INVESTIGATION OF OILWELL CEMENT MIXING: EXPERIMENTAL STUDIES AND IMPLICATIONS FOR MIXING ENERGY THEORY

Abstract

Cement is a key element of wellbore integrity. Mixing condition for cement and how its properties are affected is of great importance but often ignored in the oilwell cement design. Typical cement slurry properties such as basic rheology, thickening time, compressive strength, porosity, permeability and fluid loss can be directly impacted when mixing conditions change. Since the development of slurry properties is irreversible, selecting an appropriate initial mixing condition can be critical for short and long term cement integrity. Generally, cements are designed to perfection in the laboratory but developing similar properties in the field operation is challenging. It is common that the properties of cement slurry obtained from laboratory and field mixing do not correlate very well, which can lead to a variety of cement-job problems. More than 800 cement specimens were prepared in this study. Using various mixing procedures, we have performed extensive laboratory experiments including UCS (unconfined compressive strength), UPV (ultrasonic pulse velocity), rheology, thickening time and NMR (Nuclear Magnetic Resonance) tests on these specimens. These procedures were differed based on changing shear rate by varying rotational speed of mixing device, changing mixing time and mixing energy. We compared testing results to understand impact of each variable. Finally, empirical models based on power consumption and non-Newtonian flow characteristics of cement slurry are developed to capture the effects of mixing conditions on different cement properties. The new model is tied to a proposed scale up procedure to enable use of laboratory results in the field operations