Drilling with a Balanced-Activity Invert Emulsion Fluid in Shale: Is It Sufficient for Maintaining or Enhancing Wellbore Stability?

摘要

The great majority of wellbore stability problems encountered while drilling occur in shale formations. As invert emulsion drilling fluids (IEF) are commonly used to drill reactive shales in today’s challenging wells, better understanding is needed of shale failure with exposure to IEF. Over the years, the concept of shale stability and interaction with IEF has been linked to several theories: 1.Semi-permeable membrane theory 2.Osmotic pressure theory 3.Balanced activity theory Understanding of these individual theories is helpful to interpreting experimental data presented in this paper. The shale that is the subject of this study was cored from deepwater operations, offshore West Africa. The operator who supplied the core had previously tested core samples in air, in water, and in a triaxial test apparatus. The results from the tests are reported here. Later, experimental testing of the deepwater shale was performed with a new test apparatus that has been presented to the industry. In short, this new appartus allows for shale samples to be directly tested under downhole conditions, and results have been shared with the drilling industry. Analysis of the test results in light of “balanced activity” theory calls this theory into question. The results show that even at an activity level higher than the known shale activity level, water was able to enter the shale and weaken it. This paper uses the concept of “chemomechanical balance” to explain measured results in terms of the coupling of shale mechanical properties and IEF chemical activity and the resulting consequences for selection of activity levels for IEF. The great majority of wellbore stability problems encountered while drilling occur in shale formations. As invert emulsion drilling fluids (IEF) are commonly used to drill reactive shales in today’s challenging wells, better understanding is needed of shale failure with exposure to IEF. Over the years, the concept of shale stability and interaction with IEF has been linked to several theories: 1.Semi-permeable membrane theory 2.Osmotic pressure theory 3.Balanced activity theory Understanding of these individual theories is helpful to interpreting experimental data presented in this paper. The shale that is the subject of this study was cored from deepwater operations, offshore West Africa. The operator who supplied the core had previously tested core samples in air, in water, and in a triaxial test apparatus. The results from the tests are reported here. Later, experimental testing of the deepwater shale was performed with a new test apparatus that has been presented to the industry. In short, this new appartus allows for shale samples to be directly tested under downhole conditions, and results have been shared with the drilling industry. Analysis of the test results in light of “balanced activity” theory calls this theory into question. The results show that even at an activity level higher than the known shale activity level, water was able to enter the shale and weaken it. This paper uses the concept of “chemomechanical balance” to explain measured results in terms of the coupling of shale mechanical properties and IEF chemical activity and the resulting consequences for selection of activity levels for IEF.