion of the outer surface of the inner member, inserting the inner member into the bore of the outer tubular member, providing a drilling fluid through the bore of the drilling assembly, rotating the inner member relative to the outer member, measuring an indicator of mechanical wear between the outer member and the inner member, determining that the measured indicator exceeds a predetermined threshold level, and triggering a subsequent operation.

Abstract: A drill bit includes one or more cutters on a surface thereon configured to penetrate into a formation, at least one pad at the surface, and an actuation device configured to supply a fluid under pressure to the pad to extend the pad from the surface. The drill bit also includes a relief device configured to drain fluid supplied to the pad to reduce the pressure on the at least one pad when the force applied on the at least one pad exceeds a selected limit.

Abstract: An example method for drilling automation may comprise generating a model of a drilling system based, at least in part, on a first set of downhole measurements. The model may accept drilling parameters of the drilling system as inputs. A rate of penetration for the drilling system may be determined based, at least in part on the model. The model may be simulated using a first set of values for the drilling parameters, and a control policy for the drilling system may be calculated based, at least in part, on the rate of penetration and the results of the simulation. A control signal to the drilling system may be generated based, at least in part, on the control policy.

Abstract: Method and apparatus are presented for monitoring environmental parameters in one or more abandoned wellbores. A mandrel with a radially expandable sealing element is positioned downhole, such as in a tubing string, to provide pressure isolation. A measurement tool having one or more sensors or gauges is positioned below the mandrel to measure the environmental parameters. The data is transmitted to the surface via wire or wirelessly.

Abstract: Cables, systems, and methods are disclosed that provide for the monitoring of a cable being deployed to a borehole. Such systems may include a cable having one or more markings at regular intervals. Each marking includes marking data that may be alphanumeric characters, bar codes, ring codes, or a quick-response code. The system also includes a controller that is operable to receive, store, and transmit marking data, and a reader that is operable to read the markings and to transmit marking data to the controller. The marking data may be associated with cable data that is stored in a database to convey information about the cable, such as the origin, mechanical properties, and usage history of the cable.

Abstract: Disclosed are intelligent casing collars and cement wiper plugs used in wellbore cementing operations. A disclosed well system includes a casing string extending from a surface location within a wellbore, an annulus being defined between the casing string and the wellbore, a casing collar included in the casing string and having one or more sensors configured to measure at least one fluid property of a fluid present within the annulus, a cement wiper plug arranged within the casing string and communicably coupled to the casing collar such that measurement data obtained by the one or more sensors is conveyed to and received by the cement wiper plug, and a pulser associated with the cement wiper plug and configured to transmit pressure pulses to the surface location, wherein the pressure pulses correspond to the measurement data received from the one or more sensors.

Abstract: The subject matter of this specification can be embodied in, among other things, a system for removably attaching an optical fiber sensor loop onto a tubular member, which includes an optical fiber sensor loop having a continuous optical fiber positioned arranged in a plurality of loops, each of said loops having a first end turn and a second end turn, a first and a second turn guide each including a plurality of turn grooves increasing outwardly in increasing radii, each of said turn grooves configured to receive an end turn portion of the optical fiber, a first and a second supporting wedge each having a planar first surface configured to receive a turn guide and a curved second surface configured to be received on the tubular member, and a connector configured to couple the first mounting wedge to the second mounting wedge.

Abstract: The subject matter of this specification can be embodied in, among other things, a system for mounting a strain sensor on a tubular pipe, which includes a mechanical clamp. The clamp has a bottom flexing section having an arcuate portion terminating at a first terminal and at a second end, and a first and second upper flexing sections having an arcuate portions terminating at first terminal ends and at second terminal ends in a pivot pin assembly having a bore parallel to a central longitudinal axis of the clamp, the bore there through for receiving a removable connector. Sensor mounting arms are disposed outwardly on the first and second upper flexing sections, said sensor mounting arms including at least one receptacle sized to receive and retain ends of a strain gauge.

Abstract: A method and system for determining information about a wellbore with coiled tubing. A downhole device may be positioned within coiled tubing and run down the wellbore to determine diagnostic information about a location with the wellbore. The downhole device may store diagnostic information in a storage device that may be analyzed when the device is returned to the surface. A downhole device may be connected to the end of a string of coiled tubing that includes a diagnostic device and memory sealed in a chamber. A flow path past the chamber is in communication with the coiled tubing string permitting the flow of fluid past the chamber. A downhole device including a diagnostic device may be connected to a recess in an exterior of a coiled tubing string.

Abstract: Systems and methods for identifying formation boundaries without necessarily obtaining an azimuthal borehole image are provided. A downhole tool may be placed in a wellbore in a geological formation that has a formation boundary. First and second measurements may be obtained at a number of depths of the wellbore. The first measurement may have a first effective penetration length into the geological formation and the second measurement may have a second effective penetration length into the geological formation different from the first effective penetration length. Thus, the first measurement may detect the formation boundary at a first depth and the second measurement may detect the formation boundary at a second depth. Using a difference between the first depth and the second depth, an apparent relative angle between the wellbore and the formation boundary or an apparent formation dip, or both, may be obtained.

Abstract: A method for determining a configuration of a mechanical actuator coupled to a drilling shaft in a drilling shaft deflection device of a rotary steerable subterranean drill. The method includes receiving, at a controller, data representative of a present rotary position of a rotor of an electronically commutated deflector motor of a drilling shaft deflection device of a rotary steerable subterranean drill. The rotor is operatively coupled by a fixed-ratio transmission to a mechanical actuator comprising a drilling shaft receiver coupled about a portion of a deflectable drilling shaft of the drilling shaft deflection device. The method includes determining, at the controller, the present position of the drilling shaft receiver in dependence upon the received data representative of the present rotary position of the rotor of the electronically commutated deflector motor.

Abstract: In one example, an integrated well survey management and planning tool is implemented by a computer system. The tool can receive survey data describing a well being drilled from a surface to a subterranean geological target to be reached by drilling the well. The well is associated with a target trajectory on which the well is to be drilled from the surface to the subterranean geological target. Based, at least in part, on the survey data and the target trajectory, the tool determines an uncertainty indicator indicating an uncertainty in drilling the well on the target trajectory, and displays the uncertainty indicator in a user interface.

Abstract: One example of determining well fluid flow velocity based on vortex frequency is implemented using a well fluid flow velocity measurement system. The system includes a well fluid flow monitoring unit to determine a parameter of a well fluid flow in response to a vortex being produced in a well fluid, and provide the parameter. The system also includes a controller to receive the parameter determined by the flow monitoring unit, determine a frequency of the vortex from the received parameter, and determine a flow velocity of the well fluid based on the determined frequency.

Abstract: Fluid flux in a fluid injection and/or hydrocarbon fluid production well is monitored in a cost efficient and accurate manner by inducing the fluid flux to move an acoustic emission device in a longitudinal direction through the well, measuring a longitudinal velocity (v) of the acoustic emission device at various longitudinally spaced locations in the well using a fiber optical Distributed Acoustic Sensing (DAS) assembly which monitors the longitudinal position of the acoustic emission device against time along at least part of the length of the well, and monitoring the fluid flux at various locations along the length of the well on the basis on the measured velocity (v) of the acoustic emission device.

Abstract: A system and method for monitoring oil flow rates at multiple points in production wells using a flow sensing fiber optic cable. An illustrative system embodiment includes: a fiber optic sensing system housed within a tube suitable for a downhole environment; and a flow to signal conversion device attached to the tube and deployed in the oil flow.

Abstract: A tracer material for tracing fluid flows from a hydrocarbon reservoir. The tracer material is a particulate tracer material including a plurality of particles of a porous particulate solid material, the particles having pores containing a tracer composition characterised in that a retaining material overlies the tracer composition in at least some of the pores. The retaining material retards the release of the tracer material so that the tracer is released at a more constant rate over a longer period than in the absence of the retaining material.