Walter H. Fertl

Log-Derived Evaluation of Shaly Clastic Reservoirs

Reliable evaluation of hydrocarbon resources in shaly clastic reservoir rocks is an important, although difficult, task. This paper briefly reviews the wide variety of interpretive models that has evolved. The discussion focuses on digital shaly-sand evaluation techniques based on the Waxman-Smits model to provide information on total and effective reservoir porosity and fluid distribution, silt content, volume, type (smectite, illite, and chlorite/kaolinite), and distribution modes (dispersed, laminated, and structural) of the clay minerals present in subsurface formations. Case studies from different geologic environments present field experiences in clastic reservoir rocks that exhibit a wide range of porosity and permeability and various amounts, types, and distribution modes of clay minerals. A log-derived formation damage index is also discussed. Finally, emphasis is placed on improved reservoir evaluation of thinly laminated shale/sand intervals through the integration and enhancement of resistivity data, short-spaced dielectric measurements, and/or analytical data-blocking routines.

Volumes, Types, and Distribution of Clay Minerals in Reservoir Rocks Based on Well Logs

Discussed in this paper, is an innovative digital shaly sand evaluation approach which provides information on total and effective reservoir porosity, total and effective fluid distribution based on the Waxman-Smits equation, reservoir productivity, silt volume, and volumes, types and distribution modes of clay minerals present in subsurface ford. 52 refs.

Use of natural gamma ray spectral logging in evaluation of clay minerals

Abstract Gamma ray spectral logging can be used for qualitative and quantitative evaluations of argillaceous sediments, in both cased and open boreholes. Well-logging devices record the gamma rays emitted by potassium (K40) at 1.46 MeV, the uranium series nuclide bismuth (Bi214) emanating gamma rays at 1.76 MeV, and the thorium series nuclide thallium (Tl208) emanating gamma rays at 2.61 MeV. The information obtained can be used in strati-graphic correlations, recognition of rock types, determining the amount and type of clay minerals present in clastic formations, establishing rock potential of shales, evaluating diagenetic and catagenetic changes in clay minerals, CEC estimates, and so on.

The Use of Circumferentially Propagated Acoustic Waves in Well Logging

An acoustic logging system which uses circumferentially propagated acoustic waves for measuring several parameters in a borehole has been designed and tested. The detection of one type of wave whose particle motion is perpendicular to the borehole wall is used to indicate the presence of vertical or near-vertically oriented fractures in fluid-filled boreholes. This paper reviews the basic theoretical concepts and instrumentation design of the Circumferential Acoustilog and presents laboratory data to illustrate the validity and applications of the measured parameters. Recent improvements over previous devices of this type are discussed. One of the improvements is the incorporation of a direct fluid wave baffle which extends the range of formation velocities that can be accurately measured, and also provides the means for identifying the guided fluid wave. A recent significant improvement to the system is the addition of an orientation section which assists in determining the direction of fracture systems. Field examples from the United States, Canada, and Europe demonstrate the application of the system in varying lithologies and borehole conditions.

Experiences with Natural Gamma Ray Spectral Logging in North America

The last few years have brought on the inclusion of natural gamma ray spectral logging devices as an integral part of optimized wireline logging programs. Recorded in both open hole and cased hole environments, such logs have proven invaluable in solving the complex interpretation problems encountered in many wells. Interpretive concepts have advanced both at the wellsite and for detailed digital analyses at central processing centers. Current wellsite evaluation techniques, as well as several recently developed digital interpretive methods for shaly sands and complex and fractured reservoir rocks, are discussed. Inherent environmental constraints of natural gamma ray spectral logging devices are also highlighted.

Classification of Source Rocks on the Basis of Shale Resistivity Ratio Parameter

Abstract Source rocks can be classified on the basis of shale resistivity ratio parameter [rSR = Rsh (normal) / Rsh (observed)]. High ratios (undercompacted shales) indicate poor source rocks, whereas low ratios (well-compacted shales) may be indicative of good source rocks. In undercompacted shales, oil was not pushed out of shales by associated water due to lack of compaction.

The Continuous Carbon/Oxygen Log Basic Concepts and Recent Field Experiences

The continuous carbon/oxygen (C/theta) log, recorded by a 20-kHz pulsed neutron logging tool, locates and quantitatively evaluates hydrocarbon reserves in cased wellbores independent of the salinity of formation waters. Gamma rays emitted during neutron inelastic scattering and neutron capture provide information on relative amounts of several elements in potential reservoirs, such as C, theta, silicon (Si), calcium (Ca), etc. These data can be related to oil saturation of reservoir rocks of fresh, mixed, or unknown formation water salinities and to formation lithology. Measurement principles, tool response, test pit data, and interpretive concepts are summarized. Field case examples include hydrocarbon exploration behind pipe, monitoring of production wells, secondary and tertiary recovery schemes, evaluation of heavy oil reservoirs, tar sands, and steamfloods, and other case studies in clastic carbonate reservoirs.

Determination of Residual Oil Saturation from Geophysical Well Logs in Tertiary Recovery Projects

Abstract Residual oil saturation (ROS), a key factor in the planning of tertiary recovery projects, can be determined on the basis of reservoir engineering concepts, core data, single-well tracer tests, and specialized geophysical well-logging techniques. Uncertainty limits of conventional logging techniques are not too promising. However, several log-inject-log techniques using multiple repeat logging runs will determine residual oil saturation within the range of ROS ± (≤ 5 saturation percent). Engineered planning of both logging and injection procedures is a prerequisite for reliable log-derived ROS estimates.

Determination of true static formation temperature from well logs

Several concepts that attempt to determine static (true) formation temperature from log-derived BHT measurements with and without knowledge of drilling fluid circulation time have been reviewed. Whereas several refined models are available, the simple Horner-type technique provides quick and reliable static (true) formation temperature estimates.

Organic Carbon Content and Source Rock Identification Based on Geophysical Well Logs

Abstract The presence of organic material in potential source rocks significantly affects geophysical wireline responses. Inasmuch as the latter provide a continuous, complete, and readily available subsurface formation data record versus depth, many investigators have studied the possibility that organic carbon content and the source rock potential of subsurface strata can be identified from wireline logs. The present discussion reviews such log anomalies as observed by means of resistivity, acoustic, nuclear, natural gamma-ray spectral, and pulsed neutron measurements. The application of these well logs in open and/or cased wellbores as an indicator of the amount of organic matter contained in sediments is discussed and illustrated based on empirical correlations established with individual and/or combinations of several logging parameters. Fieldcase examples from several geographical areas are presented.