Determination of Fracture Height by Spectral Gamma Log Analysis

Abstract

Abstract Determining fracture height following hydraulic stimulation of a well is an important step in both evaluating the effectiveness of the treatment and estimating the subsequent production behavior of the well. Unfortunately, the temperature and gamma ray logs commonly used to assess fracture heights seldom yield unambiguous results. A new approach to the problem employs spectral gamma ray data to resolve some of these uncertainties. This paper outlines the method and presents data from a paper outlines the method and presents data from a field study made using the new technique. Introduction Post-treatment fracture height measurements allow Post-treatment fracture height measurements allow the engineer to judge the success of the operation and to optimize future treatments, if necessary, for other wells in the field. Fracture height information can aid in the diagnosis of post-stimulation problems such as lower production post-stimulation problems such as lower production rates or unfavorable water cuts. The data can indicate whether communication has been established between the producing formation and adjacent thief or water zones. Finally, height measurements provide a check on the accuracy of fracture design simulators used prior to the job to predict fracture geometry. Excessive fracture height implies that fracture length is shorter than the design value. Current field techniques for fracture height evaluation include temperature, spinner, borehole televiewer, passive acoustic, and gamma ray logging. Most techniques provide some direct estimates of fractured zone height at the wellbore. Fracture height determination away from the well is based on inferences. Temperature logs detect the extent of the cooled down zone. Spinner surveys provide a quantitative value of well production or injectivity along tile fractured horizons. The borehole televiewer can only be used for fracture height evaluation in open holes. Acoustical methods are hampered by inhomogeneous formation impedance and/or the need for pumping while the tool is in the hole. The most common approach for determining fracture height uses temperature and gamma ray logs. Temperature logs made before and after stimulation can be compared to define an interval cooled by injection of the fracturing fluid and thus provide an estimate of the fractured zone. If radioactively marked fluid or proppant is used, postfrac gamma ray logs will show higher levels of activity opposite where the tracer was deposited. These areas can then be equated with the fractured interval, though both techniques are subject to limitations and ambiguities. For example, the temperature log may be difficult to interpret because of low temperature contrast, flowback from the formation before and after the treatment, or fluid movement behind the casing. An obvious shortcoming of the radioactive tracer method is that the gross gamma ray log cannot distinguish between tagged material which is inside the wellbore, in channels or voids in the cement, or actually in place in a fracture. Thus, a given level of activity on the log may correspond to a small amount of radioactivity inside the borehole or to a larger amount deposited in the fracture. This paper presents a method that uses gamma ray spectroscopy to discriminate among these cases. The method was applied to four fractured wells and the field results compared to predictions based on fracture simulators. The comparison provides good agreement and offers insight into future needs for development of the technique.