Use of Mud Microloss Analysis While Drilling to Improve the Formation Evaluation in Fractured Reservoir

Abstract

Abstract Massive or small losses of drilling mud, flowing from the wellbore into surrounding formations, can be considered as good indicator of the natural fractures permeability and can be successfully used to support and integrate other detection methodologies. Most of the techniques currently used for natural fracture detection do not clearly differentiate between fractures that allow fluid flow and those that do not. Image logs, for example, can accurately locate the presence of fractures but are unable to discriminate between open and closed fractures. The most commonly used techniques to detect the mud losses consist in monitoring the level of the mud pits with acoustic, floating sensors and/or using paddles set inside the flow line that measure the return mud flow rate with a small degree of accuracy. In the last years these sensors have been partially replaced by the more accurate electromagnetic flowmeters. These tools are able to measure the mud flow rate with an accuracy of around +/- 15-20 litres/min. The mud losses are now processed in time domain and no more in depth domain, permitting a better accuracy in the identification and location of the conductive (microlosses) events. Moreover, the analysis of the microloss shape can allow the identification of the event causing the losses (natural or induced fracture, matrix porosity etc.). The main advantages of using mud losses as permeability indicators can be summarised as follows:Direct information about reservoir permeability properties on a large volume of rock,Only conductive fractures are detected,Possibility of distinguishing damaged plugged up conductive fracture from impermeable ones,Inexpensive techniqueApplication and interpretation while drilling. Some case studies are presented in this paper, showing how the microloss monitoring and analysis can help and support the conventional Formation Evaluation in the detection and location of productive fractures. Introduction Mud losses can be considered as one of the most important and critical problem occuring during drilling an oil & gas well. Consequences of circulation mud losses can be very different and variable from well to well (additional cost, uncontrolled blow out, environmental damage); the knowledge of the location of the fractures, crossing the well, has a strong technical and economical impact upon drilling, production and reservoir management strategies. Otherwise, in a fractured reservoir, the occurence of mud loss can be considered as a good indicator of the presence of natural permeable fractures. Most of the techniques currently used for natural fracture detection and localisation do not clearly differentiate between fractures that allow fluid flow and those that do not, because they do not measure fluid-flow properties directly (Dyke1). Core analysis, for example, can clearly identify the fractures but is unable to discriminate between minor and isolated fractures from the ones having high permeability. In the same way, image logs can accurately locate the presence of fractures but are unable to discriminate between open and closed fractures. The use of high resolution electromagnetic flowmeters to monitor the mud losses while drilling proves very interesting for fracture detection in that flowmeters supply a continuous and accurate detection of mud losses, providing a clear indication of the formation open fractures and/or more permeable zones crossed by the wellbore. To day, some Oil Companies, including ENI Agip, use accurate mud loss detection to solve operative problems and to improve the Formation Evaluation process. To get a true advantage in this process, a full integration between mud losses and other available techniques must be carried out.