Modelling of Productivity Decline in Geothermal Reservoirs due to Fines Migration-Induced Formation Damage

Abstract

The drilling and production history of geothermal well A indicates fines migration as a primary mechanism for productivity decline. A new method to determine movable fines and consequent permeability decline from cuttings has been developed. Both cuttings and cores are available from the analogous Ladbroke formation. Good agreement between the movable fines concentration from Ladbroke cuttings and cores validates the developed method. The method has been applied to well A cuttings for productivity decline prediction. Good agreement between mobilised fines concentrations and permeability damage from rock fragments and cores for Ladbroke formation validates the method. Good agreement between the laboratory-based mathematical modelling of fines migration in well A with the field data is observed, resulting in identification of fines migration and clogging as the formation damage mechanism. The sensitivity study shows that geothermal reservoirs are significantly more vulnerable for permeability damage by fines mobilisation than conventional oil and gas fields, since the particle attaching forces are weaker at elevated temperatures. Fines migration is recognised as a frequent cause for permeability damage and well productivity decline in conventional oil and gas wells. A mathematical model successfully predicts formation damage of studied geothermal reservoir due to fines migration and straining. The model can be used for skin prevention, mitigation and removal, which can be applied to shale, CBM and tight oil and gas reservoirs as well. The laboratory-based mathematical modelling of productivity decline in well A allows for long-term prediction of well index, for calculation of the minimum flow rate preventing fines mobilisation and for recommendations on damage mitigation.