# Patent

## Title
Oil and gas fracture liquid tracing with oligonucleotides

## Abstract

Methods of tracing fracking liquid in oil or gas bearing formations using plural unique oligonucleotide markers. Method includes pumping the plural volumes of fracking liquid, each marked with a unique oligonucleotide, into the formation, thereby defining plural fracture zones in the formation, and, pumping fluids out of the formation while taking plural fluid samples. Then, analyzing the concentration of the unique oligonucleotides in each of the plural fluid samples, and, calculating the ratio of each of the plural volumes of fracking liquid recovered for each of the plural fluid samples according to the concentration of the unique oligonucleotides present in each of the plural samples. And, then, establishing the quantity of the plural volumes of fracking liquids removed from the plural fracture zones.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to hydraulic fracturing of geologic formations in hydrocarbon wells. More particularly, the present invention relates to tracing the movement and recovery of hydraulic fracturing liquids pumped into oil and gas wells using plural unique oligonucleotides tracing compounds, which correspond with plural fracture stages and zones within a geologic formation.
2. Description of the Related Art
Oil and gas are removed from geologic formations by drilling a well bore from the surface. A well casing is inserted into the well bore, which is then perforated so that oil and gas can flow from the adjacent geologic formation into the well casing. The oil and gas may flow upwardly under natural pressure in the formation, but more commonly they are removed using an artificial lift system, such as the well known sucker-rod pump and surface-mounted pump-jack arrangement. In order to maintain production over an extended period of time, there must be sufficient formation porosity and pressure so that the oil and gas naturally flow from the hydrocarbon bearing geologic formation, through the casing perforations, and into the well casing.
As exploration has expanded into regions where there is insufficient porosity in the oil and gas bearing formations to sustain production, engineers have developed hydraulic fracturing techniques that produce artificial porosity, through which the formation oil and gas can flow into the well casing. Hydraulic fracturing is the fracturing of rock structures adjacent to the well casing perforations using a pressurized liquid pumped down the well casing from the surface. Hydraulic fracturing, or hydrofracturing, also commonly referred to as “fracking”, is a technique in which fresh water is mixed with sand and certain chemicals, and then the mixture is injected at high pressure into a well casing to create small fractures in the formation. This liquid mixture is referred to as fracking liquid. These small fractures enable formation fluids, such as gas, crude oil, and brine water to flow into the well casing. Once the fracking process is completed, hydraulic pressure is removed from the well. The formation rock naturally settles back to its original position, but the small grains of sand, referred to as proppants, hold these fractures open so as to yield the desired artificial porosity. Fracking techniques are commonly used in wells for shale gas, tight gas, tight oil, coal seam gas, and hard rock wells. The fracking process is only utilized at the time the well is drilled and placed into production, but it greatly enhances fluid removal and well productivity over the life of the well.
The sequence of events implemented to place a typical oil or gas well into production generally consists of, drilling the well bore, installing the well casing, perforating the casing, hydrofracturing the hydrocarbon bearing formation, installing an artificial lift system, recovering the hydraulic fracturing liquid, and then producing oil and gas from the well. It is significant to note that the presence of the fracturing liquid in the formation interferes with oil and gas production, and that removal of the fracturing liquid is a technical challenge for operators, and one that must be accomplished promptly, and to a reasonable degree of completion before oil or gas production from the well can commence. This disclosure is primarily concerned with the hydraulic fracturing process and the removal, or other disposition, of the hydraulic fracturing liquid (also referred to herein as “fracking liquid”). The types of wells contemplated herein include common vertical wells and wells in which horizontal drilling is used to traverse a geologic formation so as to increase productivity. In fact, hydraulic fracturing is now commonly employed in wells having horizontal bores through gas producing formations. An example of this is the Barnett Shale formation in north Texas, a region that covers approximately seventeen counties and contains natural gas reserves proven to include 2.5 trillion cubic feet, and perhaps as much a 30 trillion cubic feet of recoverable reserves.
The effectiveness of the hydraulic fracturing process, as well as the flow and disposition of the fracking liquid, is of critical importance to the well operator. Since the fracking process occurs far below the surface and is therefore difficult to monitor, any data that confirms the extent of the fractures or indicates the flow and movement of the fracking liquid is helpful in the operation of that well, and is also informative with regard to similar wells that may be drilled in the same oil field. A technique used to determine the flow and movement of the hydraulic fracturing fluid is called tracing. The tracing process involves placing a marking additive (hereinafter a “tracer”) in the hydraulic fracturing liquid before it is pumped into the well, and then monitoring the fluids subsequently recovered from the well to determine the concentration of the tracer in the well fluids recovered. The concentration of the recovered tracer is compared with the concentration originally pumped into the well, and this is used to estimate the amount of the original fracking liquid that has been recovered. Generally, once a substantial portion of the fracturing liquid has been recovered, the well is placed into production.
Fracturing liquids contain a number of additives and chemicals that are used to facilitate the fracturing process. Among these are specialized sand that is used as a proppant, a thickening or gelling agent that increases viscosity thereby enabling the water to carry the proppant into the fractures, acid used to control pH of the well, a breaking agent that later reduces the viscosity so that the fracturing liquid can be more readily recovered, and numerous other chemical treatment, the details of which are beyond the scope of this disclosure. Some consider a portion of these additives and chemicals to be environmentally questionable, and so the movement of the fracturing liquid is monitored with respect to migration of the fracturing liquids into adjacent formations, possibly including fresh water resources. Thus, it is useful to monitor migration of subterranean fluid movements by detecting the tracer in adjacent oil wells and other access points, such as nearby injection wells and water wells. The fracturing liquids also impede production of oil and gas, and operators take a number of actions to facilitate their removal. This may include chemical treatments to alter the fracture liquids to enhance their removal, and also the addition of flushing liquids to dilute or alter the nature of the fracturing liquids.
Various types of tracers have been employed in hydraulic fracturing liquids. Selection and implementation of a tracer is non-trivial because of the cost constraints and the harsh environment that oil and gas wells present. The tracing material needs to be economically feasible in large scale drilling operations, it must be readily detectable at very low concentrations using commercially available test equipment, and it must survive the extremes of pressure and temperature, and the chemical and biological environment present in an oil and gas well. It is known to use certain chemical tracer compounds, fluorescent dye tracers, radioactive isotope tracers, fluorinated benzoic acid, ionized salts, and certain other chemicals. However, the number of discrete and unique tracers that can be used in a single hydraulic fracturing job is quite limited, and is generally just a handful that would be practicable in a single fracking job. This is a significant limitation because an operator cannot monitor a complex fracking job in detail. Many jobs use only a single tracer, which only enables the tracing of the fracking liquids in total. Some jobs can use individual tracers for a few stages of a fracking job.
Thus it can be appreciated that there is a need in the art for a system and method of tracing hydraulic fracturing liquid that provides greater flexibility, greater detail, and accuracy in a reliable and cost effective manner.
SUMMARY OF THE INVENTION
The need in the art is addressed by the methods of the present invention. The present disclosure teaches a method of tracing the movement of plural volumes of fracking liquid that are pumped into an oil or gas bearing formation through a first stage of perforations in a well casing, which is coupled to a wellhead, by using plural unique oligonucleotide markers. The method includes sequentially marking the plural volumes of fracking liquid with plural unique oligonucleotides, each at a predetermined concentration, and, sequentially pumping each of the plural volumes of fracking liquid through the first stage of perforations and into the formation, which sequentially advances each of the plural volumes of fracking liquid outwardly into plural fracture zones in the formation. The method further includes pumping formation fluids to the wellhead, which may contain portions of the plural volumes of fracking liquid, while monitoring the periodic volume of the fluids pumped out. Also, periodically gathering formation fluid samples as the formation fluids are pumped to the wellhead, and, correlating each of the formation fluid samples to plural periodic volumes. Then, analyzing the concentration of the plural unique oligonucleotides in each of the samples, and, calculating the quantity of each of the plural volumes of fracking liquid recovered during each of the plural periodic volumes according to the concentration of the plural unique oligonucleotides present in each of the samples. This process establishes the movement of the plural volumes of fracking liquids out of the corresponding fracture zones.
In a refinement to the foregoing embodiment, the sequentially marking step is accomplished by metering liquid slurries that include the unique oligonucleotides into a fracking liquid blender on an ongoing basis as a fracking process is occurring. In another embodiment, the sequentially pumping each of the volumes is synchronized with plural pre-defined sub-stages of a fracking job, so that each of the sub-stages is marked with a unique oligonucleotide. In a refinement to this embodiment, the plural sub-stages of the fracking job are distinguished from one another by the quantity and size of proppant incorporated into each of the volumes of fracking liquid.
In a refinement to the foregoing embodiment, the pumping formation fluids to the wellhead step is accomplished using a down-hole pump coupled to the wellhead through a tubing string, and, the gathering plural formation fluids step further includes the step of drawing fluids from the tubing string at the wellhead. In a refinement to this embodiment, where the drawing fluids from the tubing string at the wellhead step also includes automatically, and periodically, routing the plural fluid samples to plural fluid sample vessels.
In a refinement to the foregoing embodiment, the monitoring the periodic volume of the formation fluid step further includes outputting a periodic volumetric flow value, and, the gathering plural formation fluid samples step further includes recording a corresponding periodic volumetric flow value for each.
In a refinement to the foregoing embodiment, the plural unique oligonucleotides are selected from predetermined sequences of deoxyribonucleic acid, ribonucleic acid, and locked ribonucleic acid. In another refinement, the plural unique oligonucleotides are synthesized unique DNA sequences.
In a refinement to the foregoing embodiment, the analyzing the concentration of the unique oligonucleotides step further includes concentrating at least one of the plural formation fluid samples, and, measuring the atomic mass and quantity of each of the unique oligonucleotides therein by using a matrix-assisted laser desorption/ionization with time of flight mass spectrometer.
The present disclosure also teaches a method of tracing fracking liquid in oil or gas bearing formations using plural unique oligonucleotide markers. This method includes pumping the plural volumes of fracking liquid, each marked with a unique oligonucleotide, into the formation, thereby defining plural fracture zones in the formation, and, pumping fluids out of the formation while taking plural fluid samples. Then, analyzing the concentration of the unique oligonucleotides in each of the plural fluid samples, and, calculating the ratio of each of the plural volumes of fracking liquid recovered for each of the plural fluid samples according to the concentration of the unique oligonucleotides present in each of the plural samples. And, then, establishing the quantity of the plural volumes of fracking liquids removed from the plural fracture zones.
In a refinement to the foregoing embodiment, the unique oligonucleotides are biotinylated, and the analyzing the concentration step further includes immobilizing avidin or streptavidin onto magnetic particles, and, mixing the magnetic particles with at least one of the plural fluid samples, thereby enabling the formation of non-covalent bonds between the biotinylated oligonucleotides and the immobilized avidin or streptavidin. Then, removing the magnetic particles from the sample by magnetic attraction, thereby concentrating the sample. And then, measuring the quantity of each of the unique oligonucleotides. In a refinement to this embodiment, the unique oligonucleotides are biotinylated by binding biotin to the 5′-end of the oligonucleotides.
In another refinement to the foregoing embodiment, the measuring step further includes determining the atomic mass and quantity of each of the unique oligonucleotides in the sample using a matrix-assisted laser desorption/ionization with time of flight mass spectrometer. In another refinement, the method includes agitating the at least one of the plural fluid samples with the magnetic particles to facilitate the formation of non-covalent bonds. In another refinement, the recovering step is accomplished by inserting a magnet into the sample, and then rinsing the magnetic particles prior to the measuring step.
The present disclosure teaches a method of tracing the movement of plural volumes of fracking liquid pumped into an oil or gas bearing formation through plural stages of perforations in a well casing, which is coupled to a wellhead, by using plural unique oligonucleotide markers. This method includes the steps of forming a first stage of perforations through the casing and into the formation, and sequentially marking a first group of volumes of fracking liquid with a first group of unique oligonucleotides, each at a predetermined concentration. Then, sequentially pumping each volume in the first group of volumes of fracking liquid through the first stage of perforations and into the formation, thereby sequentially advancing each of the first group of volumes of fracking liquid outwardly into a first group of fracture zones in the formation adjacent to the first stage of perforations. Next, forming a pressure seal within the casing between the first stage of perforations and the wellhead. Then, forming a subsequent stage of perforations through the casing and into the formation at a location between the pressure seal and the wellhead, and, sequentially marking a subsequent group of volumes of fracking liquid with a subsequent group of unique oligonucleotides, each at a predetermined concentration. Then, sequentially pumping each volume in the subsequent group of volumes of fracking liquid through the subsequent stage of perforations and into the formation, thereby sequentially advancing each of the subsequent group of volumes of fracking liquid outwardly into a subsequent group of fracture zones in the formation adjacent to the subsequent stage of perforations. Then, the pressure seal is removed to prepare for pumping formation fluids to the wellhead, which may contain portions of the first group of volumes of fracking liquid and the subsequent group of volumes of fracking liquid, while monitoring the periodic volume thereof. As this occurs, periodically gathering plural formation fluid samples as the formation fluids are pumped to the wellhead, and, correlating each of the plural formation fluid samples to plural periodic volumes of formation fluid. Then, analyzing the concentration of the first group of oligonucleotides and the subsequent group of unique oligonucleotides in each of the plural samples, and, calculating the quantity of each of the first group of volumes of fracking liquid and subsequent group of volumes of fracking liquid recovered during each of the plural periodic volumes of formation fluid according to the concentration of the plural unique oligonucleotides present in each of the plural samples. Thusly, establishing the movement of the first group of volumes of fracking fluid and subsequent group of volumes of fracking liquids out of the first group of fracture zones and subsequent group of fracture zones.
In a specific embodiment of the foregoing method, the sequentially pumping each volume in the first group of volumes, and the subsequent group of volumes, are synchronized with plural pre-defined sub-stages of a fracking job, such that each of the sub-stages is marked with a unique oligonucleotide. In a refinement to this embodiment, the plural sub-stages of the fracking job are distinguished from one another by the quantity and size or proppant incorporated into the plural volumes of fracking liquid.
In specific embodiment of the foregoing method, the monitoring the periodic volume of the formation fluid step includes outputting a periodic volumetric flow value, and the gathering plural formation fluid samples step includes recording a corresponding periodic volumetric flow value for each.
In specific embodiment of the foregoing method, the first group of unique oligonucleotides and the subsequent group of oligonucleotides are selected from predetermined sequences of deoxyribonucleic acid, ribonucleic acid, and lock ribonucleic acid. In another embodiment, the analyzing the concentration of the plural unique oligonucleotides step includes concentrating at least one of the plural formation fluid samples, and, measuring the atomic mass and quantity of each of the plural unique oligonucleotides therein using a matrix-assisted laser desorption/ionization with time of flight mass spectrometer.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system diagram of the hydraulic fracturing process according to an illustrative embodiment of the present invention.
FIG. 2 is a system diagram of the fracking liquid removal process according to an illustrative embodiment of the present invention.
FIG. 3 is a system diagram of the oligonucleotide marking and pumping process according to an illustrative embodiment of the present invention.
FIG. 4 is a system diagram of the formation fluid sampling process according to an illustrative embodiment of the present invention.



DESCRIPTION OF THE INVENTION
Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope hereof and additional fields in which the present invention would be of significant utility.
In considering the detailed embodiments of the present invention, it will be observed that the present invention resides primarily in combinations of steps to accomplish various methods or components to form various apparatus and systems. Accordingly, the apparatus and system components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the disclosures contained herein.
In this disclosure, relational terms such as first and second, top and bottom, upper and lower, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.
As mentioned hereinbefore, it is important to remove as much of the fracking liquid as possible prior to placing a well into production. The fracking liquid interferes with production for a number of reasons, one of which is the fact that viscosity interferes with flow of reservoir fluids into the well casing. Certain chemical treatments are included in the fracking liquid to reduce its viscosity, called breaking agents. The breaking agents operate over time such that the fracking liquid is viscous as it is pumped into the well, but less viscous when it is pumped out. The fracking liquid is pumped into the formation in several discrete stages, which correspond to several sets of perforations through the well casing, which are located at various depths within the formation. At each stage of the perforations, there are typically several sub-stages injected in the fracture process. The sub-stages may each have a different fracking liquid blend, most often including different proppant material configurations. For example, different sieve size sand or different amounts of sand added to each barrel of fracking liquid. As these sub-stages of fracking liquid are pumped in, they each define different fracture zones within any given fracture stage. Each subsequent sub-stage of fracking liquid pumped into a given stage pushes the previous stage outwardly from the casing perforations. Thus, each zone in the fracture may have a different fracking liquid profile, generally corresponding to the sub-stages. At the time this fracking liquid is recovered from the well, the individual zones drain back into the well casing and are pumped out. The operator of the well desires to understand the performance of the fracking job, including details on how individual zones have been fractured, and how the fracking liquid from each has been recovered, including the volume of liquid and the time taken for the recovery process to occur.
Wells that includes a horizontal bore into a formation commonly include ten or more perforation stages. Each stage may include from five to as many as thirty sub-stages, which corresponds to perhaps two hundred fracture zones in a given well. Ideally, an operator would like to know about the removal of fracking liquid from every zone. Unfortunately, current tracer variants are far more limited in number. It would be challenging to assemble twenty discrete tracing compounds to use in a given well, which places a clear limit on the amount of information an operator can garner during the fracking liquid removal process. The reason this is challenging is because of the extreme and hostile environment present in an oil and gas well. In addition to presenting a complex chemical environment, there is generally an acidic pH, high pressures, turbulent and shear forces, and high temperatures in a well during the fracking process. In order to function reliably, each tracer compound must survive the down-hole environment without alteration of any kind, and each tracer should not react with any chemical compounds present in the well. There can also be biological and enzymatic issues in the well that affect the tracers. In addition, the tracer compounds must be economically feasible, and must be detectible at very low concentrations (in the order of parts per billion or trillion) using commercially available test equipment. Furthermore, during the detection and measurement processes, it may be necessary to remove the tracer compounds from the well formation fluid, and concentrate them, prior to performing a test of its recovered concentration.
The present disclosure teaches the use of plural oligonucleotide compounds as hydraulic fracture liquid tracers. The present disclosure also presents specific handling and automation systems, as well as specific test methodologies. These oligonucleotides include deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and locked nucleic acid (LNA), each configured with a unique sequence that can be readily discriminated using certain mass spectrometer test equipment and methodologies.
Reference is now directed to FIG. 1, which is a system diagram of the hydraulic fracturing process according to an illustrative embodiment of the present invention. At the surface level 2, a wellhead 1 is coupled to a well casing 4, which continues downwardly to a horizontal casing 6 that was drilled and installed into an oil and gas bearing geologic formation 3. In FIG. 1, the well has been drilled and cased, and five stages 5 of perforations and fractures have been completed. The various components of the hydraulic fracturing equipment are shown on the surface 2. The hydraulic fracturing process occurs in a coordinated fashion, stage by stage 5, and zone by zone 7, until all of the zones 7 have been fractured. Each individual zone, referenced by a combination of its stage number 5 and its zone number 7, corresponds to a sub-stage of the fracturing process, and may also have utilized a distinct fracturing liquid mixture, and may have been marked with a unique tracing oligonucleotide.
At the surface 2, plural hydraulic pumps 14 force fracking liquid down the casing 4 at very high pressure. The hydraulic pumps 14 are fed mixed fracking liquid from a blender 12. The blender 12 operates on a continuous basis during each stage 5 of the fracking job, continually being fed with the various components of the particular fracking liquid mixture presently required by a fracking job specification. The fracking job specification is generated by petroleum engineers prior to commencement of the job, and its details are beyond the scope of this disclosure. With respect to this disclosure, the fracking liquid mixture components are divided into water 8, chemicals 16, sand, or proppant, 18, and tracer compounds 20. The water 8 is the largest portion of the fracking liquid, and it is pumped into the blender 12 by a water pump 10, which supplies the water 8 at a predetermined rate according to the fracking job specification. Similarly, the sand 18 is fed on a conveyor at a predetermined rate, and enters an opening in the top of the blender 12. The chemicals 16 can be fed in various manners depending on their respective material handling properties. The tracer compounds 20 are fed in precisely using a positive displacement metering pump 22. This is necessary because the concentration of the tracers 20 are so small, typically on the order of parts per million, or less.
The fracking job of FIG. 1 proceeds according to a sequential schedule. In this illustrative embodiment, that fracking schedule includes five stages 5 (labeled Stage 1 through Stage 5), each having five sub-stages that result in five fracture zones 7 (labeled Zone A through Zone E) each, for a total of twenty-five individual zones. Since each zone is to receive a unique fracking liquid blend according to the fracking schedule, and since there is just the single well casing 4, 6 to serve as the fracking liquid delivery conduit, it is necessary to sequence the preparation and delivery of the fracking liquid. Naturally, this begins with Stage 1, which is furthest from the wellhead 1. A set of perforations 26 are formed through the casing 6, accessing the formation 3 at the location of Stage 1. The surface 2 equipment is activated, and the fracking liquid, which also includes a unique oligonucleotide marker for Stage 1-Zone E, is pumped down the casing 4, 6. This liquid passed through the perforations 26 and into the formation. On a continuous pumping basis, the subsequent four zones (Zone D, Zone C, Zone B, and Zone A of Stage 1) are pumped through the perforations 26. Note that each zone receives a distinct fracking liquid mixture according the fracking schedule, and that each also receives a unique oligonucleotide marker. Also, note that the zones are pumped in reverse order, where each subsequent zone pushes the prior zone's fracking liquid outwardly into the formation, fracturing it as they progress. In other words, Zone E is pumped first, followed by Zone D, Zone C, Zone B, and Zone A. When Stage 1 is complete, a pressure seal 36 is inserted into the casing to isolate Stage 1 from the next sequence of events.
The pressure seal 36 may be a type of composite plug, as are known to those skilled in the art. Once plug 36 is in place, then the set of perforations 28 for Stage 2 are formed, and the next five sub-stages of fracking liquid with unique oligonucleotides are pumped to form the five fracture zones of Stage 2. Then, plug 38 is inserted to isolate Stage 2 from the subsequent Stage 3. This process repeats for Stage 3, with perforations 30 and plug 40, Stage 4 with perforations 32 and plug 42, and finally Stage 5 with perforation 34. Each of the five stages 5 has five zones 7, and all twenty-five of the zones have a specific fracture liquid and a unique oligonucleotide disposed within fractures just formed in the formation 3.
The nature of the stages and fractures zones depends in large measure on the nature of the formation and the petroleum engineers' plan for the extent of the fracturing job. To give this a sense of scale, some exemplary well perforation and fracturing specifics are worth considering. A well may be from 5000 to 20,000 feet deep with horizontal sections extending out to 7000 feet and more. Off-shore wells are even deeper and longer. The well is drilled and then cased with steel casing, which is commonly 5.5′′ in diameter. The bottom of the casing is closed in some fashion so that it holds pressure. Once the well is cased, the drilling rig is removed, and a “wireline crew” perforates the casing at stage locations specified by the petroleum engineers. It is common to use seven to eleven stages in a single well, but other quantities are known as well. The perforation is done with plural inverted bullet shaped copper projectiles fired with shaped charges. Each projectile makes a 0.2 to 0.25 inch diameter hole in the casing. A single stage of perforations is typically about twenty feet long, but shorter lengths are used as well, and some perforations can be over one hundred feet long.
The plugs used between stages are generally a composite material that is compressed against the interior of the well casing to withstand pressures on the order of thousands of PSI. The plugs can later be drilled out, however some have a dissolvable core, which opens after several hours to several days later. In the case of dissolvable plugs, the fracture schedule must proceed at a pace commensurate with the rate at which the plugs dissolve.
As noted above, the fracturing process creates a false porosity in the formation. This is particularly useful in horizontal wells cut through shale deposits. A fracture zone can extend three hundred feet from the well casing. The sand, or proppant, holds the fractures open after the hydraulic fracking liquid pressure is removed. Various sizes of sand are utilized in the various zones. An additive is used to gel or thicken the fracking liquid because the increased viscosity enables the liquid to carry the proppant out into the fracture zones. The number of zones in each stage is typically in the four to ten range, but the use of as many as thirty zones in a single stage is known. Thus in a large fracture job, there could be fifteen stages with thirty zones each, totaling four hundred fifty zones, each of which could be marked with a unique oligonucleotide.
With respect to the pumping and pressures applied during the fracking process, fracking liquid flow rates can run 70-75 barrels per minute with pressures well over 7000 PSI. The pumping time for a single stage can range from one to four hours. A typical fracking job can utilize 2 million gallons of fracking liquid.
Reference is now directed to FIG. 2, which is a system diagram of the fracking liquid removal process according to an illustrative embodiment of the present invention. This figure generally corresponds to FIG. 1, after the hydraulic pressure has been removed from the well and the fracking equipment has been removed. This is the recovery phase of the project, where the fracking liquid is removed from the formation. The first step is to open the plugs of FIG. 1, which can be accomplished by drilling or through the use of dissolvable plugs. This action may allow some of the fracking fluids to flow out of the well due to the pressure built up in the fracturing process, but generally, a down-hole pump will be utilized to recover the fracking liquid. As the fracking liquid is removed, it is typically mixed with formation fluids. Note that while the fracking liquids pumped into the well area generally free of gases, the formation fluids comprise both liquids and gases. FIG. 2 illustrates the fracking liquid recovery process.
In FIG. 2, a down hole pump 54 has been inserted into the casing 4, which operates to pump fluids out of the formation, up the casing 6, 4, and to the wellhead 1. In this embodiment, a sucker rod 52 driven pump 54 is employed, however, a submersible pump can also be used, as is known to those skilled in the art. The sucker rod 52 couples the pump 54 to a reciprocating pump jack drive unit 50 at the surface 2, as are well known in the art. As fluids are removed from the casing, additional formation fluids and fracking liquids flow from the formation 3 and the fracture zones 7 into the casing 6. The wellhead 1 has a piping arrangement that routes the liquids from a tubing string 56 and gases from a casing annulus 58 to a fluid outlet 60. Samples of the fluid output 60 are periodically gathered for testing. This testing includes testing for the concentration of the several oligonucleotides that were mixed into the fracking liquid as the fracturing processed occurred.
It can be appreciated that the fracture liquids in the several zones 7 generally flow into the casing on a last-in, first-out basis, and the testing of oligonucleotides may demonstrate this general trend. However, that assumption would only hold true for a uniform formation with consistent porosity and uniform formation pressures. Further, such uniform flow would require that the consistency and break-down of the fracking liquid viscosity was uniform throughout the several zones. In reality, these assumptions would be very unlikely to hold true. There are many variables that affect the nature and rate at which the fracture liquids are recovered. First is the material and consistency of the formation, and the extent of hydrocarbon and brine fluids therein. These two factors are of interest to the operator, because they are indicators of the production potential of the well and also indicate the general nature of the reserve, which influences how nearby wells might be engineered. Another factor is the content of the fracture fluid mixture in each of the several stages. There can also be problems in the recovery process where certain stages do not readily release the fracking liquid, and therefore limit production potential for the well. The oligonucleotide concentration can indicate such problematic areas, and suggest alternative treatments for mitigating them.
Ideally, the well operator's goal is to remove all of the fracking liquid from the well, so that the well only produces formation fluids. In an exemplary well, approximately 2 million gallons of fracking liquid are used, and the recovery process goal is to remove all of this so that the well can be placed into production of oil and/or gas. In a typical well, perhaps 75% of the fracking liquid is actually recovered. It is useful to understand which of the plural zones' fracking liquid has been recovered, and where the 25% of unrecovered fracking liquid might be. This is only possible if all of the fracking liquid zones have been uniquely and discretely marked. With respect to when the well is transitioned from recovery of fracking liquids to production of oil and gas, once the toe perforation start to flow back, then it can be assumed that the well is ready for production. This is because the toe perforation was the last to be fractured, and will be the last to produce. Therefore, once this perforation starts to produce, then the whole well is likely to be ready for production. The unique oligonucleotides that marked the toe perforation stages will indicate to the operator when that stage is beginning to flow.
In an exemplary embodiment, well fluid samples are taken on a periodic basis, which gradually lengthens over time. For example, during the first day of recovery, a first sample can be taken shortly after the recovery pump starts operating, and then samples may be taken at four-hour intervals. The second day samples may be taken at eight-hour intervals, then twelve-hour intervals the next day, until just daily samples are taken. This can go on for a month, or until testing shows that most of the fracking liquids have been recovered. The rate at which fracking liquid and formation fluids are pumped out of the well varies widely, based on the characteristics of the formation. This may range from 1 bbl/day to 2000 bbl/day. In the exemplary well, the recovery rate is approximately 300 bbl/day. At initial pumping, the recovered fluids are nearly all fracking liquid, but by the end of the recovery period, only a small fraction of the pumped formation fluids is fracking liquid. Again, the oligonucleotide testing procedure provides detailed information on the rate of fracking liquid recovery.
Reference is now directed to FIG. 3, which is a system diagram of the oligonucleotide marking and pumping process according to an illustrative embodiment of the present invention. This figure illustrates the equipment at ground level 62 used to pump the fracking liquid into the wellhead 64 and down the casing 65. The water flows from an input pump 76, which is supplied from a high volume reservoir (not shown), and into a blender 74. The blender 74 has mechanical agitators inside, which combine and mix the water with sand and chemicals (not shown) on a continuous basis. In the illustrative embodiment the blender 74 has a mixing volume of approximately one hundred barrels. The volume of fracking liquid flowing out of the blender 74 is measured by a flow meter 72, which is used to monitor and maintain the volumetric flows according to the fracking schedule, and for general record keeping requirements. An input manifold 70 routes the fracking liquid to plural high-pressure fracking pumps 68. The outlets of the plural high-pressure pumps 68 are combined by an outlet manifold 66, which is coupled to the wellhead 64.
As was noted hereinbefore, petroleum engineers develop a fracking schedule that itemizes the mixture components of the several zones of each stage of a fracking job. This schedule is used as the basis for adding oligonucleotides into the blending process in concert with the other blended components. The individual zones are each marked with a unique oligonucleotide. Therefore, in FIG. 3, there are plural tracer tanks 82 that each contains a unique oligonucleotide. Each of the plural tracer tanks 82 is coupled to a corresponding metering pump 84. The metering pumps 84 run at fairly low volumetric rates, so peristaltic pumps are a suitable choice for this application. The output of the plural metering pumps 84 are combined by a manifold 86 and coupled to the blender 74 or the water feed line 88 into the blender 74.
Because the fracturing process is implemented on a continuous basis, and because there is a predetermined fracking schedule, the pumping of the oligonucleotides 82 can be automated. In the illustrative embodiment, the stage schedule 80 contains a database of the volumetric flow for each zone of every stage, and also the type and concentration for each of the discrete oligonucleotides. A controller 78, such as an industrial programmable logic controller, monitors the flow meter 72 and the stage schedule 80, and then activates the appropriate metering pump 84 so that the correct amount of oligonucleotide is pumped to yield the specified input concentration, which may be approximate one to five parts per million in the illustrative embodiment. Note that oligonucleotide is produced as a fine dry power. To facilitate the metering and pumping operations, the oligonucleotides are mixed with fresh water into high concentration slurry, and are then placed into the tracer tanks 82. Agitation may be required to maintain a uniform slurry concentration in the tracer tanks 82.
Reference is now directed to FIG. 4, which is a system diagram of the formation fluid sampling process according to an illustrative embodiment of the present invention. This figure illustrates a more detailed view of the well fluid sampling system, and also shows an automated sampling embodiment. At the ground level 90, the wellhead comprises the well casing 92, a tubing string 94, and the sucker rod 96, which drives the down-hole pump. Generally, fluids are pumped up the tubing string 94, and gases flow up the casing 92 annulus. Although, the well fluids often times have a high percentage of gas content, as is know to those skilled in the art. A fluid pipeline 98 is coupled to the tubing string 94, and a gas pipeline 100 is coupled to the casing 92 annulus. Suitable valves are used, and the well fluids are output 102 to a storage or transportation system (not shown). The illustrative embodiment utilizes a sampling line 104 connected to the fluid pipeline 98, which is used to draw periodic samples of the well fluids, which would include some of the fracking liquids.
In the automated sampling embodiment of FIG. 4, the sampling is accomplished periodically and automatically using a solenoid valve 106 under control of an industrial programmable controller 110. At predetermined intervals, the controller 110 opens the solenoid valve 106 to allow well fluids to pass into the valve body 108. The valve body 108 automatically routes each sample of well fluid to a predetermined sample vessel 112. An operator periodically visits the well site to retrieve the sample vessels 112, and replace them with empty vessels. This arrangement facilitates more accurate sample gathering and less operator involvement. Once the samples are gathered, they are ready for processing and measurement of the concentrations of the plural oligonucleotides originally pumped in with the fracking liquid.
Once the samples are gathered from the wellhead, testing for the concentrations of the plural oligonucleotides is undertaken, and then calculations are made to establish the volume of fracking liquids that have been removed per sample period. These values, gathered over the several sampling periods, are then used to establish the totality of the fracking liquid recovery process, which is presented in table form for the well operator's uses. It will be appreciated by those skilled in the art that the raw well fluids are challenging to deal with, and are hard on all the instruments that are used in the sampling and measuring process. These fluids contain brine, crude oil, dissolved gases, gas bubbles, acids, solids, various well chemicals, the fracking liquid, and the oligonucleotide tracers. The raw well fluids are not ready for testing in a spectrometer, as least not on an ongoing, commercial basis.
In the illustrative embodiment, oligonucleotides are added to the fracking liquid to serve as the tracer material. In order to gather useful information in the testing process, the testing equipment needs to accurately measure minute concentrations of these materials. Additionally, these materials must survive the harsh down-hole environment. Tests conducted in developing this disclosure indicated that oligonucleotides do endure the down-hole environment and are useful for tracing fracking liquid. Oligonucleotides are short, single-stranded DNA or RNA molecules. They are typically manufactured in the laboratory by solid-phase chemical synthesis. These small bits of nucleic acids can be manufactured with any user-specified sequence. The number of potential sequences is very large. The number of sequences is four to the power of N, where N is the length of the sequence. The length of the sequence can range from 2 to 150, which equates to tens of thousands of discrete and unique oligonucleotide sequences. Each sequence has a discrete atomic mass, which is what is measured to identify unique sequences. The range of molecular weights for these oligonucleotides is from 3000 to 6500 atomic mass units.
As was noted hereinbefore, the oligonucleotides contemplated in the illustrative embodiment are DNA, RNA, and LNA. LNA is an acronym for locked nucleic acid. LNA is also referred to as inaccessible RNA, and is a modified RNA nucleotide. During synthesis, the ribose moiety of an LNA nucleotide is modified with an extra bridge connecting the 2′ oxygen and 4′ carbon. The bridge “locks” the ribose. The locked ribose conformation enhances base stacking and backbone pre-organization. This significantly increases the melting temperature of oligonucleotides, making them more tolerant in the down-hole environment. With respect to down-hole durability of these oligonucleotides, testing indicates that LNA is most durable, then RNA, and then DNA. However, DNA can be utilized down-hole and show good durability. Tests establish that DNA is thermally stable to 1000 degrees, and will not shear under wellbore pressures to at least 7700 PSI. It is expected that DNA can out-survive casing static pressure limits of 20,000 psi. The highest risk to the integrity of the DNA molecules are enzymes called DNAase. However, test samples showed that only the DNA samples sent down hole were detected in well fluid, with no byproducts from DNAase. Furthermore, testing with certain mass spectrometer test methodologies showed that DNA could be reliably detected after exposure to the down-hole environment. DNA is highly tolerant to temperatures seen down-hole, and also tolerant to a wide range of pH. While very low pH for extended periods of time can damage DNA, the down-hole environment is usually not that acidic. The down-hole pH may be in the 5-6 range, with pH of 4 being a practical low limit for acidity. However, DNA can tolerate a pH of 3 for reasonable periods of time. It would take long-term exposure to damage oligonucleotides at such pH levels.
Having established that oligonucleotides are suitable for tracing fracking liquids in real-world down-hole environments and time frames, the next hurdle to their application is recovery and testing for minute concentrations present in well fluids. Since the oligonucleotides would be destroyed by flame (gas chromatograph), the testing procedure must use a non-flame type of mass spectrometer. In the illustrative embodiment, a matrix-assisted laser desorption/ionization source with a time-of-flight mass analyzer (MALDI-TOF) mass spectrometer is utilized. This instrument tests a dry sample, so it is necessary to reduce and concentrate the well fluid sample in order to conduct the measurements of oligonucleotide concentrations. A MALDI-TOF mass spectrometer is accurate to +/−0.2%, and can readily distinguish the oligonucleotide sequences discussed herein. The output of MALDI-TOF is spectrograph style graphic, where the horizontal line distinguishes individual oligonucleotide masses and the vertical amplitude indicates the total mass of each oligonucleotide in a given test run. This data can, or course, be quantified for analysis and incorporation in the test results for the well operator.
The challenge of isolating the oligonucleotides from the other well fluid materials is addressed by biotinylation. This simplifies the recovery of the oligonucleotide in the well fluid samples and increases the overall sensitivity of the testing processes. This is accomplished by biotinylating the 5′-end of the sequence of the oligonucleotides before they are added to the fracking liquid and pumped down-hole. Biotinylation takes advantage of the fact that biotin and avidin or streptavidin (hereafter collectively referred as “avidin”) form the strongest non-covalent bond known in nature with a dissociation constant of greater than ten to the minus fifteenth power. Once the well fluid samples are collected, they are infused with magnetic particles that have avidin immobilized onto their surfaces. Of course the biotinylated oligonucleotides and avidin coated magnetic particles are strongly attracted to one another. This attraction is facilitated by agitating the mixture for a period of time to insure that substantially all of the biotin and avidin have bonded, and therefore assuring that all of the oligonuceotides have been attached to the magnetic particles.
After agitating the sample for a given period to ensure that the biotinylated oligonucleotide has had sufficient opportunity to physically contact the avidin (or streptavidin) magnetic particles, a polar magnet is inserted into the sample, which easily gathers all of the magnetic particles that have the oligonucleotides bonded to them. The magnetic particles are washed to removed well fluid residue, and further washed to collect the magnetic particles from the magnet. The magnetic particles are collected in a small volume allowing for subsequent washing with deionized water to remove any residual components from the sample solution. The magnetic particles are then ready for further preparation for analysis by, preferably, a delayed-extraction (DE) matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometer.
With respect to suitable sample sizes and test concentrations, tracers are added to the fracking liquid with a concentration in the range of one to five parts per million. The sample taken from the well fluid flow may be in the range from four ounces to one gallon, which is concentrated, dried, and then measured with a DE-MALDI-TOF mass spectrometer. Sample concentrations of eight parts per billion are reliably detected, and concentrations below one part per billion can be detected through the foregoing process. Further, the MALDI-TOF mass spectrometer can measure thresholds as low as one part per trillion.
Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.
It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.

## Claims

1. A method of tracing the movement of plural volumes of fracking liquid pumped into an oil or gas bearing formation through a first stage of perforations in a well casing that is coupled to a wellhead, by using plural unique oligonucleotide markers that have been biotinylated, comprising the steps of:
sequentially marking plural volumes of fracking liquid, correspondingly, with plural unique oligonucleotides, each at a predetermined concentration;
sequentially pumping each of the plural volumes of fracking liquid through the first stage of perforations and into the formation, thereby sequentially advancing each of the plural volumes of fracking liquid outwardly into plural fracture zones, correspondingly, of the formation;
pumping formation fluids to the wellhead, which may contain portions of the plural volumes of fracking liquid, while monitoring the periodic volume thereof;
gathering, periodically, plural formation fluid samples as the formation fluids are pumped to the wellhead;
correlating each of the plural formation fluid samples, correspondingly, to plural periodic volumes;
analyzing the concentration of the plural unique oligonucleotides in each of the plural samples by immobilizing avidin or streptavidin onto magnetic particles, and mixing the magnetic particles with at least one of the plural fluid samples, thereby enabling the formation of non-covalent bonds between the biotinylated oligonucleotides and the immobilized avidin or streptavidin, and removing the magnetic particles from the at least one of the plural fluid samples by magnetic attraction, thereby concentrating the sample, and, measuring the quantity of each of the unique oligonucleotides;
calculating the quantity of each of the plural volumes of fracking liquid recovered during each of the plural periodic volumes according to the concentration of the plural unique oligonucleotides present in each of the plural samples, and
thereby establishing the movement of the plural volumes of fracking liquids out of the plural corresponding fracture zones.

2. The method of claim 1, and wherein:
said sequentially marking plural volumes of fracking liquid is accomplished by metering plural liquid slurries comprising the plural unique oligonucleotides into a fracking liquid blender on an ongoing basis as a fracking process is occurring.

3. The method of claim 1, and wherein:
said sequentially pumping each of the plural volumes is synchronized with plural pre-defined sub-stages of a fracking job, such that each of the sub-stages is marked with a unique oligonucleotide.

4. The method of claim 3, and wherein:
the plural sub-stages of the fracking job are distinguished from one another by the quantity and size or proppant incorporated into the plural volumes of fracking liquid.

5. The method of claim 1, and wherein:
the pumping formation fluids to the wellhead step is accomplished using a down-hole pump coupled to the wellhead through a tubing string, and wherein
the gathering, periodically, plural formation fluids step further comprises the step of drawing fluids from the tubing string at the wellhead.

6. The method of claim 5, and wherein the drawing fluids from the tubing string at the wellhead step further comprises:
automatically, and periodically, routing the plural fluid samples, correspondingly, to plural fluid sample vessels.

7. The method of claim 1, and wherein:
said monitoring the periodic volume of the formation fluid step comprises outputting a periodic volumetric flow value, and wherein
said gathering, periodically, plural formation fluid samples step further comprises recording a corresponding periodic volumetric flow value for each.

8. The method of claim 1, and wherein:
the plural unique oligonucleotides are selected from predetermined unique sequences of deoxyribonucleic acid, ribonucleic acid, and lock ribonucleic acid.

9. The method of claim 1, and wherein:
the plural unique oligonucleotides are synthesized unique DNA sequences.

10. The method of claim 1, and wherein said analyzing the concentration of the plural unique oligonucleotides step further comprises;
concentrating at least one of the plural formation fluid samples, and
measuring the atomic mass and quantity of each of the plural unique oligonucleotides therein using a matrix-assisted laser desorption/ionization with time of flight mass spectrometer.

11. A method of tracing fracking liquid in oil or gas bearing formations using plural unique oligonucleotide markers that have been biotinylated, comprising the steps of:
pumping the plural volumes of fracking liquid, each marked with a unique oligonucleotide, into the formation, thereby defining plural fracture zones in the formation;
pumping fluids out of the formation while taking plural fluid samples;
analyzing the concentration of the unique oligonucleotides in each of the plural fluid samples by immobilizing avidin or streptavidin onto magnetic particles, and mixing the magnetic particles with at least one of the plural fluid samples, thereby enabling the formation of non-covalent bonds between the biotinylated oligonucleotides and the immobilized avidin or streptavidin, and removing the magnetic particles from the at least one of the plural fluid samples by magnetic attraction, thereby concentrating the sample, and, measuring the quantity of each of the unique oligonucleotides;
calculating the ratio of each of the plural volumes of fracking liquid recovered for each of the plural fluid samples according to the concentration of the unique oligonucleotides present in each of the plural samples, and
thereby establishing the quantity of the plural volumes of fracking liquids removed from the plural fracture zones.

12. The method of claim 11, and wherein:
the unique oligonucleotides are biotinylated by binding biotin to the 5′-end of the oligonucleotides.

13. The method of claim 11, and wherein said measuring step further comprises the steps of:
determining the atomic mass and quantity of each of the unique oligonucleotides in the sample using a matrix-assisted laser desorption/ionization with time of flight mass spectrometer.

14. The method of claim 11, further comprising the step of:
agitating the at least one of the plural fluid samples and the magnetic particles to facilitate the formation of non-covalent bonds.

15. The method of claim 11, and wherein the removing step is accomplished by inserting a magnet into the sample, and further comprising the step of:
rinsing the magnetic particles prior to said measuring step.

16. A method of tracing the movement of plural volumes of fracking liquid pumped into an oil or gas bearing formation through plural stages of perforations in a well casing that is coupled to a wellhead, by using plural unique oligonucleotide markers that have been biotinylated, comprising the steps of:
forming a first stage of perforations through the casing and into the formation;
sequentially marking a first group of volumes of fracking liquid, correspondingly, with a first group of unique oligonucleotides, each at a predetermined concentration;
sequentially pumping each volume in the first group of volumes of fracking liquid through the first stage of perforations and into the formation, thereby sequentially advancing each of the first group of volumes of fracking liquid outwardly into a first group of fracture zones in the formation adjacent to the first stage of perforations;
forming a pressure seal within the casing between the first stage of perforations and the wellhead;
forming a subsequent stage of perforations through the casing and into the formation at a location between the pressure seal and the wellhead;
sequentially marking a subsequent group of volumes of fracking liquid, correspondingly, with a subsequent group of unique oligonucleotides, each at a predetermined concentration;
sequentially pumping each volume in the subsequent group of volumes of fracking liquid through the subsequent stage of perforations and into the formation, thereby sequentially advancing each of the subsequent group of volumes of fracking liquid outwardly into a subsequent group of fracture zones in the formation adjacent to the subsequent stage of perforations;
removing the pressure seal;
pumping formation fluids to the wellhead, which may contain portions of the first group of volumes of fracking liquid and the subsequent group of volumes of fracking liquid, while monitoring the periodic volume thereof;
gathering, periodically, plural formation fluid samples as the formation fluids are pumped to the wellhead;
correlating each of the plural formation fluid samples, correspondingly, to plural periodic volumes of formation fluid;
analyzing the concentration of the first group of oligonucleotides and the subsequent group of unique oligonucleotides in each of the plural samples by immobilizing avidin or streptavidin onto magnetic particles, and mixing the magnetic particles with at least one of the plural fluid samples, thereby enabling the formation of non-covalent bonds between the biotinylated oligonucleotides and the immobilized avidin or streptavidin, and removing the magnetic particles from the at least one of the plural fluid samples by magnetic attraction, thereby concentrating the sample, and, measuring the quantity of each of the unique oligonucleotides;
calculating the quantity of each of the first group of volumes of fracking liquid and subsequent group of volumes of fracking liquid recovered during each of the plural periodic volumes of formation fluid according to the concentration of the plural unique oligonucleotides present in each of the plural samples, and
thereby establishing the movement of the first group of volumes of fracking fluid and subsequent group of volumes of fracking liquids, correspondingly, out of the first group of fracture zones and subsequent group of fracture zones.

17. The method of claim 16, and wherein:
said sequentially pumping each volume in the first group of volumes, and the subsequent group of volumes, are synchronized with plural pre-defined sub-stages of a fracking job, such that each of the sub-stages is marked with a unique oligonucleotide.

18. The method of claim 17, and wherein:
the plural sub-stages of the fracking job are distinguished from one another by the quantity and size or of proppant incorporated into the plural volumes of fracking liquid.

19. The method of claim 16, and wherein:
said monitoring the periodic volume of the formation fluid step comprises outputting a periodic volumetric flow value, and wherein
said gathering, periodically, plural formation fluid samples step further comprises recording a corresponding periodic volumetric flow value for each.

20. The method of claim 16, and wherein:
the first group of unique oligonucleotides and the subsequent group of unique oligonucleotides are selected from predetermined sequences of deoxyribonucleic acid, ribonucleic acid, and lock nucleic acid.

21. The method of claim 16, and wherein said analyzing the concentration of the plural unique oligonucleotides step further comprises;
concentrating at least one of the plural formation fluid samples, and
measuring the atomic mass and quantity of each of the plural unique oligonucleotides therein using a matrix-assisted laser desorption/ionization with time of flight mass spectrometer.