Formation testers are being widely used worldwide in open hole, especially in exploratory wells where fluid identification, downhole sampling, determining pressure and mobilities are very critical. Sometimes, due to tight formations, unconsolidated sands or excessive borehole washouts/breakouts, it is not advisable to pump, because with longer stationary times there is high risk of differential sticking. In this scenario, cased-hole formation tester comes into play. The whole operation involves perforating 1 foot interval in casing, then isolating the perforated interval with straddle packers (3.28 ft or 1m apart). This is followed by pumping to establish fluid type and representative sampling. Later, extended buildup is acquired for reservoir characterization until transient reaches the radial flow regime (Mini-DST).

Well A is located in XYZ Block, Pakistan and is operated by Pakistan Petroleum Limited (PPL).Pressures and mobilities were obtained with wireline formation testing probe module across open-hole which found the formation to be ultra-tight. Pumpout could not be acquired with probe module due to formation tightness, therefore the question of fluid type was unanswered. Additionally, across the prospective zone washouts were observed. Due to these factors, the well was cased to establish fluid type and reservoir characterization (permeability, skin and pressure) through cased-hole dual packer formation tester. After 10 hours of pumping, first gas breakthrough was observed on downhole fluid analyzer. Pumping was carried out for almost 22 hours in order to get representative samples. This was the longest pumping time ever on any well in Pakistan. Later, buildup was acquired since hydrocarbon presence was established (Mini-DST).

Cased-hole formation testing is very helpful when dealing with layered reservoirs as testing of each individual layer on a full-fledge DST operation would be very costly, especially if the layers end up being declared as water bearing. Similarly, conventional DST operation for ultra-tight formations can cost a lot of rig days, whereas mini-drillstem testing can conclude the same with reasonable amount of rig time. The most important benefit of cased-hole formation testing is its capability to pump for longer durations with no risk of differential sticking, thereby expanding the possibilities for formation testing and sampling.

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