Reduce Wellbore Storage Time With a Downhole Shut-in

Pressure transient testing is a form of well testing that requires a flow period followed by a shut-in period on a well. The length of shut-in time can vary significantly depending on well and reservoir parameters, such as permeability. A high-perm gas storage well may only require a 1 hour shut-in whereas a low-perm tight sandstone well may require a month-long shut-in. The longer the shut-in, the more expensive the well test and thus anything that can be done to reduce the test duration without compromising the data can be really useful and budget-friendly.

Wellbore storage, which is the first flow phase upon shut-in, will dictate how long the shut-in must be. A well must be shut in long enough to get out of wellbore storage before any meaningful analysis to be done.

Wellbore storage time is a function of several parameters, including permeability, wellbore fluid, and wellbore volume. While we can’t change the permeability, we can reduce the wellbore volume and thus reduce the wellbore storage time. This is where a downhole shut-in comes in play. If we use a downhole shut-in, such as a bridge plug with gauges BELOW the plug, then we essentially reduce the wellbore volume to just that volume between the plug and the reservoir. This method can greatly reduce the wellbore storage time and thus reduce the overall shut-in time, saving time and money!

If you are curious and want more information about wellbore storage, ​check out this article on our website​.

Run Gradient Surveys Before and After a Well Test

Gradient surveys, which are more formerly known as "static pressure gradient surveys", are a very accurate and straight-forward way to determine the fluid level in a well. This can become a very helpful data point when analyzing a well test, particularly a drawdown/buildup test. A drawdown/buildup test is performed by flowing a well at a specific rate and/or pressure, and then shutting the well in to monitor the pressure during the "buildup", and this is typically done with bottomhole pressure gauges in the well. This test can reveal all kinds of secrets about the reservoir.

Now, because you are flowing the well, often at large flowrates, there is the potential that you could bring reservoir fluid into the wellbore. This fluid influx can be an extremely valuable data point. This is where gradient surveys come in. It is not good enough to only perform a gradient survey AFTER the test, because you don't know what the starting fluid level was before the test. There could have been fluid downhole before the test, or the wellbore could have been dry. The only way to know is to run a survey before and after, so that you can measure the net change in the wellbore during the test.

If you want to learn how to run a gradient survey, check out our Well Insights topic here.

You Fall to the Level of Your Systems

"You don't rise to the level of your goals, you fall to the level of your systems." - James Clear, "Atomic Habits"

Goals are great. I love goals. They give us something to strive for and they motivate us. I would encourage everyone to have goals at work and in life. The problem with goals is that we ask too much of them. Goals alone can't keep you motivated when things get really hard or you get really busy. The thing that determines whether or not we see our goals come to fruition over the long-term is our systems and habits.

Let's say you have a goal to workout and lose weight. That goal will be good to motivate you for a little while but eventually you'll get busy and it will be hard to stay motivated. What will keep you going is a consistent habit of going to the gym every day at the same time. If you can develop a good habit of going to the gym, then it won't be motivation that fuels you and keeps your goal alive, it will be your system.

The same can be said for organizations. Big ideas and motivational speeches are great to get everyone pumped up, but it's the commitment to a good, consistent system over time that leads an organization or a team to success. Just like a chain breaks at the weakest link, a team will fall down to the level of their poorest systems.

If you want to lead a more successful team, or if you want to see your organization be more profitable, focus on improving your systems. Make more efficient workflows. Create better channels of communication with your team. Create systems that run in the background, creating operational efficiency.

I am a systems junkie and I try to apply a good system everywhere I can. I have seen huge fruit in my life and my business by focusing on better systems.

If this has piqued your interest, go ahead and order a copy of "Atomic Habits" by James Clear. It's a fantastic book and an all time classic in my opinion.

Rule of Thumb - 2:1 Buildup to Drawdown Ratio

Those of you familiar with well testing may be aware of a "drawdown/buildup" test. For those not familiar, this type of test involves flowing a well to create a pressure "drawdown", then shutting in the well to allow for a pressure "buildup". If done correctly, the pressure response during the drawdown and buildup can be analyzed to determine important things like permeability, skin factor, average reservoir pressure, and even boundary conditions.

A really simple rule of thumb for well testing is that a buildup test should be about twice as long as a drawdown. This means that if you flow the well for 1 hour, you should shut it in for buildup for about 2 hours. Often, we reverse-engineer this when designing a test. If you want a larger radius of investigation and you determine that you need a 12-hour buildup, then you should plan to flow the well for 6 hours prior to the buildup.

This rule of thumb is all about data quality and getting the most out of your test. Nothing bad happens if your buildup is too short or too long. You just might have data quality issues. If your buildup is too long relative to the drawdown, you may see some strange things happen on your derivative. These strange things can easily be misinterpreted as reservoir effects, when in reality it's just a mathematical limitation to the pressure transients.

There are a lot of other things that go into well test design as well. If you need help designing a well test, be sure to call your friendly, local well testing specialists (aka FyreRok).

The Best Salesman is a Great Product

I am a very patriotic American, but when it comes to cars, I'm a firm believer that Japanese make the best cars (not including overpriced European luxury and sports cars). Toyota and Honda make cars that simply last longer than Ford and GM. Sorry if that offends you. I love Toyota vehicles to the point that I when I am buying a vehicle, I really only consider Toyota's. The irony here is that I don't have to buy vehicles very often because my Toyotas last forever.

My current vehicle, which I've been driving for almost 8 years, is a 2006 Toyota Tundra. It recently just crossed the 250,000-mile mark, and it's still running strong. I don't figure I'll have any problem getting 300,000 miles out of this truck. It's been a great vehicle and I'd buy it again in a heartbeat if given the opportunity. I'd also recommend that if you are looking to buy a truck, you should give great consideration to a Toyota.

My 2006 Toyota Tundra. She's got some bumps and bruises, but she's still running strong.

Now why am I rambling on about pickup trucks? It's because there is a great lesson in all of this. The lesson is that a great product trumps everything else. If you don't believe me, just look at Apple. They were the red-headed step child of the tech industry for a long time, and then Steve Jobs came in and turned the priority toward creating a great product. Within a matter of years the company skyrocketed and has been dominating their industry ever since. Great products create happy customers. Happy customers turn into product evangelists who tell other people to buy a product (I'm a Toyota evangelist, if you can't tell). If you want to sell something, focus first on making a really top-notch experience for your customers, whether that be a great product or a world-class service. If you do, your customers will do the selling for you.

Many of you are not trying to sell something, but I believe this lesson still applies. If you are an engineer working on reserve analysis, focus on creating the best analysis you possibly can. Don't get too caught up in trying to promote yourself to upper management. Rather, focus on doing the best possible work you can, and providing as much value to the people around you as you possibly can. If you do, the other things will largely take care of themselves.

Also, buy a Toyota. You won't regret it.

Level-Up Your VLookups with Index-Match

I'm a nerdy engineer, and therefore it should come as no surprise to everyone that I really like Excel. I suspect many of you are nerdy Excel-lovers as well. If you are an Excel-junky you've probably come to appreciate the VLookup function, which allows you to find a value in a data set, and then return an adjacent value in a nearby column. It's an awesome tool, but unfortunately it has some limitations. If you want to really level-up your VLookup game, you need to start using Index-Match instead.

What do I mean by Index-Match? In a nutshell, these are two individual functions in Excel, "Index" and "Match". Index gives you the value in an array based on it's coordinates, and Match tells you where in an array a specified value is located. When you combine these two into a single formula, you can achieve the same thing as VLookup, but without the limitations. In other words, you get a more powerful version of VLookup.

If you've never tried the Index-Match technique before, it can seem confusing at first, so I'll give you a quick guide. Your formula should look something like this:

=Index(location of value you want to return [array], Match(value you want to lookup [cell], where the lookup value is located [array], 0))

Use this formula to create really powerful lookup functions that blow VLookup out of the water.

Invest In Information

All of us have big, costly decisions to make. Make the correct decision and you can make your company more profitable. Get it wrong and you cost the company money. Decisions are important, and therefore, having accurate information to work with also becomes important. There is major economic value in good information, and therefore we need to see it as an asset, just like a piece of machinery. Good information is something to be invested in, not an expense to be cut out of the budget.

Let's briefly talk about the difference between expenses and investments. Expenses cost you money, and after the fact you have little or nothing to show for it. The money is gone and it's not coming back. You should do your best to reduce your expenses. Investments on the other hand make you money in the future. Sure, they cost you some money upfront, but (hopefully) that money comes back to you, with profit, in the future. Investments are not something to be eliminated, rather they are something to be carefully and thoughtfully planned out.

Invest in information so that you can make more informed decisions.

Be Critical of All Data

When analyzing data, be critical and ask questions. Data should make sense and it should fit into the bigger picture. If something doesn't make sense, figure out why. Sometimes data is wrong and has quality control issues. But sometimes the data is correct and it's trying to tell you that your assumptions are wrong. Engineering problems require fitting the data together like puzzle pieces. If a certain piece doesn't fit, figure out why. Don't just throw it out because it threatens to debunk your hypothesis.

If something doesn't make sense, ask yourself why. This is a good rule of thumb to follow in both engineering as well as life in general.

Plan For The Unexpected

Let's talk about the oilfield for a minute. The heart of the oil and gas industry takes place out in the field, where there are a lot of uncertainties. First, all the work is done outdoors, in all types of weather, which means that there is a massive force impacting our work, which we have no control over. Secondly, much of our work is downhole, where we can't see what is happening. Downhole is a harsh environment with high pressure, high temperature, chemicals, scales, and a variety of fluids. Finally, in the oil patch we like to move really fast because, well, there's usually a lot of money on the line. All of this together creates a recipe for unexpected events to derail our plans.

In my time in the oil and gas industry, I've seen all kinds of bizarre, unexpected things happen, some small and some major. In fact, this kind of thing happens so often that you actually come to expect it. You learn to expect the unexpected. Plans rarely go perfectly as planned and thus we should not be surprised when unexpected things happen. Despite this, I'm always amazed when I see projects that are planned so precisely that everything has to go perfectly or else chaos ensues.

What does this mean? Well, it means that our plans should include some margin of error. When you go out to the field, pack some extra tools just in case. If you expect a project to last 5 days, plan for 7. Add a little extra money to your budget just in case. The worst thing that happens is that things go as planned and your margin of error goes "unused". If you do this, I promise you that you'll be a lot less stressed out. You can thank me later.

Remember, it's the oilfield and nothing goes as planned. Plan accordingly.

Use Temperature Sensors At The Wellhead

I spent the first portion of my career in production and I got pretty good at diagnosing and fixing production issues on gas wells. One thing I learned is that you can save yourself a lot of money and headache by identifying and fixing production issues early, before they grow and create downtime. The primary goal of every production team is to keep the wells flowing as much as possible. Measuring temperature at the wellhead is a very practical way to identify potential production issues downhole, such as salt or scale rings. These things don't form overnight and thus they can be really difficult to identify. If you have a gas well, temperature can be a really helpful diagnostic tool, because any flow restriction cause a temperature drop. If that restriction is downhole, then the temperature of the gas at surface will drop.

I am a big fan of measurement and if I was in charge of a production site I would have sensors everywhere. I would have sensors sensoring the sensors just to make sure the sensors don't stop sensoring. That's a little absurd but you get my point. Sensors provide data and data informs us of what's going on so that we can make educated decision quickly and accurately. I get that sensors cost money, but lost production costs a lot more.

A temperature sensor at the wellhead is a really practical way of identifying flow restriction downhole, quickly, so that you can get the well back to full production potential right away.

I'm not a production guy anymore, so I don't have a dog in the fight. But if I were, I'd have temperature sensors on my wellheads.

A Helpful Formula For Gas Hydrostatic Pressure

Calculating the hydrostatic pressure of a liquid column is straight-forward. In case you need a refresher, the equation is:

P-hyd [psi] = 0.052 * (Fluid Density [ppg]) * (Fluid Depth [ft])

The above equation calculates the exact hydrostatic pressure of a liquid column. But what about for a gas column? The answer is far less straight-forward because the gas is compressible and therefore the density is changing with the depth. There are iterative methods for calculating the hydrostatic pressure of gas, but these methods take some effort. With that said, someone once showed me a simple formula to estimate the hydrostatic pressure of gas, and it gets you surprisingly close to actual data. Here it is:

P-hyd-gas (psi) = 0.25 * (Depth [ft]/100) * (Surface Pressure [psi]/100)

The equation above works for 0.6 gravity gas and if you compare it to actual bottomhole data you will find that it's remarkably close.

Thank A Geologist

A major part of being a petroleum engineer is to help make good wells. While there are things you can do to make wells better, perhaps the biggest factor is something that is out of our control as engineers: rock quality. Good engineers make good wells into great wells, and bad wells not so bad. But even the best engineers can't turn bad rock into good wells. Good wells come from good rock, and there's really no exception to that.

If you are an engineer and you find yourself working with good rock, thank your geologist. Buy them a pet rock for Christmas. Show your appreciation by listening to them enthusiastically rant about things like kerogen, thermal maturity and vitrinite reflectance (smile and pretend you share in their appreciation for these things).

With that said, if you are an engineer working with bad rock, you still have a job to do. There is always optimization that can be done. Keep up the good work and keep optimizing and improving and making things better. It's what we do as engineers - it's just more fun when the geologists give us good rock to work with.

Thank you to all the geologists who gave me good rock to play with throughout my career.

Check Your Units!

We've all made the mistake of getting units mixed up. For example, putting feet into an equation that requires inches. Or mixing up Rankine and Kelvin when inputting temperature. It's super easy to make this mistake, but it can cause you a lot of headaches.

A real common one for petroleum engineers is to mix up PSIA and PSIG. It's only a difference of 14.7 psi, but it can mess up your calculations. If you see something that doesn't quite add up (literally or figuratively), go back and check your units. Especially is your pressure estimates are mysteriously about 15 psi off. I'm speaking from experience on this particular one.

If you want to see an example of how mixing up units can go very wrong, look no further than NASA's Mars Climate mission of 1999. If you've not heard of this, check out this 1 minute video:

The moral of this story is to check your units. In fact, you should probably double check them just in case.

Good, Fast and Cheap - Pick Two

Today's tip is simple, yet hard. When you are making purchasing or managerial decisions, you have three primary factors: good, fast and cheap. Unfortunately you are rarely in a situation where you can have all three. Typically, you can only have two of the three, with the third factor acting as a sacrificial lamb.

Let me give you a few examples:

You can have fast and cheap. I like to call this the "fast food option". You get it fast and it doesn't cost much, but you definitely sacrifice food quality.

You can have fast and good, but it probably won't be cheap. This is more like the "emergency open heart surgery option". It'll be fast, and hopefully the surgeon is really good, but I can promise you it won't be cheap. If it is cheap, run away!

Finally, you can have cheap and good, but it won't be fast. A classic example of this is DIY home projects. Assuming you have the skills to do the project well, you can save a lot of money by not hiring a professional. But between working a full-time job, and all of life's other commitments, these projects tend to take a long time to complete. I can speak from experience on this one!

The big takeaway with this is that if you try to get all three: good, fast and cheap, you will find yourself very frustrated and disappointed. You may get promised all three, but you probably won't get all three in the end. Pick the two that are most important and sacrifice the third.

Is there a decision where you are struggling to find all three? I take some time to evaluate which two elements are most important be willing to sacrifice the third.

Try To Avoid Packers Downhole

As you may or may not know, I spent the first third of my career so far in production. In that time I learned a lot about producing oil and gas wells, and one thing I learned is that downhole packers (between casing and tubing) really have a way of limiting your production options. The reason is that it cuts off your access to the annulus, thereby leaving your only downhole access through the tubing, which is where your reservoir fluids are being produced up. When a well has a packer, there is no easy way to provide continuous chemical injection into the well. This means that soap, corrosion inhibitor, salt inhibitor, and a whole slew of other valuable production chemicals cannot be easily injected into the well without shutting in the well. This leads to expensive capillary strings and/or other non-desirable options. For this reason, I don't like packers and I recommend trying to avoid them.

With that said, packers do serve a purpose and sometimes they are necessary. In these situations you will obviously have to install them and deal with the consequences. However, I've seen far too many times where a packer is installed as a part of the completion process, with no regard to the production impacts. I think this is silly. Production is the only point where you make money on a well, and therefore great consideration should be taken when making drilling or completion decisions that will negatively impact production. My advice is to use packers where absolutely necessary, but nothing more.

For those of you in drilling or completions, this may sound silly. Just trust me on this. Your production folks will appreciate it.

Don't Let Yourself Run Out Of Ideas

"We usually find oil in a new place with old ideas. Sometimes, we find oil in an old place with a new idea, but we seldom find much oil in an old place with an old idea. Several times in the past we have thought that we were running out of oil, whereas actually we were running out of ideas.” - Parke A. Dickey, 1958

Mr. Dickey made this quote 64 years ago, but it seems to fit perfectly with the present day. I remember being told in high school that the world was running out of oil and gas and there was no future in this industry. Then some innovative people figured out how to combine horizontal drilling with high-volume hydraulic fracturing, and BOOM, the shale revolution started! As it turns out, we weren't running out of oil and gas, we had just run out of ideas. When we embrace innovative and outside-the-box ideas, it's amazing what we can come up with.

Today’s advice is simple: be creative, think outside-the-box, and never let yourself run out of ideas.

Run A Flowing Gradient To Identify Liquid Loading During Production

You may be familiar with a static pressure gradient survey, more commonly referred to as a "gradient". The process for a gradient is simple: run in the well with bottomhole pressure gauges and record the pressure at various depths downhole. From the data you can determine both the fluid depth and the fluid density in the well. Typically this is done with the well shut in, hence the term "static". While this is helpful information to know, when we talk about liquid loading, our concerns are usually related to the production of the well. Therefore liquid loading issues tend to impact us more when we are flowing the well, not when we are shut in. This is where a flowing gradient can be helpful.

As a well flows it pushes fluid up the hole, creating hydrostatic pressure. Even wells that are unable to lift fluid to surface, still lift the fluid at least part way up the hole. When the well is shut in, the fluid then falls back down to some equilibrium point. This problem is amplified in horizontal wells because the fluid tends to fall back into the lateral when the well is shut in. Tubing that was filled with fluid during flow becomes completely empty within minutes of shut-in. If you were to run a static gradient survey on a horizontal well (without a standing valve), you would likely find no fluid in the tubing. Your static gradient survey would not be a good representation of your liquid loading issues. In this situation a flowing gradient would be much more insightful.

If you have a well with liquid loading issues, consider running a flowing gradient survey rather than a static gradient. You might be surprised at what you discover about your well when it is flowing.

If you are interested in learning more about gradient surveys, click here to check out our Well Insights on this topic.

Ask Stupid Questions First

Have you ever found yourself totally overthinking a problem? Maybe you are trying to solve a problem with really complex thinking and when you finally arrive at the solution you found that it was really a simple thing. Has that ever happened to you? No? Me neither. Engineers never overthink things.

Actually that's a lie. Engineers do this ALL THE TIME! I am no exception. Let me tell you a story.

One time I was on a completions team of mostly engineers and we were trying to figure out why the wellhead bolts appeared to be loosening during frac stages. Here's the background: A wellhead contractor would torque the wellhead bolts to spec. Then we'd frac a stage. Then they would torque the bolts again, and find that the bolts would move slightly, indicating that the bolts were not as tight as we thought. This lead to an assumption that the bolts were loosening during the frac stage. In a matter of hours we came up with all kinds of crazy theories about thermal contraction from the cold water pumping through a warm wellhead. We really thought we were onto something big, and kind of scary. Our problem however is that we forgot to ask the stupid questions first.

As it turns out, the bolts were not loosening at all. The appearance of loosening bolts came from the fact that the wellhead contractor had a crew change, which meant the new crew used a different torque wrench that was calibrated slightly different. This means that when they torqued to 700 ft-lbs, they actually got 705 ft-lbs, thus causing the bolts to move a tiny bit. It turns out the there really was no problem at all and the whole thing was giant, meaningless fire drill. The best part of the story is that the person who found the solution was the only guy on the team who was NOT an engineer. Thus he didn't overthink the way that engineers tend to do. Every time I look back on this story I laugh.

Here's my point: start with the simple solutions first. Ask silly questions and rule those out before moving on to more complex theories. This will often help you solve your problem more quickly and you're less likely to feel like an idiot at the end.

Understand The Limitations Of Echometers

Echometers are a neat tool that uses sound to determine (among other things) the depth of fluid downhole in a well. Similar to radar or seismic, an echometer sends a sound pulse down a well and measures how quickly that sound bounces back. Coupling this data with some math, we can estimate the depth of fluid in a well. It's a cheap, fast and neat way to answer the question, "How much fluid is in my well?" With that in mind, it's important to understand that echometers have limitations. The fluid level calculated by an echometer has some error in it. I've shot echometers on the same well, back to back, and received different fluid level calculations, sometimes off by 50 feet!

This is not an indictment on echometers, but rather my point is to say that you need to understand the limitations of this technology and only use it when appropriate. If you are simply trying to get an estimate of fluid level, or perhaps just trying to figure out if any fluid is downhole at all, then an echometer is a great tool for this application. However, if you are trying to determine your bottomhole pressure for a reserve analysis, then an echometer is simply not going to cut it. In this case you need a lot more accuracy than +/- 50 ft of fluid. If this is the case, then I'd recommend running bottomhole gauges in the well and performing a static pressure gradient, which will result in a much more accurate pressure measurement.

In short, echometers are a cheap and easy way to estimate your fluid, but it's important that you understand their accuracy and make sure sure you are using the appropriate tool for the job.

Don't Assume All Skin Is Due To Damage

I'm sure you've heard the mantra before, "What happens when you assume? You make and @$$ out of U and ME." That has stuck with me, partially because I've been burned more than once for making silly assumptions, and I'm sure I'm not the only one. We could probably have an entire Engineering Tip just on the topic of assuming, but today I want to get a little more specific.

For those who are not aware, when a petroleum engineer talks about skin, they don't mean the stuff that covers your body. They are talking about "skin on a well", which is a measurement of how good or bad a well is performing relative to it's maximum potential. Simply put, more skin is bad, less skin is good. And we calculate skin through well testing.

With that in mind, here is where I'm going with all of this. When we test a well and get a skin factor, it's easy to assume that the skin is due to damage and that the skin can be removed with a simple workover treatment. While that may be true, there are other forms of skin that can occur on a well, and a simple acid treatment might not due the trick. Other factors of skin include partial penetration, spherical flow, turbulence, inclination and perforations. When you see skin on a well, it's important to take a deeper look and try to understand what might be causing that skin. If you have a well with partial penetration, an acid treatment is not going to solve the problem. The only thing that will remove that skin is deepening the well, which may or may not be practical.

My point here is this, damage is a major factor that contributes to skin on a well, but it's not a guaranteed that all skin is caused by damage. Take a deep look at things and see if there is a bigger issue going on. If not, then by all means go forth and remove the damage. Just don't assume, or you may find yourself haunted by the voice in your head asking, "you know what happens when you assume, don't you?"

If you are interested in learning more about skin on a well, check this out.