An Interwell Water Flood Study: Flow Pattern and Reservoir Heterogeneity Evaluation of Tamabra Formation of Poza Rica Field Area 1

Abstract

Abstract A successful reservoir flood depends on prior knowledge of the flow stream within a reservoir and a proper injection/production pattern.A northern field formation in Mexico, which currently includes 44 injectors and more than 150 producers, has been under water flood since early 1965. Due to the loss of injection fluid and low sweep efficiencies a chemical interwell tracer program was designed and implemented to fully investigate the flow stream within the current injection/production pattern and to evaluate reservoir heterogeneity.However, based on the results of seismic and well test analysis and due to the large number of active injection and production wells, the entire field was divided into four sections for this interwell tracer study.This paper presents the interwell study results for the upper section of the north field, which includes seven injectors (A to G) and 36 producers (1 to 36).A detailed interwell study over a 10 month period indicated the existence of massive high-permeability channels as well the existence of a major fault initiating from this section and extending downward into the lower section of the field.Also, the results indicate the existence of several transverse faults along the main fault.Detailed graphical results to identify the flow stream, a comprehensive interpretation of the heterogeneity of the formation, and tracer travel time are also presented. Introduction Secondary recovery is a process in which reservoir fluid is mobilized and is moved from an injection well toward a production well.The success of this process greatly depends on the knowledge of reservoir continuity and uniformity in terms of fluid transmissibility and how much of the reservoir fluid volume can be contacted by the injection fluid.Techniques such as seismic, geological deposition and reservoir simulation provide valuable information about the feasibility of secondary recovery projects.The heterogeneity of a well may be studied by various methods such as well logs, formation permeability and conductivity data, pressure transient and seismic analysis.However, the actual distribution of reservoir fluid transmissibility can only be evaluated from pressure transient and interwell tracer data.Also, a more supportive method to fully describe communication between the injectors and producers is to trace the interwell flow line of injected fluid during a flood project. Interwell tracer testing consists of injecting either a radioactive or chemical tracer(s) into an injection well(s) during a gas or waterflood job and subsequent sampling of production well(s) for a prescribed period of time. Samples are analyzed for tracer content, which will delineate communication between the injection and production wells.Tracer testing provides detailed information on the characteristics of the flow-pattern in the reservoir, which include flow barrier, preferential flow-path, and pore volume swept at breakthrough. Tracer testing in the petroleum industry started in 1962 and is a well-established technique to evaluate interwell communication as well as reservoir heterogeneity.[1–7]The primary use of interwell tracers is to precisely identify interwell communication and reservoir continuity.Pressure transient tests can provide information about reservoir continuity, but thief zones are difficult, if not impossible, to detect.This is due to the fact that pressure transient tests provide an arithmetic average for reservoir total transmissibility over the tested thickness,[8]while tracer testing provides a direct communication evaluation. In a tracer job, as tracer is injected into the reservoir, the imbibition process, due to the preferential wetting phase, may be large.The injected tracer may then imbibe or be trapped in the pores.As a result of trapping, only a small quantity of tracer is dispersed in the flood front. For this reason, the partition coefficients of tracers must be known prior to injection, if tracer is used for residual oil saturation evaluation.Although the principle of tracer application for this purpose is similar to that of interwell tracing, the interpretation of data is somewhat different.It is out of the scope of this paper to discuss tracer's entire applications, and therefore, focus will be on the direct communication between the injection and production wells and heterogeneity evaluation of the formation.