Abstract

The objective of the study was to test a new methodology that combines non-destructive, high-precision analytical techniques to detect and quantify formation damage introduced by drilling fluids.

Reservoir core material and standard Berea sandstone samples were exposed to synthetic oil based and water based drilling fluids under downhole conditions simulated by radial drainage, dynamic filtration experiments. Alterations in the rock pore structure were investigated using non-destructive methods including 10-MHz Nuclear Magnetic Resonance (NMR) relaxation measurements, NMR Imaging and Computed Tomography (CT) Scanning. Backscattered Electron Imaging (BSEI), core analysis and mercury porosimetry provided additional, quantitative information on baseline rock properties.

Results show that several types of damage can be identified and quantified. Mud solids (barite) caused shallow, partially irremovable internal filtercakes. Polymers accumulated deeply inside core plugs and are considered to be potentially damaging. The synthetic oil based fluid introduced surfactants into the rock which could not be removed by backflooding.

The study demonstrates the high potential of the methodology to generate comprehensive data on the type and distribution of components that can cause formation damage in reservoir rocks.

Introduction

Formation damage is the impairment to the productivity of hydrocarbon bearing rock formations caused by the combination of mechanical and chemical activities required to drill or complete wells or stimulate reservoirs. Wellbore damage occurs where introduced or released particles, water, emulsions or scaling products alter the hydrocarbon delivery system. Recognition and treatment of formation damage typically rely on core displacement tests in combination with traditional core analysis and petrographic analysis methods. Coreflood experiments are useful indicators of the changes in the baseline permeability, yet, they provide no direct insight into the causative and triggering mechanisms controlling damage. At present, any evaluation of the causative mechanisms combines coreflood experiments with petrographic procedures like secondary electron microscopy, cryogenic electron microscopy, thin section observation, X-Ray diffraction or X-Ray Fluorescence". Such approaches have either observational value, i.e. they are non-quantitative, or they may require to break the rock material prior to the analysis.

CT-Scanning, on the other hand, is a well established quantitative and non-destructive analytical method in formation damage analysis. Furthermore, NMR relaxation measurements (cf. Attard et al.), as well as NMR-Imaging have been applied to investigate a large range of petrophysical phenomena, transport processes and chemical reactions inside rocks. Earlier applications of MR-Imaging in non-destructive formation damage analysis have, for example, been reported by Fordham et al. Basan and Pratt combined Backscattered Electron (BSE) Microscopy with image analysis to both identify drill mud components in pore structures and quantify their abundance. Using any of these methods alone, will produce information limited to the individual character of the method. In combination, however, these non-destructive methods will provide comprehensive, quantitative data on the type and distribution of formation damage in a rock.

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