Injection Water Treatment Starts In The Producing Wells
Authors: Sloat Ben 
Published: December 2007

Introduction

People and industry need water. To manufacture a ton of paper requires 2,400 bbl of water. 180 bbl of water are needed to make a ton of steel and, although we are not used to thinking of oil in tons, if we did, it would take 70 bbl of water to produce one ton of waterflood oil. Every living creature and plant on the race of the earth has to have water and we humans will go through two barrels of it every day.

With world water supply constant and world water demand increasing at an alarming rate it is extremely important for every water consumer to match quality with intended use. Certainly all of the water needed to make a ton of steel does not need to pass U.S. Public Health Standards nor is pumping drinking water into the ground to produce waterflood oil a good match of quality and use!

Fortunately, water flooding has actually created a demand for bad water.

Instead of having a nasty salt water disposal problem on its hands the oil industry has a challenging opportunity to turn a liability into a profit. The technology required to process and reuse produced water is now at the point where costs can be estimated with confidence and profit forecasts have real meaning. And since most waterflood producing wells will lift ten times as much water as oil during their economic life we can say with tongue in cheek that "opportunity" is not only at hand but is overflowing on a few projects.

Four major water quality problems associated with reinjecting produced water are discussed in this paper. Mechanical and chemical procedures for defining each problem, correcting it and then monitoring water quality to guard against any recurrence of the trouble are presented.

THE SEPARATE SYSTEM MYTH

How many times have you heard the statement "our supply water won't mix with the formation References and illustrations at end of paper. water so we are going to keep them separate" - - A neat trick - - the only trouble is that it can not be done. Even in a "once through" system where a single supply water is used for all injection requirements and every barrel of produced water is disposed of separately, the supply well water and the original formation water have plenty of chance to mix in the vicinity of the producing wells. Collecting the return water and injecting it into a few input wells out on one end of the pattern does not change conditions in very many of the producing wells at the time of water break-through. In fact, a split system forces water troubles to localize in the producing wells where they are not only expensive to handle mechanically but also have an important bearing on oil production.

FIELD CASE HISTORY

Proceeding on the premise that there is no easy way to design around produced water problems the next step before starting a flood is to have the various source waters analyzed, mixed in all proportions with the formation water for mineral compatibility and then thoroughly checked for organic growths and bacteria.

Table 1 shows typical supply water and Red Fork formation water analyses for Creek County, Okla. floods. Fig. 1 was prepared using the chloride, hardness and calcium values for the two waters and then checked initially by actually analyzing known mixtures of the two waters.

Hydrogen sulfide was not found in any of the original formation water samples and was never detected in the water supply well during three years of regular monitoring. Nevertheless, as water broke through to the producing wells hydrogen sulfide started to show up in bleeder samples, Fig. 2. As the percentage of supply well water in the produced fluid increased, so did the hydrogen sulfide values. Cultures for sulfate reducing bacteria set up from produced water samples which contained H2S developed as many as 80 colonies per milliliter. And again, neither the original formation water nor the supply well water ever cultured positive for sulfate reducing bacteria.