Cloud Storage and Data Analysis Put Valuable Information Into the Hands of All Injection Well Operators
Every year, the Environmental Protection Agency performs a survey of injection wells nationwide. The most recent publicly available numbers from the EPA’s Underground Injection Well Inventory listed almost one-half million injection wells across the United States in 2016, and that number continues to grow.
Injection wells are deservedly hailed as the safest and most practical method to handle the massive amounts of wastewater brine produced by the oil and gas industry. They are also growing in popularity as a cost-effective and environmentally friendly option for handling challenging liquid municipal wastes as well.
While the immense usefulness of injection wells in waste management can hardly be overstated, if they are to live up to their promise, they must be carefully monitored. Regular routine monitoring, coupled with a rigorous understanding of injection science, is the only way to assure the long-term integrity and safety necessary for safe storage of wastewater brine and simultaneous protection of drinking water sources.
OILFIELD INJECTION WELLS
The history of injection well technology dates back into the 1930s. While the highly specialized and controlled wells of today bear little resemblance to those early experiments, the basic concept remains the same.
Injection wells send waste fluid or slurry underground into naturallyoccurring, porous rock formations. Injection wells serve many purposes across several sectors, although their use is most common in the oil and gas industry. Generally, oil and gas industry injection wells complement oil production and help manage of oilfield wastes.
In older oilfields, residual oil and natural gas are recovered using an injection well. Typically, injected fluid displaces or thins remaining oil and gas to make it available for extraction. Injection fluids may consist of freshwater, brine, steam, polymers, inert gasses or carbon dioxide. Often, an injection wells are located in the middle of a production field to help bring up residual oil and gas that might otherwise go unrecovered.
The enormous volume of waste, particularly liquid wastes, created by oil and gas production imposes significant costs on an oil and gas operator. In fact, the limiting factor on total recoverable reserves in a hydrocarbon reservoir is the cost of lifting and disposing of produced water. Well completion flowback, produced water, naturally occurring radioactive material (NORM) as well as drilling muds and cuttings, all require specialized treatment and disposal solutions.
Ideally, waste liquids are collected, treated, and recycled to be used again in drilling and hydraulic fracturing. However, there are always some volumes of wastewater and waste materials to be contained and they are best contained in secure disposal injection wells.
The majority of oil and gas waste injection wells are operated to dispose of filtered produced water. However, a small number are developed to handle wastewater, spent drilling muds and solid wastes such as drill cuttings. Here, materials are blended to create a slurry which can then be injected into a disposal well where they are contained in fractures, caverns, karsts, or other underground spaces.
PROTECTING THE ENVIRONMENT
Properly managed injection wells offer a wide range of environmental benefits. Unfortunately, they also can evoke unwarranted suspicion among the public.
Injection wells have been associated in the public with induced seismic activity and drinking water contamination. Like many complex subjects, injection well risks may be overblown or misrepresented in the news media, leading to mistrust and opposition in a community.
A fact remains, however, that when things go wrong with an injection well, particularly one that manages hazardous wastes, real threats to resources or public health are present. For these reasons, operators must make environmental and corporate responsibility paramount.
With the potential hazards to public health and the environment inherent in operation of an injection well, federal, state and local regulators hold operators to high standards for safety and compliance. Regulatory enforcement may vary from one region to another, but the cost of non-compliance can be steep and can extend beyond notices and fines. Expensive civil lawsuits, damage to a company’s brand, or even revocation of its license to operate are potential penalties for non-compliance.
WELL TESTING IS CRITICAL
Testing of Injection wells is critical at all stages, from well construction through ongoing operations. A wide range of tests offer valuable data about what’s happening underground. These data, when paired with modern analysis techniques, provide engineers and operators valuable insights into an injection well’s integrity, safety, and performance. In addition, testing can offer a glimpse into a well’s future conditions with the help of advanced modeling technologies.
TYPES OF TESTS
Leak Off Test (also known as Pressure Integrity Test or PIT)
• When: During well construction.
• What: Measures the strength of an open geologic formation and its “fracture pressure” or “fracture gradient” before a well is completed.
• Why: Leak off tests are integral to the safe drilling and operation of an injection well. The pressure at which fluid enters the formation informs the operator of the maximum injection pressure or maximum mud weight while drilling without circulation loss from excessive leak off.
• How: The well is shut-in and then fluid is gradually pumped downhole. As fluid volume increases, wellbore pressure on the geologic formation increases. At a certain pressure, some fluid flows into the rock, which is known as the “leak off.” If high enough, fluid pressure will fracture the rock, creating new pathways as seen in injection test pressure trends.
Step Rate Breakdown Test (also known as SRT)
• When: During well construction.
• What: Measures the injection rate versus the pressure variation.
• Why: Step rate testing is the standard method of finding the maximum injection pressure without fracturing the formation. If the formation is already fractured, the test can determine fracture closure pressure. Testing can identify the ideal injection parameters, including injection pressure and flow rate.
• How: The injection well is shut-in or stabilized at a reduced but constant injection rate. Next, a series of constant-rate injections begins. Each injection is of equal duration, but the rate of injection is increased incrementally from low to high.
• When: At completion of a new well, or after a well is operated at constant flow rate after a long shut-in.
• What: Injectivity tests indicate of how much fluid an injection well can hold, also known as the “total permeability” of the well, expressed as an “injectivity index rating.”
• Why: Understanding the permeability of a well can help determine well productivity, or if stimulation is needed to improve permeability of the underlying geologic structure.
• How: A pressure gage is lowered to a major permeable zone within the reservoir. Water is pumped into the well at a series of rates, increasing the rate successively each time. Well pressure vs. time is recorded and each pumping rate is held constant until the pressure stabilizes. At each pressure stabilization, the next pumping rate is begun and held constant until a stable pressure is reached. After three or four sequences, pumping stops and the well is returned to its natural pressure level. Results are graphed into an injectivity index rating.
Pressure Fall-off Test
• When: Systematic pressure fall-off tests are conducted annually or biannually but can also be done after each injection. It is good practice to measure fall off of pressure at the end of each injection to collect data on pressure transients.
• What: Measure variations of surface pressure and/or bottom pressure after fluid injection stops.
• Why: Data from pressure fall off tests can be used to determine many important well properties. See the section on Pressure Transient Analysis below for more detail.
• How: Water is injected at a stabilized rate for a period of up to 24 hours. The well is then shut in and pressure fall off rate is recorded until it is stabilized.
PRESSURE TRANSIENT ANALYSIS
Pressure transient analysis looks at data gathered from injection well tests, such as the fall off test described above. Pressure transient analysis can be performed whenever there is a sudden change in the injection rate (like shutting-in the well), assuming pressure data is collected. For example, after an injection in a mini fall-off test, analyze data for linear and radial flow to determine fracture and formation properties.
Such properties include:
• Fracture closure pressure
• Fracture dimensions
• Formation conductivity
• Fracture linear flow
• Formation permeability
• Reservoir pressure
• Reservoir boundaries
• Fracture dimensions (length, height)
• Fracture storage
• Fracture conductivity
• Fracture face skin
• Total skin
• Size of a damaged inner zone with secondary fractures (if it develops)
The results can be a valuable source of information for operators and help engineers understand how a well is performing and whether it may be at risk.
REAL-TIME DATA MONITORING
Periodic testing during the life of a well is indispensable. Taken on their own however, such tests are not enough to offer an accurate picture of a well’s overall performance. Constant monitoring is key to predicting problems before they happen. With real time monitoring, operators can detect and assess potential problems, including fracture height growth potentially leading to containment loss, or damage to reservoir permeability which could lead to capacity loss. Even if test results point to a significant issue, real-time monitoring buys time for a more effective response.
Injection data monitoring and analysis is most commonly performed in the field, which limits its usefulness. Instant visibility into well safety and health is provided by remote analysis which can be performed anywhere and is considered a best practice. However, serious challenges to remote analysis, such as data transmission and access, must be overcome. Until recently, remote real-time monitoring was limited to a small number of remote Real Time Monitoring Centers (RTMC) but that is changing.
To remotely monitor an injection well in real time, the operator needs:
• The ability to stream live data from field sites to a centralized data repository.
• The ability to save data streams to a dedicated data server for technical analysis, ideally from Internet enabled devices.
• Enough computing power and analytical software to display real-time data streams while performing necessary analysis.
The cost and logistical demands of remote real-time monitoring have traditionally been too costly for smaller independent operators, but the landscape is changing. Innovations in technology are making these tools available.
HOW IT WORKS
Injection well field sites are often data-rich environments, with measurements being taken in real time on site. Such measurements include:
• Flow rate
• Surface treating pressure
• Annulus pressure
• Injection fluid density
• Injection fluid viscosity
• Solids concentration
• Bottomhole injection pressure (with a downhole gage)
The real-time monitoring involves observing and analyzing data collected during both injection and well shut-in. Monitored injection-pressure and -rate data provide the basis for pressure fall off analysis. Fall off analysis can:
• Evaluate injection formation pressure and stress change.
• Estimate formation permeability and skin factor.
• Determine injection-induced fracture geometry at the end of each slurry injection batch.
Injection rate and pressures must be monitored closely during injection so if abnormal changes are observed, corrective actions can be taken. A sudden or gradual increase in injection pressure may indicate well or fracture plugging, which could indicate well integrity problems or violate permitted values.
Similarly, an abnormal decrease in pressure could indicate a well leak or fluid breaking out of the permitted zone and therefore, loss of containment. Analyzing collected fall off data during well shut-in helps determine crucial reservoir and fracture parameters necessary for safe injection operations and assurance. If fall off analysis yields an unanticipated fracture geometry, advanced fracture simulations can model fracture growth in three dimensions under actual injection conditions.
WELL MONITORING SOLUTIONS
The insights provided by real-time remote monitoring are so valuable that it should be widely available to operators of all sizes and budgets. Fortunately, recent innovations have placed this technology within reach of most operators.
The remarkable advancements in cloud storage and computing in recent years open the door to new levels of computing power. It is now a cost effective, flexible option for storing data and the computational resources needed for real-time analysis. The cloud, with the help of webbased applications, now makes it possible for users to access data on demand, anytime, anywhere, from any device with Internet access.
In the past, dedicated network data servers, locallyinstalled desktop software, and expensive remote monitoring centers were required to collect and parse data streams from field sites. Now, such processes are being replaced by light, convenient web applications powered by the cloud. These applications are platform and hardware independent and accessible through a browser using any web-enabled device such as a phone, tablet or laptop.
Remote, real-time injection well monitoring solutions are available to even the smallest operating companies. For example, Advantek's @SSURE® is a real-time monitoring and analysis web-application that provides access to real-time injection data and analysis from anywhere. Also, the company’s @SSURE® for Surveillance product monitors actual injection events in real time and uses guided logic to generate automated alerts to prompt engineering interventions ahead of troublesome operations. It can also mine historical injection data to improve predictions of operational upsets under various injection conditions.
A CRITICAL TOOL
Robust injection-well testing and monitoring are vitally important. Remote real-time data analysis also offers a critical tool for assurance of fracture containment and well integrity. Such results make it possible to optimize injection parameters, maximize injectivity, extend well life and minimize risk of creating unexpected fractures.
Breaches, while rare, can have potentially catastrophic consequences for communities and the environment Additionally, a catastrophic failure exposes the operator to significant legal and financial liability if regulatory obligations have not been met.
Conversely, a good testing protocol is much more than a defensive strategy. When injection wells are monitored and the data analyzed in real time, operators can detect early warnings and alert on- and off-site staff of operational upsets ahead of time. This helps minimize disruption and stem any resulting financial losses.
Data carries gifts for those with the ability to see them. It is possible to reap the rewards, both operational and financial, of safe, optimized injection wells with proper testing and analysis of field data in real time.
Authored by Omar Abou-Sayed