Numerical simulation study on the mechanism of releasing ultra-deep water shallow gas by drilling pilot holes

Abstract

This study proposed an operation mode that actively releases shallow gas from an ultra-deep water shallow gas reservoir by drilling pilot holes. This operation mode can help discharge underground shallow gas in advance and better guarantee the security of the subsequent drilling operation. The flow law of shallow gas in the release process was simulate according to the field data of the Shenhu area in the South China Sea to study the mechanism and feasibility of actively releasing shallow gas in ultra-deep water area. The findings of the study revealed that shallow gas and seawater driving each other behavior (DEO behavior) in the shallow gas reservoir and borehole zones (underground zone: refers to the zone comprising the borehole and shallow gas zones) existed in the early release process, and the DEO behavior exhibited an obvious cyclic phenomenon. Simultaneously, the morphological development of the shallow gas spewing into the static seawater and real seawater zones was studied to obtain a clearer understanding of the gas morphological development in seawater. Based on the above findings, further investigations revealed that the operation that released shallow gas with 8-3/8″, 9-5/8″, and 12-1/4″ pilot holes in the target field did not affect the buoyancy of the seawater or sink the platform; there was almost no fluctuation to the sea surface. This study examined and compared the shallow gas flow law and remaining shallow gas volume in the reservoir with time under the application of 8-3/8″, 9-5/8″ and 12-1/4″ pilot holes. Consequently, it found that under the condition of target field larger hole sizes exhibited lower collapsing and dispersing heights (the distance between the last shaped shallow gas bubble about to burst and the mud line) and faster shallow gas release velocity. Furthermore, a numerical simulation study on the operation mode of releasing shallow gas from an ultra-deep-water shallow gas reservoir by drilling pilot holes can provide theoretical support for subsequent shallow gas treatment decisions in ultra-deep-water drilling construction.