The Use of Tracers in Diagnosing Interwell Reservoir Heterogeneities - Field Results

Abstract

A suite of "preferred" gas and water tracers is verified for field use in identifying interwell sweep problems and providing information useful to the design, control, and interpretation of a tertiary oil recovery process. Introduction In fluid injection projects, the channeling or by-passing of injected fluids through fractures and high permeability stringers results in poor reservoir sweep efficiency and low oil recovery. When the injected fluid is water, channeling problems have a less severe impact on the flood economics because water is relatively inexpensive and can be recovered and recycled through the reservoir to recover additional oil. However, many of the improved oil recovery processes employ expensive fluids, such as surfactants, micellar fluids, and solvents that must produce oil during a single pass of a relatively small volume produce oil during a single pass of a relatively small volume through the reservoir. It is important to identify and correct any serious reservoir heterogeneities that would lead to channeling and inefficient use of expensive improved recovery fluids. Some knowledge of the near wellbore reservoir heterogeneities can be derived from well logs and core permeability data. Pressure transient and pressure pulse tests are useful in detecting interwell fractures and in determining interwell communication. Other information sometimes is available from prior waterflood performance. A supportive method to determine reservoir interwell anatomy and how a reservoir would perform in an improved recovery process is to trace the interwell flow of injected water during an initial waterflood. For the past several years, the results of 20 tracer programs conducted in reservoirs undergoing programs conducted in reservoirs undergoing waterfloods, gas drives, and alternate water-solvent injection have become available to the author. These tracer programs provided a proving ground and opportunity to programs provided a proving ground and opportunity to screen the performance of numerous water and gas tracer materials. The results also helped arrive at a suite of "preferred" tracers for waterfloods and gas drives. This paper discusses the use of chemical and radioactive tracers to identify sweep problems in a tertiary miscible pilot area in West Texas, two potential micellar pilot areas in Wyoming, a Wyoming waterflood, and a hydrocarbon miscible project in Alberta, Canada. Discussion Information Obtainable From Interwell Tracing Specific information obtained from tracing the interwell flow of injected fluids through a subterranean formation and how this information is derived from the tracer data is discussed. This type of information is the objective of every oilfield tracing program and is useful in the design, control, and interpretation of subsequent tertiary oil recovery processes applied in that field. 1. Volumetric Sweep: The volume of fluid injected at an injection well to breakthrough of the traced fluid at an offset producer is indicative of the volumetric sweep efficiency between that pair of wells. Very small injected volumes to breakthrough (relative to the interwell pore volume) would indicate the existence of an interwell open fracture (or a very thin high permeability stringer) and would give an idea of the volume of that channel. Knowledge of channel volume is important to the sizing of a remedial treatment. JPT P. 1410