Chemoporoelastic characterization of Ghom shale

• Chemoporoelastic parameters required for modeling swelling behavior of a water sensitive rock are introduced.
• Mineralogical and apparent properties of Ghom shale are described.
• Techniques and procedures for testing shales, analysing the results and calculating the parameters are explained.
• Shale pore pressure in response to hydraulic and chemical loading is measured.
• Chemoporoelastic properties of Ghom shale are described and discussed.

Swelling and deterioration of shales are the main causes of wellbore instability and associated problems in the drilling of hydrocarbon exploration and production wells. Shales have some unique characteristics that impact the wellbore stability. A few physiochemical theories are proposed for describing drilling fluid–shale interactions. The main problem in using these theoretical developments for evaluating potential borehole instability problems and developing realistic predictive models is the lack of experimental data on the shale properties. This paper presents results of study to characterize the chemo-poroelastic parameters of the illite rich Ghom shale in the laboratory. These parameters include a wide range of mechanical properties, porosity, permeability, solute diffusion coefficient, swelling coefficient and reflection coefficient. The required samples are prepared from an undisturbed outcrop block of Ghom formation.The nitrogen gas injection method was utilized to measure the porosity of the shale sample. The pore volume was determined using the Boyles law technique. The permeability of the sample was calculated from transient analysis of the water injection pressure data. Consolidated drained and undrained triaxial tests were carried out to evaluate the required mechanical properties of Ghom shale. A diffusion cell was designed to measure the diffusion coefficient based on the concept of solute flow due to chemical potential gradient. A free swelling test was also conducted to investigate the swelling behavior of the rock samples in fresh water. Finally, a pore pressure transmission test was performed to quantify the osmotic properties of the shale in contact with a water-based fluid through calculation of the reflection coefficient.