A Review of Waterflood Filtration
Authors: Kirk Joseph W. 
Published: December 2007

A basic review of waterflood filtration, this paper covers the history of filtration, theory, different types of filters and types of filter media. Gravity, pressure, diatomaceous earth and cartridge-type filters are described and their operation discussed. Filter media, such as sand, and coal are presented, and drain systems are discussed. Many of the common operating problems are also listed and described.

Introduction

For the engineer who must design a filter plant, the selection of the proper media, underdrain, etc., can be a bewildering task. Unfortunately, the theory of filtration has not advanced to the point where it can be of significant help. Experience thus becomes the engineers' best hope; however experience does not always assure optimum results, but it at least narrows the field.

The following discussion is to

provide a description of the materials commonly used in the oil field and the advantages and disadvantages of each and provide guidelines that will aid the engineer in operating the unit that is installed.

History of Filtration in the Oil Fields

The first two filters for the clarification of water to be used for injection purposes were installed 30 years ago in the Bradford-Allegany fields. These filters were of the pressure type and contained a graded sand and gravel bed. They were soon followed by gravity filters which were constructed of wood, steel or concrete and contained either sand or anthracite coal as the filter media. In 1937 the first pressure-type filter to contain a graded bed of anthracite coal as the filter media was put into service.

Theory of Filtration

Filtration may be defined as the process of passing a liquid containing suspended material through a suitable medium in such a manner as to effectively remove the suspended solids from the liquid.

Basically this is a mechanical action, and the larger suspended particles will not pass through the interstices of the filter media due to their size; however this action alone may not effect complete clarification, since some of the suspended solids are small enough to pass through the filter media.

If there is appreciable turbidity in the filter effluent, coagulation may be necessary.' Coagulation, which may be employed in open pits, tanks or even in the line prior to the filter, tends to agglomerate the smaller suspended material and forms floc particles which are large enough to be retained by the filter media. Usually in the coagulation process sufficient retention time is provided to allow the larger and heavier coagulated particles to settle out and to prevent some of the unreacted coagulant passing through the filter and precipitating in the distribution system. For optimum filter operation, the water from the settling basin or tank contains some floc particles. Initially these particles penetrate the filter media through the numerous voids to a depth of from 2 to 4 in, The actual depth of this penetration into the media depends, to a great extent, upon the rate of filtration, the thoroughness of the pretreatment and the effective size of the filter media. As the filtration process is continued additional suspended solids are trapped in this upper portion of the media. This results in the formation of a thin blanket of sludge, commonly called the Schmutzdecke. The Schmutzdecke is a bulky mass of material composed of floc particles and bacteria and contains millions of small capillaries.

When the water is properly pretreated and some floc particles are continuously being carried over to the filter, a good Schmutzdecke is formed initially and filtration continues to improve during the filter cycle.

Normally it can be expected that with good filter operation all particles larger than 45 microns will be removed, although in many cases effective removal of smaller size particles is obtained.

It is apparent that the presence of floc particles in the water entering the filter contributes to more effective filtration. On the other hand, it is seldom desirable for the turbidity of the incoming water to be in excess of 10 ppm. If the amount of floc entering the filters is too large, the length of filter cycle will be substantially reduced.

JPT

P. 1220ˆ