How Realistic is the Calculated Cementing Displacement Efficiency?

Abstract

Primary cementing is a crucial task in the completion of oil and gas wells, as it is potentially meant to provide zonal isolation, and prevent uncontrolled flows and environmental hazards. Much research has been conducted to find the key techniques for obtaining the maximum displacement efficiency during cementing operations. Yet, it appears that the industry could benefit from more investigations on the complications involved in displacement processes. In this work, a methodology is proposed in an attempt to obtain qualitative and quantitative predictions of displacement efficiency. This method, which appears to complement previously existing methods, introduces a combined analysis of instability of the interface between the two fluids with an analytical solution of fluid displacement flow in eccentric annuli. The analytical solution enables the time-dependent calculation of interface location and provides a quantitative judgement on the volume fraction of the displaced fluid left in the annular space. On the other hand, the instability models provide an insight on the degree of cement contamination, and guidelines on how to minimize the amount of inter-mixing. The proposed approach was implemented for several displacement cases and the results were evaluated by both Computational Fluid Dynamics (CFD) simulations and experimental tests. Instability of the interface in all the cases was studied and the analysis provided more in-depth understanding of the effect of different parameters on displacement efficiency. Considering that in the existing analytical models, including the one presented in this work, the interface between the two fluids is supposed to be sharp, the calculated volume fraction of displacing fluid can be not necessarily a proper representative of the real displacement efficiency. It was observed that there can exist cases where the volume fraction of the displacing fluid did not necessarily indicate an inefficient displacement, whereas the instability analysis suggested that the corresponding design had to be avoided. This was also validated by CFD simulations. Moreover, the instability model can provide more information about the critical values of design parameters and propose optimized designs for the improvement of displacement efficiency. The present work provides a versatile tool that enables quantitative determination of displacement efficiency, along with an enhanced judgement on the amount of inter-fluid mixing and cement contamination. The novel approach of coupling the instability analysis with displacement flow calculation not only offers improvements on displacement designs, but also assists to avoid any undesirable outcomes caused by ineffective cement placement.

Cement Placement: An Overview of Fluid Displacement Techniques and Modelling

“…Running the primary displacement flow in turbulent for cement placement has not been proven to provide better displacements. As earlier discussed, some works such as the ones of Foroushan et al [39,40] and Yerubandi et al [31] suggested that keeping the flow rate below a certain level provided better displacements. McLean et al [23] concluded that the turbulent flow did not offer a better displacement, and thinning the cement to run it in turbulent regime caused additional channeling.…”

“…In a case study, they showed that for a displaced fluid with density = 1546 Kg/m 3 , flow behavior index = 0.785, consistency index = 0.209 Pa-s, and flow rate = 50 gpm, in an annulus with the size of 3 inch × 2.5 inch, the density ratio must be at least 1.28 if the displacing fluid has the same viscosity, and the ratio of the apparent viscosity in a concentric annulus must be at least 1.26, if the densities are the same (see Figure 18). According to the discussions by Foroushan et al [40], increasing the flow rate increases the chance of interface instability. They suggested that there exists a maximum flow rate above which the interface becomes unstable.…”

“…The Displacement and Mixing Facility (DMF) at the University of Tulsa (Tulsa University Drilling Research Projects) was designed and built by Durmaz [38] for the experiments of fluid displacement in pipes for the purpose of studying plug cementing operations. The test section was made of a transparent 2" pipe with a length of 35 ft. Later, this facility was modified and re-built by Foroushan et al [30,39,40] to accommodate the experiments of fluid displacement in annuli. The outer pipe of the annulus was made of an acrylic pipe with inner diameter (ID) of 3" and the inner pipe was aluminum with an outer diameter (OD) of 2.5", which provided a diameter ratio of 1.2.…”

“…Their test section could be positioned in all inclination angles, ranging from 0 degrees (vertical) to 90 degrees (horizontal). To adjust the eccentricity, Foroushan et al [40] used two in-house adjustable centralizers, which, to minimize the flow disturbance, were placed before the first pressure transducer from the inlet and right downstream the last pressure transducer. However, it should be mentioned that both test section pipes were gradually deformed with time and expecting that they could have a perfectly concentric annulus or full control on the eccentricity appeared to be too ideal.…”

“…Following the idea of flow in a narrow slot, Foroushan et al [13,39,40] proposed a model for displacement of fluids in narrow eccentric annuli at different inclination angles. Through the conservation of momentum, they solved a Poisson-type of differential equation with an irregular boundary, because of the unwrapped annulus, for each fluid region.…”