Rock physical and chemical alterations during the in-situ interaction between fracturing fluid and Silurian organic-rich shales in China

Abstract

Hydraulic fracturing has been proved to be an effective way to unlock shale gas resources. During this process, fracturing fluid comes into contact with shale reservoirs and interact with the rocks. This could cause alteration of the pore systems in shales, thereby affecting gas transport and production. To illustrate this geochemical process and identify the physical and chemical alterations of shales, fluid-shale interaction at in-situ conditions is studied for typical marine organic-rich shales from the Silurian Longmaxi Formation in the Sichuan Basin, using actual fracturing fluid and deionized water over a 5-day period. The minerals and pore structures before and after the reaction were examined by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), QEMSCAN, computed tomography (CT) scanning, nitrogen adsorption testing and organic geochemical analysis. Results showed that, the fracturing fluid and deionized water caused indiscernible changes to the organic matter, but a variety of alterations on carbonate minerals. Calcites and dolomites were markedly dissolved, but quartzs, albites and illites remained relatively unaltered. More dolomites crystals were precipitated in the deionized water-shale interaction than in the fracturing fluid-shale interaction, and some of the fractures seemed to be filled by flocculation of friction reducers or deposited mineral particles. The fracturing fluid could enlarge the pore volumes and improve pore connectivity, and the invasion depth of fracturing fluid after 120 h’ was around 60 μm. The dissolved pores and loosening/shedding of particles detected by CT increased through time, and the calculated 3D pore volume increased by 67% at 24 h, 117% at 72 h, and 430% at 120 h compared to that of the initial sample. The XRD and nitrogen adsorption data indicated that new pore volumes increased with higher contents of carbonate minerals in the sample and longer treatment by fracturing fluid. The mineral and pore-scale alterations of shale reservoirs indicated that fracturing fluid-shale interaction (FFSI) enlarged the reservoir reconstruction volume, increased well performance during shut-in period and should be considered as part of the well treatment optimization process.