Oil production optimization without natural gas constraints: The harvesting of upstream natural gas

Abstract

Almost always oil production is accompanied by the surface production of associated gas. In the case of under-saturated reservoirs where the gas–oil-ratio (GOR) is expected to be relatively constant, the gas production rate from the field is expected to increase according to any increase in oil production rate. For saturated reservoirs the gas production rate over time increases because the solution gas–oil ratio is a function of pressure. More gas is released from the oil as the pressure drops.With the above phenomena in mind, it means that any plan to maximize reservoir oil production must be followed by a plan to handle the associated gas. In the case where gas production rate is at maximum capacity as dictated by surface facility limits, it can be inferred that oil production rate may have to be held below a certain level.If gas is “harvested” at its raw state, i.e., out of the primary separator and before it is treated, the benefits would be far more than the obvious saving of the costs from treating the gas. Compressed natural gas (CNG) en route to onshore or to a centralized LNG facility for example becomes then a tool of primary gas management.In a study performed on a theoretical reservoir and modeled after a West African deep-water field which has approximately 2 billion barrels of oil in place, twenty (20) producers, six (6) gas injectors and twelve (12) water injectors, it was determined that an additional 193.5 million barrels of oil could have been produced over ten years had there not been a limit to the gas production rate to be handled. The maximum field gas production rate per day in this study was 1060 MMSCF/d, of which 415 MMSCF/d was re-injected daily into the reservoir. The maximum field oil production rate was 250,000 STB/d and the maximum field water production rate was 80,000 STB/d. This work shows the advantages of eliminating production constraints imposed by natural gas production. With gas harvesting, the field oil rate can be optimized to ensure that the reservoir is producing at its true potential.