This invention relates to an improved method of logging a well in a subterranean formation which has been subjected to a fracturing operation. More particularly, the invention relates to a method of logging a well in a subterranean formation which has been fractured by injecting fluid and proppant material tagged with at least one radioactive material into the formation through the well.
Fracturing subterranean, e.g., hydrocarbon-bearing, formation is well known as a technique to impart porosity to the formation or otherwise render the formation more suitable for economic recovery of the mineral values contained therein. Formation fracturing often involves injecting relatively large volumes of an aqueous fluid and proppant material, e.g., sand, mixture at relatively high pressure through a wellbore into the formation.
Post-treatment fracture height measurements allow the drilling engineer to judge the success of the fracturing 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 rates or unfavorable water cuts. The data could 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 design.
Current field techniques for fracture height evaluation include temperature logging, spinner surveys, wellbore televiewer, and passive acoustic and radioactive 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 the fractured horizons. Seismic methods, such as acoustic methods and the like, are hampered by inhomogeneous formation impedance and/or the need for pumping while the tool is in the hole. The televiewer can only be used with accuracy in open holes.
The most common approach for determining fracture heights 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, post frac 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.
Both temperature and gamma ray logging 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. Currently, gamma-ray logs are provided by gross or total gamma-ray detectors which count gamma rays without distinguishing between the differing energy levels of individual gamma rays. One substantial shortcoming of the current radioactive tracer method is that the gross or total 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 total gamma-ray log may correspond to a small amount of radioactivity inside the borehole or to a larger amount deposited in the fracture. Clearly, it would be advantageous to provide a more discriminating well logging method using radioactive tracers.