Application of UBD and MPD Improved Discovery of Fractured Granitic Basement Hydrocarbon Reservoirs

Abstract

Extensive discoveries of basement hydrocarbon reservoirs have been made in many places of fractured granite and carbonate basement in the world. Important hydrocarbon findings were achieved in fractured granitic basement in Chad and Indonesia by means of UBD and MPD technologies. The granitic basement in Chad and Indonesia featured with hydrostatic pore pressure gradient with narrow density windows and well developed fractures. The pore pressure coefficient of the basement of Chad was predicted in between 1.02-1.06, and an underbalanced drilling (UBD) technology with a micro-foam drilling fluid was used to make an attempt on reducing drilling fluid losses; the pore pressure coefficient of the basement of Indonesia was estimated to be 1.04, and an underbalanced managed pressure drilling (UB-MPD) technology with a synthetic based drilling fluid was utilized to avoid drilling fluid losses and in favor of hydrocarbon discovery. Different drilling technologies or modes received different results although drilling in same fractured granitic basement with similar pore pressure. Losses and kicks continued almost all the time during drilling, coring and wireline logging in some wells during UBD in Chad. Losses happened as soon as the rig pump started while overflow occurred no sooner than the rig pump stopped. However, the potential problem of losses and kicks was completely controlled by utilization of UB-MPD technology in Indonesia. No losses were found during underbalanced managed pressure drilling, tripping, connection, and circulation. Nevertheless, both basement hydrocarbon reservoirs in Chad and Indonesia have been obtained important discovery. Crude oil returned to the surface during UBD in Chad and abundant natural gas produced during UB-MPD in Indonesia. Both UBD and UB-MPD technologies are effective to gain the discovery of fractured granitic basement reservoirs. The underbalanced MPD technology, a precisely pressure controlled drilling system, is able to accurately control the annular pressure profile throughout the wellbore, therefore it could effectively achieve safe drilling in narrow density window and cut non-productive time. It is proved to be more effective and safer in drilling of fractured granitic basement.