Applying Water Chemistry to Recovery
Authors: Martin Waylan C. 
Published: December 2007

American Institute of Mining, Metallurgical and Petroleum Engineers, Inc.

This paper presents the established and theoretical influence of the chemical and physical properties of water on ultimate recovery of crude oil. Establishment of useful data and their consequent evaluation in regard to the effect of filterable solids and potential precipitates on recovery are covered. A review of experiences and current research on pressured water effects on the reservoir are presented, with special concentration on calcium sulfate.

Introduction

The primary objective in the presentation of this paper is to review field experiences and laboratory results of the chemical and physical studies involving secondary oil recovery by water repressuring. There has not been adequate opportunity to effectively correlate these conditions with the existing literature that reports data both directly and indirectly related to the subject matter.

The remoteness of the producing interval and the use of comparatively small core samples for studies have left us with the inability to accurately duplicate formations and their natural conditions. There particularly seems to be a limitation on the data of producing intervals in regard to the natural connate water [hereafter used as a reference to the total original interstitial water in the reservoir], the chemical characteristics of the matrix, and the sensitivity of the matrix to the foreign water introduced in an injection program and the changes that result therefrom. This paper makes a meager effort to reveal some of the conditions encountered and the interpretation of their implications.

FILTERABLE SOLIDS IN INJECTION WATER

A factor that is frequently puzzling in a waterflood project is the significance of the filterable solids. The author suggests the use of this term "filterable solids" as describing that material suspended in the water that has particle sizes in excess of 0.5mu [one micron equals 0.001 mm] in diameter. This generally differentiates the significant suspended solids from smaller suspended particles such as colloids, which are not filtered out by normal millipore filter tests and are not considered of significance in a flood water. This also separates these filterable solids from suspended liquids such as oil.

The interpretation of the significance of filterable solids in injection waters is relatively complex and involves a multitude of factors. The nature of the solid is considered the primary factor in such an evaluation. It becomes necessary to include consideration of the particle sizes, the relative number of the different sizes, and the physical characteristics of the particles. Another factor of considerable concern in regard to these particles is the permeability of the combination at higher pressures. Also to be considered in this regard are the interstices in the formation through which these particles must pass. This in itself involves considerable complexity in view of the different sizes of interstices in a single zone and the relative distribution of these sizes. This is further complicated by the variation in interstics from one zone to another in the same producing interval. It is immediately apparent that this subject is entirely too broad to allow detailed coverage in the paper; therefore, it will be covered more generally by relating actual conditions and the implications.

In making massive numbers of filtration tests at the injection wellhead pressure, it is immediately apparent that, if the producing interval were as tight as a millipore filter [0.45mu], then the injection well would plug almost immediately in many instances.