Qualitative Pressure Profiling to Determine Wellbore Contacts with Hydraulic Fractures
Authors: Nelson D.G. , Economides M.J. 
Published: December 2007

This work discusses a field-based pressure profiling method which allows direct evaluation of two important indicators of hydraulic fracture treatment quality - the presence of multiple fractures and vertical coverage. Individual or groups of active perforations are systematically hydraulically isolated with a conventional dual packer assembly. Short-term pressure changes are measured with downhole memory pressure gauges and comparisons of the pressure response behavior of the isolated intervals are made. Methodology, field test results, and implications are presented in this paper. Field tests show two types of perforation pressure response. Data can show which perforations are connected to fractures and whether multiple fractures have been created. One response ranges from 0 to 2 psi/minute and the other ranges from 3 to 30 psi/minute. The pressure profile results are compared to, and are consistent with, less direct interpretations from production logs and tiltmeter fracture mapping.

Introduction

Classic hydraulic fracturing models (PKN, KGD, radial, and variants), that assume linear elastic fracture mechanics and single-plane fractures that originate and/or are contained in a single layer, often fail to capture observed field behavior. Several phenomena may cause "abnormal" fracture initiation and propagation.

The fracture plane is supposed to be normal to the minimum principal stress. It has been postulated by almost every investigator in the field that in the overwhelming majority of petroleum applications, fractures are vertical and normal to the minimum horizontal stress.

However, even in the simplest, albeit elegant, description of the state of stresses for a given depth, the only stress that remains unchanged over large areas is the absolute vertical. Decrease in the reservoir pressure, because of production-induced depletion, or increase in the reservoir pressure because of fluid injection, may alter the interrelationship among the stresses.

Shallower reservoirs are particularly susceptible to such phenomena. In the process of original formation deposition, the well-known Poisson translation might have dictated a predictable relationship between vertical and horizontal stresses. However, subsequent geologic events such as glaciation, erosion, diagenesis and tectonic or geothermal phenomena may alter this relationship considerably.

At an extreme, the minimum horizontal stress, presumed to be "locked in place", may become larger than the vertical stress and, thus, a hydraulic fracture becomes horizontal, lifting the overburden. In the past, the critical depth, above which horizontal fractures are created, was reported by many to be in the range of 0-1500 feet. However, it would not be difficult to envision far greater depths in highly overpressured, tectonically compressed, and/or lightly overburdened formations. More important is the case where a formation is near this critical depth and where a fracture plane, initially propagating normal to one stress may change course by 90 degrees and become normal to another stress. This change of direction can happen when the difference in the magnitudes of the two stresses is less than the net pressure of the propagating fracture. Fracture geometries such as these, one of which is referred to as "T"-shape, are not uncommon.

For vertical fractures, in reservoirs approaching stress isotropy in the horizontal plane, the net pressure, added to the minimum horizontal stress may exceed the seating pressure of transverse natural fissures, causing excessive leakoff. In many cases, both the latter phenomenon and "T"-shape fractures may bring about a severe dehydration of the fracturing fluid slurry leading to premature screenouts.

P. 251