Abstract

Core analysis of poorly consolidated and unconsolidated formations iscomplicated by the inherent potential for induced rock matrix disturbance andexpansion. These alterations can occur after release from natural confiningstresses during coring and core retrieval. However, more representative corescan be obtained by carefully designed and controlled coring programs.

The single most significant laboratory problem imposed by unconsolidatedformations is maintaining the reservoir rock texture and pore geometry 1n theabsence of natural confinement. This problem can be minimized by recentimprovements in handling and confinement methods.

An improved unconsolidated formation confining technique has been developed, tested and has proven to be well suited to production laboratory scale volume.This method overcomes many restrictions imposed by other techniques. The methodis rapid and produces portable and competent samples upon which reproducibleporosity data can be obtained. The plugs can be conveniently retained in theirpositive pressure confinement mounting for subsequent specialized testing andcan be observed through the mounting sleeve.

Data obtained using the improved confining technique in unconsolidatedreservoir material are reported and discussed. Applications includeconventional core analysis, permeability refinement, porosity log verificationand investigation of formation resistivity factor, capillary pressure andrelative permeability relationships.

Introduction

Core analysis of poorly consolidated and unconsolidated Lower Cretaceous heavyoil bearing (10–25 ° API) formations encountered in Alberta and Saskatchewancan be problematic. Rock matrix mechanical strength is limited by the degree ofcementation and, as a result, matrix deformation and expansion can potentiallyoccur at any time after in-situ confining conditions are modified. The twoprominent areas of concern in maintaining grain-to-grain relationships arecoring/core retrieval and laboratory preparation/testing.

The potential for matrix disturbance is at particular significance duringcoring and core retrieval if precautionary measures are not taken to preventcore damage. Coring hardware and techniques have been developed andsuccessfully utilized to recover unconsolidated core under diverse conditions.These improvements in coring and core handling practices permit excellent corerecovery and, most importantly, recovery of essentially unaltered corematerial. Given the fact that the ultimate value of core analysis is dependentupon and limited by core quality, careful control of the coring program cannotbe overemphasized. To this end, practical coring recommendations for improvedcore recovery and core quality are included for reference In Table 1.

The second area of concern is maintenance of sand grain contact and orientationthroughout laboratory testing. The potential for core alteration duringlaboratory sampling and core confinement has been recognized.1,2 Inmost cases artificial consolidation or mechanical support is necessary tomaintain representative matrix and pore geometry conditions throughout theanalytical process. Although many techniques have evolved that have resulted inimprovements in core analysis and special core analysis data, the requirementto produce more versatile and representative artificially confined samples hasprovided the impetus for continued research and development. Currently utilizedanalytical methods and confinement techniques in Canada are intrinsicallyinadequate for all applications and. as a result, impose data gathering/datautilization restrictions.

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