Formation Damage Due to Aqueous Phase Traps in High Permeability Reservoirs and its Impact on Production Enhancement – Experimental Study

Abstract

Matrix acidizing, stimulation and other aqueous or solvent based pumping treatments such as scale squeezes, sand consolidation and organic deposition removal techniques play an ever-increasing role in maximizing value of high margin, high rate Deepwater wells. Near wellbore permeability impairment due to aqueous fluids capillary trap is a well-studied phenomenon in low permeability reservoirs but has not received much attention in high permeability oil reservoirs. During the execution of some of these different pumping treatments, an apparent formation damage was observed during execution in the form of lower productivity index (PI) post-treatment. In most cases, the PI impairment did not recover after the wells were brought online. In one case, the PI impairment slowly improved over time and fully recovered after the equivalent of ~1000 PV of the oil flow back. This temporary damage, in turn, created issues in terms of treatment design and execution often blurring the cause of damage and thereby affecting chemical and diverter selection and placement design variables. A laboratory study was undertaken to understand the nature of the damage. The results obtained from laboratory experiments to understand the impact of brines on effective permeability to oil are presented in this work. The results of the study are outlined below: Pore throat size distribution and degree of heterogeneity are principal factors controlling initial, short-term damage. Effective permeability reduction is related to non-uniform displacement by an alternate phase (oil or water), leaving less connected pores unswept. Long term damage depends on the flow rate / capillary number (Nc):  High Rate / High Capillary Number results in short-term damage becoming permanent,  Low rate / Low Capillary Number leads to gradual recovery over a long oil flowback period.  Mutual solvents were not effective in removing the observed damage.