Logging Method for Determining Horizontal Velocity of Water in Oilfield Formations

Abstract

A new borehole measurement technique has been developed for determining horizontal velocity of waterflowing in a reservoir. Neutrons are used to make a radioactive sodium tracer in formation salt water streaming around an unperforated cased observation well. Monitoring of gamma rays from the tracer provides data from which the horizontal velocity can be calibrated. Introduction In the enhanced recovery of oil, many techniques employ the injection of water or chemical solutions into the reservoir formations. To flood the reservoir effectively, horizontal continuity must exist between injection and producing wells, and good vertical conformance of the injected fluids must be maintained.Intervals inferred to be correlative from log data may be separated from one well to the next by reduced permeability caused by natural factors such as formation tensing, shale barriers, or faults. Reduced permeability also can be caused by factors resulting from production operations, such as migrating fines, swelling clays, emulsion blocking, scale and paraffin deposition, and sand production.Conversely, situations can arise where a zone may carry away excessive injection fluids. Such thief zones can be caused by channeling into adjacent beds or by fractures in the reservoir. When planning the injection of costly chemicals into a recovery pattern, it is essential to determine the magnitude of the problems well in advance. problems well in advance. Radioactive injection surveys, well-to-well pressure testing, and chemical tracer surveys can pressure testing, and chemical tracer surveys can provide useful data. These techniques are somewhat provide useful data. These techniques are somewhat qualitative in layered reservoirs and, in the case of tracer surveys, can require several weeks to obtain definitive results. Consequently, there is a need for a system to measure specifically the horizontal flow of water in selected zones of a reservoir.To meet this objective, a new nuclear logging technique for measuring horizontal water velocity in cased-off formations has been developed and field tested. At present, the system is limited to station measurements in the borehole. A radioactive tracer, sodium-24, is manufactured within the formation salt water by neutron irradiation. Over a period of time the radioactive tracer is monitored as it drifts away from the borehole. From these data, the horizontal water velocity is computed. Method and Theory Fluid now velocities in a typical reservoir range from about 0.1 to 1.0 ft/D (0.03 to 0. m/d). In some pilot projects, a monitor well close to an injector can have projects, a monitor well close to an injector can have fluids passing at several meters per day. In our method to measure the horizontal velocity, an unperforated cased borehole penetrating the formation of interest is used, since undisturbed streamline flow around the borehole is required.Neutron irradiation of a reservoir formation produces several radioactive isotopes that emit produces several radioactive isotopes that emit gamma rays with characteristic energies. Some of these are listed in Table 1. JPT P. 675