A tracer device for processing fluid which is injected into a structure enclosing a borehole in a bedrock or a casing arranged in the borehole, is described. A method for tracking an occurrence of a processing fluid which is injected into a structure is also described.

Injection of chemical inhibitors (scale inhibiting substances) in producing structures close to a weiibore is often used in the gas and oil producing industry in order to keep production restraining scaling of deposit products under control. Such well processing may well be carried out 2-4 times per year in wells exposed to troublesome scaling. The inhibitors are in most cases dissolved in water and are pumped into the actual structures in a reservoir. The chemicals are adapted to be deposited onto the surfaces of the rock structure in order to slowly be emitted to the well fluid and thus inhibit scaling for instance in a production tubing and appurtenant downhole tools.

The producing, petroleum carrying layers have a production rate higher than the inhibitor injection rate. The injection rate is often controlled by the pump capacity of the injection system, or possibly by limitations in the well, for instance the fracturing pressure. When a well comprises different production zones with different characteristics, the achieved injection rate may show great variations, where typically a zone with a large reservoir pressure receives a small inhibitor amount, whereas a zone with small production may receive large amounts of inhibitors. This phenomenon may appear in both vertical and horizontal wells. An example of outflow of production fluids and inflow of processing liquid in different zones in a highly permeable, horizontal well is illustrated in the table below:

Sum Zone 1 Zone 2 Zone 3

Well Production index 190 120 25 45

(Sm3/day/bar pressure dif- ference)

Reservoir pressure (bar) 295 297 300

Production Bottom hole pressure (bar) 265

Production (Sm3/day) 5975 3600 800 1575

Distribution among zones 100% 60% 13% 26%

Injection Bottom hole pressure (bar) 300

Injection (m3/day) 675 600 75 0

Distribution among zones 100% 89% 11% 0%

In order to avoid insufficient processing, an increased injection pressure is often used, or possibly the viscosity of the inhibitor fluid is changed. To increase the viscosity has been tried to be highly successful in many cases, but there is a risk that the structures will be damaged. In many cases it is not possible to use increased injection pressure due to limitations to the equipment used, or because of the strength of the rock(s). Thus it is desirable to improve the control of the inhibitor processing, such that excessive use of means like pressure and viscosity in order to be on the «safe» side, is avoided.

The invention has for its object to remedy or to reduce at least one of the drawbacks of prior art, or at least to provide a useful alternative to prior art.

The object is obtained by features disclosed in the description below and in the subsequent patent claims.

To the extent the phrases «upstream» and «downstream» are used in the subsequent description and in the accompanying patent claims, these phrases are to be seen relative to the normal flow direction of fluid from an underground structure and to a surface installation, even if a processing of a portion of a borehole at times comprises fluid flow from the surface installation and into the underground structure. «Up- stream» thus means a more distant position than a «downstream» position relative to, for instance, the surface installation.

The invention provides a downhole arrangement where a number of trace element containers are pre-installed in a tubing extending through a portion of a borehole which is to be subjected to a processing. An example of such a tubing is a production tubing or a liner and/or screen(s) for the production tubing. The containers each hold a trace element different from the trace element in the other container(s). The trace elements are indissoluble in formation fluids, typically oil or formation water, but are dissolved and released as they come into contact with a processing fluid, for instance a fluid which is injected together with a scale inhibitor and is carrier of the scale inhibitor. The trace elements may have these features themselves, or they may be encapsulated in a material with such features, or they may be chemically bound to a carrier, the chemical bond ceasing to exist only in contact with the processing fluid.

By analysing the fluid flowing up from the borehole, for instance the fluid produced in the well after accomplishing the inhibitor processing, it will be possible to verify which parts of the borehole that have been supplied with processing fluid by mapping the occurrence of trace elements present in the outflowing fluid, occurrence of processing fluid at a trace element container having resulted in the actual trace element being transported out of the well by the processing fluid. The processing fluid which returns, may be completely or partly mixed with other fluids, for instance formation fluids flowing out from a well.

The arrangement may also be used for mapping of how other well processing has been carried out, for instance how a fracturing processing (pumping in of a fluid under high pressure for fracturing the rock structures) has been carried out, as the trace elements may be dissolvable also in contact with a fracturing fluid. A further example of an area of application is the tracking of how squeezing of a sealing material has evolved, for instance by a sealing operation of a cemented portion between a borehole wall and a casing.

Typically retrieval of a trace element from a container far down/out in a well indicates that the processing has been too aggressive, whilst retrieval of trace element only from the container closest to the wellhead indicates that the processing has not reached the core zone of the borehole, as no trace elements from this area are retrieved in the fluid flowing back from the borehole.

The trace element may be provided encapsulated in one or more carbonates or oxides for releasing as the processing fluid with low pH-value is brought into contact with the trace element container during outflow from the borehole.

In a first aspect the invention more specifically relates to a tracer device for processing fluid injected into a structure enclosing a borehole in a bedrock or a casing arranged in the borehole, characterized in that a portion of a fluid line extending through the structure, is provided with at least two spaced apart trace element containers each holding a trace element different from the trace element in the other container(s), the trace elements being dissoluble in contact with the processing fluid and indissoluble in contact with a formation fluid.

The trace elements may be contained in an encapsulating material which is dissoluble in contact with the processing fluid and indissoluble in contact with the formation fluid, or are chemically bound to a carrier, the chemical bond being unaffected of the formation fluid, and the trace elements are chemically releasable in contact with the processing fluid.

At least one trace element container may be arranged downstream of the structure which is the target for the injection of the processing fluid.

At least one trace element container may be arranged upstream of the structure which is the target for the injection of the processing fluid.

The trace element containers may each be formed as a pipe section provided with one or more compartments arranged on the pipe section wall and be in fluid communication with a fluid channel in the pipe section, the fluid channel forming a portion of a flow path for the formation fluid and/or the processing fluid away from the structure.

The encapsulating material holding the trace elements may be formed from at least one or more carbonates, or at least one or more oxides.

In a second aspect the invention more specifically concerns a method for tracking of an occurrence of a processing fluid which is injected into a structure enclosing a borehole in a bedrock or a casing arranged in the borehole, characterized in that the method comprising the following steps:

a) filling at least two trace element containers with a trace element, the trace element in one of the trace element containers being different from the trace element in the other container(s), the trace elements being dissoluble in contact with the processing fluid and indissoluble in contact with a formation fluid;

b) arranging spaced apart trace element containers in a portion of a fluid line extending through the structure which is target for the injection of the processing fluid; c) letting the processing fluid come into contact with the trace element as the processing fluid flows away from the structure;

d) measuring the occurrence of said trace elements downstream of the structure; and e) using the occurrence of said trace elements as indicator for occurrence of processing fluid injected upstream of each of the trace element containers.

The trace elements may be held in an encapsulating material which is dissoluble in contact with the processing fluid and indissoluble in contact with the formation fluid, or are chemically bound to a carrier, the chemical bond being unaffected by the formation fluid, and the trace elements are chemically releasable in contact with the processing fluid.

The trace elements may be released on dissolution of the encapsulating material by use of pH-regulating means in the processing fluid.

In the following an example of a preferred embodiment which is illustrated on the accompanying drawings, is described, where:

Fig. 1 shows a principle drawing of a subsea borehole extending horizontally in a structure, and where the borehole is connected with a floating surface installation via a riser;

Fig. 2 shows in a larger scale, a principle drawing, partly cut through, of a portion of a production well for hydrocarbons, where a liner is provided with a tracer device in accordance with the invention provided with three spaced apart trace element containers in a production zone of the well;

Fig. 3 shows in a larger scale a perspective drawing of a pipe section provided with a multi-sectional trace element container;

Fig. 4 shows in a larger scale a principle drawing of a trace element encapsulated in an encapsulating material; and

Fig. 4b shows a principle drawing of a trace element carrier where a trace element is chemically bound to the trace element carrier.

In figure 1 the reference numeral 1 indicates a subsea bedrock where a production well 2 is established in a borehole 21 extending partly horizontally through a producing structure 11 in the bedrock. The borehole 21 is connected to a surface installation 4 via a riser 41 known per se. In the horizontal portion of the production well 2 a tracer device 26 in accordance with the invention is arranged, as a number of trace element containers, herein shown a first, a second and a third trace element container 261, 262, 263, are arranged spaced apart from each other in the longitudinal direction of the production well 2. The reference numeral 28 indicates an area where a processing fluid is injected into the producing structure 11.

Reference is now made to figure 2 where a portion of the production well 2 is shown in greater detail. Except from where the borehole 21 extends through the producing structure 11, the borehole 21 is lined in a known way per se with casing 23 which is partly cemented to the surrounding bedrock 1 by means of cement 24. Both the producing structure 11 and the cement 24 are examples of structures which in certain situations require processing in order to improve the productivity, prevent leakage etc., for instance in the form of fracturing or injection of scale inhibitors in the producing structure 11, or squeezing a sealing material in through formed openings (not shown) in the casing 23, in a cement structure 24 which does not show sufficient density to prevent leakage in an annulus 12 between the bedrock 1 and the casing 23.

A production tubing 22 forms a continuous fluid channel 221 from the production well 2 and to the surface installation 4, possibly to another, not shown reception plant.

From an end portion 231 of the casing 23 a liner 25 extends outwards in an unlined portion of the production well 2. The liner 25 comprises a fluid channel 251 which via inlet openings, for example one or more screen(s) 27, form a fluid connection between the producing structure 11 and the fluid channel 221 of the production tubing 22. The liner 25 is provided with a tracer device 26 which in the embodiment shown is constituted by a first, a second and a third spaced-apart trace element containers 261, 262, 263 in the longitudinal extension of the liner 25.

Reference is then made to figure 3, where a trace element container 261 is shown in greater detail. On a sub 266 a number of trace element compartments 2662 are arranged protruding outwardly from a sub wall 2661 and are evenly distributed over the circumference of the sub 266. Each trace element compartment 2662 is provided with a fluid communication opening 2663, here shown as a split in the sub wall 2661, such that there is a connection between the trace element compartments 2662 and a fluid channel 2664 in the sub 266.

The fluid channels 221, 251, 2664 form a flow path 3 (see figure 2) for formation fluids 29 which are produced in the production well 2, possibly the formation fluids 29 mixed with said processing fluid 28 when this in larger or smaller concentration returns from the structure 11, 24 which has been treated.

The first trace element container 261 holds a trace element 264' which is different from a second trace element 264" which is held in the second trace element container 262, and the third trace element container 263 holds a third trace element 264'" which is different from the first and the second trace elements 264', 264". The trace elements 264', 264", 264"' are preferably distributed to all the trace element compartments 2662 in the respective trace element containers 261, 262, 263.

In an embodiment (see figure 4a) the trace element, here shown as the first trace element 264', is encapsulated in an encapsulating material 264a which is indissoluble in contact with the formation fluid 29, typically oil or water, but may be dissolved by the processing fluid 28 when this shows certain features. The encapsulating material 264a may for instance be a carbonate which is dissolved when it is exposed to an environment with a certain acidity.

In another embodiment (see figure 4b) the trace element, here shown as the first trace element 264', is chemically bound to a carrier 265. The chemical bond is not affected by the carrier's contact with the formation fluid 29, but by the processing fluid 28 when this shows certain chemical features.

When the structure 11, 24 which is to be treated, is added processing fluid 28 through suitable means (not shown) thereto, the residual amounts of the processing fluid 28 will flow back from the structure 11, 24, for instance mixed with the formation fluid 29. If the injection of the processing fluid 28 was not extensive enough, for example by being injected into an area around the first trace element container 261 only, the backflow of processing fluid 28 will only include the first trace element 264'. Correspondingly, a more extensive injection which also comprises the area around the second trace element container 262, will result in backflow of processing fluid 28 via both the first and the second trace element containers 261, 262 which thus gives off the first and second trace elements 264', 264" respectively. Finally an even more extensive injection, typically an injection more extensive than planned, will result in the processing fluid 28 also flowing back through the third trace element container 263, and the backflowing processing fluid 28 will bring with it all types of trace elements 264', 264", 264'".

On analysing of the occurrence of trace elements 264', 264", 264"', typically based on sampling of the mixture of formation fluid 29 and processing fluid 28 which is received at the surface installation 4, possibly at another suitable location, an operator gets a good indication of how the processing of the structure 11, 24 has been carried out.