RESERVOIR CHARACTERIZATION USING OPEN-HOLE, CASED-HOLE, AND PRODUCTION LOGGING

Based on a workshop co-sponsored by PTTC's West Coast Region on April 29, 1999, in Long Beach, CA, and April 30, 1999, in Bakersfield, CA.

BOTTOM LINE

Wireline logs historically have been— and continue to be— the most critical and widely available tools for characterizing and managing reservoirs. Reservoir producibility and economics are being improved with new technologies for gathering and interpreting formation properties in open holes and behind pipe, coupled with new technologies for measuring reservoir fluid flow within, as well as behind, casing.

PROBLEM ADDRESSED

New wireline tool technologies and new reservoir-wide, pattern-recognition approaches to wireline log analysis can lead to a vastly improved evaluation of formation properties (both in open hole and behind pipe). Their application can also lead to improved production logging (determination of fluids entering and moving through the borehole). Many independent operators, however, have not had experience in applying these new tools and approaches to improve their bottom lines. These approaches are applicable to California’s diatomite reservoirs, which have historically been difficult to characterize using wireline logs.

KEY WORDS:

Open-Hole Logging, Cased-Hole Logging, Production Logging, Logging-While-Drilling, Diatomite Reservoirs, Reservoir Characterization, Reservoir Management

SPEAKERS

Zaki Bassiouni, Louisiana State University

Steve Halvorsen, Schlumberger Oil Field Services

Tim Quinlan, Schlumberger Oil Field Services

TECHNOLOGY OVERVIEW

Real-time information advantages provided by loggingwhile-drilling include early detection of geologic markers, directional control for increased drilling efficiency, pore-pressure measurement, and pre-invasion formation evaluation. Post-drilling wireline technologies to consider include open-hole and cased-hole formation evaluation, in addition to technologies focused on identifying and quantitatively measuring fluids moving within the wellbore and behind pipe.

For open-hole logging, one must consider determining shale volume from gamma ray response or establishing the reservoir-specific relationship between formation factor and porosity. The emphasis, however, should be placed on discovering the unique relationships between reservoir properties and wireline response (i. e., development of a petrophysical model) by using cross plots, Pickett plots, and other pattern recognition approaches. The steps in developing a petrophysical model are: (1) identifying the reservoir properties of interest (i. e., primary properties), (2) identifying properties measured by logs (i. e., secondary properties), and (3) establishing the primary to secondary property relationships.

Diatomite reservoirs in California are excellent examples of where the application of traditional “universal formulas” for open-hole wireline log analysis have been unsuccessful. The reservoirs are composed primarily of accumulations of the siliceous shells of microscopic organisms and have unique properties. The silica they contain is of a different density from that of traditional quartz sand, which confounds porosity interpretation by traditional density log techniques. Although the hollow shells result in high porosity, their small size keeps permeabilities low, in the millidarcy range. Adding a mixture of varying amounts of clay and quartz sand further complicates the interpretation by traditional methods. New pattern recognition techniques, designed to match porosities and saturations observed in core samples, have placed wireline logs back in a position of importance in characterizing these reservoirs.

Technologies for determining formation properties are revolutionizing reservoir management. A number of tools that may be used individually or in a complementary fashion make it possible to accurately identify watered-out zones and bypassed pays. Pulsed-neutron logs take advantage of the differential thermalneutron-capture cross-sections of salt water, oil, and gas for identifying formation fluids behind pipe. Carbon/oxygen logs have similar capabilities for differentiation, but are not subject to many of the restrictions associated with the pulsed-neutron tool. They operate reliably in the presence of waters of fresh, mixed, or unknown salinity.

Sonic logs perform similarly in both open-hole and cased-hole applications, and, when used in conjunction with neutron logs, are very good at detecting the presence of natural gas. Density logs in open holes are very effective, since the recent development of appropriate methods to correct for borehole environmental effects. Still in the prototype stage are through-casing resistivity measurement tools capable of measuring potential differences in the nanovolt range. These tools show promise of measuring Sw behind pipe over a range of salinities and porosities.

A number of tools may be useful for determining the effective isolation of producing zones and determining flow behind pipe. The gamma ray log is often capable of detecting water flow into a producing zone behind casing by measuring increased radioactivity due to scale buildup. Oxygen activation logs, employing high-energy “fast” neutrons, can dynamically detect water movement behind pipe. Temperature and noise logs are useful for detecting gas movement. Pulsedneutron logs also may detect gas channeling.

Additional production logs can evaluate the nature and quantity of fluids by zone and the movement of fluids within the wellbore. They also can help to monitor the reservoir depletion process and/or measure the effectiveness of fracturing, acidizing, or other treatments. Gamma-ray logs can detect localized scale buildup at fluid entry points through the casing. For gas entry, the noise log serves the same purpose. Again, the gamma ray log used with radioactive tracers may be useful for describing fluid movementswithin the wellbore.

Mechanical flow meters— used in combination with other fluid identification devices measuring properties such as bulk-fluid density or dielectric constant or in conjunction with pulsed-neutron and carbon/oxygen logs— yield critical information on multiphase-flow geometry, relative-phase proportions, and volume of flow within the wellbore. If only two phases are present, a single fluid-identification device, if properly selected, may be adequate to accurately evaluate the holdup (relative proportions) of each phase. If three phases are present, at least two fluid-identification devices are required to make a similar assessment.

LESSONS LEARNED

Wireline logs gather a large amount of information quickly and at reasonable cost. They measure formation propertiesin-situand identify where other tests should be performed. However, they measure needed parameters only indirectly, and logging data sometimes require involved interpretation techniques.

Reservoir-by-reservoir logging solutions are necessary, not magic applications of “worldwide average” equations for interpretation. It is always important to develop and apply techniques that relate observed properties from various sources (e. g., cores, fluid samples, and seismic as well as wireline log data) for each reservoir/formation encountered.

CONNECTIONS:

Zaki Bassiouni
Chairman, Department of Petroleum Engineering Louisiana State University
CEBA Bldg., Room 3523, Baton Rouge, LA 70803
Phone 225-388-6040, Fax 225-388-5990, E-mail pezab@lsu.edu

Tim Quinlan
Schlumberger
4900 California Ave., Suite 401A,
Bakersfield, CA 93309
Phone 661-864-4700, E-mail halvorson1@slb.com

Steve Halvorsen
Schlumberger Well Services
4212 Admiral Doyle Dr.,
New Iberia, LA 70560
Phone 318-365-9998, Fax 318-265-8357, E-mail quinlan1@slb.com

For information on PTTC’s West Coast Region and its activities contact: 
Iraj Ershaghi, Director, Petroleum Engineering Program, HEDCO-316
University of Southern California, Los Angeles, CA 90089-1211
Phone 213-740-8076, Fax 213-740-7982, E-mail ershaghi@archie.usc.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

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