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5 THINGS TO CONSIDER WHEN PLANNING A DFIT

A Diagnostic Fracture Injection Test (DFIT) is an important type of well test that is used to determine key fracture and reservoir properties. A DFIT is performed by pumping a small amount of fluid into a well, typically about 20 bbls, and monitoring the surface and bottomhole pressure for a period of time after shut-in. By analyzing the bottomhole pressure in the hours, days or weeks after injection, you can gain an understanding of in-situ stress, initial reservoir pressure, and in some cases, permeability. This information is critical for creating accurate frac designs, reservoir models, and decline curves.

The goal of any DFIT should be to get the most accurate analysis possible, and an accurate analysis requires accurate data. A little planning goes a long way when it comes to obtaining accurate data.

With that in mind, here are 5 things that should be considered when planning a DFIT:

1) What is the test objective?

This may sound obvious, but you’d be surprised at how many well tests are performed with an unclear objective. Clearly understanding your objectives, and more specifically, the parameters you hope to gain, will give you clarity as to where to spend your budget and where it is appropriate to cut costs.

For example: If you are primarily interested only in breakdown and fracture pressure, then it may make sense to run your test with only surface pressure gauges. Afterall, when you frac a well, you are typically only measuring surface pressure. In this scenario, the surface pressures are what you are most interested in anyway. However, if your objective is to understand your pore pressure so that you can have more accurate decline curves and EURs, then it makes a lot of sense to spend the extra money for downhole gauges. Trying to gain a precise measurement of pore pressure from surface gauges is difficult at best, and usually results in inaccurate calculations.

Clearly defining your test objectives upfront will help you make better decisions that will result in a more meaningful test analysis.

2) What is the anticipated maximum bottomhole pressure?

Your maximum pressure will occur at formation breakdown. Bottomhole pressure will be the surface breakdown pressure plus hydrostatic pressure. In deep, high pressure formations, bottomhole pressure can peak at extremely high pressure during breakdown, sometimes in excess of 15,000 psi. If you plan to run bottomhole gauges, this maximum pressure will need to be factored in and bottomhole gauges with a high-pressure transducer will need to be used, so as not to overpressure the transducer, which can cause damage to the gauge.

3) Try to use fresh water as the wellbore fluid

Hydrostatic pressure has a major impact on a DFIT analysis. For this reason, it is recommended that fresh water is used as the wellbore fluid, as this will result in a consistent and reliable hydrostatic pressure. Additionally, if you want to estimate things like pipe friction or leak-off rates, it will be necessary to know the fluid density and viscosity. These parameters are known quantities in fresh water, but can vary significantly in brine.

If fresh water cannot be used (for instance, the hole is filled with brine), it is recommended that a static pressure gradient be run in the hole prior to pumping the DFIT. This will allow you to calculate the hydrostatic pressure and density, which will yield more accuracy in the analysis.

4) Try to avoid pressure-activated sleeves

A common completion method nowadays involves pressure-activated sleeves. These sleeves shift open when the casing is pressure tested. While this is a good cost-saving technique, it does present a challenge with a DFIT. Often, the pressure required to activate the sleeve is greater than the breakdown pressure. This means that when the sleeve shifts open, the formation breaks down. Typically this activity is occurring before the DFIT is being pumped. The result is that the formation is already broken down when the DFIT is pumped, and there is no way to determine how much fluid went into the formation during breakdown. While the impact may be small, it could result in a less accurate analysis.

It is recommended that whenever possible, a well be perforated ahead of pumping a DFIT. By doing this, the formation will break down during the DFIT, not prior to it. This creates a lot more control over the amount of fluid going into the formation as a part of the test. Obviously, to do this, the decision to test a well must be made prior to running the casing, as the casing will need to be designed for perforation, with no sleeves installed.

5) Measured data is always better than calculated data

If a certain data point is critical to the analysis, then measuring that data point directly is important to get a meaningful analysis. Calculating a data point leave too much room for variation as it relies on assumptions, and we all know what happens when we assume!

In the case of DFITs, this is especially true to bottomhole pressure. If bottomhole pressure is going to be a critical data point, and the room for error is minimal, then measuring the bottomhole pressure with downhole gauges is a must. There are simply too many variables that can impact bottomhole pressure if it is calculated from surface (hydrostatic pressure, friction, fluid level). The most accurate way to obtain bottomhole pressure is to bypass all of these variables and measure the pressure directly.

DFITs are a great way to understand a reservoir, and they are particularly meaningful in unconventional reservoirs, such as shales, where frac treatments are instrumental to the well, and initial reservoir pressure is very difficult to obtain. Proper planning and consideration is crucial to getting a meaningful analysis on a DFIT.

FyreRok works with oil and gas operators on design, execution, and analysis of DFITs. For more information about how we can help, contact us here.

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