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    HOW TO RUN A DELIVERABILITY TEST

    A deliverability test is a simple type of well test that is used to determine a well’s production potential or “deliverability”. Whereas more complex well tests rely on pressure transient analysis to determine reservoir characteristics such as permeability and boundaries, a deliverability test looks simply at production data (flowrate and flowing pressure) and is used to develop deliverability components, C and n. The values of C and n are then plugged into the deliverability formula to create a well-specific deliverability equation:

    The deliverability equation above is used to describe how much flowrate should be expected for a specific pressure drop. It can also be rearranged to determine how much pressure drop is necessary to achieve a desired flowrate. In order to use the deliverability equation above, a well’s specific C and n values must be determined, which requires a deliverability test. Absolute Open Flow (AOF) can also be calculated with the above equation, once C and n have been determined.

    Generally speaking, there are three different methods for running a deliverability test: flow-after-flow, isochronal, and modified isochronal. Let’s take a brief look at these three methods.

    Flow-After-Flow

    A flow-after-flow test involves flowing a well at multiple different flowrates (typically 3 or 4 drawdowns), one after another, until a stabilized pressure is reached. There are no shut-in times on a flow-after-flow test. For a high-permeability well, which will stabilize quickly, a flow-after-flow test can be a quick and efficient way to test a well. The biggest limitation to this method is that each pressure drawdown must reach stabilization individually, which can create a very time-consuming test for lower-permeability wells.

    Isochronal

    The isochronal test method was developed for lower-permeability wells with longer stabilization times. The idea is to flow the well at multiple different rates for the same length of time, with shut-ins between each flow period to allow the well to build back up to full reservoir pressure. Only one stabilized drawdown is required, which can save a lot of time. By keeping each drawdown the same length of time, the points can be plotted together to determine the log-log slope which correlates to 1/n. Then, the single stabilized point can be used to move to the straight line to the point of stabilization.

    Modified Isochronal

    Similar to the isochronal test, the modified isochronal was developed to further shorten the length of test time for lower-permeability wells. In a modified isochronal test, each drawdown and buildup period must be the same length of time. One flow (typically the last) much reach stabilization. The big benefit to a modified isochronal test is that well does not need to build up to reservoir pressure before beginning the next flow. This can save a lot of time on wells that build pressure slowly.

    How To Get Started

    If you are new to well testing and want to get started with a deliverability test, the most practical test to start with is a modified isochronal test. With this type of test, you stand the best chance to get meaningful data with the least amount of test time. Here is a simple test procedure to get started:

    1.       Flow the well at a near-constant rate for 1 hour

    2.       Shut in the well for 1 hour

    3.       Flow the well again for 1 hour at a different flowrate

    4.       Shut in for 1 hour

    5.       Flow the well again, at a 3rd flowrate, until the pressure reaches stabilization (less than 0.25% pressure drop per 15 minutes)

    6.       For more accuracy, flow at a 4th flowrate for 1 hour (not necessary but gives more confidence)

    7.       Plot your data points on a log-log plot (flowrate vs. ΔP^2) and use the stabilized point to obtain your C and AOF

    Obviously, every well is different and requires different flow times. The procedure recommended above is just a starting point. Some wells will be stabilized before 1 hour while others will not. Make adjustments where necessary, but just be sure that each drawdown and buildup period is the same length of time.

    As with all well testing, you will get the best results if you measure bottomhole pressure during the test with downhole gauges. However, a deliverability analysis is a bit more forgiving than a pressure transient analysis. If you are looking to reduce costs, you can run a deliverability test with just surface gauges and still get a meaningful answer. This is not the case with pressure transient testing, as the tolerance for accuracy is much tighter, and therefore bottomhole pressure measurement is imperative. For this reason, some people see deliverability testing as a more practical way to gain insight into their well.

     

    Do you want to get started on a well testing project but don’t know where to start? Maybe you are trying to decide between a deliverability test or a more in-depth pressure transient test. Regardless of your situation, FyreRok is here to help. We specialize in well testing and we are happy to guide you through this process. Reach out to FyreRok today and let us help you get started.

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