Mini DST to Characterise Formation Deliverability in Unconventional Reservoirs

Characterization of reservoir deliverability is fundamental for the economic development of any field. In the Bakken, a need exists for reliable pressure transient tests to provide effective formation permeability of the combined fracture-matrix porous formation. This effective permeability can then be compared to laboratory measured core permeability of the matrix rock samples. This comparison is the basis for planning early production options and subsequent decisions for EOR alternatives. In the Bakken this understanding is particularly important because of the influence of massive hydraulic fracture stimulation on reservoir performance. Determining well deliverability potential by conventional drill stem tests (DST) or traditional wireline formation tests (WFT) in the past has resulted in mixed success in the Bakken. On the other hand, the mini-DST has definitely increased reliability and the success rate of pressure transient tests. The operation of mini-DST tool requires much less time than the classic DST, and multiple zone tests can be conducted to assess individual zone deliverability. The Mini-DST tool uses the conventional Wireline Formation Tester (WFT) configured with a dual-packer module and downhole pump. Tests are conducted by inflating the dual-packer module to isolate a 3-foot interval of the wellbore. Then, formation fluid is pumped out from the packer-isolated wellbore interval followed by a pressure buildup in the interval. Some simple overlay comparisons, as well as conventional pressure transient analysis, are used to interpret the drawdown and buildup pressure responses. In this paper we present several field tests which were analyzed both by conventional pressure transient analysis and numerical simulation. The analyses have provided insight into a better understanding of the flow mechanism in the Bakken both during primary production and in forecasting various improved and enhanced oil recovery proposals. The experience can also serve as a basis for test design in similar low-permeability reservoirs.