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.

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.

Run More Efficient Gradient Surveys

Engineering Tip: Limit Your Gradient Surveys To 4 or 5 Stops​
Static pressure gradient surveys are a great way to understand your hydrostatic pressure. The idea here is to run in the hole with downhole gauges and record the pressure at various depths. This information can give you some key information about your bottomhole pressure, fluid level, and fluid density/pressure gradient. This information is extremely useful when optimizing production from a well.

The mistake many people make is that they run a lot of unnecessary gradient stops. You can calculate everything you need from four survey points, two in fluid and two in gas. A lot of people run gradient surveys with 7, 8 or 9 survey points. This is a bit of a waste of time as there is no additional information gained by all of those extra survey points. The only thing you gain from all of those extra survey points is a larger invoice!

Ideally, I like to run a gradient survey with 4 or 5 five-minute stops. It looks like this:

Point 1: TD minus 1 foot (fluid)

Point 2: TD minus 25 feet (fluid)

Point 3 (optional): TD minus 50 feet (fluid) - gives you one extra fluid point just for consistency, but not necessary

Point 4: Half way between surface and TD (gas)

Point 5: Surface (gas)

The survey points above will allow us to calculate both a fluid gradient and a gas gradient, which tells us everything we need to know. Running 7, 8 or 9 survey points does not add anything to the final calculation, it just adds time and money.