Tracer-Test Interpretation in Naturally Fractured Reservoirs

Abstract

Summary This paper presents a solution for the inverse problem, or interpretation, to the flow of tracers in naturally fractured reservoirs. The model considers cubic block geometry. The Rosenbrock method for nonlinear regression used in this study, allowed the estimation of up to six parameters for this cubic block geometry. The nonlinear regression for the three cases was carefully tested against synthetical tracer concentration responses affected by random noise, with the objective of simulating as close as possible step injection field data. Results were obtained within 95 percent confidence lirnits. The sensitivity of the inverse problem solution on the main parameters that describe this flow problem was investigated. The main features of the nonlinear regression program used in this study are also discussed. The procedure of this study can be applied to interpret tracer tests in naturally fractured reservoirs, allowing the estimation of fracture and matrix parameters of practical interest (longitudinal fracture dispersivity a, matrix porosity, fracture half-width w, matrix block size d, matrix diffusion coefficient D2 and the adsorption constant kd). The methodology of this work offers a practical alternative for tracer flow tests interpretation to other techniques. Introduction Current hydrocarbons production comes importantly from naturally fractured formations. The behavior of these reservoirs is quite different from that of "homogeneous" - conventional - reservoirs. The complex matrix-fracture interaction of these systems makes their characterization a challenging task. Among the different tools currently available to accomplish this endeavor, tracer test interpretation is taking an ever increasing role. These interwell tracer tests have significantly contributed to the better understanding of the fluid flow in these systems. Radioactive and chemical tracers have been used for many years in groundwater hydrology to analyze the movement of water through porous formations, but their use in geothermal and petroleum reservoir engineering is more recent. It has been recognized, as already stated, that tracer test interpretation, in addition to well-to-well pressure transient tests, is a very important contribution towards accomplishing the characterization of naturally fractured reservoirs. As concluded by these authors, these two testing techniques are complementary, not competing. There are several papers that discuss the flow of tracers in naturally fractured reservoirs. For a review of the recent work the papers of Rarnirez et al. may be consulted. Most of these studies deal with the direct problem (i.e., predicting the tracer response behavior from the knowledge of pertinent reservoir and tracer parameters).

Characterizing Partially Fractured Reservoirs by Tracer Injection

“…The applications of tracers to study naturally fractured reservoirs have been the subject of numerous studies (Wagner 1977, Tester et al 1982, Ramirez 1993, Shinta et al 1993, Daltaban et al 1994, Ramirez et al 1994, Sato et al 1994, Zellou et al 1995, Maroongoog et al 1995, Deng et al 1995, Wattenbarger et al 1995, Almeida et al 1996. Despite all these studies (only few cited here), the response of tracers in partially fractured reservoirs is yet to be investigated.…”

Detailed numerical evaluation of diffusion convection equation in layered reservoirs during tracer injection

“…Ramirez et al investigated the response of the tracer test in matrix fracture medium and tried to estimate fracture opening and also matrix diffusion coefficient in different geometries 7 , 21 . Samaniego and Rodriguez used the tracer data in an observation well to estimate some reservoir parameters in a fractured reservoir using a trial method 22 . Qasem et al showed that parameters such as porosity and permeability and their relationship significantly affect tracer results in fracture-matrix systems 23 .…”

Fracture Characterization Using Flowback Water Transients from Hydraulically Fractured Shale Gas Wells

“…Many researchers used RTA and PTA to characterize the fracture network − because production data are available for almost every well. Micro-seismic monitoring is also broadly used in the field to characterize the fracture network during and after the hydraulic fracturing operations. − Tracer test is another method commonly used to characterize the fracture network. − Each of these three methods has its strengths and limitations. No one method is solid enough to be applicable to all wells, especially when dealing with field data.…”