Monday, July 31, 2017

Well Testing Design and Analysis chapter 3

The final interpretation step, called history matching or verification, uses the model established in the second buildup to predict presure response throughout all four periods of the test and confirms that the model satisfactorily accounts for all data. This may result in more parameter adjustment because every period must now be matched simulatneously, even though the second flow period is planned intentionally long to minimize the influence of previous periods.

The design phase not only maps out the mechanis of a test, but also ensures that, once underway objectives are met. For example, the progress of the planned transient can be followed at the wellsite and compared with that forecast during the design. To avoid the costly mistake of rigging down before the transient indicates a desired feature, wellsite validation of data during the test remains a must. This is best accomplished with surface readout of downhole gauges and enough computing power at the surface to produce approriate plots, notably the log-log diagnostic plot. If the reservoir response is quite different from the assumed in the design, wellsite diagnosis permits an instant correction of the job, perhaps a lengthening of the transient, to ensure optimum use for the data. In certain cases, real-time readout is not feasible and downhole recording must be used. Data validation can still be performed onsite right after retrieving the gauges. 

Layered reservoir testing (LRT) was originaly conceived to investigate production wells. Recently in offshore Congo, AGIP used the technique to evaluate a layered  reservoir encountered by an exploration well. Conventional testing of individual pay zones in an exploration well would normally call for a separate DST-perforation run for each zone. But using layered reservoir testing, AGIP obtained reliable kh, skin and productivity index values for individual zones with only one trip in the hole, at considerable cost savings. 

The drawback of using an LRT in the exploration setting is that production from different zones commingles, ruling out representative sampling from different pay zones. Fortunately, a recent tecnological provides a solution. Samples of extraordinary reliability may now be obtained from any number of zones using the new wireline-conveyed MDT Modular Formation Dyanmics Tester, but this has to be planned in advance because the sampling takes place in open hole. 

AGIP's innovative use of layered reservoir testing (LRT) in an exploration well occured offshore Congo. The goal of the test was to evaluate two producing layers only a few meters apart with only one trip into the hole, a much less costly undertaking than the usual two trips. The LRT technique was originally developed for testing production wells, in which several layers produce commingled-in this case, LRT is a must to evaluate each layer's dynamic properties. 

source : Oilfield review magazine 


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