Horizontal respect distance for hydraulic fracturing in the vicinity of existing faults in deep geological reservoirs: a review and modelling study

Abstract

Hydraulic fracturing is widely used in the petroleum industry to enhance oil and gas production, especially for the extraction of shale gas from unconventional reservoirs. A good understanding of the vertical distance which should be preserved between hydraulic stimulation and overlying aquifers (potable water) has been demonstrated as being greater than 600 m (2000 feet). However, the effective application of this technique depends on many factors; one of particular importance is the influence of the fracturing process on pre-existing fractures and faults in the reservoir, which, however, to date, has had little analysis. Specifically, the identification of the required respect distance which must be maintained between the hydraulic fracturing location and pre-existing faults is of paramount importance in minimizing the risk of felt, induced seismicity. This must be an important consideration for setting the guidelines for operational procedures by legislative authorities. We investigate the respect distance using a Monte Carlo approach, generating fifty discrete fracture networks for each of three fracture intensities, on which a hydraulic fracturing simulation is run, using FracMan Ò . The Coulomb stress change of the rock surrounding the simulated injection stage is calculated for three weighted source mechanisms combining inflation, strike-slip and reverse. The lateral respect distance is obtained using values from literature of the amount of stress required to induce movement on a pre-existing fault. We find that the lateral respect distance is dependent on fracture intensity and the failure threshold. However, the weighting of the source mechanism has limited effect on the lateral respect distance.

Hydraulic Fracture Injection Strategy Influences the Probability of Earthquakes in the Eagle Ford Shale Play of South Texas

“…We noted that ~10% of the HF wells in our eight focus regions targeted the Austin Chalk Formation (265 wells) rather than the Eagle Ford Formation (2,229 wells) where a lower percentage of wells correlated with seismicity (7.1% vs. 19.2%, supporting information, Table S2). The difference in earthquake probability appears to be a result of the geographic distribution of HF wells as the Austin Chalk is only targeted in a narrow zone where the seismicity was located (supporting information, Figure S7), supporting the widely held notion that proximity to susceptible faults is a critical factor for induced seismicity (e.g., Moeck et al, ; Pawley et al, ; Skoumal, Brudzinski, et al, ; Westwood et al, ). Although the lack of sufficient earthquake depth resolution prevents interpretation of where the seismicity is occurring, we note that carbonates like the Austin Chalk are more likely to produce induced seismicity than shales like the Eagle Ford (e.g., De Pater & Baisch, ; Lei et al, ; Sone & Zoback, ).…”

Real‐Time Imaging, Forecasting, and Management of Human‐Induced Seismicity at Preston New Road, Lancashire, England

“…Various options have been suggested to regulate induced seismicity. Fault respect distances (Westwood et al, 2017) require an operator to avoid known faults in the subsurface.…”

Faults and associated karst collapse suggest conduits for fluid flow that influence hydraulic fracturing-induced seismicity

“…However, for HF, this reasoning is complicated by additional conceptual hurdles. For example, HF wells inject into impermeable formations to stimulate reservoir productivity (17); thus, reasonable pressure perturbations are restricted to the stimulated reservoir zone proximal to the well bore [up to hundreds of meters (18)(19)(20)(21)]. Furthermore, geomechanical modeling reveals that poroelastic changes from HF are transmitted only locally (20).…”