Shear Dilation Diagnostics: A New Approach for Evaluating Tight Gas Stimulation Treatments

Abstract

Many tight gas reservoirs require fracture stimulation to achieve commercial outcomes. These reservoirs can often be characterized geologically and geomechanically by high deviatoric stresses and hard, naturally fractured rock. Stimulation treatments in such reservoirs may create complex fracture networks from a combination of shear and tensile failures. Water fracs can be used in environments where shear failure is anticipated to dominate. However, few practical modeling tools exist [1] to identify the dominant failure modes prior to or during stimulation and [2] to evaluate the effectiveness of such treatments. This paper seeks to provide the engineer with a suite of tools capable of achieving these two goals. Firstly, this paper presents a dual porosity, pressure dependent permeability reservoir simulation model that was devised to honor shear failure mechanisms [also called shear dilation] using basic geological characterization. The assumptions of this model and the pragmatic selection of first order effects are discussed. Using the results of this simulation model, three families of diagnostic tools are presented. The first category is that of Treatment Diagnostics (TD) which include bottomhole pressure evaluation, injectivity and fall-off analysis. The second approach is called Seismic Based Reservoir Characterization (SBRC), which uses the microseismic to determine the stimulated rock volume (SRV) as well as provide estimates of the initial and stimulated fracture network properties. The third category is Post Treatment Diagnostics (PTD), which incorporates the evaluation of pressure drawdown characteristics. Finally, this paper compares these individual approaches and provides a workflow to evaluate data on future wells. Introduction Under some geological conditions the stimulation design for tight gas reservoirs may need to account for complexity caused by shear dilation mechanisms. In these cases, treated water with minimal or no proppant concentration [water fracs] can be the most effective stimulation method. However, few simulation models exist which account for this failure mode. The principal motivation for this study was firstly to develop a reservoir simulator to account for the first order effects of shear dilation. It was envisaged that, because of the complexity of the model, quantitative measurements of the pre and post stimulation reservoir properties would require direct history matching with the simulator. Despite this limitation, the aim was also to use the simulator to develop qualitative field-based diagnostic tools to determine the level of shear dilation contribution and to determine the effectiveness of such treatments where shear dilation dominates. Shear Dilation Concept Shear dilation is the event of sliding which results in self-propping due to irregularities or asperities on rock surfaces (Fig. 1). The self-propping effect can be a significant component driving stimulation effectiveness in some reservoirs1. Studies have shown that this failure mechanism can be created at pressures less than the minimum horizontal stress2. The geomechanical rock properties required for shear dilation to occur are high deviatoric stress, high rock strengths, brittle rock with a low tendency towards plastic deformation [high Young's modulus and low Poisson's ratio] and abundant low strength rock fabrics or natural fractures.

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“…Thus, in order to properly select locations for hydraulic fractures, factors such as near wellbore stress condition, wellbore direction, direction of principal stress etc. should be considered with cautious [21]. Horizontal well is popular in unconventional gas stimulation because it can greatly increase the contact area between fracture and reservoir.…”

Developing Guidelines for Selection of Appropriate Fracture Models in the Numerical Simulation of Well Performance Behavior for Liquid Rich Ultra-Low Permeability ULP Reservoirs

“…Proppant is either placed in theopened fractures or a self propping effect known as "shear dilation" can occur. Shear dilation is caused by the irregularities and asperities on the rock surface that prevent the fracture from completely closing (Chipperfield et al 2007).…”

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“…To account for such stress-dependent permeability upon depressurizing the reservoir, Raghavan and Chin [88] proposed a formula that considers permeability as a function of the pressure. Other researchers such as Raghavan and Chin [88] and Chipperfield et al [89] proposed other relationships defining how permeability is related to the stress state. Other modeling approaches including fractional derivative formulation have also been implemented (e.g., [90]).…”