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SPE/DOE 8938 EVALUATION OF FRACTURED RESERVOIR ROCKS USING GEOPHYSICAL WELL LOGS

SPE

by Walter H. Fertl, Dresser Atlas

© Copyright 1980, Society of Petroleum Engineers This paper was presented at t~e ! 980 SPE/DOE SXmposium on Unconventional Gas Recovery held in Pittsburgh, Pennsylvania, May 18-21, 1980. The material is subject to correction by the author. Permission to copy ISrestricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy., Dallas, Texas 75206

ABSTRACT
Recent economic forecasts place recoverable crude oil reserves from fractured reservoir rocks in excess of 40 billion stock tank barrels. Hence, over the past few years the petroleum industry has exhibited an ever increasing interest in fractured reservoirs. The present discussionsummarizes wireline logging techniques applicable in the detection of fractured reservoir rocks and reviews their advantages and possible limitations.

Basically, fracture porosity--which frequently ranges from less than 0.05% up to 5.5% -- is primarily controlled by the width, length, area, spacing, and surface roughness of the fracture. Natural fracture systems, their porosity, orientation, morphology,spacing, and continuity are much dependent on the mode of origin. Fracture systems fall into two main categories, artifical (i.e. induced, shear, extension, tensile) fractures and natural ones. Recently, natuval fracture systems (1) have been classified as tetonic features, which are structure related, (2) regional fractures, (3) contractional fractures, which are not restricted to geologicstructures, result from desiccation, syneresis (tension, extension, "chickenwire" fractures), thermal gradients (columnar jointing in igneous rocks), mineral phase changes (calcite to dolomite), and (4) surface-related fractures, which may result from unloading, release of stored stress and strain, unsupported boundaries, and weathering effects. Furthermore, natural fractures may be open or healed.Healed or sealed fractures are filled with minerals (such as calcite) precipitated over geologic time from circulating subsurface waters. Nevertheless, sealed fracture systems are frequently weakness places favorable to hydraulic well stimulation. Concentrating a properly engineered well stimulation effort at a closed natural fracture system present at and within vicinity of the wellbore oftenestablishes communication with an open fracture system at some distance from the wellbore.(2)
RESPONSE OF WELL WGS IN
FRA~D

INTRODUCTION
Natural fracture systems may be encountered in various lithologies, including sandstones, shales, carbonates, igneous and metamorphic reservoir. rocks. Natural fracture systems not only control performance and state of depletion in reservoirs under primary,secondary, or tertiary recovery schemes; greatly control wellbore pattern of production and injection wells; affect exploration concepts, drilling operations, casing seat selection, cementing and completion techniques, etc. Natural fracture systems can be identified, studied, and evaluated by proper selection and combination of several techniques. These include surveys from space, areal reconnaissance,surface geology, seismic information, core analysis, downhole cameras, inflatable packers, geophysical well analyses, well testing, and production behavior. The present discussion focuses on the use of well logs to identify the presence, determine extent, and/or evaluate the porosity of open, natural fracture systems and, hence, highly permaable zones. References at end of paper

FORMATIONSPresence of a single natural fracture or a massive fracture system 5~n cause minor to significan t departures (3,4, 5, J from the "normal" well log response. Such anomalies may be recorded by mechanical, electrical, acoUStic, nuclear, radioactive, and temperature logging devices as a function of borehole conditions, .drilling mud characteristics and fracture-related reservoir properties.
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