Changes in Porosity Due to Acid Gas Injection As Determined by X-Ray Computed Tomography
Authors: Vickerd M.A. , Thring R.W. , Arocena J.M. , Li J.B. , Heck R.J. 
Published: March 2010

There is an increasing trend to dispose acid gases (H2S and CO2) generated from natural gas processing by geologic sequestration. Industry and government must carefully assess the compatibility of the formation matrix and fluids, with the intended injection stream, to ensure safe and efficient facility operations. We investigated cores subjected to acid gasflooding using X-Ray computed tomography (CT) analysis. The changes in pore morphology, pore size, and pore distribution were examined. Results indicated that the porosity roughly doubled and pore size increased substantially. Visually distinct areas of the core suggested that changes in porosity were non-uniform, probably due to the natural heterogeneity in the rock and/or regions of nonuniform flooding due to lack of pore interconnectivity in the matrix. Detailed CT image analysis revealed traces of halite (NaCl) probably due to the desiccation of the initial water saturation by the highly under-saturated injected gas. Our results suggest that changes in porosity are not attributed to dissolution processes but due to fine dislodgement and desiccation. Consequently, the permeability and porosity increases were the result of physical processes rather than chemical ones.

Introduction

Acid gas injection into geologic formations is an alternative way to dispose the undesirable waste stream produced from natural gas processing. This technology allows sour gas disposal to be economically viable and also provides a more environmentally friendly option compared to the conventional flaring method. To ensure successful disposal in a safe and efficient manner, adequate disposal information such as the compatibility between the fluid and the reservoir matrix is needed. It is a regulatory requirement to prove the formation and injection fluid compatibility as stated in government disposal guidelines(1)

The rock permeability and porosity are the most important properties that control the fluid flow necessary for a successful injection operation, and they are typically assessed through core sensitivity tests(2–4). Alteration of the formation porosity or permeability during acid gas injection occurs mainly through chemical or mechanical mechanisms(3) such as desiccation, dissolution, and fine dislodgement. Usually these mechanisms are assumed to impose detrimental effects on the permeability of the matrix, but in some cases, benefits result from injection operations due to increases in permeability and/or porosity(2)

Desiccation

Prior to the actual injection, the acid gas stream, once separated from the natural gas stream, undergoes the processes of compression and dehydration. Dehydration removes water in the gas stream to reduce the possibility of hydrate formation. Once the undersaturated acid gas stream enters the formation via the injection well, desiccation of the formation occurs upon contact with the irreducible water present in the pore spaces(2, 3). Reduction in water saturation increases injectivity due to lessening of adverse permeability effects associated with the presence of the initial water in the porous media(2). A concentrative effect occurs if the initial irreducible water is highly saturated with soluble salts or if the initial water is reduced to significantly low levels such that the remaining water is supersaturated with dissolved ions.