Viability of surface testing for wells with high concentrations of CO2 in the wellbore.
Performing pressure transient analysis tests from the surface provides a no risk and low cost alternative to traditional down-hole testing. However, this relies on being able to accurately convert the surface pressure to bottom-hole conditions. Data Retrieval Corp. has built its reputation by conducting hundreds of comparison tests with down-hole gauges over its 25 year history. In doing so, DRC has been able to greatly expand the number of wells that are testable from the surface. In the past, wells with high concentrations of CO2 would have been deemed poor candidates for surface testing, due to the difficulties in modeling the phase behavior of CO2. However recent opportunities to run tests in wells with high CO2 concentrations in conjunction with down-hole gauges have lead to advancements in our models that allow surface testing of wells with much higher concentrations of CO2. One such well presented here will show that surface testing is a viable option for wells with almost 32% CO2.
The main difficulty associated with high CO2 wells is the unique phase behavior of the CO2. Proper modeling of wellbore conditions during flowing and shut-in periods is of utmost importance due to the extreme temperature and pressure sensitivity of the CO2. This is especially true during unstable flow periods, or when going from flowing to shut-in conditions as temperatures and pressures are changing most rapidly at those times. It is not uncommon for the CO2 to go through multiple phase changes in the wellbore. In the past this has prevented high CO2 wells from being tested from the surface. Conducting these comparison tests in conjunction with down-hole gauges has allowed DRC to improve our models to handle the difficult phase behavior of the CO2 and how its density rapidly changes during unstable or rapidly changing flow.
Proper thermal modeling is also especially critical during the shut-in because improper modeling of the wellhead temperature would result in a build-up curve that is distorted, which would lead to an incorrect analysis. The cooling of the wellhead upon shut-in is a phenomenon known as thermal decay. When a well is shut-in, the heat being brought to surface by the produced fluids is cut-off. This results in cooling at the wellhead, which causes the density of the fluids to increase. This is sometimes seen as decreasing pressure at the wellhead during the build-up. DRC has developed a proprietary model for this phenomenon that accurately models the changing wellhead temperature and thus accounts for the increasing density. By employing DRCís proprietary model for thermal decay, accurate build-up pressures are calculated, resulting in correct build-up curves and analyses.
Figure 1. Cartesian Comparison Plot of DHG vs SPIDR
The well being presented makes 10-15 MMSCF/D gas and has a CO2 concentration just less than 32 mol%. It can be seen in Figure 1 that the SPIDR calculated bottom-hole pressure matches up very well with the three down-hole pressure gauges. There is a difference of about 15 psi during the build-up and about 25 psi during the flowing period. It is important to note that the flowing period was of a fairly short duration, and thus the well wasnít able to reach thermal stability, making it more difficult to model from surface. Figure 2 presents a semilog comparison plot of the build-up data and shows that the shape of the build-up curves also match up very well. This conversion was done ìblindlyî, or without the down-hole gauge data, and as such the first pass at modeling the thermal decay would require a bit of fine-tuning to generate results that perfectly overlay the with the down-hole gauge data. However, even without further adjusting the models, it can be seen in Table 1 that the reservoir characteristics determined from the SPIDR data very closely match those determined from the three separate down-hole gauge datasets. While the numbers are not an exact match, they are close enough to allow the operator to make the same decisions about their reservoir as the down-hole gauge results would. By taking these results and further fine-tuning the conversion models, DRC will be able to provide a viable alternative to running down-hole gauges for all the wells in this field.
Figure 2. Semilog Comparison Plot of DHG vs SPIDR
Table 1. Analysis Comparison of DHG vs SPIDR
Pressure transient testing from the surface is a viable alternative to traditional down-hole gauge testing for an increasingly larger number of wells. DRC continues to push the limits of what is deemed testable from surface, and comparisons with down-hole gauges on wells such as this allow us to take the limits even farther. By continually advancing our models to account for higher concentrations of CO2 we present a unique opportunity to an increased number of operators to conduct pressure transient tests in the most safe, cost effective manner available on the market today.
