A Geomechanical Study of Refracturing Based on Microseismic Observations - Case Study of Haynesville and Eagle Ford Wells

Abstract

Microseismic monitoring of refracturing of depleted horizontal wells frequently shows a concentration of microseismic activity at the heel area when no mechanical isolation is used. This observation suggests that a considerable length of the well toward the toe does not benefit from refracturing and remains unstimulated. Different completion techniques, ranging from injecting diverters to using mechanical intervention methods, are usually used to avoid the localized stimulation and to enhance the treatment effectiveness. However, often overlooked is the effect of the reservoir rock's mechanical characteristics and how they contribute to the treatment results.In this study we investigated the potential contributing factors to the observed microseismic response: i) fluid pressure drop along the lateral, ii) diverter ineffectiveness, and iii) stimulation of pre-existing fractures versus developing fresh fractures from new perforations. Estimation of pressure losses along the well for the common casing diameters and fracturing fluids indicates that a high pressure gradient develops along the well during refracturing. It results in significantly higher injection pressures at the heel than at the toe, leading to higher discharge rates at the heel. If the added diverters fail to seal off the perforations at the heel area, this condition persists throughout the treatment and causes the localized stimulation of the rock, as is usually observed by microseismic monitoring.The numerical simulation of refracturing indicates that under a non-uniform treatment pressure profile and in the absence of effective diverters, the initiation and propagation of new hydraulic fractures is unlikely. The dominant stimulation mechanism is the shear failure of natural fractures, driven by the increase of fluid pressure by injection of fluid through the old perforations. This result is consistent with the observed long delay in microseismic response to refracturing and the increasing event counts as pumping continues.Based on these findings, we developed an alternative refracturing method that aims at increasing the reservoir effective complexity and enhancing the conductivity of the pre-existing hydraulic fractures uniformly along the well. The proposed method consists of a prolonged low-pressure and low-rate pad stage to pressurize the reservoir, followed by a high-pressure injection stage to stimulate the pressurized natural fractures and to place proppant in the new fractures. Critical to the success of this method is to avoid a high pressure contrast along the well. This can be achieved by proper selection of injection pressure and fluid viscosity with respect to the reservoir stresses and pressure, and the well characteristics. Numerical simulations indicate that the proposed method can considerably enhance the efficiency of refracturing, at no additional cost compared to the common refracturing methods.

Review of Shale Oil and Gas Refracturing: Techniques and Field Applications

“…For example, in a shale gas well in the Woodford block, the construction pressure gradually increased, but there was basically no increase in the post-fracturing production [30]. Agharazi et al [41] argue that, in the presence of many perforated clusters, it is difficult for refracturing to generate new fractures. As gas continues to be produced after the initial fracturing, Mohr's circle of formation rocks shifts to the right, increasing the effective stress acting on the weak planes of the rocks, thus making it more difficult to generate new fractures.…”

Well Performance Simulation and Parametric Study for Different Refracturing Scenarios in Shale Reservoir

“…The effective stress increases while the reservoir pressure depletes, in which, in turn, the porosity and permeability of shale reservoir change due to rock matrix deformation. The stress sensitivity will further reduce the flow capacity of the fracture system in the stimulated area [21], which is a critical factor for production prediction and optimal designation of restimulation. The existing complex fracture network is properly characterized in this numerical model; in addition, the solid deformation effect and complex gas flow behavior are taken into consideration.…”

Research on stress alternation effects and fracture reorientation for refracturing treatment

“…Withdrawing liquid from underground formations increases the effective stresses but potentially reduces the total stresses. 14 The altered-stress concept has been shown to be valid in both field tests and finite-element calculations. 15 In the oil fields, liquid production from the underground formation causes the depletion of pore pressure so that the net effective stress on the rock may be increased.…”