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Tradeoffs in 3D Seismic Acquisition Between Shallow and Deep Objectives or Value of 3D High Density (Infill) Seismic Survey to Improve Economic Results of CBM Wells from Cow Creek Unit

Nancy J. House
EnCana Oil & Gas (USA), Inc., Denver, CO

3D seismic data provides much needed lateral information in the inter-well space not possible through extrapolation of well information. However, the survey must be designed to provide the required coverage at the depth of interest. Seismic energy spreads away from its source in a spherical pattern or a cone spreading downward from the shot. Fold, or redundancy is developed by utilizing the overlap of these cones of energy produced by each shot and recorded into a 3D spread, or pattern of receivers on the surface Fold or redundancy is depth dependent as the cones of seismic energy spread out further as they go deeper. While shallow data may be easy to obtain and good quality, it is often cost prohibitive to increase the source and receiver sampling effort to the level required providing as little as 6 or 7 fold at shallow depth, 1000’ or less. In a survey designed for much deeper targets. One of the early bids for the Cow creek survey from PGS Onshore was designed to provide a nominal fold of 6-8 at a depth of approximately 1000’. The cost estimate for this survey was prohibitively high at a cost of over $1.8 million for acquisition alone. At that time the decision was made to pursue the deep data while preserving the option to come back at a later time to fill in the survey at a higher density. The higher density infill program would be designed to obtain better resolution of the deeper reservoirs and increase the possibility of obtaining some usable data at the shallow Mesa Verde level to improve the outcome of the shallow low risk CBM wells.

Concurrent to the planning and design of the 3D survey to image a deeper higher risk target, teh operator was planning 8 CBM wells in a 2 square mile area near the western edge of the survey. Would be possible to provide the higher density coverage in a 2.5 to 2 square mile area to image three Mesa Verde coals with thickness ranging from 5 feet to 25 feet and the intervening sandstone, siltstones? A plan was devised to infill the survey in a 1.75 square mile resulting in a nominal fold of 1 in most of the bin locations at a depth of 1000 feet. The approximate cost of this additional effort centered in section 12 T 12N R 92W would be between $50,000 and $60,000 based on the cost for additional, shotpoints and receivers outlined in the Western contract. The cost of each well contained within the survey area is $50,000 and the expected Value of a good well vs. poor well is summarized in the table below.

Cost $50,000/well
Well Value-High Producer Value Low-Producer
Well 32-12 $500,000 $200,000
Well 42-12 $500,000 $200,000

This paper will evaluate the value of the seismic information by estimating how the uncertainty the amount of gas to be produced by a given well would be affected by the information added by the higher density 3D survey shown in figure 1. The conditional probability that seismic could predict ‘better locations’ by addressing one of the key uncertainties, coal thickness, can be estimated forward modeling the seismic response of the expected ranges of coal thickness’ and determining those which will be detectable by the higer density infill seismic. With the conditional probability that the 3D seismic data will be able to detect the coals estimated, Bayes Theorem of probability can be invoked to calculate the posterior probability that a thicker coal will be encountered in one location rather than another based on the observed response of the seismic data.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado