Abstract

Since 1975, RWE-DEA has carried out numerous successful polymer field projects. In the reservoir units where injection had been initiated in the late seventies, production is close to being terminated or can be estimated rather precisely by extrapolation of the concentration-versus-time profile. Polyacrylamide loss is calculated on the basis of injection and production data and amounts to 15 to 24 or 25 to 53, respectively. Taking into account several correction factors, characteristically specific field retention values are in the range of 8 to 14 or 13 to 28, respectively.

Field retention values are compared to polymer retention values determined in laboratory flood tests on packs of crushed reservoir core material and surface sands. As a general trend, polymer losses observed in laboratory tests are higher than those experienced in field applications. Possible interpretations are discussed.

Introduction

Sixteen years ago, our company started the first polymer flood project, which involved injection of a polyacrylamide solution into a well drilled in the middle block of the Oerrel-Sued field in North-West Germany. The project was small but technically and economically successful and justified the application of the process to other, similar blocks of this field and to adjacent fields.

Since then, eleven polyacrylamide projects have been carried out in the fields Oerrel-Sued, Hankensbuettel-Sued and Hohne with polymer injection still going on in two projects presently. Some of the follow-up projects were definitely larger in scale than the pilot project in the Oerrel middle block. Up to now, a total volume of more than 5,000 metric tons emulsion polymer - correspond into so: 1,600 metric tons polyacrylamide - has been injected. The polymer was supplied in the form of an emulsion of about 1/3 active matter, 1/3 oil, and 1/3 water. In a first step, the emulsion was inverted by means of an activator in continuously operating, fully automatic units. In a second step, the stock solution thus obtained was diluted to the designed concentration.

The success or failure of a polymer project depends on various parameters such as mobility ratio, recovery factor at the beginning of polymer flooding, and rock wettability. An essential parameter in this connection is polymer loss in the reservoir during flooding, which may be due to adsorption, filtration, polymer retained in fluids not produced, polymer trapped in dead-end pores, or polymer clinging to rock material, among other factors. This paper presents a comparison of the retention values obtained in laboratory flood tests and actual polymer losses in selected field projects.

Projects suited particularly for our purpose were those where polymer injection had been started in the late seventies and polymer recovery was either nearing completion or could be estimated by extrapolation of the concentration/time profile. The volume of partially hydrolyzed polyacrylamide so far recovered in all projects is 330 metric tons or 21 per cent of the total amount injected.

POLYMER DETERMINATION IN THE PRODUCED FLUIDS

The amount of polymer recovered was determined on the basis of regular analyses of water cut and polymer contents of the produced fluids. Polyacrylamide consists of a chain of carbon atoms, with branches on every second atom. The branches are either amide or acrylate groups. The amide/acrylate ratio is known to have an impact on the polymer tolerance toward brine and on the viscosity yield in fresh water.

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