Abstract
Coal powders, as cuttings, invade the drilling fluid along a coal seam during coalbed methane development, thereby changing the properties of the drilling fluid. Therefore, this work aims to investigate the influence of coal powders on drilling fluid performance. The powders of lignite, anthracite, and contrasting shale were added to a water-based polymer drilling fluid. Then, the rheology, filtration, lubricity, and adhesiveness were measured, and the natural degradation, as well as the wettability were further evaluated. The results show that some parameters of the drilling fluid, including viscosity, lubrication coefficient, adhesion coefficient, contact angle, and surface tension, increase after adding coal powders, while other parameters, such as filtration loss and natural degradation, decrease. Compared with lignite and shale, anthracite powders, with the lowest mineral content, exhibit the smallest change in the rheological property, lubricity, adhesion, and natural degradation of the drilling fluid. Moreover, the content and size of the coal powders generally have opposing effects on the drilling fluid. When the coal powder content reaches 3 wt.%, the surface tension and contact angle of the drilling fluid show more evident changes than other parameters. Based on the analysis of the stress intensity factor, the drilling fluid with coal powders exceeding 100 mesh can reduce the capillary force in microfractures, and in combination with other factors (such as reduced filtration loss and sealing and supporting of the microfractures), improves wellbore stability. Therefore, coal powders with suitable particle sizes and concentration levels are expected to become a new drilling fluid material to protect coal field reservoirs.
Highlights
With the development of the global economy, the demand for energy is increasing (Boycheva et al, 2020; Kivi et al, 2018; Li et al, 2017)
The screened lignite, anthracite, and shale sample powders were added into the prepared water-based polymer drilling fluid according to a certain mass fraction, the ZNN-D6 sixspeed rotary viscometer was used to record the readings of 600 rpm and 300 rpm (h600 and h300, respectively), which were used to evaluate the rheological properties of mud and measure the six-speed values to calculate apparent viscosity (AV), plastic viscosity (PV), and yield point (YP)
When coal powders are mixed into a water-based polymer drilling fluid, the performance of the drilling fluid changes, most of the indexes change minimally
Summary
With the development of the global economy, the demand for energy is increasing (Boycheva et al, 2020; Kivi et al, 2018; Li et al, 2017). Unconventional energy sources, such as coalbed methane (CBM), shale gas, and tight sandstone gas, have replaced some conventional energy sources (Akhtarmanesh et al, 2013; Lu et al, 2017; Shen et al, 2019; Wu et al, 2020; Zhong et al, 2011). PetroChina Coalbed Methane Co., Ltd has been continuously intensifying the exploration and development of CBM, and it has achieved an increase in the average annual production and sales equivalent to 50 Â 104 t of oil, with an annual output of 200 Â 104 t upon the completion of the construction of an unconventional gas field in 2019 (Wen et al, 2019)
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