Gas and oil shales vary considerably in terms of reservoir pressure, temperature, mineralogy and local stresses, however the key wellbore construction related issues are borehole stability, hole cleaning and rate of penetration. With many shale plays being in environmentally sensitive and/or densely populated areas, the traditional drilling fluids designs utilizing either strong chemical inhibition or conventional invert emulsions may not meet future environmental requirements. A new fluids approach to wellbore construction utilizing environmentally acceptable water based fluids has been taken to meet these demands.
This paper presents the steps taken in the design and formulation, plus results from the testing, of an environmentally and economically acceptable fluid that meets stringent criteria for fluid performance, wellbore stability and drilling performance. This novel water-based drilling fluid is based on fresh water where fluid properties such as rheology, fluid-loss, lubricity and shale stability are controlled with chloride free and environmentally-friendly products. The unique mechanism of shale inhibition that is the performance key to this technology uses nano-sized particulates designed to plug the pores and micro-cracks present and induced in unconventional shale formations. Laboratory testing on several oil and gas shales, evaluating shale permeability reduction on fluid exposure, has demonstrated several orders of magnitude of reduction in fluid permeability both in non fractured and naturally microfractured shales. This translates to no fluid invasion into the shale and stability of the wellbore. This newly developed environmentally friendly water-based drilling fluid for unconventional shale drilling is currently under field trial and verification.
