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.

In this paper, a new tool to be placed in the bottomhole assembly (BHA) is introduced for the purpose of counteracting heave-induced down-hole pressure oscillations in offshore drilling. The tool contains a control choke, sensors and computing capability, and will operate autonomously with minimal implications for standard drilling procedures. A small-scale prototype is developed and successfully tested in laboratory experiments showing significant reduction in down-hole pressure oscillations. Realistic, full-scale cases investigated using computer simulations also show good performance. The paper gives a description of the small-scale lab as well as the mathematical model used for the computer simulations. A full-size autonomous choke prototype is currently being constructed for further testing under more realistic conditions. Design aspects of the prototype are presented in the paper.

Summary A novel method of utilizing simulations of surge and swab induced by floating rig heave is presented in this paper. The intended applications are in well planning and follow-up of drilling and completion operations. We focus on rig heave during drill pipe connections when the rig's heave compensator cannot be engaged. The method consists of: (1) estimating a dynamic, well- and operation-specific, rig heave limit based on surge & swab simulations at different depths in a well and (2) clearly communicating the dynamic rig heave limit to the rig crew and onshore organization as a simple metric. We present cases where this novel methodology has been tested during the drilling and completion of two offshore wells in Norway, and we elaborate on the operators’ view of the method's advantages. We conclude that complementing the traditional fixed rig-specific heave limit with the dynamic one that is based on the properties of the actual well and the actual drilling/completion parameters offers an opportunity to improve management of risks related to breaching well pressure margins or damaging downhole equipment and to reduce costs through reduction of weather-related non-productive time. We show that the dynamic rig heave limit may differ significantly from well to well and also throughout the same well depending on the kind of operation in the well, depth in the well, well geometry and other parameters related to well and operation properties. Our conclusion is that care should be taken when generalizing a maximum allowed rig heave value as is the industry practice today. The benefits of utilizing dynamic well-specific rig heave limit should be assessed during well planning for any well drilled and completed from a floating rig. Well planning software existing today does not offer this functionality.

Summary This paper describes measured and simulated downhole pressure variations ("surge and swab") during drill pipe connections when drilling an ultra-deepwater well offshore Brazil on the Carcará field. Floating rig motion caused by waves and swell ("rig heave") induces surge and swab when the drill string is suspended in slips to make up or break a drill pipe connection and topside heave compensation is temporarily deactivated. This is a known issue in regions with harsh weather such as the North Sea, where pressure oscillations of up to 20 bar have been reported during connections. Recorded downhole drilling data from the Carcará field reveals significant pressure oscillations downhole (in the same order of magnitude as in the North Sea) each time the drill string was suspended in slips to make a connection in the sub-salt 8 ½" section of the well. Mud losses were experienced around the same well depth and they might have been caused by surge and swab. Measured surge and swab pressure variations have been reproduced in an advanced proprietary surge and swab simulator that considers rig heave, drill pipe elasticity, well friction, non-Newtonian drilling mud, well trajectory and geometry. Moreover, findings in this paper suggest that surge and swab was in fact significantly higher than recorded by the MWD (Measurement While Drilling) tool. The true magnitude of surge and swab is not captured in the recorded MWD data due to low sampling frequency of the downhole pressure recording (one measurement every six seconds, a standard downhole pressure sampling rate used on many operations today). This work shows that surge and swab during drill pipe connections on floaters may challenge the available pressure window for some wells even in regions with calm weather such as Brazil. Managed Pressure Drilling (MPD) is a technique that improves control of the downhole pressure. It is, however, not possible to compensate fast downhole pressure transients, such as heave-induced surge and swab, using MPD choke topside. This is due to the long distance between the choke and the bit, which translates into a time delay in the same order of magnitude as typical wave and heave periods. A downhole choke combined with continuous circulation is one of potential solutions. Surge and swab during drill pipe connections can result in a loss or an influx and should be considered in the well planning phase when mud weight, section lengths, etc. are selected.

Harsh weather conditions result in severe heave motion on floating drilling rigs. The drill string is heave-compensated during drilling ahead, however, when slips are set during drill pipe connections, the topside heave compensation system is disabled, and the drill string is then moving up and down together with the rig. Pressure oscillations below and around the drill bit, induced by such movement, are known as "surge & swab". These oscillations can be in the order of 20 bar or more during drill pipe connections in the North Sea and pose a serious challenge to drilling of wells with narrow pressure margins from floating rigs in harsh weather environment. A managed pressure drilling (MPD) choke at the surface cannot be used to control rig heave-induced surge & swab due to the fast nature of the pressure oscillations in question, stochastic character of the sea waves that cause them and long time-delay between topside choking and bottomhole response. Continuous Circulation System (CCS) might be able to reduce the pressure oscillations somewhat by maintaining constant mud flow during connections. Computer simulations and laboratory experiments were previously used to investigate a novel method for attenuating surge & swab by utilizing an autonomous choke to be installed in the bottom-hole assembly (BHA). The bottomhole pressure oscillations can be effectively reduced through dynamic in-situ control of the mud flow through BHA by a downhole choke. This paper presents the downhole choke system and the results from the first pilot trial conducted in a mud flow loop at Ullrigg test rig in Stavanger utilizing a full-scale version of the choke. The prototype was subjected to drilling mud with flow rates up to 2500 lpm and differential pressure up to 250 bar to investigate its ability to accurately control the flow while at the same time withstanding the demanding conditions. Satisfactory choke valve characteristics were obtained, indicating ability to control the flow with sufficient precision. Flow testing resulted in severe erosion of carbon steel components while wolfram carbide components were able to withstand the erosive nature of the flow. The test also uncovered challenges related to operation of the choke with high differential pressure and flow rates which could later be related to the motor, used to control the downhole choke assembly. The next phase of the project is to design a downhole prototype and test it in an onshore test well to achieve Technology Readyness Level (TRL) 4 qualification ("ready for first use offshore"). The final goal is to qualify the downhole choke together with MPD and a Continuous Circulation System (CCS) for use on floating drilling rigs in harsh weather environment.