Open Hole Drill Stem Tests (DIT's) are conducted with a drilling rig in open hole (hence the name). Total lest times are approximately twenty four hours while on bottom test times average six hours. The results of the tests are typically used to? ensure sufficient reservoir quality exists to justify running casing and conducting a completion program. However, there are many examples of DSTs that have yielded incorrect results and as a result DSTs are considered to be somewhat unreliable. The intent of this paper is to demonstrate a new approach that improves the reliability and accuracy of DST analysis.
A typical open hole DST consists of a 5–10 minute initial flow period, 60–90 minute initial shut-in period, and one hour main flow period and a two to four hour final shut-in period. The intent of the initial flow is to relieve the mud hydrostatic pressure that is trapped under the packers. The objective of the initial shut-in is to capture an initial reservoir pressure. The final flow withdraws a known quantity of reservoir fluid and creates a pressure transient into the reservoir. The final shut-in records the rate at which the reservoir pressure recovers in response to the reservoir fluid withdrawal. The final buildup is analyzed to determine the reservoir flow capacity and the deliverability potential of the reservoir.
However, due to the short flow periods there are a significant number of open hole DSTs that recover very little, if any reservoir fluids. Subsequent analyses of these DSTs results in reservoir deliverability estimations that are significantly lower than the actual post completion results. In the past the explanation for these discrepancies has been that open whole DST's are unreliable. In fact, open hole DST's are reliable if they are analyzed for the correct fluid. If the formation is relatively tight or a lot of drilling mud filtrate has invaded the reservoir, all that will be recovered during the DST is mud filtrate. When the filtrate production rate is used in the test analysis, the discrepancy between the DST predictions and actual post completion results disappears.
To demonstrate; this, a case study was done of three open hole DSTs and subsequent post fracture completion tests for the Montney reservoir in Northern Alberta.
The downhole pressure plot for the Montney open hole DST on Manir 16-35-71-4W6 (attached as Figure I) illustrates the test sequence. An initial flow and shut-in were not conducted on this test in order to have a longer final shut-in (six hours). The flow period was one hour long and recovered minimal gas (less than 10 m3/d) and 0.176 m3 of drilling fluid. The downhole recovery recorder plot (attached as Figure 2) illustrates the fluid entry into the test string when the downhole valve was open. There is an abrupt fluid entry that occurs immediately after the test valve is opened (TVO). After that me rate of pressure increase continually decreases until the end of the flow period. When the test valve is closed (TVC) the pressure ceases to increase.