A cradle-to-grave approach in well completions
Completion of an oil & gas well is a critical part of the well construction process. In this operation, a tubular string, along with associated hardware and fluids, is installed into the wellbore to serve as a well barrier and to transport hydrocarbons from downhole to the surface.
The harshness of the downhole environment into which the string is installed and the criticality of the barrier that it provides often necessitate the use of specialized equipment and procedures to avoid a reduction in the string’s service life, or worse, a loss of well integrity. Advances in technology and in our understanding of the downhole environment provide us with the opportunity to further enhance well integrity and extend the life of wells.
Let’s look at some current philosophies in this regard and present ideas on how the industry may improve upon them.
Completion tubulars and well integrity
Well integrity is a phrase not taken lightly in the oil & gas industry. It can be defined in many ways, but at its core is the desire to prevent the uncontrolled release of well fluids (e.g., oil and gas) that may have a detrimental impact upon the safety and wellbeing of people, property and environment. This is achieved through the proper application of technology, knowledge and procedures aimed at preventing this from ever occurring.
Unfortunately, despite our efforts, well integrity incidents do occur. One industry-sponsored study found that 18% of wells drilled encountered well integrity incidents with most occurring in years five through nine of well life. The cause of these failures was identified to be the tubular string in nearly 40% of recorded incidents. As a result, the tubular string warrants significant attention when it comes to preserving well integrity.
To complete a well, a tubular string that is specially designed for the task is installed as the innermost element, as shown in Figure 1. The string is installed by connecting each individual tube to the rest of the string through premium threaded connections. Once installed, the completions tubular string plays a pivotal role in safe and successful operations.
First, it provides a communications path for hydrocarbons to flow from the downhole pay zones to the surface. This downhole environment and the transported fluids are often highly corrosive and thus require the completions string to be produced from corrosion resistant alloys (CRAs).
As a well barrier, the tubular string also protects the surrounding well elements from corrosion and contains the fluids which may be produced at elevated pressures and temperatures. As history has shown, a failure of the completions tubular string, be it a failure of the tube body or the threaded connections, may have devastating effects upon the overall integrity of the well.
Life of a CRA tubular
CRAs often are used in oil & gas completions where their unique ability to resist harsh well environments makes them the ideal choice for tubular construction. All CRAs begin life at a mill during primary metal manufacturing. From there, the life of a CRA tubular may follow a long and circuitous route to its ultimate end, as shown in Figure 2.
After manufacture, the tubular is shipped from place to place, threaded and coupled, placed into storage, installed, and potentially removed before being reworked and placed back into storage for eventual re-use. This culminates in the disposal or recycling of the tubular once its useful life has drawn to an end.
At each point along this path, opportunity exists for damage to the tubular. Damage may be physical or metallurgical in nature and while this damage may not appear to affect anything more than aesthetics, the integrity of the tubular may be compromised under certain conditions.
So, what are the conditions that potentially can compromise a completions string? First, failure may be purely mechanical, such as the improper make-up of the threaded connection joining adjacent tubulars within the string. This may arise as a result of an incorrect evaluation of connection integrity, which when combined with the conditions of the downhole environment, lead to a leak from the connection. Or, failure may be the result of corrosion.
Downhole environments are known for high pressures and high temperatures (so-called HPHT wells) and often contain corrosive species such as H2S and CO2. Despite having high corrosion resistance against these conditions, CRAs still may be prone to corrosion if proper care and handling procedures have not been observed.
The procedures for care and handling of CRA completion strings vary across the industry, considering such factors as alloy, well environment and intended use. A few precautions are common to most procedures. These include minimizing or eliminating physical damage (e.g., die and insert impressions, scratches), minimizing contact with non-CRA metals (e.g., pipe racks, wire rope slings) and adhering to detailed connection make-up criteria.
A large portion of our current efforts in preserving string integrity is focused upon the installation and removal processes. During installation and removal, load is transferred to and from the string via toothed dies and inserts. The impressions left behind in the CRA tubulars by the teeth have been well-documented as preferential initiation sites for corrosion, especially stress corrosion cracking (SCC) and sulfide stress corrosion cracking (SSCC), an example of which is shown in Figure 3.
So strong is the causal link between tooth impressions and corrosion initiation that it is common practice to use low-marking dies and inserts, or even completely nonmarking technologies, when installing or removing CRA completions strings. The anticipated impact of these impressions often is determined through material qualification testing performed during the planning stage.
Precautions to prevent material transfer to the CRA tubulars also are commonly observed. These include storage in specially outfitted pipe racks designed to eliminate metal-to-metal contact, the use of fabric lifting slings instead of wire rope, and even the use of non-ferrous gripping elements or nonmarking equipment. Such measures are aimed at preventing the transfer of iron and carbon to the surface of the CRA where their deposits also can provide preferential sites for corrosion initiation.
Connecting, or “making-up,” of the premium threaded connections also is afforded significant attention. In this process, data are acquired from the torqueing equipment and are displayed graphically as a plot of torque versus turns. Upon completion of the make-up, a technician evaluates the graph by applying pre-established acceptance criteria along with experience and judgment. He or she must disposition the connection as acceptable or rejectable, diagnose the cause if rejectable, and carry out correction if necessary. Failure to do so accurately and reliably can lead to improper make-up of the connection with serious implications on well integrity.
A cradle-to-grave philosophy is one that considers all interactions with a subject from the moment of its birth to the moment of its death. For a completions string, this requires accounting for all interactions with the string from its creation at the mill to its eventual disposal. Adopting such an approach provides us with the opportunity to extend the service life of completions strings while simultaneously improving well integrity.
Starting with an overview of the CRA tubular life cycle, we again look to Figure 2. This figure is but one of many potential paths a completions string may follow throughout its life. Regardless of the exact path taken, it is apparent that there are many touch points involved, each with the potential to compromise the string. Our initial goal should be to reduce the number of touch points. This may be achieved through consolidation of processes, thereby reducing transportation and handling requirements.
The impact of each touch point also may be reduced or eliminated by applying the same care equally throughout the lifecycle. For example, as has been mentioned, much of our focus historically was placed upon installation and removal. There, the use of low-marking and non-marking technologies often is required to address the negative effects of tooth penetration impressions on the tube body.
During these operations, much more aggressive gripping appliances may be used, leaving behind deep tooth impressions in the tube body and the coupling. It is obvious that if we are concerned with the tooth impressions imparted during installation and removal, then the same concerns and precautions should be considered in all operations, including threading and bucking.
One might assume that material qualification testing would include potential effects from all operations in the tubular life cycle. Instead, testing often is limited to determining only the effects of tooth penetration imparted during installation and removal, and in some instances, even this is omitted.
As a result, qualification testing may not always provide a true representation of what is being installed in the well. With advances in corrosion testing, we are now able to assess even the smallest effects upon tubular integrity. These new methods which include electrochemical corrosion testing, as shown in Figure 4, provide quantitative results, allowing us to determine such parameters as corrosion rate and resistance to polarization. The data obtained also can be fit to corrosion models to evaluate the lifecycle of the tubular and corresponding reductions to the useful life as a result of various factors.
Opportunities also exist to improve integrity at the threaded connection. Recent, successful deployments of Big Data analytics and machine learning technologies for connection evaluation (e.g., Frank’s International’s iCAMTM technology) have realized significant improvements in accuracy and reliability over human technicians. This effectively removes subjectivity from the process and generates results that are more consistent with optimal parameters. Incorporation of process automation into the overall connection make-up operation provides even more benefit with gains not only in consistency, but also in efficiency, cost savings and safety.
Looking to the future
As we look to the future, it must be with the understanding that the majority of the “easy oil” has been found. Operators consistently are targeting reservoirs that are considered more complex because of their depth, higher downhole pressures and temperatures, or the corrosive nature of the downhole environment. As the industry focuses more on these wells, a more comprehensive cradle-to-grave philosophy should be implemented to ensure that the highest levels of integrity are maintained throughout the service lives of the wells.
There is yet another question that should be asked. Can we extend service life even further, adding directly to the bottomline value proposition of the well? The answer is yes. This will require innovative solutions that go beyond traditional methods and practices of the past. The digital revolution that currently is enveloping the industry will provide the necessary foundation that will allow technologies once thought not applicable to the oil & gas industry to thrive.
The increased acceptance and implementation of automated and semi-automated processes will increase consistency, repeatability and reliability throughout the well construction process. Big Data analytics and visualization will allow us to capitalize on the innumerable amount of data created every day. Artificial intelligence and machine learning will identify trends not readily perceptible to our human minds and provide more accurate and consistent data-driven decisions and recommendations.
Looking at these technologies from a holistic viewpoint starts to paint the picture of an immersive, intelligent rig environment that will increase well integrity, while at the same time increasing repeatability and reliability. This will have the added benefits of lower costs and higher efficiency during the entire well construction process.