Advancements in SPIDR Technology
By Chad Cluver |
Thu, 30 Sep 2010
At Data Retrieval Corp. we strive to provide our customers with the best technology and service available in the industry. We “push the envelope” on the types of wells that can be successfully tested from the surface and we have recently made a number of enhancements to our algorithms for converting surface data to bottom-hole conditions. We have also learned a number of "best practice" testing procedures to optimize the operating conditions necessary for obtaining valid results. This brief article will serve to summarize those best practice procedures as well as our recent enhancements to conversion algorithms.
We are testing an increasing number of wells with high condensate yields that are well below the dew point during shut-in, both at surface and even in the reservoir. During flowing periods, all produced fluids (gas, condensate, and water) are being lifted out of the wellbore, however upon shut-in some of the condensate may drop out of solution. A PVT analysis of the well stream is used to calculate how much drops out. If this is not done then there will be a scalar offset in the pressure data. In cases where PVT data is not available or it is out of date, we have worked on revising our condensate dropout model via comparison to empirical data to more accurately reflect wellbore conditions during the build-up when significant phase changes can occur. This has resulted in a more accurate modeling of the BHP which then reduces the error bar in calculated values of skin and P*.
Wells flowing at high rates (greater than 100 MMSCF/D) are becoming more common candidates for surface testing. Operators do not want to risk lost tools in wells that are such high performers. Like any model, when you operate at the limits you incur an increased amount of error or uncertainty in the output. The same was true for our conversion model, in that we were seeing higher error at flow rates above 100 MMSCF/D than what we had deemed acceptable for successful surface testing. We worked with several operators to incorporate actual downhole gauge data from very high rate wells (100-150 MMSCF/D) into a new frictional model for use in these high rate wells. What we have seen thus far is greatly improved accuracy and consistency in calculating flowing BHP’s at those high rates. We are also now applying what we have learned from this into our general model to improve consistency and minimize error under all flowing conditions.
We have found that a multi-rate test followed by a pressure buildup is the preferred test sequence for many operators. We have learned that if a multi-rate test is desired (to determine non-Darcy skin for example), it is best to perform the multi rate test after the buildup. A long, stable flow period before shut-in results in the best test results for an analysis to determine reservoir characteristics. Often the flow periods of the multi-rate are not of sufficient duration for pressures and temperatures to properly stabilize in the tubing and the reservoir. This not only makes modeling the surface data to bottom hole conditions especially challenging if not impossible, but it introduces unnecessary uncertainty into the analysis. If the multi-rate is performed after the PBU, then good reservoir characteristics can be determined prior to gathering the additional data in the multi-rate. This lesson also applies to tests conducted with down-hole gauges. We have also seen the use of process control or “automatic” chokes during the flow periods. This results in “saw-tooth” pressure responses which are sub-optimal for pressure transient testing applications. We recommend the disconnection of the automatic choke and the use of fixed choke settings during transient testing periods.
We have made several important enhancements to our conversion algorithms that now allow us to test an even wider range of wells from the surface. Data Retrieval Corp. continues to lead the industry in the capturing of high quality, high frequency wellhead pressure data, and the accurate conversion of that data to bottom-hole conditions. We continue to successfully test an increasing number of wells that were once thought impossible to test from surface, and continue to back up our work by offering a free comparison test when conducted simultaneously with down-hole gauges. Data Retrieval Corp. offers complementary well test planning consultation, and works with our customers to ensure test objectives will be met and that quality data will be captured. Additionally, we will evaluate any well as a candidate for potential surface testing without charge.