MesaVerde Group Reservoirs -Field Trip and Workshop

Based on a workshop co- sponsored by PTTC's Southwest Region and the New Mexico Bureau of Geology and Mineral Resources, May 21-23, 2003, Albuquerque, New Mexico.

BOTTOM LINE

The Mesaverde Group contains significant remaining natural gas in the San Juan Basin. Sophisticated geological models are necessary to predict infill locations based on depositional facies. Less sophisticated models ignore stratigraphic complexities, reservoir facies frequency and scale-dependent attributes that are critical to understanding the reservoir distribution. The most productive facies are channels and beach ridges. Carefully screened outcrop analogs to subsurface reservoirs can be used to model the formations of the Mesaverde Group at the scale of infill wells, and to more accurately predict reservoir distribution. Outcrops demonstrate how new reserves can reside between wells within medium- to small-scale reservoir compartments. Scale-dependent attributes, distribution of depositional facies and the sophistication of the geological model largely control the success of locating successful infill wells. Infill potential of Mesaverda formations is a function of the orientation and aspect ratio of channels and beach ridges, the frequency of channels, and the flow barriers between layers. Optimal stimulation procedure for the Lewis Shale in this area is a single-stage, 150,000 lb. foamed linear gel, hydraulic fracture treatment.

PROBLEM ADDRESSED

The Mesaverde Group is a significant natural gas producer in the San Juan Basin. The reservoirs are more complex than they are often portrayed. Complexities include geological aspects (complex stratigraphy, diverse depositional environments, and geographical variations) as well as regulatory aspects that contradict stratigraphic relationships.  

KEY WORDS:

Coalbed Methane, Geological Model, Lewis Shale, Menefee Formation, Mesaverde Group, Outcrop Analogs, San Juan Basin, Well Stimulation

SPEAKERS

Sedimentology, Stratigraphy, Log Interpretation Strategies, Brian S. Brister, NM Bureau of Geology and Mineral Resources, NM Institute of Mining and Technology and William Peabody, Peabody Consulting.

Reservoir Modeling, 
Tom Engler, Petroleum Engineering Department, NM Institute of Mining and Technology

Coalbed Methane Potential of the Menefee Formation, 
Steve Hayden, District Geologist and Deputy Inspector, New Mexico Oil Conservation Division, district III

TECHNOLOGY OVERVIEW

Operators expressed interest in developing natural gas reservoirs in the Mesaverde Group of the San Juan Basin in a 2001 survey. Although often portrayed as simple fractured blanket sandstones, these reservoirs are complex. Simple computer reservoir and stimulation models often do not capture the complexity of the formations in the Mesaverde Group.

Past Production/Expected Future Trends
Blanco Mesaverde Pool (field) was discovered in 1927 and has grown in response to pipeline capacity, decreased well spacing, and price increases. Cumulative production is greater than 10 Tcf and there is an additional 7 Tcf of known reserves. At this time more than 4,900 completions yield 0.75 Bcf/day. It is expected that within the next 20 years about 4,300 additional completions will occur, and many will have dual completion in the Dakota Sandstone. By statute, production from the Lewis Shale is considered part of the Mesaverde. The Menefee Formation is behind pipe in thousands of wells.

In the San Juan Basin, Upper Cretaceous Mesaverde (MV) Group Reservoirs include the formations located between the Mancos Shale and the Lewis Shale. Problems in the MV include transitional upper and lower contacts, a broad range of depositional environments, geographical variations in formations, and statutory definitions that contradict the stratigraphy. For example, much of the Lewis Shale (not MV in the stratigraphic sense) is considered MV by statute. However, law excludes the La Ventana Tongue (Chacra sands), which is stratigraphically part of the MV. Finally, the complexity of the MV reservoirs is often much greater than generally acknowledged.

It is obvious then, that the Mesaverde Group can be subdivided more than one way. The authors of the workshop suggested that the best way to subdivide the MV is to use criteria that optimize exploitation of reserves. Using the proper outcrops, field-based observations allow the complexities of reservoirs to be demonstrated, mapped and quantified. Outcrop analogs allow the reservoir to be understood on the scale of 80-acre infill wells or less and can help operators understand "anomalously good" reservoir units and predict their distribution.

Mesaverde Field Trip: Examples of Complex Reservoir Architecture.
A field trip provided "hands on" examples of the stratigraphic and reservoir relationships faced by operators in the San Juan Basin. The field trip was designed to demonstrate where new reserves could reside between wells and behind pipe in existing wells using examples from the Menefee Formation of the MV. Insight from the visited outcrops was used during subsequent exercises to demonstrate how to improve models of MV reservoirs. The field trip resulted from ongoing efforts by New Mexico Tech in the San Juan Basin. In a 2001 report, New Mexico Tech suggested to the US Bureau of Land Management that more than 4,300 new subsurface completions (mostly infill wells) will occur in 20 years. Production modeling suggests that these wells will access new reserves and accelerate production from the MV.

Mesaverde Correlations
A correlation exercise was conducted using the stratigraphic knowledge gained during the field trip. Stratigraphic complications at the top and base of the Menefee (seen on the field trip) helped to demonstrate what might be "missed" by using simple correlations. The correlation exercise graphically portrayed that the scale-dependent attributes of correlation are extremely important for medium- to small-scale reservoir compartments. It is because of this feature that infill drilling usually adds to the reserve base. It is important to understand the scale and distribution of generic stratigraphic units that are most likely to add new incremental reserves during infill operations.

Examples from the Menefee Formation
Infill well development is sensitive to the degree of natural fracturing, the quality and quantity of pay available and pressure depletion. Simplistic approaches to modeling gas recovery, however do not capture the details of the Mesaverde, particularly the Menefee. For this reason, one section of the workshop demonstrated the impact of the level of sophistication of the geological model on the production performance.

Using a numerical flow model, a development history was planned which follows the typical sequence observed for the Mesaverde. As suspected, estimates of gas-in-place are dependent on the number, type and size of individual layers and their petrophysical properties. Initial findings from the geological work suggests infill potential of MV formations is a function of the orientation and aspect ratio of channels and beach ridges, the frequency of channels, and the flow barriers between layers.

Another key point to infill well development is the magnitude of pressure depletion in existing producing horizons. The Point Lookout Formation is a primary MV target across the region. Low gas recovery in about half of the studied wells in this formation was due to pressure depletion from existing wells. After 50 years of production, low recovery of gas-in-place (GIP) from the Beach Ridge and Menefee intervals of the MV indicate significant remaining gas reserves.

Stimulation in the Lewis Shale within the San Juan Basin
The Lewis Shale within the San Juan basin has become an important and economically producible source of gas in the San Juan Basin. The generally productive area for the Lewis Shale is in the north-central part of the Basin. The Lewis is between 1,000 and 1,500 ft thick throughout the basin. Although called a shale, the Lewis is a sandy siltstone containing four dominant lithofacies: (1) organic mudstone (highest TOC and adsorption values), (2) laminated sandy mudstone (characterized by microfractures and microporosity), (3) bioturbated facies (low TOC but abundant microporosity), and (4) submerged sand ridge facies (has some conventional matrix porosity).

The Lewis shale is undeveloped in hundreds to thousands of wells producing from the Mesaverde Group and deeper reservoirs. Various hydraulic fracture designs have been used to complete the Lewis Shale. Production data from 313 Lewis wells was analyzed to determine the optimal design parameters. Optimal stimulation procedure for the Lewis Shale is a single-stage, 150,000 lb. foamed linear gel, hydraulic fracture treatment. Additional slickwater and linear gel treatments are recommended, although the data set for this application is small.

CONNECTIONS:

Brian S. Brister
NM Bureau of Geology and Mineral Resources
NM Institute of Mining and Technology
801 Leroy Place
Socorro, NM 87801-4796
Ph: 505-835-5378 
Email: bbrister@gis.nmt.edu

Tom Engler
Petroleum Engineering Department
NM Institute of Mining and Technology
801 Leroy Place
Socorro, NM 87801
Ph: 505-835-5207
Email: engler@nmt.edu

Steve Hayden
District Geologist and Deputy Inspector
NM Oil Conservation Division, district III
1000 Rio Brazos Rd.
Aztec, NM 87410
Ph: 595-334-6178 ext. 14
Email: shayden@state.nm.us

For information on PTTC’s Southwest Region and its activities contact:

Dr. Robert Lee, Director, Petroleum Recovery Research Center,
New Mexico Institute of Mining and Technology
801 Leroy Place-Campus Station
Socorro, New Mexico 87801

Martha Cather, Coordinator
Phone: 505-835-5685 Fax: 505-835-6031
Email: martha@prrc.nmt.edu

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