Gas Methods of Enhanced Oil Recovery on the Oil Fields of South Iraq

摘要

Geological studies of oil and gas fields in South Iraq, from our point of you, revealed the possibility of successful application of gas injection into the structure crest zone in order to increase oil recovery. Analysis of geological structures of oil deposits along with assessment of hydrocarbon gas resources available in the region has been undertaken. Existence of massive geological structures, regionally extensive and effective cap rocks, non-permeable and suitable for gas technologies, volumetric, physical and chemical properties of the reservoir and oil, proximity of gas resources in deeper intervals was crucial in identifying prospective fields. Zubair oil field, situated in Mesopotamia region of South Iraq, has been selected and tested for gas injection technique without compression in order to increase the recovery factor of the field. Depth difference of up to 2000 metres between the oil and gas deposits as well as the distance of only 30 kilometres between them make it possible to organise bypass of gas into the oil field without compression. Based on the material balance formulae a system of differential equations, describing dynamic changes of oil and gas saturated pore volumes and reservoir pressure in relation to different rates of gas injection, has been worked out. Daily oil production greatly depends on these varying rates of gas injection. Calculations have revealed that gas injection rate of 1 million cubic meters per day can increase daily oil production by 40%, while injection rates of 2 and 3 million cubic meters per day enhance production by 2.4 and 3.3 times respectively. It is worth to mention that these calculations were carried out assuming constant bottom-hole pressure and constant number of oil producing wells. Therefore, results of the studies suggest that bypass of gas from gas field with high reservoir pressure into the pool roof of oil field can be an effective method of improved oil recovery.