Summary

The most important contributor to improved oil recovery on mature fields is drilling of infill wells. Managed–pressure drilling (MPD) and continuous–circulation–system (CCS) techniques can be used for improved control of bottomhole pressure when drilling wells in depleted fields with narrow pressure windows, but rig heave is a challenge when drilling from floating drilling units. Rig heave, caused by sea waves, induces downhole pressure oscillations that could exceed the operational pressure window. These oscillations are called “surge and swab,” and occur during tripping in and tripping out of the borehole, as well as during drillpipe connections, while the drillstring is suspended in the slips. Downhole choking was introduced as a method to reduce downhole pressure oscillations induced by the rig heave, and the concept was tested at laboratory scale and using computer simulations (Kvernland et al. 2018). The simulations were performed using a purpose–developed software that uses such input variables as wave height, pump flow, drillpipe movements, rig characteristics, and drilling–fluid properties, along with well design, drillpipe, and bottomhole–assembly (BHA) data, to simulate downhole pressure induced by rig heave. The simulator is designed to model dynamic interactions between the drilling fluid and the drillstring in a rigorous manner, which gives it the ability to accurately predict rapid downhole changes, such as those induced by ocean waves.

In this paper, we provide an overview of the surge–and–swab simulator, describing its capabilities and limitations. Data from drilling a North Sea well are then used to validate the simulations performed using the software. The well used as an example in this paper was drilled conventionally from a floating rig. The downhole pressure variations recorded during three different drillpipe connections are compared with simulated downhole pressure. The simulations are performed on the basis of the recorded rig heave as well as the actual drilling–fluid, well–design, and drillpipe data. Results show that there is a good correlation between simulated and actual measured downhole pressure. The surge–and–swab simulation software is then used to simulate the same drillpipe connections using three different techniques and combinations of techniques used for improved downhole pressure control: (1) MPD, (2) MPD combined with CCS, and (3) MPD combined with CCS and a downhole choke. Results show that rig heave–induced downhole pressure variations are reduced to a level that is considered acceptable for drilling a well with a narrow pressure window for the last two cases, whereas use of backpressure MPD alone is not sufficient. The combination of MPD and CCS reduced surge and swab for two out of three drillpipe connections. For the third and deepest connection, the surge–and–swab pressure increased. The largest reduction in significant downhole pressure variations occurs when MPD and CCS are combined with downhole choking.

Future work will consist of further developing the surge–and–swab simulator so that it will be possible to use it in well planning and as real–time decision support during drilling operations. The simulator will also be developed to include the possibility of simulating various well completion operations such as running casing and liners. The next hardware development phase consists of designing and building a complete downhole tool for testing in a well.

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