Effective stress history and the potential for seismicity associated with hydraulic fracturing of shale reservoirs

Abstract

The influence of effective stress history on reservoir stability and the response to hydraulic fracturing are simulated for two hypothetical shale reservoirs. It is numerically demonstrated that the effective stress history influences the present-day stability of faults and natural fractures. Any assumption of a homogeneous stress state is shown to be unrealistic in the majority of fractured shale reservoirs. The simulations demonstrate that, depending upon the effective stress history, shear displacements can be induced by stress changes associated with stimulation without a change of fluid pressure in the destabilized fractures. Elastoplastic shale and other fractured reservoirs are shown to have a memory of past geomechanical states. These numerical findings demonstrate the value of interpreting reservoir effective stress history when planning fluid injection. A previous case history is summarized to illustrate the overwhelming dependence of stress history analyses on a wide spectrum of earth science disciplines. It also illustrates the application of stress history analyses to activities other than hydraulic fracturing. The current inexperience of effective stress history description means that new approaches must be identified to optimize the necessary multidisciplinary investigations.

The nature of stress state: numerical and laboratory experiments and some field observations

“…Plane-strain, planview simulations of a 5 km × 5 km shale reservoir are used to identify characteristics of the stress distribution in discontinuously fractured rock. The simulation procedures are similar to those reported by Harper (2014) to demonstrate the influence of stress memory on the stability of faults and fractures. Some observations and implications drawn from these simple numerical representations of stress state are evaluated in the light of published information and some experiments previously conducted on intact sandstone core samples.…”

“…Brady et al (1986) described the stresses so generated as 'locked in'. Harper (2014) further investigated this model, pointing out that the stability of a fault structure will typically be influenced by such surrounding stresses so that any description of fault stability which assumes a planar frictional surface in a homogeneously stressed medium can be misleading.…”

“…Many cycles were then imposed to allow the model to approach equilibrium. More details on the model and simulation procedures are given in Harper (2014).…”

Geological controls on artificial fracture networks in continental shale and its fracability evaluation: A case study in the Yanchang Formation, Ordos Basin, China

“…The aim of shale reservoir hydraulic fracturing is to form a nonplanar, asymmetric, multi-branch fracture network (Yu et al, 2014;Harper, 2014). During hydraulic fracturing, two types of deformations are formed in shale reservoirs.…”

Can Repetitive Small Magnitude-Induced Seismic Events Actually Cause Damage?

“…Intraplate regions are primed to be susceptible to induced earthquakes, as the geological stress states are already at critical stress conditions: the geological structure is in a quasistatic equilibrium state [3,[5][6][7]. Recent advances in fluid-based geoengineering activities, hydraulic fracturing/ enhanced recovery technology, population growth, and worldwide energy consumption rates are suspected of contributing to the exponential increase in seismic activity within these historically aseismic regions by altering deep lithology effective stress states, which result in subsurface shear failure [1-3, 5, 6, 8-20].…”