Empirical analysis for the characterization of geo-mechanical strength and pressure regime:Implications on hydraulic fracturing stimulation

Among the several activities involved in oil exploration are the determination of hydrocarbon in-place and mechanical competency of the oil reservoir.The pressure regimes of the formation have also become vital properties which must be well known to ensure preliminary awareness of the hydraulic fracturing.This study seeks to adopt a prediction strategy of the overall geo-mechanical competency and strength of the formation,using a less stressful computational process and an empirical analysis,developed using three wells from ED BON area in parts of Niger Delta.Elastic constants such as Poisson Ratio,Young's,Shear and Bulk moduli which are the parameters for characterizing rock mechanical properties were estimated,as well as the subsurface formation pressures and the associated fracture gradient using P-wave sonic and density logs.The results from the analysis showed that there is correlation between elastic strength and fracture pressure.

Fault reactivation potential and associated permeability evolution under changing injection conditions

Understanding the hydraulic and frictional sensitivity of fault to different injection conditions is one of the efficient ways to provide useful implications for fault reactivation potential.Numerical simulations of fractured reservoir have provided information on how fault behaviour varies under changing hydromechanical properties and injection conditions.A coupled hydro-mechanical model which can represent the elastoplastic behaviour of a fault was employed to predict and quantify the effects of varying injection positions and injection rates on permeability response and potential of fault reactivation under isothermal injection.We examine the sensitivity of seismic event magnitude and timing to variations in both pressure perturbation and stress as injection location changes.We generate results for two scenarios:one with changing injection position but with uniform injection rate,and another scenario with increasing injection rate at the same injection position.We observed that the potential of fault reactivation is affected by the hydraulic diffusivity potential of the fluid pressure,and this mechanism is mediated by a function of the injector position and injection rate.As the velocity of fluid transmission increases,increasing fluid pressure impact pore pressure elevation and reduced effective stress.However,an injector position where there is low diffusivity causes low pore pressure build-up rate,incapable of inducing shear failure,and thus,permeability enhancement is retarded in this case.Accordingly,the injection rate variation influences the rate of pore pressure build-up,the timing and magnitude of induced seismic events.This is also reflected in the permeability evolution as a response to the variations in the magnitude of fault openings and cracks.This changing injection conditions however influences the timing required to reach the critical peak friction point as pore pressure build-up rate and sensitivity to loading response change.Hence,with changing position and rate of injection,the evolution of fault permeability appears to be intrinsically controlled by a condition which favours elastoplastic deformation and fracture failure,with slip distance increasing with high injection rates.

Controls of fault geometry and thermal stress on fault slip modes:Implications for permeability enhancement and injection-induced seismicity

Fluid pressurization within the fault zone generates increasing pore pressure and stress change which is liable to create shear and/or brittle fractures within the reservoir volumes and subsequently generating earthquakes of varying magnitudes.Here,we explored time-dependent fault weakening processes in the fault zone which are dependent on several factors,including the rate of cold-water injection,modes of injection(hydromechanical(HM)and thermo-hydro-mechanical(THM)interactions),and changing fault spatial configurations using data from Niger Delta Basin.The variation in the stability of different fault models in withstanding stresses induced by HM and THM fluid interactions is evident.Fault permeability enhancement and the behaviour of slip event under isothermal and non-isothermal conditions revealed that stress and pore pressure perturbations have a first order control on the rate of fault dilation and compression.It is observed that the progressive cooling of the reservoir induced thermal stress which induced the timing of slip by unloading the fault to earlier seismic rupture in the non-isothermal case,and accelerates the magnitude of the fault reactivation and the accompanied induced seismicity.Owing to increased tendency of shear failure during injection,fracture opening through shear dilation is more enhanced in THM simulation as the fracture permeability is significantly higher than in HM.This effect becomes increasingly more dominant with intermediate fault angle and joint orientation.Certain fault/joint configurations which were resistant to shear failure under isothermal injection had their frictional resistance broken by thermal stress.The results also indicate that there is higher pore pressure build-up in THM than in HM as the injection rate increases and reservoir temperature drops during cold injections..This study has demonstrated the importance of fully characterizing the fracture geometries and configurations of normal faulting regime in addition to fluid injection conditions when developing fractured reservoirs to mitigate seismic risks and hazards that could result from early fault reactivation.