The study of mass transfer in gas shales and the optimization of hydraulic stimulation processes via additives

2013-10-01Gas shales contain an abundance of natural gas. Low-permeability ("tight") shale is commonly found domestically in the Appalachian Basin but it has yet to be fully explored. A major limitation to the efficient extraction of this "tight gas" is the current lack of knowledge of the physics and mass transfer properties involved in production from such low-permeability porous materials. Without an improved fundamental understanding and appropriate modeling and simulation, forecasting of natural gas production from these formations will be inaccurate and unreliable, and operators must continue relying on "trial and error" in their development strategies. With the demand for natural gas continuing to surge, an urgent need, therefore, exists today to improve the efficient drilling and completion of new wells as well as for re-completion of existing wells in order to meet this increased demand. As a result, we not only need to drill and complete more wells, we also need to improve production efficiency on a per well basis. ❧ In this study we have characterized the properties of gas shales and the fundamental mass transfer mechanisms associated with the gas production process; we have developed, in addition, a new method to reduce water-blockage related formation damage. A set of techniques have been employed systematically during our shale characterization work, which provide us with ways to better understand the unique physical and chemical properties of these shale materials. These formations, for example, are characterized by a very low and anisotropic permeability, with the horizontal permeability being much higher than the permeability in the vertical direction. Our experiments using BET with these shale samples have revealed that they possess multiple levels of porosity, ranging from microporous and mesoporous regions (characterizing the sample's matrix) to macropore/microfracture domains. ❧ We have also designed and carried out high-pressure gas depletion experiments in order to study the gas production behavior of these shale samples and the various mass transfer mechanisms that prevail. Furthermore, we have utilized the experimental knowledge generated to form the basis of a simple continuum-type model to study gas depletion from these materials. Our high-pressure system was designed in such a way as to mimic the actual formation's pressure conditions, so that we can independently control the pressure parameters to study the resulting production behavior of the shale core sample. The analysis of the experimental data indicates that the gas that depletes during the early stage is free-gas from the macropore/microfracture domains (as well as the macrofractures that may also be present in these small size, lab-scale samples). The gas depleted at longer times is via a desorption-diffusion-viscous flow process from the shale matrix. We use numerical simulations to analyze the model in order to understand and interpret the experimental behavior we observe during our lab-scale, high-pressure depletion experiments. The combined experimental and numerical studies show that for the shale core sample investigated, the production rate (after the initial production period), as well as the ultimate production potential are both highly dependent on the porous structure of the formation. ❧ During hydraulic fracturing, water-blockage related formation damage often takes place caused by water trapped in the formation that blocks the path-way for gas molecules to escape. As part of the study, therefore, we have systematically investigated the effectiveness of a novel surfactant, selected from an extensive survey among commercially available surfactants, in reducing capillarity of the formation. Besides employing contact angle measurements and spontaneous imbibition experiments to study the surfactant, we have also carried out forced imbibition experiments, with which we are able to study the fluid flow-back behavior under the presence of the novel surfactant. We have also conducted static and dynamic experiments in order to investigate the surfactant loss due to the presence of commonly used proppants. Our lab-scale investigation have shown that via the addition of the surfactant, the capillarity of the formation was significantly reduced; this then means that use of this particular surfactant in the field may result in less water-blockage related formation damage