Full-Physics Thinking in Unconventional Plays

Abstract

Unconventionals, perhaps more than other plays, demand consideration of process interactions. Geomechanical interactions occupy a central role in Unconventionals: geohistory, and the mechanical processes that operate, creates pre-cursor conditions; manufacturing the reservoir is a dominantly mechanical activity; and during reservoir production, mechanical interactions play a governing role. Classical methods of geomechanical interpretation and analysis fail to address the physics interactions, and can lead to incorrect deductions and decisions. These difficulties arise because the classical approaches assume that rock stress is an independent parameter and can be assigned a value. That view is physically impossible. The key point is that the concept of stress can be expressed in multiple ways –the most important one is that stress is the specific (mass/volume-related) elastic energy. Using this “take” on stress, we examine some important aspects of Unconventional reservoirs, focusing on hydrofracture stimulation. We assess some notions that inhibit understanding and interfere with the discovery of better practices. The hydrofracture process involves injecting a medium (usually water-based) into perforations, aiming to create new openings in the rock mass that will allow better hydrocarbon flow. The injected fluid pressure (an energy measure) and volume define the energy input. Some energy is consumed in making new discontinuities, and in shifting rocks. Where is the rest? As discontinuities open, the adjacent rocks become strained, typically in ways that lead to local contractions and volume loss, so their stress (elastic energy) state increases. In poro-elastic terms, the pre-existing pore fluids gain some of this added energy, so have higher pressures. Calculations show that injected fluids do not invade the pore system of the matrix rocks, and therefore, those not yet recovered in flowback must be located in newly-created (or enhanced) openings – typically fracture-like features. After one hydraulic fracture stage, the subsurface state is considerably altered, with impacts on subsequent stages. After multiple stages, the state is characterised by high energy levels that work against the maintenance of the permeability created by the stimulation activities. The full-physics interactions, expressed in terms of energy components and partitioning, lead to new insights, and provide a framework within which new operational practices can be contemplated.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015