|
|
EXPLOITING TIGHT GAS SAND SWEET SPOTS |
| PTTC Home | Solutions From the Field |
Based on a workshop cosponsored by PTTC’s Rocky Mountain Region on April 26, 1999, in Denver, CO.
Reservoir characterization is needed to find sweet spots in Green River Basin tight gas reservoirs of Wyoming. The importance of fractures and depositional facies are different for the Frontier Formation (Table Rock) and the Almond Formation (Siberia Ridge).
A wide variety of techniques were used to characterize Frontier (Table Rock Field) and Almond formation (Siberia Ridge Field) to find tight gas sweet spots within the eastern portion of the Greater Green River Basin. Fracturing appears most intensive near several small faults or lineaments at both fields. However, its significance is different for each setting. Permeability is enhanced by a factor of about two parallel to east-west fractures.
Tight Gas, Greater Green River Basin, Fracture Analysis, Horizontal Wells
Horizontal Well from Table Rick Field, Frontier Formation
Lee Krystinik and Frank Lim, Union Pacific Resources Co.
Natural Fracturing in Horizontal Core, Table Rock Field, Frontier Formation
John Lorenz, Sandia National Laboratories
Sweet Spot Identification, Siberia Ridge Field
Stephen Sturm, Schlumberger Holditch-Reservoir Tech. and Randal Billingsley, BP-Amoco
Almond Formation Reservoir Characterization, Siberia Ridge Field
William Clark, Schlumberger Holditch-Reservoir Tech.
Natural Fracture Characterization from Borehole Data
Lesley Evans, Schlumberger Holditch-Reservoir Tech.
Two characterizations of Frontier and Almond Formation sand intervals can be compared from several case studies.
Table Rock Field, Frontier Formation
Union Pacific Resources’ Rock Island 4-H well, located on the north plunge of Table Rock Field, Sweetwater County, Wyoming, was the culmination of a project with the US Department of Energy (DOE) Federal Energy Technology Center
(FETC) and Gas Research Institute (GRI). The goal was to find technologies to produce significant tight gas resources from southwestern Wyoming.
Milestones of the project include:
Core and log information and production tests from the GU #4 well (about 1.5 miles to the east-northeast), combined with 3-D seismic, were factors in selecting the well’s site. The highly fractured reservoir is primarily developed in hummocky cross-stratified, very-fine grained, lower-shoreface sandstones with about 10% porosity, 0.025 md permeability. Fractures range from being open to partially filled with mineral cements and bitumen. Fracturing appears most intensive near several small faults penetrated by the well. Open, east-west fractures occur along the entire length of the well.
Horizontal drilling for low permeability gas allows large well spacings, improving per well recoveries and reducing the environmental impact. The Rock Island 4-H horizontal well reached 16,784 ft (14,950 ft TVD) in the Frontier formation at 270° F and 10,000 psi. Attempted coring between 15,424-15,951 ft recovered 77 ft, or 96% recovery. The horizontal lateral used a medium radius curve (14°/ 100 ft) to reach an 88° deviation from vertical and geosteered 1,750 ft within a 20-ft target window.
Natural Fracture Analysis in the Rock Island #4-H Well
Fracture analysis of 78.2 ft of near horizontal core from the well found 76 natural fractures. The first two cores were taken from an upper marine unit, the third from a lower marine unit. Fracture spacing varied significantly.
Two sets of fractures are present in the core:
The east-west fracture set is mineralized with quartz druze, a black bituminous lining, and a late-stage kaolinite layer. The younger, north-south fractures are mineralized, with quartz druze having larger, clearer crystals, and local patches of late-stage calcite. The east-west fractures formed first, as a response to overpressure, and were reactivated later during local thrust faulting. About 30% of east-west fractures have been reactivated by right-lateral to oblique shear. The north-south extension fractures may be contemporaneous with reactivation of the first set, related to extension over the crest of the thrust-related flexure.
About half of the fractures of both sets have obvious remnant open apertures despite mineralization and should be good permeability pathways at reservoir conditions. No coring-induced fractures were found in the horizontal cores. Plugs taken along and across the fractures suggest that permeability is doubled, but that the north-south fractures do not significantly affect reservoir permeability.
Siberia Ridge Field, Almond Formation
Several studies have created interdisciplinary models to determine sweet spots in the low permeability Almond Formation
gas-bearing sandstones at Siberia Ridge. The field is located on the northeast flank of the Wamsutter Arch in Sweetwater County. The top of the Almond dips
2-2.5° toward the northeast at the field with depths between 9,300 to 11,900 ft.
Long-term reactivation of northeast trending lineaments related to basement faults appears to control thickness, depositional pattern, and reservoir quality in the Almond. Several of the northeasttrending lineaments compartmentalize the field.
Within the field, the Almond can be divided into a brackish to nonmarine “Main Almond” and an upper Almond marine sequence. The Main Almond is approximately 450 ft thick, comprised of discontinuous, tidal flat and tidal creek sandstones surrounded by coastal plain and fluvial siltstones, shales, and coals. Sandstones in the Main Almond—a major source of gas produced from the Almond—are compartmentalized below well spacing and have poor reservoir quality (0.001 md).
The upper Almond is a 30-50 ft thick transgressive marine sequence, characterized by laterally continuous linear shoreline “bar” sandstones that are eroded by northeast trending channels of tidal to fluvial origin. Net sandstone varies from 5-30 ft, with an average of 0.019 md permeability and average of 10.3% porosity. Almond wells within Siberia Ridge typically produce an average of 1.44 bcf, with sweet spot wells at >3.74 bcf.
Most of the net sandstone (using an 8% porosity cutoff) and gas production at Siberia Ridge is from the upper Almond. Net ØH and net KH are also best in the upper Almond, where depositional facies control petrophysical properties. Previously, fractures were believed to be a major influence on production from the Almond. Now, “matrix” properties are believed to be at least as, if not more, important. Although fractures cannot be ignored, the facies (especially the upper Almond channel sandstones) are more important than fractures for high gas production.
Sweet spots in the Siberia Ridge were defined as wells with 180 days of on-stream production >130 mmcf gas. They tend to be the best long-term producers in the field. About 80% of these wells were completed in the upper Almond channel. Other wells with high cumulative production are in Upper Almond shoreface/ foreshore sands, located on or near lineaments, suggesting that some of these are fracture swarms. The presence of fractures is also supported by cores, image logs, and seismic 3-D analysis.
Lee Krystinik,
Union Pacific Resources Co.
MS 3900, PO Box 7,
Fort Worth, TX 76101
Phone 817-321-6702, Fax 817-321-6705 E-mail l.f.krystinik@uprc.com
John Lorenz, Geotechnology Research
Sandia National Laboratories
Div. 6112, Albuquerque, NM 87185-5800
Phone 505-844-3695, Fax 505-844-7354 E-mail JCLOREN@sandia.gov
Stephen Sturm and William Clark
Schlumberger Holditch-Reservoir Technologies
3609 S. Wadsworth Blvd. Suite 500,
Denver, CO 80235
Phone 303-985-9900, E-mail ssturm@slb.com and
wjclark@slb.com
For information on PTTC’s Rocky Mountain Region and its activities contact:
Roger Slatt, Department Head, Geology/ Geological Engineering
Colorado School of Mines, Golden CO, 80401-1887
Phone 303-273-3822, Fax 303-273-3859, E-mail rslatt@mines.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.
Petroleum Technology Transfer Council, 2916 West T. C. Jester, Suite 103, Houston, TX 77018
Toll-free 1-888-THE-PTTC; Fax 713-688-0935; E-mail hq@pttc.org;
web www.pttc.org
| PTTC Home | Solutions From the Field |
|
We encourage your comments, please send us email at: hq@pttc.org or use our Feedback Form. Copyright © 2004 Petroleum Technology Transfer Council |