Interwell Tracers - An Effective Reservoir Evaluation Tool: West Sumatra Field Results
Authors: D&apos;Hooge John A. , Sheely Clyde Q. , Williams Billy J. 
Published: December 2007

Summary
Conoco successfully has used four different radioactive tracers simultaneously to determine the direction of fluid movement in a sequence of Pennsylvanian-age sandstone benches within the northern half of the West Sumatra Unit. Radioactive tracer material introduced at the injection well and identified at the producing well provides the flow direction and rate of movement of injected fluids. This information will be used to maximize sweep efficiencies and optimize depletion plans in this multipay, discontinuous reservoir.

Introduction
Production in the West Sumatra Unit, Rosebud County, MT, originates from one or more of four stratified and lenticular sandstone benches within the Lower Tyler formation of Pennsylvanian age. The sandstone benches, which are the result of stream channel deposits, alternate with shale and/or limestone and are bound on top and bottom by unconformities (Fig. 1). Thus, the correlation of individual sandstone benches and the continuity between wells becomes a matter of interpretive judgement. Moreover, the heterogeneous nature of the Lower Tyler sand poses considerable difficulty in matching producing responses to offset injection. Therefore, it was decided to implement an interwell radioactive tracer program and use results of the program to maximize sweep efficiencies and optimize depletion plans in the unit.This paper describes the implementation and initial results of an interwell radioactive tracer program conducted in the northern half of the West Sumatra Unit. The basic design of the tracer program, injection of the tracer, sampling and rate measurements, and analyses of the produced water are described. The results of the program clearly demonstrate that radioactive tracers can be used effectively and economically to diagnose reservoir pay continuity and fluid movement. The test further confirms the advantage of radioactive tracers over chemical tracers.Conoco used four radioactive tracers - carbon- 14, cobalt-57, cobalt-60, and tritiated water - to resolve the relationship of injectors to offset producers in the field. Results of the program have identified both reservoir continuities and discontinuities as well as probable areas of poor sweep efficiency. In several instances, geologic interpretations were changed as a result of the tracer study. Further monitoring of tracer concentrations will allow the calculation of the optimal secondary input volumes that should be directed toward the respective offset producers.

Theory
The primary objective of an interwell tracer program is to define the direction of fluid movement from a particular injection well to an offset producer. Assessment of channeling effects, volumetric sweep efficiencies, fluid transmissibility, reservoir permeabilities, and permeability distribution are all secondary goals. Before the program can be initiated, an estimate of the fluid response time between injector and producer is required to decide whether the project is feasible time-wise. A method for determining the time required for tracer-tagged fluid to flow from injector to producer was used for this purpose. According to this method, the time required is mainly a function of the effective permeability to the injected fluid and the total pressure drop from injector to producer, assuming the most permeable continuous layer between producer and injector will give the fastest response.
JPT
P. 779^