Foam is gaining increasing applications in both drilling and cementing. Typical applications include drilling operations in naturally fractured formations and depleted or patially depleted reservoirs. The unique pressure profile created by foam makes it an ideal drilling fluid in many areas where a narrow operating window exsits between pore pressure and fracture pressure. While rotary drilling is the most widely used drilling technique, there has been no study on foam drilling with drill pipe rotation. This study was undertaken to identify the effect of pipe rotation on pressure losses and velocity profiles of foam flow in both concentric and eccentric annuli. The effects of pressure, foam quality and pipe eccentricity were also investigated.
For the first time, pipe rotation effects on foam flow were experimentally studied on a flow loop consisting of a pipe viscometer and an annular test section. Pipe rotary speeds were varied from 0 to 400 RPM with foam qualities ranging from 60% to 90% at different flow rates. Computational Fluid Dynamics simulations were performed to compare with experimental results. A flow-through rotational viscometer was used to investigate foam rheology at higher pressures and verifiy the results of the pipe viscometer.
Results show that pipe rotation increases pressure drop by more than 30% for low quality (below 70%) foams in a concentric annulus. It has no noticeable effects on medium quality (70%-80%) foams, and slightly decreases pressure drop for high quality foams. In an eccentric annulus, pipe rotation clearly shifts the maximum axial velocity core along the pipe rotation direction. When there is a cuttings bed, however, the velocity core shifts to a direction that is opposite to the pipe rotation direction, which has a tremendous impact on hole cleaning. Unlike incompressible fluids, the absolute pressure also affects foam rheology and pressure drop for a given quality foam. The results of this study are particularly useful for drilling and cementing design calculations for underbalanced and managed pressure drilling operations.
Introduction
Compared with conventional and aerated drilling fluids, foam is a compressible and relatively homogeneous mixture. A great flexibility for pressure control is thus possible. Areas where there is a high demand for control of multi-pressure systems will find useful applications for this flexibility. Foam can be an excellent candidate for managed pressure drilling operations. The unique curved pressure profile produced by foam flow in a well makes foam particularly useful in many areas where there is a narrow operating window between pore pressure and facture pressure gradients; e.g., in Deep Water Drilling. While great efforts are being placed on Dual Gradient Drilling,1 foam may serve as a "Continuous/Multiple Gradient Drilling fluid" by properly controlling the surface foam properties, back pressure and flow rate.
Figs. 1–2 show an example of drilling an offshore well using conventional fluid, dual gradient and foam. Seven casings are required to seal a certain interval below the mud line if a conventional, incompressible fluid is used. With dual gradient drilling, the number of casing requirements reduces to three. If one looks at the possibility of using foam instead, as shown in Fig. 2, it is possible that one casing can seal the whole interval because the pressure profile of foam lies within the narrow window of the curved pore pressure and fracture pressure profiles. The unique pressure profile created by foam at least provides a starting point to consider future foam applications in any area where a narrow operating window exists between pore pressure and fracture pressure, which is clearly not limited to deep water drilling. Admittedly, there are many other factors to consider while drilling an offshore well.
