gohfer 9.1.3 2019

GOHFER, which stands for Grid Oriented Hydraulic Fracture Extension Replicator, is a planar 3-D geometry fracture simulator with a fully coupled fluid/solid transport simulator. GOHFER was developed by Dr. Bob Barree of Barree & Associates in association with Stim-Lab, a division of Core Laboratories. GOHFER has been continually refined using established formulations that have been verified in Stim-Lab’s laboratory and in the field.

A regular grid structure is used to describe the entire reservoir, similar to a reservoir simulator. The grid structure allows for vertical and lateral variations, multiple perforated intervals as well as single and bi-wing asymmetric fractures to model the most complex reservoirs. GOHFER allows modeling of multiple fracture initiation sites simultaneously and shows diversion between perforations. The grid is used for both elastic rock displacement calculations as well as a planar finite difference grid for the fluid flow solutions. Fluid composition, proppant concentration, shear, leakoff, width, pressure, viscosity and other state variables are defined at each grid block.

The in-situ stress is internally calculated from pore pressure, poroelasticity, elastic moduli and geologically consistent boundary conditions. The width solution is fully 3-D allowing shear decoupling and local displacements are controlled by local pressures and rock properties. Screenouts consider localized leakoff and proppant holdup and are not assumed to be caused by pad depletion or insufficient width. Fracture extension is based on a smoothly closing tip model and eliminates the fictitious singularity at the tip as well as the stress intensity factor.

The fracture extension and deformation model in GOHFER is based on a formulation that expects the formation to fail in shear and be essentially decoupled. Most models assume linear-elastic deformation of a fully coupled rock mass. In shales the assumption of shear is critical. Along with this GOHFER accounts for pressure dependent leakoff, transverse storage in horizontal and off-angle fracture components, and pressure dependent modulus of the bulk rock mass. All these things happen as the formation is stressed to the shear failure point. The result is higher treating pressures, smaller fracture heights, and more difficulty in placing proppant. The proppant transport model in GOHFER also includes modeling of non-uniform solid and liquid velocities, solid holdup, and variable transmissibility linked to fracture offsets and irregularities.

GOHFER allows geologic structure to be included in the modeling to simulate fracture growth in complex folded and faulted regions. Fluid and proppant injection is automatically redistributed at each timestep to model simultaneous injection into multiple perforation sets or clusters in limited-entry or horizontal well treatments. Perforation erosion and variable near-well tortuosity are modeled based on simulation of both laboratory and field studies.

GOHFER models both horizontal longitudinal and transverse fractures (multiple transverse planar fractures off of a single wellbore). GOHFER can represent the 3D stress tensor, non-orthogonal fractures, fracture reorientation, wellbore tangential stresses and breakdown conditions. It handles stress shadowing or fracture interference of multiple transverse fractures. Perforations can be selectively opened or closed throughout the simulation to model either stage by stage, multiple cluster treatments with fracture interference calculated between clusters as well as interference between stages (stage stress shadowing). Ball drop treatments may also be simulated.

GOHFER accurately models fracturing results as verified by radioactive tracer, micro-seismic and tilt-meter surveys. It is the only model that is backed by more than 20 years of laboratory research in all major areas of transport and mechanics. GOHFER includes the ability to import micro-seismic data to compare with simulation results.