Marvin Rourke

Challenges of Wireline-Formation Testing and Fluid Sampling in Tight, Low-Permeability Gas Reservoirs: A Case Study from Saudi Arabia

Saudi Aramco’s operations cover a wide range of challenging environments, some of which are multilayer gas reservoirs, consisting of thinly-laminated shaly sands with limited lateral and vertical extensions. Evaluations based on conventional log data often prove to be inconclusive. Wireline formation testers are routine sources of reservoir pressure and fluid samples that address uncertainties in these formation evaluations. Low permeability reservoirs pose numerous challenges for obtaining successful pressure measurements and fluid samples, including supercharging effects and “tight” tests, where no pressure measurements or samples can be obtained. In some cases, tight reservoir permeability prevents an adequate and competent mud cake buildup, which can prevent sealing of the conventional rubber probe type pads. In this paper, the authors share the different approaches employed under some of these challenging conditions in getting representative pressures and samples. Proper selection of tool configuration and appropriate pad and packer design were key success factors. In the paper, we reviewed the learning from some previous attempts with limited success. A pre-job modeling was done to predict the expected rates and drawdowns. We used dual ports within the straddle packer testing interval, which permits the segregation of gas and the filtrate. This allowed the formation fluid to be sampled faster than a single-port packer design or probe type tool. We also demonstrate how a combination of pad type probes and dual packer helped to improve the success rate and reduced our pumping time during sampling. This paper reviews cases of testing in tight gas reservoirs. It also demonstrates how we improved the success rate of our pressure testing and sampling by combining modern wireline formation testing (WFT) functionalities and effective pre-job planning. This approach is helping us meet the challenge of obtaining representative pressures and fluid samples in low permeability formations.