Through Casing Fractures Mapping Using Acoustic Measurements: A Case Study in Najmah-Sargelu Formation, West Kuwait

Abstract

The Jurassic Najmah-Sargelu formations in Kahlula field of West Kuwait is a tight carbonate fractured reservoir. In such tight environment, the fractures are the most contributing to oil production. Therefore, fracture characterization is required to optimize reservoir production. However, because of the difficult drilling environment, the acquisition of open-hole logging data is not guaranteed. Moreover, as the conventional imaging tools (resistivity or ultrasonic) can only be operated in open-hole, any fracture evaluation would be a significant challenge after the well is cased. This paper presents a method how to characterize geological features such as natural fractures, faults and beddings in cased-hole, in the absence of conventional open-hole image logs. The Deep Shear Wave Imaging (DSWI) is the method used to characterize fractures network in this case study. The method consists of using through casing borehole acoustic measurements to map fractures, or any other geological features such as faults and beddings. Standard wireline acoustic and cement bond log services were logged in a cased well covering Najmah-Sargelu formations. The acquired acoustic data are monopole, in-line dipole and cross-line dipole waves. The dipole waves were used to map the fractures, and the other geological features, crossing the wellbore and none-crossing the wellbore when they are in the area of investigation. The cement bond log data were acquired to evaluate cement bond quality behind casing for well integrity purpose, and also for well completion purpose. The fracture mapping technique is basically the analysis of the very weak wave signals reflected back from the formation when the transmitted waves hit a reflector with an acoustic impedance contrast. This reflector can be a fracture, a bedding plane, or a fault. The low attenuation property of the flexural waves allowed us to see fractures up to 100 feet away from the wellbore. The analysis allowed locating the fractures along the wellbore, their extension in the formation, their dip angle, and their location relative to the wellbore. Well-X sidetrack in Kahlula field was selected to evaluate DSWI technique. If the result is promising then it will be implemented widely across West Kuwait fields. The benefit of using Well-X is that it already had open-hole image logs in the main bore to validate DSWI results. The result illustrated that fractures picked from DSWI matched well with what has been observed from the main bore of Well-X. Consequently, the DSWI result were used to select the most fractured intervals for testing and completion design in Well-X sidetrack. The tested interval showed commercial oil production with minor water cut. The successful application of DSWI enabled the deployment of this technique in West Kuwait fields to replace open-hole image logs in case the acquisition of openhole logging data cannot be conducted due to harsh well conditions.