Early quantification of the well's ability to effectively drain the productive reservoir will result in improved economic performance. The effective drainage area of a tight gas well is primarily controlled by reservoir geometry, rock and fluid properties and well completion efficiency. This paper will present theoretical effective drainage areas for wells completed in blanket sands (radial drainage) and channel sands (linear drainage). The work will focus on the relationship between reservoir geometry, effective gas permeability, porosity and fracture half-length for dry-gas reservoirs.
The relationship of effective gas permeability and fracture half-lengths on the effective drainage area will be presented as a result of this work. Typical Appalachian Basin producing reservoir properties will be incorporated into the simulation work tailoring this analysis to those producing environments. The cases presented in this paper will include permeability ranging from 0.01 md to 1 md, reservoir geometries ranging from radial flow to channel widths from 250 ft to 1000 ft and fracture stimulation half-lengths up to 300 ft. For consistency, all stimulation cases will be based on an assumed 200 md-ft fracture conductivity.
The effective drainage area of tight gas reservoirs can best be determined through production analysis. The paper discusses two methods of determining the effective area for dry-gas reservoirs. In tight gas reservoirs (reservoirs with effective gas permeability less than 0.1 md), a significant amount of time is required before the pressure transient is affected by all of the boundaries of the reservoir. The no-flow boundaries affecting the well's pressure transient behavior can be physical boundaries or boundaries due to offset production. Therefore, the effective drainage area of a tight gas producer will also be a function of natural boundaries, drilled well spacing, effective gas permeability and completion efficiency.
The efficient well spacing for tight gas producers can be defined in many ways. For the purpose of this paper, the efficient spacing is defined as the well spacing required to attain eighty-percent recovery of the in place gas over thirty years. An alternate definition considered by the authors was the well spacing required to maximize the well's thirty year recovery. The simulation results for these cases are also presented. However, these cases resulted in very low recovery of the in place volume.
For the same reservoir geometry the efficient well spacing increases with both permeability and fracture half-length. This work demonstrates that as a system becomes more linear in nature, the effective drainage area is reduced for the same reservoir and completion parameters.
The following is a brief summary of two production analysis techniques that can be used to determine the effective drainage area of a producing well. This is accomplished by first estimating the well's drainage volume. The volume is then used to estimate the well's drainage area from the volumetric equation. Both techniques combine gas material balance and flow equations to estimate the well's effective drainage volume. The assumptions common to both techniques are as follows:
Well production rates and pressures are accurate.
The flowing bottomhole pressure can be accurately estimated from surface pressures if not measured.
The reservoir drive mechanism is volumetric depletion.