Abnormally High Fracturing Pressures in Step-Rate Tests

Abstract

SummaryStep-rate tests (SRT's) are commonly used to infer formation parting pressure. The fluid injected is usually an incompressible Newtonian fluid, such as water. A theory was developed to describe the increase in wellbore pressure (i.e., fracturing pressure) once parting pressure is exceeded. The theory is based on a Perkins-Kern geometry (confined-height, elongated fracture) and assumes the limit of large leakoff coefficient and/or time. The theory applies for two different injection profiles. It was compared with measurements of wellbore pressure in eight field SRT's and one constant-rate injection test at the Nevada Test Site. Four tight gas sandstones, three liquid-saturated coals, one liquid-saturated sandstone, and one volcanic tuff are included. In most of the cases studied, the observed net wellbore pressure is two to nine times higher than predicted from conventional fracturing models. In seven cases, fracture geometry appears to be consistent with the Perkins-Kern model, a finding confirmed by modeling with a planar, fully 3D simulator. In the two cases that are inconsistent with the Perkins-Kern geometry, the theoretical prediction is based on fully 3D modeling. The statistical mean of discrepancy between observed and predicted net wellbore pressures is 3.9, with a one-standard-deviation range of 1.7 to 8.9.The data set is used to discriminate between the hypotheses put forward to explain the discrepancy. Poroelastic effects, tip plugging by fines in sandstone formations, the multistranded nature of fractures, and fracture-surface roughness or waviness are not viable explanations. The explanations most consistent with the bulk of the data are high friction losses in constrictions within a fracture and apparent fracture-toughness effects. Of the two, we favor the first. The results presented here are further independent evidence for abnormally high pressures in induced fractures, such as previously reported in larger fracture stimulations in tight gas sands and in coalbed fracture stimulations.