PORE PRESSURE EVOLUTION AND CORE DAMAGE: A COMPUTATIONAL FLUID DYNAMICS APPROACH

摘要

During tripping out of the hole,core is submitted to a relatively sudden decrease in pressure which leads to fluid expansion and movement out of the pore space into the borehole.It is widely recognized that the rapidly expanding fluids can generate fractures where the tensile stress created exceeds the tensile strength of the rock.Ideally,tripping rates should be established for each particular core during the planning phase,but common practice tends to be based on generic rules of thumb.The resulting schedules can lead to unjustifiably long tripping out operations just”to be in the safe side”,or core damage if the process is too fast.There is a lack of clarity and consensus regarding tripping schedules which impacts on both the integrity of the core and the economics of coring.With high daily rig costs,a more scientific and quantitative approach,tailored to particular core and reservoir-fluid characteristics,is required.This is particularly important for coring shale gas reservoirs.In this paper we describe the application of Computational Fluid Dynamics(CFD) to model transient pressure differentials in a gas reservoir core during retrieval.The model was constructed to represent a 3D cylindrical core.Flow of fluid within the porous media and its restrictions were calculated and compared for different pressure and time scenarios.Rock failure criterion has been based on the principle that the maximum tensile stress in the core is at maximum equal to the rock’s tensile strength.The results support and confirm empirical evidence that the pressure differentials created in a core during core retrieval are very low for relatively high permeability rock(300 mD).As the core permeability decreases the time required for the core internal pressure to approach the external pressure increases,requiring longer equilibration times.The model realisations demonstrate that a tensile failure criterion is more likely to be reached during the final last stages of the trip.The extreme case of a very low permeability rock(shale) of 0.00001 mD shows high differential pressures throughout the maximum simulated trip time of 12 hours,suggesting that pore pressure release damage is expected in shales during normal,economic tripping operations.The methodology can be adapted and extended to include the effects of mud cake,as well as multiphase flow modelling(relative permeability) for core recovery in oil reservoirs.