Field

The present specification relates to a tracer method for hydraulically fractured hydrocarbon wells.

Background

The use of tracers to monitor aspects of the performance of hydrocarbon wells is an established technique. The tracers may be: (i) water tracers, in that they are predominantly soluble or dispersible in water; (ii) oil tracers, in that they are predominantly soluble or dispersible in the hydrocarbons; or (iii) partitioning tracers, in that they are soluble or dispersible between both water and hydrocarbon phases.

Some tracing methods will employ more than one type of tracer and use the difference in behaviour to deduce properties of the hydrocarbon formation. For example, partitioning and water tracers may be injected into a production well along with injected water and then monitored as they are subsequently produced from the well. The time difference between the production of the water tracers, which are produced with the returning injected water, and the partitioning tracers, whose production is delayed by their interaction with the hydrocarbons in the formation, can be used to deduce parameters relating to the local remaining hydrocarbon content of the formation. Alternatively, applications may use only water tracers. For example, water tracers may be introduced in an injection well and their presence monitored at adjacent production wells in order to obtain information about the flux of water from the injection well to the production well.

In addition to injection techniques, it is also known to introduce tracers into a well by including them in articles placed into the well. By detecting the rate of tracer production over time, information can be deduced about the performance of the hydrocarbon well.

Tracers should be detectable in small to very small quantities, for example at levels below 100 parts per billion (ppb), preferably at levels of 50 ppb or lower, more preferably at levels of 10 ppb or lower, and most preferably in the parts per trillion (ppt) range (that is, at levels less than 1 ppb). The levels are typically determined on a mass/mass basis. The tracers should also be environmentally acceptable with low toxicity for insertion into the ground and usage, for example, in reservoir applications, but they must also be species that are not naturally present in the ground in such quantities as to contaminate the results of a tracer study.

Typical detection methods include gas chromatography - mass spectrometry (GC-MS), gas chromatography - mass spectrometry - mass spectrometry (GC-MS-MS), liquid chromatography - mass spectrometry (LC-MS), liquid chromatography - mass spectrometry - mass spectrometry (LC- MS-MS) and high-performance liquid chromatography (HPLC), which can typically detect very low concentrations of the tracers in the produced fluids. It is desirable that tracers should be detectable in low quantities and also that they can be reliably distinguished from other tracers and species which are naturally present in reservoir fluids.

The use of hydraulic fracturing for stimulating hydrocarbon wells is also an established technique. A wellbore is drilled, perforated, and then the surrounding formation is hydraulically fractured by high- pressure injection of fracking fluid through the perforations in the wellbore to create cracks in the surrounding formation through which hydrocarbons can flow. The fracking fluid generally comprises water and a proppant (e.g., sand) which functions to hold induced fractures open. The fracking fluid may include one or more further additives such as rheology modifiers to optimize parameters such as viscosity.

Typically, the hydraulic fracturing process is performed in stages. A first stage of the hydrocarbon well towards the toe of the wellbore is perforated, the first stage is hydraulically fractured, and then the first stage of the hydrocarbon well is sealed with a plug. Moving up the well towards the heel, the steps of perforating, fracturing, and plugging can be repeated multiple times. During this process, it is usually desirable in any given stage to focus all the fracturing within that stage without the fracturing extending across into other stages. This aids in optimizing the fracturing within each stage. If the fracturing is constrained within a stage, it is designated as in-stage hydraulic stimulation. If the fracturing extends across into other stage, it is designated as cross-stage hydraulic stimulation. Crossstage hydraulic stimulation results in crossflow between different stages.

One method to verify cross-stage or in-stage stimulation in a multi-stage hydraulically fractured oil and/or gas well is to use radioactive tracers. Different radioactive tracers can be added to the different stages. After completing the multi-stage fracturing process, a gamma logging tool can be inserted within the wellbore to detect the location of the radioactive tracers and determine whether they have remained in the stages in which they were placed or whether they have moved into other stages indicating cross-stage hydraulic stimulation. For example, radioisotopes having different energies can be used to label different stages prior to fracturing and a gamma logging tool can be inserted into the wellbore after fracturing to detect the presence and location of the different energy radioisotopes and determine whether they have moved cross-stage.

It is an aim of the present specification to provide an improved method of verifying cross-stage or instage stimulation in a multi-stage hydraulically fractured hydrocarbon well.

Summary

According to the present specification there is provided a multi-stage hydraulic fracture and tracer method for a hydrocarbon well to detect cross-stage or in-stage stimulation. The method comprises: hydraulically fracturing a first stage of a hydrocarbon well using a first aqueous fracture fluid; adding a first water tracer to the first stage; sealing the first stage with a first plug; hydraulically fracturing a second stage of the hydrocarbon well using a second aqueous fracture fluid; adding a second water tracer, which is different to the first water tracer, to the second stage; sealing the second stage with a second plug; drilling out the second plug; testing aqueous fluid flowing out of the second stage after drilling out the second plug for the presence of the first and second water tracers, wherein if both the first and second water tracers are detected it is determined that there is cross-stimulation between the first and second stages, whereas if only the second water tracer is detected and not the first water tracer then it is determined that there is no cross-stimulation between the first and second stages.

This method differs from that described in the background section in that it uses water tracers instead of radioactive tracers. The method also differs from that described in the background section in that it uses backflow of aqueous fluid after drilling out a fracture stage plug, such that the tracers can be detected / measured at the surface without requiring a tool to be inserted down the well to detect the tracers in-situ within the stages of the well. As such, the method has the advantages of avoiding radioactive tracer use and the necessity to push a tool into the well to take tracer measurements. It should also be noted that while water tracers are known per se for use in monitoring water flow in a hydrocarbon well system, this methodology is specifically targeted at detecting cross-stage and instage stimulation in a multistage hydraulically fracture well and more specifically uses a sequence of aqueous fluid backflow samples as the stage plugs are sequentially drilled out in order to detect crossstage and in-stage stimulation.

In practice, more than two stages can be hydraulically fractured and sealed with a plug in a sequence moving from a toe end of the hydrocarbon well towards a heel end of the hydrocarbon well, each stage being provided with an associated water tracer. Each stage of the hydrocarbon well can be perforated prior to hydraulically fracturing the stage. This aids in forming pathways for the hydraulic fracturing fluid to enter the rock formation surrounding the wellbore in a target stage. After forming the series of stages, the plug associated with each stage can be drilled out and aqueous fluid flowed back and tested for tracer content in a sequence moving from the heel end stage of the hydrocarbon well towards the toe end stage. For any given stage, if only the water tracer associated with that stage is detected and no water tracer from other toe-side stages, then it is determined that there is no crossstimulation between that stage and other toe-side stages. Alternatively, if one or more water tracers from one or more toe-side stages are detected, it is determined that there is cross-stimulation between the stages associated with the detected water tracers. Tracer data for all of the stages can be collated and a report generated showing the extent of cross flow for the entire hydrocarbon well.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figures I to 5 show a schematic representation of the steps involves in a multi-stage hydraulic fracture and tracer method according to the present specification.

Detailed Description

As described in the summary section the present specification provides a multi-stage hydraulic fracture and tracer method for a hydrocarbon well to detect cross-stage or in-stage stimulation. The method implements a sequence of hydraulic fracturing stages. At each stage an associated water tracer is added, and then each stage is sealed with a plug within the wellbore such that the stages are sealed off from each other within the wellbore. Multiple stages can be perforated, hydraulically fractured, and sealed with a plug in a sequence moving from a toe end of the hydrocarbon well towards a heel end of the hydrocarbon well, each stage being provided with an associated water tracer. The plug associated with each stage is drilled out and aqueous fluid flowed back (through the same wellbore through which the fracture fluid was pumped) and tested for tracer content in a sequence moving from the heel end stage of the hydrocarbon well towards the toe end stage. For any given stage, if only the water tracer associated with that stage is detected and no water tracer from other toe-side stages, then it is determined that there is no cross-stimulation between that stage and other toe-side stages. Alternatively, if one or more water tracers from one or more toe-side stages are detected, it is determined that there is crossstimulation between the stages associated with the detected water tracers. Tracer data for all of the stages can be collated and a report generated showing the extent of cross flow for the entire hydrocarbon well.

It should be noted that aqueous fluid can backflow out of a stage as a result of pressure within the stage which is released when a plug is drilled out. In this case, no additional pumping may be necessary. Furthermore, the backflow of aqueous fluid has the dual purpose of carrying tracer to the surface for testing and also removing solid debris from the well system, e.g., debris from the drilled- out plugs.

Each water tracer can be added to its respective stage by injecting into the hydrocarbon well in the aqueous fracture fluid. The water tracer may be injected into the hydrocarbon well in the aqueous fracture fluid for at least 50%, 60%, 70%, 80%, 90%, or throughout a duration of the hydraulic fracturing step for each stage of the hydrocarbon well. Furthermore, a rate of addition of the water tracer into the aqueous fracture fluid can be varied depending upon a fracture fluid pump rate. For example, the rate of addition of the water tracer can be varied in order to dose the aqueous fracture fluid with an even amount of tracer concentration while still using a variable fracture fluid pump rate to optimize the hydraulic fracturing procedure in any given stage.

The tracers in the different stages should be distinguishable to detect cross-stage stimulation. As this is likely to be between adjacent stages, each stage of the hydrocarbon well may be provided with a water tracer which is distinct from at least the water tracers provided in adjacent stages. For example, two, three, four, five, or more different water tracers may be used in a sequence of stages and then the sequence of tracers repeated for the next set of stages and so on. Alternatively still, each stage of the hydrocarbon well may be provided with a water tracer which is distinct from the water tracers in all the other stages of the hydrocarbon well. That is, a unique tracer is provided for each and every stage. Which of these methods will be used in practice will depend on the availability of unique tracers and the number of stages in the hydrocarbon well system.

Sufficient tracer must be added to each stage so as to be detectable in samples of flowback fluid. A total mass of each tracer is determined by summing a volume of all of the aqueous fluid to be used across all the stages of the hydrocarbon well and multiplying by a target chemical tracer concentration within the water to give a total mass of each tracer needed per stage.

Figures 1 to 5 show a schematic representation of a multi-stage hydraulic fracture and tracer method for a hydrocarbon well. The method comprises:

Figure 1(a) - A hydrocarbon well 10 is drilled.

Figure 1(b) - A first stage of the hydrocarbon well is perforated 12.

Figure 1(c) - The first stage of the hydrocarbon well is hydraulically fractured 14, and a first water tracer is added.

Figure 1(d) - The first stage of the hydrocarbon well is sealed with a plug 16.

Figure 2(a) - A second stage of the hydrocarbon well is perforated 18.

Figure 2(b) - The second stage of the hydrocarbon well is hydraulically fractured 20, and a second water tracer, which is different from the first water tracer, is added.

Figure 2(c) - The second stage of the hydrocarbon well is sealed with a plug 22.

Figure 3(a) - A third stage of the hydrocarbon well is perforated 24.

Figure 3(b) - The third stage of the hydrocarbon well is hydraulically fractured 26, and a third tracer, which is different from the first and second water tracers, is added.

Figure 3(c) - The third stage of the hydrocarbon well is sealed with a plug 28.

Figure 4(a) - The third stage plug is drilled out 30.

Figure 4(b) - The third stage plug debris is flowed back to the surface 32.

Figure 5 - A water sample is taken during flowback and tested for tracer content: Figure 5(a) illustrates the situation when there is no cross-stage leakage (tracer 34 from the third stage only detected); Figure 5(b) illustrates the situation when there is leakage from one lower stage (tracer 36 from both the third stage and the second stage detected); and Figure 5(c) illustrates the situation when there is leakage from two lower stages (tracer 38 from all three stages detected). The steps illustrated in Figures 4 and 5 can be repeated to drill out the second plug, flow back, sample, and test for tracers. The steps illustrated in Figures 4 and 5 can then be repeated again to drill out the first plug, flow back, sample and test for tracer. It should also be noted that while the illustrated example includes three hydraulic fracturing stages, this is for illustrative purposes only and in practice the method may involve more than three hydraulic fracturing stages.

The method of the present specification is intended to replace current radioactive tracer methods that are used with a gamma logging tool that is pushed into the well to detect the presence of different energy radioisotopes that have been added into each stage. The present specification provides an alternative method using water chemical tracers that do not require any use of radiation or well intervention, as surface samples can be used to determine the required measurements. As such, this invention eliminates radioactive tracer use and the necessity to push a tool into the well to take the necessary measurements for detecting cross-stage stimulation in multi-stage hydraulically fractured hydrocarbon well systems.

A number of unique chemical-based water tracers known to survive the harsh environments of an oil and gas reservoir can be selected for use in this methodology. Such chemical-based water tracers are known in the art. The number of unique chemicals can be selected to match the number of stages to be monitored within the well. Furthermore, the total mass of each tracer can be determined by summing all of the water to be used across the whole wellbore stimulation program and multiplying by the targeted chemical tracer concentration within the water to give the total mass needed per stage.

During the stimulation of an individual stage within the well, one of the unique chemical tracers can be added to the frac water throughout the whole stage duration. The rate of addition can be varied depending upon the overall frac fluid pump rate in order to dose the water with an even amount of tracer concentration. Tracer addition can be stopped when flush water is added to displace the frac fluids into the fracture network of the stage.

A plug is added into the well and set just downstream of the fracked stage to isolate the stage. The next stage is then targeted, and this process is repeated for each stage using one unique chemical tracer in each stage. Once all the well stages are completed, a tool is lowered into the well and the last plug towards the heel of the well is drilled out. Solids are flowed to the surface using pressure from within the stage. A sample of aqueous fluid is taken at the surface based upon volume between the stage and surface. The next plug is then drilled out and a sample take again when the well has flowed to surface to remove the solids. This process is repeated at each flowback.

All water samples can then be sent for tracer analysis using known water chemical tracer techniques, e.g., either using liquid chromatography or gas chromatography. If no cross-flow is present between stages, then the only tracer to be detected within a sample should be the tracer from the opened stage. Tracer from stages lower down the well will have been prevented from flowing up the well due to the plugs in the wellbore. If tracer from stages lower down the well are detected, then these must have circumvented the plugs through fractures crossing into the opened stage. As such, it is possible to detect the presence of cross-flow, identify which specific stages are contributing to cross-flow, and quantify the amount of cross-flow from the relative amounts of tracer in any given sample. A report can be generated giving detailed information about the multi-stage hydraulically fractured hydrocarbon well system. Based on learnings from analysis of such reports, the present methodology can aid in optimizing hydraulic fracturing procedures and also aid in optimizing the operation of such multi-stage hydraulically fractured hydrocarbon well systems.

While this invention has been described with reference to certain examples and embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.