Application of mini-falloff test to determine reservoir parameters and optimize fracture designs in a tight gas field

This paper provides examples of using small injection tests on tight gas wells in Jonah field to determine reservoir pressure and transmissibility. Prior to Mini-Falloff (MFO) test application in Jonah field, reservoir pressure and permeability of each sand body were obtained from log-derived correlations; these correlations in the presence of depletion have been determined to be invalid. The incorrect reservoir pressure and permeability will generally result in wrong fracture geometry and pre-frac rate prediction. The MFO's were designed as small injection tests prior to fracture stimulation to correct these anomalies. The After Closure Period following the MFO injection and shut-in contain the reservoir pseudo-linear and pseudo-radial flow periods. Analysis of the pseudo-radial flow period provides reservoir transmissibility (kh/μ) and initial reservoir pressure (Pr) in a manner similar to a Horner analysis. These two-reservoir parameters play a crucial role in the optimization of fracture design and rate prediction. For a tight gas sand, the amount of time required to reach pseudo-radial flow was thought to be quite large. However over sixty percent of all the valid tests performed so far reached pseudo-radial flow within a shut-in time of 24hrs. A total of eighteen MFO tests were performed in the Jonah field Lance formation from April to October 2003. Two of the tests were invalid due to extremely high depletion and surface pressure gauge malfunction. The remaining sixteen valid data sets show various ranges of depletion in twelve sands tested (0.36-0.52psi/ft) while the two remaining sands were overpressured (> 0.7psi/ft). Permeability results in general ranges from 0.004 to 0.08md. The Jonah field log-derived correlations were validated and improved with the MFO results.