Managed Pressure Drilling is one of several techniques gaining traction on critical wells with very tight pore and fracture pressure windows. In order to stay within these narrow mud weight windows it is critical to understand the bottom hole pressures. Some efforts have been made to combine surface modeling with downhole measurements during the actual drilling phase utilizing data sent to surface by mud pulse telemetry. However, during liner running and cementing operations this data is generally not available. Due to the narrow annular clearances around the liner and the heavier slurries used during cementing the chances of exceeding the fracture gradient are greatly magnified. To compound this during most cementing operations in managed pressure situations the surface measurements, and therefore the associated models based off them may bear no relationship to what is happening downhole, particularly as the cement can be effectively in freefall inside the pipe. Measurements can be, and are, compared both before and after the actual job, but during the critical time of the actual cement job and displacement there is often no indication of what is actually happening downhole.
As stated above the telemetry system most utilized during drilling is mud pulse telemetry. However, as these systems occupy the pipe bore, and require a full mud column inside the pipe with a flow rate above a minimum threshold, then they are not used during cementing and liner running operations. To overcome these limitations a distributed measurement and acoustic telemetry network was run incorporated in the drillstring. This network sends data including annular and bore pressures, both from directly above the liner-running tool, but also distributed back along the string to surface. Acoustic telemetry transmits through the steel wall of the drillstring and therefore is independent of flow or fluid and can send data whilst tripping as well. Additionally the tools are fully through bore allowing the passage of cement darts and slurries with no real restriction. As the network also supplies interval measurements, then the passage of the different stages of the cement job can be seen as they move both down the pipe and back along the annulus.
This paper will show, through actual field results, what is really happening downhole in the critical time of running the liner to bottom and during the managed pressure cement job. Results will be compared with traditional surface measurements. We will describe how we used downhole and interval calculations of friction factors and equivalent circulating densities to model what is happening in and around the liner during the cement job. We will discuss the application of the data, and the resultant algorithms and calculations derived from this to affect subsequent jobs and to improve decision-making and therefore the safety and efficiency of these critical cement jobs.
As can be imagined the techniques thus described are new to the industry for real-time applications and further work is anticipated in delivery and interpretation of the results to further enhance this type of solution. A view forward will also be given in the conclusions on where this technology has the potential to go.