AN INSIGHT OF THE SUBSURFACE THROUGH BOREHOLE IMAGES CASE STUDY OF MW-34 MENENGAI GEOTHERMAL FIELD, KENYA

Abstract

Borehole imaging has been one of the fast-evolving technologies in well logging. The understanding of geological formations and logged formations below the surface has greatly improved since the advent of borehole imaging tools. The presence of natural fractures provides important pore spaces and fluid flow conduits. Insights can be provided into the subsurface fracture (natural and induced) detection and characterization using image logs. Menengai Geothermal field is quaternary volcano located in the central part of the Kenya rift which hosts a high temperature geothermal system. No core has been acquired from any of the wells drilled in this field. Therefore, to have a better understanding of the subsurface, a few wells were selected for borehole imaging. There are different methods of acquiring borehole images that include; Optical , acoustic and micro-resistivity. In this study, acoustic boreholeimaging devices were used to image MW-34, a vertical well drilled in NE part of the Menengai Geothermal field in Kenya. with the QL 43 ABI device. This device is also known as "borehole televiewers." It operates with pulsed acoustic energy so that it can image the borehole wall within drilling fluids. Using WellCad Software to analyze the borehole image we were able to recognize structures through irregularities on the borehole wall. The structures were captured from the image by fitting sinusoids to discontinuity traces. Numerous low acoustic amplitude fractures, which appeared as dark sinusoids, were noted in the depths ranging from 1370-1380 m and from 1530-1550 m. A few large apparent apertures fractures have been noted with the largest of 1160.69 mm in size, some lithological contacts have also been noted and appear to be low acoustic amplitude at 1180 m and 2275 m among other depths. Generally, the fracctures have a strike orientation of NE-SW.