Measurement of fluid properties in organic-rich shales
Jung, Chang Min
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The primary objective of this study is to develop and improve water-based drilling fluids and fracturing fluids for organic rich shale reservoirs by using nanoparticles and to gain fundamental insight into water and oil flow in shales. This dissertation presents results for several shale formations in the US, namely the Barnett shale, the Eagle Ford shale, the Utica shale, and the Bakken shale. The research discussed here presents new methods for studying the interaction between various fluids and organic-rich shale and the development of proper methods to measure apparent and relative permeability of shale. First of all, we show how the petrophysical properties of shales are changed when they are poorly preserved. Experiments were performed to measure important petrophysical properties such as porosity, density, weight change, hardness, wave velocity and permeability before and after shale samples dried-out. The large differences in shale properties between preserved and un-preserved samples as reported herein, clearly indicate that shales should be preserved at the well site and tested with a standard procedure ensuring minimum alteration of fluids from the shale. Failure to follow a standard procedure leads to measurements that do not reflect the “true” or in-situ properties of the shale. Instead, the measurements can be a factor of 2 or 3 different from the “true” value. The shale handling, preservation and measurement techniques and procedures presented here can be used as a standard protocol for studying organic rich shales. Next, we discuss how fracturing fluid can change the petrophysical properties of shale. Among the various petrophysical properties, the fluid permeability is chosen to determine the effect of the fracturing fluid on the shale. Experimental procedures are presented to suggest how to measure the shale permeability. To measure the fluid permeability, the Pressure Penetration Technique (PPT) was developed and used. The reference permeability with sea water brine was measured and then fracturing fluid was injected into the shale. The brine permeability was re-measured to see the effect of exposure to the fracturing fluid, and experimental data show the permeability change due to fracturing fluid plugging the shale. Next, we focus on the development of a Water Based Mud (WBM) system for organic-rich shale. Drilling through a shale formation can result in borehole instability problems, and this is known to add substantial costs to the operation. This is because conventional drilling fluids tend to interact with clay minerals in shales, and the mechanical properties of rock are changed by clay swelling. To reduce the interaction between water-based muds and shales, we need to reduce water invasion into the shale. The addition of nanoparticle additives to water-based drilling fluids can significantly reduce the invasion. We report results for shale permeability and pressure penetration though shales using different fluids: brine, base mud and nanoparticle based muds. To better define the effect of nanoparticles, we used different concentrations of nanoparticles in the mud. From the large reduction in permeability and the pressure response results, we clearly show that nanoparticles act as good shale inhibitors to ensure wellbore stability during drilling. Experimental studies used to measure the relative permeability of shale. Such measurements have never been done before. Due to the extremely low permeability of shale, it is very difficult to measure the relative permeability of shale directly. We propose a method of relative permeability measurement using NMR (Nuclear Magnetic Resonance) spectroscopy to measure fluid saturations and a RPC (relative permeability measurements under a confining pressure) set-up to conduct the displacement. RPC set-up is a newly developed forced injection set-up using the unsteady-state method. The in-situ fluid saturation was successfully measured with NMR, and the set-up was also useful for measuring the relative permeability of shale. It yielded continuous results from the Bakken shale tests.