The shale gas boom has been a game changer for the energy industry. Many drilling problems related to wellbore instability are caused by incompatibilities between the drilling fluid and the shale formations encountered. This problem is not new, and the drilling industry has developed many effective testing methods for evaluating the compatibility. However, these useful methods are not effective for unconventional resources, as unconventional shale is very different from traditional swelling shale. This paper presents recently developed testing methods for on unconventional shale. The new testing methods presented in this paper include: a) Indentation test to evaluate shale mechanical property degradation as a function of drilling fluids’ chemistry and rheology over time; b) Scratch test with repeated passes to evaluate the depth of shale mechanical property degradation; c) Fluid penetration test to evaluate drilling fluids’ penetration rate into the shale and sealing of micro-fractures in fissile shale; d) Wellbore stimulator to evaluate drilling fluids’ chemistry, rheology, and filtration effects on wellbore stability under downhole pressure and temperature. Test results on the Eagle Ford and Marcellus shale are presented to illustrate the effectiveness and usefulness of these testing methods for evaluating drilling fluids’ effects on unconventional shale and wellbore stability.


The shale gas and oil boom has been a game changer for the oil and gas industry, with an investment of billions of dollars and creation of over two million new jobs [1]. Many thousands of shale gas and oil wells have been drilled every year. In the Eagle Ford shale play alone, over 4,000 new wells were drilled every year since 2012 [2]. Much experience has been gained and much progress has been made from drilling such a large number of wells [3], resulting in improved drilling efficiency from year to year. Wellbore or shale instability is still the largest cause of non-productive time in shale oil and gas drilling operations [3]. The design, evaluation, and selection of the most suitable drilling fluid is central to managing shale instability and to achieving drilling cost reduction.

Managing shale instability is not new to the drilling industry, yet oil/gas shale is quite different from traditional shale. The industry has been dealing with shale instability since the beginning of oil well drilling and has developed many effective and fit-for-purpose testing methods for testing and evaluating shale and fluid interaction [4]. These methods are often effective and useful in selecting the most suitable drilling fluid for highly or moderately reactive shale. When these traditional methods are applied to oil and gas shale, little or no differences with various fluids are seen from the traditional tests, resulting in the conclusion, according to conventional understanding and knowledge, that gas or oil shale is non-reactive shale and drilling fluids do not make a difference. Drilling experience in unconventional resources showed a clear difference of drilling fluids on wellbore stability and non-productive time [3, 5, 6].

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