Modified Solvent-Based Mud Acid Stimulation of Condensate Gas Reservoirs: Case Study of UUU Cluster in West Niger Delta

Abstract

Matrix acid stimulation has been successfully applied to remove formation damage in the high-permeability Niger Delta sandstone oil and gas reservoirs since the mid-eighties. The predominant acid treatment fluid of choice is expectedly the regular mud acid (RMA) – hydrochloric acid and hydrofluoric acid (HCl:HF combination acid). Fluid formulation is however challenging for gas wells due condensate banking and water block development in the near-wellbore area at pressures below dew point pressure. Candidate wells were identified for stimulation after well test data analysis and production performance evaluation. This paper focuses on selection and application of a modified solvent-based RMA to remove drilling and completion fluid-induced formation damage in addition to production-related damage in high-rate, gas-condensate wells in a cluster of western Niger Delta fields. A low-strength alcoholic mud acid was selected primarily based on bottomhole temperature (< 220 °F), formation mineralogy – over 70% quartz with low clay content (< 5%) for the high-permeability hydrocarbon formations. The alcohol-based solvent – methanol was included in all stage treatment fluids (preflush, main treatment and overflush) for condensate bank removal and improved well production performance. Treatment fluid injection and soak treatment was applied at minimum threshold of 100 gallons per foot across the perforations using coil tubing. The results indicated improved well-inflow performance for the old gas-condensate compared to the newly completed well after adequate clean up. Average production gain realised in the aged wells was over 50%. This paper has confirmed the applicability of this modified RMA for near-wellbore formation damage removal without increasing the producing water gas ratio (WGR) in gas wells.