Formation fluid characterization using wireline formation testers (WFTs) in very tight formations has always been challenging. Due to low formation permeability/mobility, it is often possible to miss out on potential hydrocarbon-bearing zones because the WFTs are not able to sustain pumping.

In this paper, a novel technique has been proposed and demonstrated that will prove very useful in fluid characterization and sampling of ultra-tight formations where the conventional dual packer approach fails.

Currently, the most suitable inlet module for use in very low mobility zones is the straddle packer, (commonly known as dual packer and used as same for further reference in this paper). This type of an inlet in a formation tester is used to straddle approximately 1 m of the formation interval using two inflatable elastomer packers. This technology has been used since 1988. However, due to the minimum interval pressure constraint and high amount of mud solids present in the initial cleanup, formation fluid in low-mobility zones often remains uncharacterized.

Fluid identification and/or sampling with a dual packer in ultra-tight formations also has limitations such as high cleanup volume to be pumped out from the straddled interval and other constraints such as the minimum interval pressure, which can be due to maximum differential pressure rating of packer/mandrel or the minimum inlet pressure requirement of the pump. This limitation becomes more challenging in ultra-tight formations. Due to this reason, formation fluid identification cannot be carried out in such tight zones, even using a dual packer module.

The technique described in this paper facilitates identification of small traces of hydrocarbon in the fluid by replacing the interval mud with a clean solids-free external fluid via internal pumping within the flowline of the tool into the sump (straddled) interval. The external fluid will recharge the depleted interval pressure, and pumping can be resumed. This recharging and pumping cycle can be repeated multiple times, if required and as needed based on the response of the interval pressure and fluid analyzer.

The goal of this technique is to create a more ‘optical friendly’ environment in the interval where even trace amounts of pumping induced hydrocarbons can be identified by downhole fluid analyzers, thus, allowing fluid mapping in previously neglected low mobility zones. This visibility can be effectively seen with the pressure response in the straddled interval during recharging as well. If there is low mud filtrate invasion and presence of hydrocarbon in the formation, then there is good chance of identifying traces of hydrocarbon on the fluid analyzer, which can result in characterization and hence discoveries of new pay zones.

The proposed workflow was applied to one of the low-permeability oil fields of India. Based on the formation responses and results, similar applications of this technique are in field-testing stages worldwide.

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