To optimize production in naturally fractured carbonates, it is important to locate and describe these fractures insitu. A combination of log measurements, core data and production surveys can be employed to locate and orient the fractures and assess their overall contribution to reservoir performance in fractured formations. These log techniques include the imaging of the wellbore with the Formation Micro Scanner © log (FMS) and the Borehole Televiewer (BRTV). This paper is a discussion on wellbore imaging with the Formation Micro Scanner (FMS) © and Borehole Televiewer (BRTV) ©. The image data is compared to detailed core analysis, where available, and used to determine the length orientation and probability of fractures as a function of porosity. The Digital Sonic Tool (SDT) © full waveform data is also discussed in locating fractures and in establishing the technique of the Stoneley wave attenuation as a means of identifying permeable zones.
In low porosity, tight formations, natural fractures are the primary source of permeability which affect both production and injection. The open fractures are not reservoir by themselves and do not contribute much to porosity but they provide an increased drainage network to any porosity. They may also connect the wellbore to zones of better reservoir characteristics.
Whatever the conclusions inferred from fracture location techniques, only dynamic measurements by pressure and production results can tell whether or not a fracture or fractured zone will produce. The productivity of the well will also vary widely with lateral and vertical extent of the fractures, fracture width, and matrix porosity and permeability.
The oil industry has gained experience in fracture finding over the years from many disciplines. For example, anomalies and fuzzy zones on seismic sections, extrapolation of outcrop studies to the subsurface, pressure interference between wells, unexplained overachiever wells, lost circulation zones during drilling, poor core recovery, high drilling rates and anomalies on most log responses, have been, correlated to the existence of fractures.
In order to optimize the technique of fracture identification, it is important to detect fractures insitu. Although, characteristics associated with fractures can be identified from anomalies on various wireline logs, it is advantageous to Image the borehole to not only detect the fractures but to determine their orientation and angle and density.
To accomplish this, several examples are discussed in which wireline imaging techniques using Schlumberger's Formation Micro Scanner (FMS) © log and Borehole Televiewer (BHTY) © are utilized. In addition, to the imaging and the standard resistivity and porosity data, Array Sonic (SDT) © log data provides acoustic data for fracture location along with the Stoneley wave attenuation referenced for relative fracture permeability or producibility. These wireline techniques are discussed, first, in technique, and second, with examples, with the imaging and acoustic techniques compared to the detailed core analysis, where available, as well as the Stoneley permeability approach compared with flowmeter data also where available. A new technique utilizing Stoneley reflections for a higher vertical resolution in locating fractures is also discussed.
