Abstract

Solids characterisation forms an integral part of many studies related to petroleum engineering, e.g., coreflood evaluation assessment of fines migration and characterisation of precipitates; and is often completed using routine analytical methods such as Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). These methods, however, cannot provide sub-micron detail of morphology and chemistry of solids and cannot be applied readily to characterise non-crystalline solids or solids present in low quantities. In this study, ATEM has been applied to examine solids isolated from produced fluids from an oilfield. The method has the potential to give early indication of production problems like sands production and scaling. It has also been used effectively to characterise scale inhibitor precipitates, which are non-crystalline, from corefloods and 'batch' experiments. Although ATEM cannot be viewed as a routine method since it requires specialised equipment and expertise, it can provide critical information.

Introduction

SEM and XRD are conventional routine solids characterisation techniques applied in many studies related to petroleum engineering, including reservoir evaluation, coreflood experimentation and applied research.

Formation damaging solids or precipitates range in size from millimetres to nanometers and can be both crystalline and/or non-crystalline in nature. Both SEM and XRD have limitations in providing comprehensive characterisation. The main limitation of SEM is its inability to characterise very fine solids (<1.0 m) or provide chemical analysis of sub-micron particles. XRD will only aid in the identification of crystalline solids. Solids which are amorphous (non crystalline precipitates/colloidal solids) will remain undetected, as will components present in minor amounts.

Analytical Transmission Electron Microscopy (ATEM) is a high resolution technique which can characterise in detail, both crystalline and non-crystalline solids in the size range from nanometers to microns. It has the capability to provide quantitative chemical, morphologic and crystallographic analysis of isolated sub-micron sized solids.

Solids Studied

In this study ATEM has been applied to determine morphology and chemistry of a variety of solids from produced fluids, evaluation corefloods and experimental studies.

Filters from produced fluid samples taken from separate producing wells of a North Sea oilfield have been examined. These formed part of a comprehensive analysis programme of both fluids and solids being undertaken on a new field prior to water injection.

Corefloods are often undertaken to evaluate the most effective application of scale inhibitor for a particular oilfield. Routine experimental procedures include fluid analysis of effluent and whole rock XRD. In this study, effluents were filtered through <0.02 m filters as the coreflood progressed and resulting 'trapped' solids examined by ATEM.

Scale inhibitor squeeze treatments are routinely undertaken by adsorption methods which applies a thin monolayer of inhibitor onto exposed mineral substrates. An alternative approach, by the emplacement of inhibitor precipitate within the rock porosity, has been evaluated by coreflood and 'batch' experimentation for application in a North Sea oilfield.

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