A two-part series focusing on enhancing natural evaporation rates
As dealers are always looking for their next deal and wildcatters continue searching for the next big play, our industry often punts future problems down the road. We’ve become accustomed to being able to out-politic or out-science our opposition to keep our industry vibrant and profitable.
As states and environmental groups have recognized the correlation between seismic activity and oil-field injection wells, we can expect a challenge to our customary way of doing business and a demand for an alternative to oil-field injection wells for the disposal of produced waters.
Our industry’s water professionals are aware of this issue and have been asking, “What is a cost-effective, practical alternative to oil-field injection wells for the disposal of high TDS, total dissolved solids, waters?”
The water from deep inside the earth and brought to the surface with oil production has acquired characteristics that make the disposal or reuse of that water a challenge. The water that we receive is left over from ancient seas and—
• is salty to the point that reuse options are limited
• filled with metals, chemicals and bacteria that can lead to corrosion of the wellbore and surface handling equipment
• can readily produce hydrogen sulfide
These physical and chemical characteristics and potential life-safety issues limit the use of conventional methods of wastewater treatment and disposal, and are challenging our industry to develop a method that is cost neutral, beneficial or a non-impact to the environment.
INJECTION DISPOSAL WELLS
Oil-field injection disposal wells have met industry needs for many years, providing a method for getting rid of water that is not easy to deal with. However, the volumes we receive with the production of unconventional plays by deep horizontal wells have greatly increased the need for large-volume saltwater disposal wells.
Disposal wells, when operated properly, are a very safe and efficient way to dispose of salt water. However, the need to apply higher pressures or cycle those wells off and on to force the water into the receiving formation can increase the costs of drilling new wells through those formations and cause casing problems in existing wells.
More noticeable to the regulators and the public is that existing faults in the immediate area may move to reduce pressures forced upon them. If we are going to continue to be good at what we do in dealing with produced waters, we have to be prepared for any future regulations on decreased allowable disposal volumes, a moratorium on future disposal wells in areas of heightened seismic activity and an increase in costs for produced-water disposal.
Our industry has been recycling produced waters and has become highly efficient at using the high TDS waters for carrying proppant downhole during the fracturing operations. Recycling and reusing water are practical, but as volumes of produced water continue to increase, new technologies or options for disposal will be crucial.
We can’t send of this water into any state or federal waterways. We can’t pursue conventional beneficial reuse of the waters without incurring significant treatment costs, and we may be prevented from injecting it into the earth if decreasing seismic activity becomes a charge of our regulators.
NEW DISPOSAL TECHNOLOGIES
Fortunately, there are technologies today that can reduce our capacity needs for oil-field waste disposal at a price point that would not destroy our industry if and when we have to institute alternative disposal methods.
These technologies are impressive, and the inventors and their backers continue to work to improve efficiencies and drive down the cost of operation. These technologies include:
• acoustophoresis, where the cavitation of air bubbles produces enough energy to momentarily break the saltwater bonds
• cryo-distillation where we separate heavier frozen salt waters from lighter fresh waters
• several different presentations of electrically changing molecular structures to remove the salts from the water
All these technologies will lead to the beneficial reuse of waters, whether for agriculture, reducing the salt loads carried in many Western rivers, reducing the salt loads in estuaries along the coast or municipal consumption.
These beneficial reuse options could lead to additional profit centers for water midstream companies and producers or, at a minimum, cost-sharing public-private partnerships between the oil industry and municipalities.
EVAPORATION—MOTHER NATURE’S SAFE DISPOSAL METHOD
Another beneficial reuse opportunity that reduces the volumes of water to be injected is evaporation. Evaporation was Mother Nature’s first beneficial reuse project and can take waters that have been out of the hydrologic cycle for millions of years and return them to the hydrologic cycle in a form beneficial to mankind and nature.
It is Nature’s way of handling the safe disposal of bad waters. Unfortunately for us, her time frame for evaporation and ours are not in sync. She has a much longer return on investment period and isn’t really concerned about EBIDTA (earnings before interest, taxes, depreciation and amortization).
Evaporation is the transfer of water from the liquid to the vapor state. The rate, which is what we need to know in performing a cost evaluation of a proposed project, from a water surface is proportional to the difference between the vapor pressure at the surface and the vapor pressure in the overlying air (Dalton’s law).
In still air, the vapor-pressure difference soon becomes very small, and evaporation is limited by the rate of diffusion of vapor away from the water surface. With that said, evaporation is completely dependent upon the atmospheric conditions surrounding your area of interest.
Evaporation depends on water temperature, air temperature, air humidity, air velocity above your water surface and the amount of energy available to maintain water temperature.
EVAPORATION RATE FORMULAS
There are many different formulas for the estimation of Mother Nature’s evaporation rates. One of the most common is this EPA formula:
• E = Evaporation Rate (Gallons/Day)
• A = Water Surface Area (ft2)
• W = Wind Speed Above water (mph)
• P = Water's Vapor Pressure (mmHG) at Ambient Temperature
• T = Temperature (°F)
This formula is one of the simpler calculations available to estimate evaporation rates from the surface of a water body, and there are many more available to the project designer.
However, each calculation is specific to a geographical area as you have to consider the climate and physiography of the water body and its surrounding area. The evaporation of water is driven by the energy available and the ease at which water diffuses into the surrounding atmosphere.
SUBWAY TRAIN ANALOGY
Imagine a subway train with H2O molecules lined up at the edge of the platform, milling about because heat and energy are driving them to move. As an empty subway car pulls up, some of the water molecules are pushed off the platform into the car. Once that initial car is full, another car pulls up to accept more molecules and carry them away. If the car already has some passengers in transit, fewer molecules can be added as the doors open.
This is a way to visualize how the available energy in the water forces molecules to move and the collision of molecules pushes some through the edge of the surface and into the air available along the water’s surface.
The prevailing atmosphere can accept a certain number of water molecules based upon how much water is already being carried in the air. Turbulence caused by wind and thermal convection transports the vapor from the surface layer and permits evaporation to continue. Evaporation of a pound of water at 70 degrees Fahrenheit requires about 1050 Btu (585 cal/g at 20°C), and unless a heat supply is available, there can be no evaporation.
Boiling (pun intended) evaporation down to something that we can use in the real world includes defining where we are, the prevailing climate conditions, how much space we have to work with and the cost of local energy sources. The Penman formula and weather data for a geographical area from NOAA are used to calculate the rate of Mother Nature’s evaporation per day.
Once we establish that rate, it is a pretty simple, backof-the napkin calculation to determine how much storage area we would need to guarantee our ability to accept the produced-water volumes for any given area. However, as the sizes of storage areas increase, so does the cost and environmental impact.
For example, bird netting reduces the natural evaporation rate as it interrupts the steady flow of fresh non-saturated air moving across the top of a surface. Also, we have to remember that 50,000 TDS waters are going to leave behind 17.5 pounds of solids per evaporated barrel that we are going to have to dispose of or find a reuse market for. Although Mother Nature’s method would work for us, who needs more Great Salt Lakes spread across America?
BIONIC IMPROVEMENTS TO MOTHER NATURE
Like the introduction to the “Six Million Dollar Man” television show from the ’70s, we have the technology to improve Mother Nature by adding some bionic components to her repertoire. If we look at the very basic parameters of evaporation being available energy to the water and how much capacity is in the air to accept new water molecules, we can begin to add technologies to improve those performance parameters.
Some of the more common technologies include sprayers and misters. Old-school sprayers force water into the atmosphere exposing more water-surface area to the less humid air moving across the water surface, increasing the space in which molecules have to move from the stored waters into the local atmosphere. If not properly managed, sprayers can cover the area surrounding your stored waters with salt, and if you look closely at the surrounding boneyard, you are likely to see a bunch of dead equipment encrusted to the point of failure with salts.
Misters increase the surface area of water exposed to the atmosphere by aerosolizing the waters into very fine droplets. These aerosolized plumes can be more effectively directed along the surface of the pond, minimizing the environmental risk of overspray.
We can pretty simply change the interface between surface water and atmosphere by physically increasing the surface areas of the waters we are working to evaporate.
The next parameter we can enhance in Mother Nature’s formula for increasing evaporation is the addition of energy to get the water molecules moving. Mother Nature relies on the net solar radiation at the water’s surface and the heat energy stored in the water to move the water molecules.
As season’s change, so does the amount of available energy for evaporation. Adding energy in the form of heat or concentrating solar energy on the surface of a water body will increase evaporation. Misters operating inside a heated structure will evaporate more water than misters operating in ambient conditions.
If we can change the amount of surface area available and can add heat to increase the movement of water molecules, we’ve increased the rate at which water wants to leave the stored-water body. Our next task is to make sure that there is enough unsaturated air available to accept those water molecules.
We cannot build a pond inside tall storage walls or in a humid area with very little wind velocity and expect to maximize our enhancements to natural evaporation. We can build structures that encourage and direct the movement of winds across the surface of storage ponds.
COSTS FOR HELPING MOTHER NATURE
There is an obvious cost to improving Mother Nature’s ability to take away waters. Construction of permitted storage facilities, the power needed to operate and/or add heat, and the costs of solids disposal all depend on the type of system installed.
The operational costs (exclusive of capital build) of the pilot systems I have evaluated in my work that were effective and mitigated environmental risk operated between $0.05 to $0.55 per evaporated barrel of produced water. The capital build (in Texas) of the two most effective systems as compared to the replacement of a 15,000 bpd saltwater disposal well (approximately 5,500,000 bbls/ year) are as follows:
Both these systems would provide 15,000 bbl/day of saltwater disposal. The adder that a lot of people miss in their economics is the extra cost you would need to include for the disposal of the salts accumulated through evaporation. In enhanced evaporation, you can increase the TDS levels of water to a point where you essentially cut off evaporation.
There is a point in the process where it is no longer economically feasible for you to continue to evaporate, and you have to do something with the salts. We could use existing injection wells that would now be operating at a much slower injection rate, or we can dry the salts and haul them to a landfill.
In West Texas, 5,500,000 bbls/year of evaporated produced water would yield approximately 110,000,000 pounds of salt. Our industry is probably not willing to spend the cost to dispose of dried salts at this time. (The West Texas cost of 11,000 loads per year to the landfill is approximately $2.40 per barrel of water evaporated.)
If we don’t have the space or a convenient, cost-effective way to handle the disposition of salts to allow Mother Nature to conduct business in a manner that she is used to, we can force her hand by adding heat energy to boil off produced waters.
INNOVATIVE FORCED EVAPORATION SYSTEMS
There are several systems on the market today that can evaporate waters, eliminating the need for a saltwater disposal well or wells to dispose produced waters in a given geographical area. These systems continue to be improved upon, and the inventors in this industry have developed some unique methods of capturing heat from nearby industrial equipment and improving the efficiency of applying the required heat energy to water.
These “forced” evaporation systems offer unique opportunities that conventional enhanced-evaporation systems can’t take advantage of—the ability to recapture fresh waters for reuse in an area immediate to the process and the opportunity to use steam to power turbines for the production of power. These “forced” systems require more energy than the enhanced-evaporation system, but offer the user some opportunities to recapture costs or generate an optional revenue stream.
As the systems require more energy for operation, the operating costs are higher than the enhanced-evaporation systems, but the capital costs could be comparable depending upon location and infrastructure.
There are several systems on the market today that can evaporate waters, eliminating the need for a saltwater disposal well or wells to dispose produced waters.
The solids disposal requirements of these “forced" systems wouldn’t be any different at the landfill, but would require a continuous removal system as compared to a batch system that would be utilized when removing solids from large storage areas. In Part II of this article, we are going to compare the success/failure and costs associated with several case studies of alternative disposal methods.
In summary, we cannot stick our heads in the sand mimicking our methods of produced-water disposal and expect the ability to dispose of waters in oil-field injection wells to continue unhindered for decades.
As there are many alternatives to disposal available for the ultimate disposition of produced water to the industry, solely relying on evaporation is probably short sighted, and we need to be focused on a method or combination of methods that work best within our geographic areas, regulatory climates, cost structures and the physical/chemical properties of our waters.
Authored by Michael Neese