Abstract
Properties of shale in an acid environment are important when acid or CO2 is injected into geologic formations as a working fluid for enhanced oil and gas recovery, hydraulic fracturing and reduced fracture initiation pressure. It has previously been shown that acid fluids can enhance the formation conductivity and decrease the hardness of shale. However, less is known about the effect of dilute acid on the adhesion properties of shale. In the study, shale samples are characterized in detail with advanced analysis. Adhesion properties of shale via dilute acid treatment were revealed by atomic force microscopy (AFM) for the first time. Results indicate that acid treatment can greatly enhance adhesion forces of the shale surface. After acid treatment, the average adhesion forces show a platform-like growth with an increase in loading force. Through analysis of results from AFM, scanning electron microscopy, and X-ray diffraction, we affirm that the enhanced adhesion forces are mainly from increased specific surface area and reduced elastic modulus. The results presented in this work help understand the adhesion properties of shale oil/gas present in an acidic environment, which have great significance in unconventional resources development.
Highlights
As an important alternative for conventional energy sources, shale gas development has been a great success with the wide application of horizontal drilling and multistage hydraulic fracturing (Collell et al 2015; Tang et al 2018)
With an increase in loading force, the adhesion force of pre-acidizing sample remains relatively stable for every position, revealing that the test occurred in the elastic range
For acid reacts with carbonates preferentially, and only two drops of diluted HCl were used in the adhesion force test, we focus on the mineralogical and topographical changes to clarify the effect of dilute acid treatment on adhesion properties of shale
Summary
As an important alternative for conventional energy sources, shale gas development has been a great success with the wide application of horizontal drilling and multistage hydraulic fracturing (Collell et al 2015; Tang et al 2018). To get high conductivity fracture network and reduce the fracture initiation pressure underground, acidizing technology (Qi et al 2012) and supercritical C O2 fracturing (Huang and Hu 2018; Ishida et al 2012; Jiang et al 2016; Ribeiro et al 2017) were introduced in the development of shale reservoirs, which would create a weakly acidic environment in shale formations. For Longmaxi shale selected for the experiment, acid-soluble minerals account for nearly 10% and distribute in the rock matrix heterogeneously. Carbonates (calcite and dolomite) can account for up to 40% of shale (Dai et al 2014; Yang et al 2016). Further research on the acid/shale reaction becomes necessary
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