Evaluation of Fractured Reservoirs

Efforts were made to find improved means for locating fractures penetrated by a wellbore and for estimating fracture reservoir volume. The four approaches to the problem utilized acoustic logs, porosity estimates from different sources, transient pressure and fluid flow data, and resistivity logs. Acoustic amplitude attenuation and acoustic variable-intensity interference patterns have been used to locate fractures. Although amplitude logs have often proved useful for fracture detection, they are frequently inconclusive when used alone. This is due partially to variations in amplitude caused by factors other than fracturing. The variable-intensity interference patterns produced by borehole discontinuities in casing and in open-hole fractured sections were found to be quite useful in detecting fractures; but, like amplitude logs, they are not definitive by themselves. A technique has been developed that uses porosity estimate comparisons to evaluate fractured reservoirs. If this technique is used with acoustic variable-intensity interference patterns, it may be helpful for delineating fractured zones and for estimating fracture porosity. Transient pressure behavior observed for a fractured reservoir was found to be in agreement with that theoretically predicted for linear flow systems. Since this behavior is markedly different from that of a homogeneous reservoir, interpretation of transient pressures may provide a means of recognizing fractures. Other helpful techniques employing pressure and fluid flow data are comparison of calculated kh's for injection and withdrawal of fluids, comparison of calculated kh's for different rates of injection, and calculation of k/phi's from pressure and log data, where k is permeability, h is producing thickness, and phi is porosity. Some of these techniques also present possibilities for calculation of reservoir volumes. The fourth approach to fracture detection showed that, under certain conditions, an induction log can be used to detect a resistivity anomaly opposite a fractured zone. Although some of the techniques discussed show promise of being helpful, further study will be required before evaluations of fractured reservoirs become as satisfactory as evaluations of "normal" porosity reservoirs. Introduction Experience has shown that the presence of a fractured system can improve the productivity of hydrocarbon reservoirs. In some cases, fracture void space also supplies a significant portion of the total porosity. However, quantitative determination of the contribution of fracture systems to production from petroleum reservoirs has proven difficult. This is due in part to lack of consistently successful techniques for locating and usefully describing fracture systems penetrated by a wellbore. This paper presents the initial results of a research program on the use of borehole measurements for evaluation of fractured reservoirs. The objectives of this research are to find improved means forlocating fractures penetrated by a wellbore, andestimating in-situ reservoir volume contained within fracture systems in communication with the wellbore. This progress report will describe our attempts to date to use four types of data for fractured reservoir evaluation:acoustic logs,porosity estimates from different sources,transient pressure and fluid flow data, andresistivity logs. ACOUSTIC LOGS Amplitude Logs The acoustic amplitude log is one of the most widely used measurements in attempts to detect fractures. SPEJ P. 28ˆ