Itnproving Producibility With Cased-Hole Wireline Techniques

Abstract

Abstract Many oil wells thoughout Alberta are being worked over to increase production＿ Often, the open-hole. logs do not provide adequate information jor planning these workovers. This is because reservoir conditions have changed since the initial evanluation and completion, and there is uncertainty as to the present extent of water encroachment. Measurements though casing, complemented by the open-hole log data and down-hole production measurements, can serve to locate the remaining oil zones and indentify their producibility. Computer techniques can be used to simplify interpretation procedure and combine all of the data into a usable presentation. Examples presented here are localized to the. Beaverhill Lake reservoirs of north-central Alberta. The methods described have broad applicability to other reservoirs throughout Alberta. Introduction IN THE MANY oil wells throughout Alberta to beworked over for increased production, there is often uncertainty as to which intervals have been watered out by a flood front. The open-hole log data (when available) are no longer completely representative of the oil in place. As a result, many old wells are being re-evaluated with cased-hole wireline techniques to locate the remaining producible oil zones. measurements with the Dual Spacing Thermal Decay Time tool(1,2) (TDT*-K), which can be run through tubing, provide the required porosity and water-saturation information. Thermal neutron decay time (7), like resistivity, increases with decreasing porosity or water saturation (i.e., with increasing oil in place), and vice versa. Quantitative methods have been published(J) for using thermal neutron decay time (T) and porosity to obtain water saturation behind casing. In the event that open-hole log data are not available, the ratio measurement from the TDT-K system can be converted to apparent porosity. Ratio, which is derived from the count rates of the two detectors, increases with porosity. Comparison of the values from the cased-hole logs with the results from the original open-hole logs serves to define changes in reservoir conditions and to locate the remaining oil zones. Log-derived values of porosity and water saturation can be used to verify the productibility of these zones prior to perforating.(4,5) These re-evaluations, using cased-hole logs, are providing a new information base for planning workovers. They have been used with considerable success in reefal limestone reservoirs of north-central Alberta, Enough wells have now been evaluated to establish the interpretation procedure. Computer techniques have been used to combine the open-hole and cased-hole log data and to document the results in an easily usable presentation. Although the examples discussed here are localized, the methods described have broad applicability to other reservoirs throughout Alberta. Identification of Oil-Bearing Intervals Behind Casing Wells in the Judy Creek Field are typical of those requiring cased-hole evaluation prior to workover. Production is from the Swan Hills member of the Beaverhill Lake Formation. The field is being waterflooded using a flank-peripheral injection scheme.* Porosity and permeability vary throughout the Bection and water advances preferentially along the most permeable paths from the injectors to the producing wells. When recompleting watered-out wells, there is uncertainty as to which unperforated porous intervals are still oil-bearing.