Detection and Characterization of Fractures in Naturally Fractured Reservoirs

Abstract

Abstract Imaging log, sonic log, optical microscopy, NMR, MRI, and tomography have greatly enhanced the possibility of measuring fracture geometric characteristics. However, the fracture hydraulic width and permeability can only be evaluated from dynamic data revealing production or adsorption along the wellbore. Different analytical approaches accounting for mud rheology and their application to several field cases where mud losses were recorded with electromagnetic flowmeters are presented to evaluate the fracture hydraulic width from mud loss measurements. Observation of mud loss evolution in time also revealed whether a single fracture or an intensely fractured zone had been intercepted by the drilling bit. Results are discussed with respect to imaging log data, core analyses, and well test interpretation results. In general, a very good correspondence was found between mud loss occurrence, fracture detection from imaging log, core data, and productivity tests. Introduction Fractured reservoirs are difficult to simulate due to the presence of tectonic discontinuities, intersecting a generally almost tight matrix, that strongly influence the fluid flow pattern in the producing formation. Technologies such as imaging log coupled with microfracture analyses of conventional cores with optical microscopy, NMR, MRI, and tomography have greatly enhanced the possibility of measuring the fracture geometric characteristics. However, the fracture hydraulic width and permeability can only be evaluated from dynamic data revealing production or adsorption along the wellbore. The use of high resolution electromagnetic flowmeters to monitor mud losses during drilling appears very attractive for fracture detection and characterization in that flowmeters provide an accurate, continuous recording of mud adsorption, thus giving a clear indication of the formation conductive fractures or more permeable zones intersected by the wellbore.(1,2) Only at a later time, production logs can indicate primary fluid entry points and transient pressure analysis allow estimation of the fracture or fracture network permeability. Appropriate data collection at early stages is crucial to identify and characterize faults and fractures affecting the reservoir productivity and performance, and integration of all available information probably represents the only effective approach to achieve a good understanding of fractured reservoirs in order to develop effective management strategies.(3,4) Mud loss identification The exposure of open natural fractures by the drilling bit produces a sudden decrease of the outlet mud flow rate. According to Dyke et al.(2) losses through matrix permeability or into natural fractures can be distinguished by the characteristics of the loss. Losses through pores start slowly and gradually increase as drilling proceeds whereas losses into natural fractures show a rapid initial increase in loss rate followed by a gradual decline in time. Such loss progressions with time give rise to the typical responses in the mud tank level, as shown in Fig. 1.