Tritiated Water As a Drilling Mud Tracer In Beaufort Exploration Wells
Authors: Millar R.H. , Buckles R.S. 
Published: March 2010

In the Beaufort sedimentary basin, the salinity of native formation waters is unusually low and highly variable. This has made it difficult to distinguish between invaded mud filtrate and formation water when interpreting the results of limited-volume tests. Examples are presented to demonstrate the need for a reliable drilling mud tracer. The advantages of tritiated water for this purpose are discussed and procedures for tracing mud systems are described. Besides providing a means of quantitatively determining the relative amounts of recovered filtrate and formation water, the technique can also give an indication of the radial extent of filtrate invasion. Example applications of the tracer technique are presented for a number of Mackenzie Delta exploratory well tests.

INTRODUCTION

FORMATION WATERS in the Beaufort sedimentary basin have relatively low and highly variable salinities even at considerable depths. This makes it difficult to establish the true salinity of undiluted formation water, and the relative amounts of formation and mud filtrate waters recovered from limited-volume tests. A knowledge of formation water salinity (and hence resistivity) is important in exploration situations for improved log interpretation and for formation fluid mapping. Without good salinity data, formation evaluation contains an added element of uncertainty, and sometimes cost, incurred, for example, through more extensive testing.

An obvious solution to this problem is to introduce a tracer completely foreign to the subsurface environment. A suitable quantitative tracer should ideally be a substance which is:foreign to normal subsurface fluid systems,water soluble,chemically and biologically stable at reservoir conditions,not susceptible to adsorption on mud solids or formation rock,low in evaporation rate,not detrimental to mud properties,not detrimental to geochemical hydrocarbon analyses, andsubject to rapid, simple and accurate analysis at the well site.

Virtually all chemical tracers are deficient in at least some of these requirements. Those for which routine methods of analysis are available generally exhibit enough adsorption, evaporation or reaction losses to render them unquantitative. Common chemical ions such as chloride, carbonate, bicarbonate and sulphate are similarly unsuitable, as they are usually present in the mud in unpredictable, widely variable concentrations.

For short-term wellbore tracer applications, the nitrate ion is perhaps the best all-round chemical tracer available to the industry(1,2,3). However, even the nitrate ion is known to undergo chemical and bin-logical alteration with time at formation conditions. The nitrate method is best suited to tracing just prior to drilling and testing an objective horizon. It can he cumbersome when the mud filtrate is discoloured (perhaps due to lignosulfonate additives) and the normal visual colorimetric test becomes impossible.

In frontier exploration wells, there may be many zones of interest from surface casing to final total depth. Testing is usually delayed for many weeks while drilling continues casing is run and deeper zones evaluated. Accordingly, if a mud tracer is to be effective, uniform concentrations must not only be maintained in the mud system at all times, but also, on migrating into a permeable formation