An Extended Mini-DST Depth of Investigation Using a New Formation Testing Tool and Innovative Analytical Model

Abstract

Drillstem tests (DSTs) and mini-DSTs are established reservoir dynamic evaluation techniques used to gather pressure transient data. A DST is still the preferred option for field-scale evaluation because the depth of investigation (ri) in a mini-DST is generally limited to tens of feet. Based on recent technology advancements being deployed, a review of the parameters affecting ri analytically and numerically is essential as these values have significance on operational efficiency. The most recent analytical models have shown that the production rate has a major effect on ri estimates, and this paper will focus on the most recent innovations in both hardware and analytical models that can extend the mini-DST data. Specifically, in-depth comparisons of new analytical models with van Poolen and Kuchuk’s equations will be presented. The discussion will extend to key hardware components with focus on pumping rate and gauge resolution. Finally, the new approach will be validated using data from restricted locations, from a thick high-permeability sandstone, and from a thick low-permeability carbonate. The van Poolen equation shows that for a fixed buildup time, a formation tester rate of 10 bbl/d compared to a DST rate of 1,000 bbl/d would result in the same radius of investigation, given that the equation does not account for flow rate. Kuchuk’s equation and the new analytical models show that flow rate has a direct impact on detectable ri—the higher the flow rate, the higher the detectable ri. Pressure transient analysis (PTA) was used to illustrate the impact of flow rate and gauge resolution on data quality, particularly on the portion of the flow regime relevant for detectable boundaries. In addition, a PTA plot superimposing flow rates from measured downhole data is combined with an idealized gauge to help to determine the magnitude of the apparent gauge resolution and its impact on the detectable ri. Both the new and traditional radius of investigation equations show that data acquired with the new formation tester tool obtains a deeper depth of investigation due to excellent data quality. The results are compared to conventional formation testers with limited flow rate and lower gauge resolution. This paper presents a new approach with a rate-dependent radius of investigation, compared to traditional approaches of van Poolen and Kuchuk. The superior data quality from the new formation tool leads to a substantial improvement in estimated ri.