This paper presents successful huff and puff process in a thick, inclined oil pool with compositional-gradient. Jake South field in Sudan contains two sets of oil and gas accumulations. The upper major pay includes thick (net pay of 200 ft), under-saturated oil inclined zones (dip angle 15-18 degrees), with compositional variation (23.82-32.6° API) and initial GOR of 62-105 scf/STB. The lower pay covers 12 gas-condensate and light oil (36-39° API) zones. The main technical challenges are attic oil recovery and strong aquifer encroachment. The solution is that high-pressure gas-condensate, high-GOR oil and high-pressure nitrogen areto inject into low-pressure oil zones for huff and puff process and avoid early water breakthrough since start-up of field development. The related technical aspects are below:
In-house studies, including solubility and viscosity reduction test, 1D and 2D visible microscopic test, and long-tube physical simulation of huff-n-puff process, confirmed that gas-condensate could dissolve into oil, resulting in foamy oil with formation volume factor of 1.23 and viscosity reduction of 50% of initial value.
In-situ huff and puff process. Single tubing string with isolation packer completion is run. To exploit attic oil in high-permeability (horizontal permeability of 2,990 md, and vertical permeability of 80% of horizontal permeability) zones, gas-condensate zones and upper two-thirds of main oil ones are perforated. High-pressure gas-condensate (condensate 60.9° API) travels up the tubing, and injects into oil zones, soaks for 2-3 weeks to allow the gas to migrate up-dip from the well and to displace up-structure oil downward towards the producer, then goes back on production. After gas/condensate zones deplete, high-GOR light zones and the additional main oil zones are opened, high-GOR light-crude and high-pressure nitrogen were co-injected into main-oil zones for huff-n-puff scheme, light oil working as a solvent to dilute the remaining lower-API oil. Injection pressure and injection volume of hydrocarbon gas and nitrogen are optimized to ensure that the well could flow naturally during the puff process.
Multi-well huff and puff process. Huff and puff processes were conducted simultaneously at five up-dip producers to contact new oil, the watered-out, low-productivity down-dip wells were shut-in for pressure maintenance via gas injection to mitigate water influx into the oil zones.
Field production performance illustrated that initial well production at flowing conditions from five producers ranged from 3,600 to 13,800 BOPD, with well production boosting 7- to 27-fold. The average off-take rate achieved 4.0-6.6% of OOIP per year. No early water breakthrough occurred at the prolific producers. Compositional simulation confirmed that ultimate recovery factor for oil could exceed 50%. The innovative, low-cost strategy that improves oil recovery is constructive and valuable to development of those similar oil fields.