Minidrillstem Tests To Characterize Formation Deliverability in the Bakken

Abstract

Summary Characterization of reservoir deliverability is fundamental for the economic development of any field. In the Bakken, there is a need for reliable pressure-transient tests to provide the effective formation permeability of the fracture/matrix system in this formation for deliverability calculations. This effective permeability can then be compared with laboratory-measured core permeability of the matrix rock samples. This comparison is the basis for planning early-production options and subsequent enhanced-oil-recovery (EOR) decisions. This comparison is particularly important because of the influence of massive hydraulic-fracture stimulation on reservoir performance. Determining well-deliverability potential by conventional drill-stem tests (DSTs) or traditional wireline formation tests (WFTs) in the past has resulted in mixed success in the Bakken. On the other hand, the mini-DST has increased the reliability of pressure-transient tests because the formation interval of interest is produced at a precise constant rate into a chamber between two inflated packers with negligible wellbore-storage effects. Also, the operation of a mini-DST tool requires much less time than the classical DST, and multiple zones can be tested sequentially to assess the individual-zone deliverability. The mini-DST tool uses the conventional WFT configured with a dual-packer module and downhole pump. Tests are conducted by inflating the dual-packer module to isolate a 3-ft interval of the wellbore. The formation fluid is pumped out from the packer-isolated wellbore interval to conduct pressure-drawdown and -buildup tests in the interval. An overlay of all the pressure-drawdown and -buildup results from various intervals is compared on a single plot to identify the most productive interval. Finally, conventional pressure-transient analyses are performed to interpret all pressure-drawdown and -buildup tests. In this paper, we present several field tests that were analyzed both by the preceding procedure and by numerical simulation. The analyses of several mini-DST results have provided insight into a better understanding of the flow mechanism in the Bakken both during primary production and in forecasting various improved-recovery and EOR proposals. The results can also serve as a basis for test design in similar low-permeability reservoirs elsewhere.