Analysis Discrepancies when using PTA Software

By Chad Cluver | Tue, 19 May 2009

Data Retrieval Corporation has built its reputation over the past 25 years by not only providing a cost effective, no risk method to capture high resolution, high accuracy wellhead pressure data, but by also then accurately converting this data to bottom hole conditions for an increasingly wide variety of gas and gas condensate wells.  DRCís proprietary method for this conversion is what makes us an industry leader in surface well testing.  However, in addition to the capture and conversion of the wellhead pressure data, DRC provides a complimentary analysis on the data (build-up, drawdown, two-rate, etcÖ).  The analysis method employed by DRC is the MDH Semilog analysis, an analysis method that may be different than what may commonly be done by operators today such as derivative type-curve matching using commercially available Pressure Transient Analysis Software (PTA Software).  One of the reasons for this is that surface data often results in ìnoisierî derivative plots, which makes derivative type-curve matching more difficult.  Another reason is that DRC rarely has all of the information about a reservoir that needs to be put into derivative type-curve matching software which would also make the analysis less accurate, and can cause the derivative plots generated by DRC and the PTA analysis software to differ.  Analysis of the data based on these plots is then also subject to an individualís interpretation of the data, which allows for further discrepancies.

Because of these differences, it is not uncommon for an operator to derive a different analysis of the data than what is provided by DRC.  This is especially important when running a comparison against a downhole gauge, because it can then appear that there is an error in the conversion, when in fact this is not the case.  It is important to make sure that the data is being loaded into the PTA analysis software properly, that the flow periods are properly accounted for, and that the appropriate plots are being generated for comparison purposes.  DRC provides the most accurate conversion of wellhead pressure to bottom hole conditions available.  Instead of comparing DRCís analysis with an analysis done on downhole gauge data using a completely different analysis technique by a different person, it is important to compare the actual pressure data.  If the converted data matches well with the recorded downhole data, then an analysis should be done on both by the same person using the same analysis software and technique, so that the analyses can be compared on an ìapples to applesî basis.  When this is done, it is seen that the converted data provided by DRC allows for the same reservoir characteristics to be determined as by the downhole gauge.

DRCís proprietary conversion algorithm remains unmatched in the industry today, and allows for a wide range of well types to be tested from the surface.  DRC continues to offer a free trial to new customers in order to prove the technology, where a SPIDR and conversion of the data captured will be provided for free during a test in which a downhole gauge is also ran, provided DRC is provided the downhole gauge data after the report is completed.  When this data is properly compared, and the analyses are properly done and then compared, it is seen that DRC offers downhole gauge results without most of the cost and none of the risk of going downhole.