UNDERSTANDING NATURALLY FRACTURED RESERVOIRS

Primarily based on the following PTTC workshops: “The 3rd Naturally Fractured Reservoir Symposium,” held October 28, 1997, in Socorro, NM (Southwest Region)

“Oil Recovery from Naturally Fractured Reservoirs: Field Studies, Modeling, and Analytic Methods,” held October 16, 1998, in Midland, TX (Texas Region)

“Secondary Natural Gas Recovery in the Appalachian Basin: Application of Advanced Technologies in a Field Demonstration Site, Henderson Dome, Western Pennsylvania,” held November 19, 1998, in Pittsburgh, PA (Appalachian Region)

BOTTOM LINE

The detection of the subsurface characteristics and distribution of fractures is critical to petroleum reservoir profitability. Cost-effective data can be gathered that enables predictive models to maximize production from fractured reservoirs.

PROBLEM ADDRESSED

Natural fractures have influenced reservoir performance in the fine-grained (siltstone) reservoirs of submarine fan origin in the Permian Spraberry Trend. While the remaining resource is known to be large, there is little information on the actual fracture system other than what has been inferred from past pulse and tracer tests, observed from a few vertical cores, and predicted from reservoir simulation. The economics of a proposed CO 2 flood project require that the fracture system be described as accurately as possible to obtain the most reliable recovery forecasts.

The abundance and orientation of fractures correlates with production in the sandstones of the Lower Silurian Medina Group in western Pennsylvania, the Yates field in west Texas, the Gilbertown field area in Alabama, and the Uinta Basin in Utah. The direct measurement of fractures, and prediction of their characteristics from indicators of regional stress, are needed to explore and develop reservoirs in these areas.

KEY WORDS:

Horizontal Core, Fracture Characterization, Anisotropy, Dual Porosity, Microfractures, Borehole Imaging, Seismic, Reservoir Characterization, Natural Fractures

TECHNOLOGY OVERVIEW

In the west Texas Spraberry reservoir, 400 ft. of oriented horizontal core were obtained from two pay zones vertically separated by 140 ft. The study of natural fractures in core yielded estimates of fracturing episodes, changes in stress fields through time, and diagenetic episodes. A microresistivity borehole imaging log run over the same horizontally cored intervals gave an independent estimate of the distribution and importance of the fractures. Long-spaced sonic logs and micro-frac treatments yielded additional estimates of current stress profiles.

In a western Pennsylvania Henderson Dome area study, a new, low-cost methodology was developed to obtain information on larger-scale fractures that significantly affect gas reservoir performance. This involved examining the orientation and other characteristics of microfractures within rock matrix grains obtained by rotary sidewall coring.

Seismic shear (S) waves are required to image fractures in the subsurface. Because the heavily wooded terrain of the Henderson Dome area is not conducive to successful application of vibroseis techniques generally used for this purpose, the project tested alternative methods to either generate seismic shear waves directly or to take advantage of the naturally occurring conversion of conventional P waves to S waves in the subsurface.

In the Gilbertown Field area in Alabama, second derivative structure maps (displaying the rate of change of curvature related to subtle bends in the the Gilbertown graben system faults) showed a correlation to fracture distribution and well productivity.

LESSONS LEARNED

1. In reservoirs with natural fractures, the fractures are the primary control for production and recovery characteristics. Without understanding the fracture system, reservoir performance can never be accurately predicted. Using several techniques for agreement on fracture properties greatly increases confidence in the exploration, development, and exploitation of these reservoirs. As always, calibration of techniques against known situations leads to the best results.
2. Making assumptions about fracture properties with little or inconclusive evidence (such as distributions observed only in vertical cores) can be dangerous. Even in reservoirs of uniform lithology with no major associated structure, fracture orientations may be complex and varied in different parts of the reservoir.

FIELD RESULTS

Horizontal core in the Spraberry Trend in west Texas, revealed multiple fracture sets, each with its own distribution characteristics with respect to lithology, spacing, strike, and mineralization. In some instances, mineralization in certain fracture sets distinguishes them in terms of age and supposed diagenetic events. Some sets in the upper pay zone are mineralized, although they have identical orientation to non-mineralized sets in the lower pay zone. A borehole imaging log run over the horizontally cored intervals agreed on the orientation of various fracture sets encountered, but was, in general, too optimistic, since it identified 33% more fractures than are visible in the core. This discrepancy remains unresolved, but may be due to coring-induced fractures on the borehole wall.

A microfracture examination in thin section allowed operators to predict zones of high fracture quality in the Henderson Dome area. Project activity also resulted in a new method for orienting rotary sidewall cores. The successful design of shaped explosive charges to generate S waves demonstrated seismic as a viable tool for imaging fractures in the Henderson Dome area. The reliance on natural P-to-S-mode conversion at shallow depths from conventional shot-hole explosives also confirmed the value of seismic data. The ability to locate zones with abundant fractures using these techniques will make the difference between extending new fields vs. drilling wells that produce some natural gas but prove to be uneconomic.

An analysis of fracture patterns in the Yates field in west Texas helped avoid drilling several unnecessary injection wells and observation wells. It used hierarchical fracture models, clustering and neural net techniques, and hydraulic parameter analysis, coupled with a linkage to reservoir models. This represented a cost savings of more than $3 million in implementing a gravity-assisted thermal segregation process.

In Utah’s Uinta Basin, surface fracture data, combined with well log data and waterflood performance data from existing fields, served as input to a visualization technique and numerical modeling that led to defining “sweet spots” or preferred drilling locations.

CONNECTIONS:

Spraberry Trend:
Paul McDonald, Pioneer Natural Resources Co.
PO Box 3178, Midland, TX 79702
Phone 915-683-4768

Henderson Dome Area -Microfracture Study:
Steve Laubach, Bureau of Economic Geology
Box X, University of Texas, Austin, TX 78713
Phone 512-471-1534, Fax 512-471-0140, e-mail laubachs@begv.beg.utexas.edu

Henderson Dome Area -Seismic Study:
Bob Hardage, Bureau of Economic Geology
Box X, University of Texas at Austin Austin, TX 78713
Phone 512-471-1534, Fax 512-471-0140, e-mail hardageb@begv.beg.utexas.edu

Yates Field Study:
Violeta Ivanova, Schlumberger Austin Research Center
8311 N. FM 620 Rd., Austin, TX 78726
Phone 512-331-3710

Gilbertown Field Area Study:
Jack Pashin, Geological Survey of Alabama
PO Box O, Tuscaloosa, AL 35486-9780
Phone 205-349-2852, Fax 205-349-2861, E-mail jpashin@ogb.gsa.tuscaloosa.al.us

For information on PTTC’s regional resource centers and activities contact:
Appalachian: Douglas Patchen, Program Director, Appalachian Oil & Natural Gas Research Consortium,
West Virginia University, NRCCE-Evansdale Dr., PO Box 6064, Morgantown, WV 26506-6064
Phone 04-293-2867 x-5443, Fax 304-293-7822, E-mail dpatch@wvunrcce.nrcce.wvu.edu

Southwest: Robert Lee, Director, Petroleum Recovery Research Center,
New Mexico Tech, 801 Leroy Pl., Socorro, NM 87801
Phone 505-835-5685, Fax 505-835-5210, E-mail lee@prrc.nmt.edu

Texas: William L. Fisher, Bureau of Economic Geology, University of Texas at Austin,
University Station, Box X, Austin, TX 78713-2924
Phone 512-471-0209, Fax 512-471-0140, E-mail wfisher@mail.utexas.edu

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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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