Evaluation Of Supercritical CO 2 Displacement Shale Gas In Shale

Recent years has witnessed a rapid growth in the development of shale gas exploitation, as a result, shale gas is considered to be an important unconventional natural gas resource. However, unlike conventional gas reservoirs, shale gas is characterized by its unique existence form including free gas, adsorbed gas and dissolved gas. Currently, free gas contributes to the most of the yield in the early stage, whereas the adsorbed gas was largely exploited during the late period. In conclude, it is of great importance to do some research on shale gas absorbability and find a way to put it into use in the shale gas exploitation.Due to the fact that the adsorption capacity of CO2 is stronger than that of CH4, CO2 plays an important role in the exploitation of shale gas. Under the guidance of competitive adsorption theory, some tests on displacement efficiency of CH4 by super critical CO2 has been run on long core flooding apparatus to find flow behavior occurred in the matrix. From the results acquired from the tests, the flowability of CO2 and CH4 in nanopores is evaluated in super critical condition. At the same time, the cause of flowability difference between the two gases is undermined. Above all, we gain some insights into the mechanisms involved in replacement process, which, in turn, can be benefit for the exploitation process. Some of the results are:(1) CO2 shows a greater advantage in the adsorption tests.Shale is inclined to adsorb more CO2 than CH4 under the same experimental environment, which shows that the injected CO2 accelerates the desorption process of CH4.(2) The amount of adsorbed CH4 differ among samples:once the temperature is settled, the maximum adsorptive capacity increases with the rise of pressure; whereas, given different temperatures, the adsorptive capacity shows a reverse tendency to temperature; in addition, the amount of TOC content also contributes to different adsorptive capacities. The absorbance varies in an identical way to that of TOC content. What's more, adsorptive capacity is also different among clay minerals.(3) Long core experiments witness a recovery ratio of 41% in depletion process, among which matrix contributes 23.2%. In contrast, the test considering replacement between CO2 and CH4 reaches a higher final recovery ratio of 31.8%. In addition, experiment result shows a great deal of similarity with numerical simulation, which, verifies the existence of competitive adsorption between CO2 and CH4.(4) In shale matrix, the pressure transmissibility and flowability of super critical CO2 is smaller than that of CH4, providing a more favorable condition for the replacement and displacement of CH4.(5) The overall mechanism in the replacement process can be concluded as follows:the stronger adsorptive capacity of CO2 on the surface of shale, the slower diffusibility of CO2 in micro pores, filtration resistance due to increased viscosity as well as the occurrence of "counter-dissolveness" in the displacement process. As a result, a piston like displacement is achieved which strengths the efficiency of exploitation.