In another life I worked as the production/operations manager for a good sized independent oil producer. We operated about 425 wells, and during my time there, drilled 100 + wells, of which about 35 were hydraulically fractured. This is intended to give a simplified backdrop to evaluate the cited article. You can decide for yourself if there is anything hysterical about it.
A little geology is necessary to understand some of the process. Generally, an oil producer is looking for an underground structure to form a trap to collect oil or gas underground. Visualize a producing stratum as an undulating layer of rock underground. There are a number of different types of traps, but basically you are looking for a place where the particular underground stratum or strata creates a place for oil or gas to collect, within the rock itself. Bear in mind we are talking about processes that occurred over geologic time, hundreds of millions of years. One type is an anticline, where the stratum forms an underground hill. Seismic analysis can locate some traps, but precise mapping is usually done by well logging, running various instruments down a wellbore to take readings. There are many different types of instruments used, but one of the first and frequently used for mapping structures is a natural gamma ray tool. These are instruments that are run down along the well on a cable and take various measurements of the sides of the wellbore. All strata/soils have some degree of natural radioactivity. Shales, because of their chemical composition are more radioactive than most productive strata, which are generally limestone/dolomite or sandstones. Shales are relatively impermeable, so they tend to create an impermeable cap over most structures. Basically, the gamma ray log will produce a graphical output strip of radioactivity vs depth and because the shales create spikes on the graph, you can use them as geologic “markers,” and map and determine the depth to a particular horizon from well to well, sometimes over hundreds of miles, and see where they rise and fall, forming hills and valleys. The radiation they are talking about is actually the naturally occurring radiation in some of the elements that have a small percentage of radioisotopes that compose the clays that compose the shales.
The second thing a geologist looks for in a productive stratum, besides the presence of hydrocarbons, is the porosity and the permeability of the material. There is no “pool” down there. The oil or gas exists within the pore space within the rock. Look at a piece of chalk or limestone, they are probably about 3-6% porous, the hydrocarbons are located within those miniscule pores. Once you find porosity, a place for fluid to collect within the rock, you then have to see if it is permeable, or just how well those pore spaces are connected up, to allow the gas or fluid to flow through the rock. Shales are much less porous and much less permeable, probably by an order of magnitude, but I do not have any hard numbers to quantify how much as there was no shale production where I worked. The oil and gas are under pressure, sometimes considerable. There are several mechanisms that move oil and gas thru the rock. One is called solution gas drive, much like letting the fizz out of a bottled drink, the wellbore allows the gas to expand, driving it to the well. Usually oil and gas exist with brine, generally many times more saline than seawater. The oil and gas generally separate to some degree to the top of the structure. The brine, despite being a fluid can be compressible slightly, and the expansion of the brine when a well is drilled at the top of the structure can also drive the oil and gas to a wellbore. It can be a combination of both. Produced brines in an oilfield are disposed of re-injecting into disposal wells or into other areas of the producing formation, thousands of feet below any potable water.
When we drilled a well, we were required to “case” the well. The well would be started with a “native” drilling mud, that is fresh water and the fine particulates ground up in the drilling process. The mud is pumped down through the drill pipe and bit and circulated back up the outside to the surface, cooling the bit and carrying with it the drill cuttings which were allowed to settle out in pits or tanks. The mud was continuously re-circulated. We were required to drill down, several hundred feet, to well below any strata known to have potable water. We would then set the surface casing, usually, in our area, 8-5/8” pipe, within a 13-14 inch well bore. The pipe would then be cemented in place, by pumping cement down the inside of the pipe, until it circulated clean cement up to the surface. This was usually plain Portland cement, no aggregate, basically a grout. It sets within seconds once it stops moving. It is given time to cure and develop strength. The purpose of the surface casing is to isolate each of the strata and to protect any fresh water bearing strata. Then a smaller bit would go back into the well, inside the casing, out the bottom, and the well would be drilled to its final depth. This drilling is done with native mud until they get near the producing zone, when they “mud up,” adding generally natural materials such as corn starch and bentonite ( a kind of clay) to increase the viscosity of the mud and to help it carry the drill cuttings from that depth. If they are in an area where the formation pressure is substantial, then “weighting” compounds are added to the mud, generally materials like ground walnut shells or barite (a natural, but heavy, material), to add to the weight of the column of fluid to counteract/balance the formation pressure. Mechanical blow out preventers are installed where needed to give them the capability to seal off the well bore at the surface. Once at total depth, the wells would then be “logged.” There might be intermediate casing strings that could be needed depending on the depth of the well and the conditions, but generally, the “production” casing would then be set and similarly cemented, to several hundred feet above any producing strata. Generally, the next step is to locate the productive zones within a well and then to perforate those sections of the casing. This is done by wireline tools which carry an array of small explosive "shaped" charges that cut a hole, about 3/8” in diameter, through the casing, the cement and into the producing formation, probably to about 20” or so in depth. A better explanation of logging, tools and perforation process could probably be found at the Schlumberger website, they are probably the leader in this field.
Hydraulic fracturing is used to compensate for low permeability of the rock and to thereby increase production. Basically, the object is to create a vertical fracture (picture a knife blade) through the producing rock and propping it open, exposing the face of the producing rock to what will become a high permeability channel, allowing more face of the rock to drain to the crack and then to the wellbore. The rock will fracture along the plane of the rocks least compressive strength. In a well deeper than 2500 feet, the overburden, the weight of the soil and strata above the producing zone, will prevent a horizontal fracture. There is considerable engineering that goes into designing a “frac job.” There are other ways of doing it, some techniques involve using nitrogen-based foams instead of gelled water because they clean up easier.
The process involves pumping a fluid into the well, through the perforations and into the formation at enough pressure to generate a fracture in the rock, followed by a tapered mixture of “proppant” generally sand, but in exotic wells involving extreme depths, pressures and temperatures, might be small spheres of sintered bauxite or other materials. The height of the fracture is regarded as being a function of the rate the fluid is pumped into the well. The overall length is determine by the amount of fluid, but is also influenced by the rock properties and how fast the fluid pressure bleeds off into the rock face. The fluid we used was water gelled with guar gum, the same stuff we use to thicken salad dressings. There are catalysts that cause it to “crosslink” and become about the consistency of the green Slime we used to play with as kids. The initial 20-30% of the fluid is pumped into the well as a “pad,” just gel, to initially generate the crack, this is followed as a continuous process, by adding proppant to the gel, e.g.,1# per gallon, for so many gallons of gel, then 2# per gallon, on up, starting with a fine sand and then usually shifting to a coarser sand. The sand we used was called Brady sand, SiO2, like beach sand, but each grain was almost spherical. The sand would carry down the crack until the gel bled off into the face of the rock, the fracture would stop growing and the sand would be packed, propping the crack apart enough to create a high permeability channel. There are chemicals added to reverse the gel and the fluid is eventually swabbed back out of the well. Then the production equipment is then installed, usually through a third string of pipe, typically 2-3/8” steel tubing to bring the production to the surface.
Well density and spacing, or the acreage for each drilling unit, and the pattern for drilling, is usually set by whatever regulatory body is involved, based upon the nature of the producing zone and intended to efficiently drain the reservoir. Oil wells can be spaced as close as 5-10-20-40 acres where I worked. Gas wells are generally spaced less densely, one well each160-320-640 acres is common. A “section” is a square mile, 640 acres. Operating companies will “pool” acreage from several owners, to form a drilling unit, but may also be constrained by what acreage they can include in a unit by the pattern established by the spacing rules. The owners of the acreage usually agree to share their interest in the production in the proportion to their acreage in the unit, but this can be a subject of negotiation.
I’d be curious to know the circumstances of a drilling rig burning down, that is generally avoidable by good engineering and pressure control practices. I’d be surprised to hear about a blowout in shale production, because if you had the kind of pressures and flow rates required for a blow out, you would not need to fracture a well. On the other hand, if the rig was a workover rig, repairing an already producing well, and they weren’t careful controlling the pressures, I could see a rig fire. The difference between the kinds of rigs is not always appreciated by someone not in the business. Having flammable water from a drinking water well caused by a gas well doesn’t make much sense, I’d start looking for a nearby gas station with leaking tanks first. There would be about a mile of vertical separation in addition to the cementing. I’d like to know the dates of the anecdotal stories because while there are historical accounts from the earliest days of drilling, a century ago and longer, where wells were allowed to leak and overflow into streams, when there were no environmental concerns and fires burned on the surface, but there is absolutely no reason for it today.
A little geology is necessary to understand some of the process. Generally, an oil producer is looking for an underground structure to form a trap to collect oil or gas underground. Visualize a producing stratum as an undulating layer of rock underground. There are a number of different types of traps, but basically you are looking for a place where the particular underground stratum or strata creates a place for oil or gas to collect, within the rock itself. Bear in mind we are talking about processes that occurred over geologic time, hundreds of millions of years. One type is an anticline, where the stratum forms an underground hill. Seismic analysis can locate some traps, but precise mapping is usually done by well logging, running various instruments down a wellbore to take readings. There are many different types of instruments used, but one of the first and frequently used for mapping structures is a natural gamma ray tool. These are instruments that are run down along the well on a cable and take various measurements of the sides of the wellbore. All strata/soils have some degree of natural radioactivity. Shales, because of their chemical composition are more radioactive than most productive strata, which are generally limestone/dolomite or sandstones. Shales are relatively impermeable, so they tend to create an impermeable cap over most structures. Basically, the gamma ray log will produce a graphical output strip of radioactivity vs depth and because the shales create spikes on the graph, you can use them as geologic “markers,” and map and determine the depth to a particular horizon from well to well, sometimes over hundreds of miles, and see where they rise and fall, forming hills and valleys. The radiation they are talking about is actually the naturally occurring radiation in some of the elements that have a small percentage of radioisotopes that compose the clays that compose the shales.
The second thing a geologist looks for in a productive stratum, besides the presence of hydrocarbons, is the porosity and the permeability of the material. There is no “pool” down there. The oil or gas exists within the pore space within the rock. Look at a piece of chalk or limestone, they are probably about 3-6% porous, the hydrocarbons are located within those miniscule pores. Once you find porosity, a place for fluid to collect within the rock, you then have to see if it is permeable, or just how well those pore spaces are connected up, to allow the gas or fluid to flow through the rock. Shales are much less porous and much less permeable, probably by an order of magnitude, but I do not have any hard numbers to quantify how much as there was no shale production where I worked. The oil and gas are under pressure, sometimes considerable. There are several mechanisms that move oil and gas thru the rock. One is called solution gas drive, much like letting the fizz out of a bottled drink, the wellbore allows the gas to expand, driving it to the well. Usually oil and gas exist with brine, generally many times more saline than seawater. The oil and gas generally separate to some degree to the top of the structure. The brine, despite being a fluid can be compressible slightly, and the expansion of the brine when a well is drilled at the top of the structure can also drive the oil and gas to a wellbore. It can be a combination of both. Produced brines in an oilfield are disposed of re-injecting into disposal wells or into other areas of the producing formation, thousands of feet below any potable water.
When we drilled a well, we were required to “case” the well. The well would be started with a “native” drilling mud, that is fresh water and the fine particulates ground up in the drilling process. The mud is pumped down through the drill pipe and bit and circulated back up the outside to the surface, cooling the bit and carrying with it the drill cuttings which were allowed to settle out in pits or tanks. The mud was continuously re-circulated. We were required to drill down, several hundred feet, to well below any strata known to have potable water. We would then set the surface casing, usually, in our area, 8-5/8” pipe, within a 13-14 inch well bore. The pipe would then be cemented in place, by pumping cement down the inside of the pipe, until it circulated clean cement up to the surface. This was usually plain Portland cement, no aggregate, basically a grout. It sets within seconds once it stops moving. It is given time to cure and develop strength. The purpose of the surface casing is to isolate each of the strata and to protect any fresh water bearing strata. Then a smaller bit would go back into the well, inside the casing, out the bottom, and the well would be drilled to its final depth. This drilling is done with native mud until they get near the producing zone, when they “mud up,” adding generally natural materials such as corn starch and bentonite ( a kind of clay) to increase the viscosity of the mud and to help it carry the drill cuttings from that depth. If they are in an area where the formation pressure is substantial, then “weighting” compounds are added to the mud, generally materials like ground walnut shells or barite (a natural, but heavy, material), to add to the weight of the column of fluid to counteract/balance the formation pressure. Mechanical blow out preventers are installed where needed to give them the capability to seal off the well bore at the surface. Once at total depth, the wells would then be “logged.” There might be intermediate casing strings that could be needed depending on the depth of the well and the conditions, but generally, the “production” casing would then be set and similarly cemented, to several hundred feet above any producing strata. Generally, the next step is to locate the productive zones within a well and then to perforate those sections of the casing. This is done by wireline tools which carry an array of small explosive "shaped" charges that cut a hole, about 3/8” in diameter, through the casing, the cement and into the producing formation, probably to about 20” or so in depth. A better explanation of logging, tools and perforation process could probably be found at the Schlumberger website, they are probably the leader in this field.
Hydraulic fracturing is used to compensate for low permeability of the rock and to thereby increase production. Basically, the object is to create a vertical fracture (picture a knife blade) through the producing rock and propping it open, exposing the face of the producing rock to what will become a high permeability channel, allowing more face of the rock to drain to the crack and then to the wellbore. The rock will fracture along the plane of the rocks least compressive strength. In a well deeper than 2500 feet, the overburden, the weight of the soil and strata above the producing zone, will prevent a horizontal fracture. There is considerable engineering that goes into designing a “frac job.” There are other ways of doing it, some techniques involve using nitrogen-based foams instead of gelled water because they clean up easier.
The process involves pumping a fluid into the well, through the perforations and into the formation at enough pressure to generate a fracture in the rock, followed by a tapered mixture of “proppant” generally sand, but in exotic wells involving extreme depths, pressures and temperatures, might be small spheres of sintered bauxite or other materials. The height of the fracture is regarded as being a function of the rate the fluid is pumped into the well. The overall length is determine by the amount of fluid, but is also influenced by the rock properties and how fast the fluid pressure bleeds off into the rock face. The fluid we used was water gelled with guar gum, the same stuff we use to thicken salad dressings. There are catalysts that cause it to “crosslink” and become about the consistency of the green Slime we used to play with as kids. The initial 20-30% of the fluid is pumped into the well as a “pad,” just gel, to initially generate the crack, this is followed as a continuous process, by adding proppant to the gel, e.g.,1# per gallon, for so many gallons of gel, then 2# per gallon, on up, starting with a fine sand and then usually shifting to a coarser sand. The sand we used was called Brady sand, SiO2, like beach sand, but each grain was almost spherical. The sand would carry down the crack until the gel bled off into the face of the rock, the fracture would stop growing and the sand would be packed, propping the crack apart enough to create a high permeability channel. There are chemicals added to reverse the gel and the fluid is eventually swabbed back out of the well. Then the production equipment is then installed, usually through a third string of pipe, typically 2-3/8” steel tubing to bring the production to the surface.
Well density and spacing, or the acreage for each drilling unit, and the pattern for drilling, is usually set by whatever regulatory body is involved, based upon the nature of the producing zone and intended to efficiently drain the reservoir. Oil wells can be spaced as close as 5-10-20-40 acres where I worked. Gas wells are generally spaced less densely, one well each160-320-640 acres is common. A “section” is a square mile, 640 acres. Operating companies will “pool” acreage from several owners, to form a drilling unit, but may also be constrained by what acreage they can include in a unit by the pattern established by the spacing rules. The owners of the acreage usually agree to share their interest in the production in the proportion to their acreage in the unit, but this can be a subject of negotiation.
I’d be curious to know the circumstances of a drilling rig burning down, that is generally avoidable by good engineering and pressure control practices. I’d be surprised to hear about a blowout in shale production, because if you had the kind of pressures and flow rates required for a blow out, you would not need to fracture a well. On the other hand, if the rig was a workover rig, repairing an already producing well, and they weren’t careful controlling the pressures, I could see a rig fire. The difference between the kinds of rigs is not always appreciated by someone not in the business. Having flammable water from a drinking water well caused by a gas well doesn’t make much sense, I’d start looking for a nearby gas station with leaking tanks first. There would be about a mile of vertical separation in addition to the cementing. I’d like to know the dates of the anecdotal stories because while there are historical accounts from the earliest days of drilling, a century ago and longer, where wells were allowed to leak and overflow into streams, when there were no environmental concerns and fires burned on the surface, but there is absolutely no reason for it today.
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