Practical Reservoir Analysis and Drainage Area Considerations

Knobloch, Timothy S., Jerry James, and Eugene Huck
James Engineering, Inc., Marietta, OH

Historically, many operators have utilized state minimum requirements as the basis for determining the placement of infill oil and gas wells. State requirements were often based upon the 1785 public land grid system of township, section, and range that resulted in the development of 40 to 640 acre drilling units. Other state requirements were based upon formation depth, lease unit setbacks, drilling unit requirements, spacing orders, or the distances to certain structures. However, it appears that the effects of offset well production upon reservoir pressure depletion, recoverable reserves, and deliverability may be a secondary consideration for operators when selecting potential well sites.

Experience shows that operators utilizing only state minimum requirements often ignore pressure depletion effects on natural gas reservoirs, and oftentimes achieve less than desired results. This paper reviews the classifications of oil and gas reserves and the well spacing regulations of Pennsylvania, Ohio, West Virginia and Kentucky, and effect of the Darcy flow equation upon reservoir pressure, pressure transients, production, and reserves. A Morgan County, Ohio case study including a historical review of well reserves time, initial reservoir pressures, and the effect of depletion are also presented.

The authors conclude that the pressure depletion problem may not be isolated to southeastern Ohio and may lead to the booking and drilling of reserves that may not be there. The authors stress the need to thoroughly analyze traditional reservoirs and deeper horizons and to develop a checklist for reviewing acreage including drainage area considerations.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004