Thesis (M.Eng.)--Chulalongkorn University, 2010

Typically, oil reservoirs in Pattani Basin in the Gulf of Thailand are highly faulted, relatively small compared to other reservoirs elsewhere, and most of the time they are multiple and stacked. In order to make the marginal reservoirs economically attractive, there is limited development option and almost by default the slim monobore completion is selected to justify the small reserves. Basically, this monobore completion allows one single well to accommodate as many hydrocarbon zones as possible. Most of the time, all zones are perforated and produced commingledly. It is generally observed that natural flow periods of these small reservoirs are short. In some cases, these monobore oil wells can be completed with conventional gas lift to extend its production or increase recovery factor. However, in some cases, both capital and operating costs of gas lift have a great impact on these economically burdened fields, especially the offshore environment. As a result, it is not always economic to drill and complete oil wells with conventional gas lift and many monobore oil wells are completed without gas lift for economic reason. Therefore, the gas zones in these monobore oil wells without gas lift become very important because these gas zones, if managed properly, can provide additional in-situ gas to increase or optimize the well’s GLR; thus increased oil production rate or reserve recovery factor. This thesis is to study some pre-determined variables that affect the oil recovery factor using the in-situ gas lift technique in the monobore oil wells with commingled production and compared to the monobore oil wells with the conventional gas lift. It can be concluded that the recovery factor of oil wells using the in-situ gas lift is very comparable to that of the oil wells with conventional gas lift. The higher recovery factor can be achieved from having the deeper or the thicker in-situ gas zone or the time-lapsed perforation schedule of the in-situ gas zone; however, this is not always true for increasing permeability