Converting non-SPIDR Wellhead Pressure Data

By Adam Swartley | Tue, 1 Dec 2009

When DRC was first founded our primary goal was to provide a digital wellhead pressure (WHP) gauge that would allow customers to collect data at surface of a higher quality than anything available in the industry.  This data could then be used for well test analysis by converting it to bottomhole conditions using DRC’s proprietary models.  Over the years we had many requests to convert and analyze data that is captured by non-SPIDR WHP gauge hardware.  We were always very hesitant to agree to work with this data.  The main reason for our hesitation was that we had no way to verify the quality of this data.  It is then impossible to determine if any anomalies in this data or deviation from true bottomhole pressure (BHP) measurements is due to error in our models or due to the low quality of the gauge itself.

As the technology to capture WHP data has advanced over the years, we have noticed that more and more operators are installing sophisticated surveillance systems on their wells.  These surveillance systems can include a good quality WHP gauge.  Because of these advances we are increasingly working with data captured by non-SPIDR gauges.  There are still difficulties and pitfalls in using this data, but in some cases the operational issues with getting a SPIDR to the well site (i.e. remote locations and countries where importing equipment is impossible) will outweigh the negative impact using this data may have on our analysis.  One major benefit is that if you miss getting a SPIDR on your well for a shut-in, you can simply send us your historic data and still get a useful analysis.

Before we agree to perform a test using the operators existing hardware to capture data, we must first view a sample data set.  We will use this data set to determine the resolution, accuracy, and sampling frequency of your hardware.  Typically the most important factor to consider is the sampling frequency.  The higher the permeability of your well, the more frequent sampling we will require.  For instance if you expect that your well had a permeability above 10 mD, we would require at least 1 sample every 10 seconds.  The example below shows a data set that was sent to DRC for conversion.  You can see in this semilog plot that the early period of the build-up is masked due to low sampling frequency.  Luckily this gauge did sample at a high enough rate (1 sample every 10 seconds) for us to obtain a good analysis of the later time data.

Gauge resolution is also a major concern.  For instance if your hardware can only sample pressure data with a resolution of  5 PSI, it will be impossible for us to provide a credible analysis.  The example below highlights the resolution of a poor quality gauge versus that of the SPIDR.  Again, a higher permeability environment where pressure changes are very small in relation to the time, makes having a high resolution gauge crucial.

One of the biggest drawbacks in using other WHP hardware is that typically these gauges will not be thermally compensated for ambient temperature effects.  Day to night temperature swings will cause fluctuations in the readings of a non-compensated gauge.  The SPIDR is thermally compensated and there will be virtually no pressure response to day to night temperature changes.  The plots below show a data set converted by DRC that exhibits these temperature effects every 24 hours.  In the linear plot these effect appear to be negligible, but the accompanying derivative plot is more revealing.  These fluctuations may or may not affect your analysis depending on their severity and your well conditions.  In this case a basic analysis of skin, permeability and P* were not affected.

While there are difficulties and drawback in using non-SPIDR hardware, there are also operational and logistical benefits that may outweigh other considerations.  In these cases we encourage you to contact us so that we can discuss your situation and check the quality of your data. We have found that this type of arrangement is especially suitable in regions of the world where it may be difficult to import a SPIDR.