Evaluation Of Cement Fluid-Loss Behavior Under Dynamic Conditions

Abstract

Abstract The fluid-loss behavior of cement slurries and its relation to rheological properties is being examined using a dynamic fluid-loss apparatus. This instrument has the capacity to test slurry dehydration by sequential displacement of drilling muds, spacers, chemical washes and cement slurries through a drilled hole in a formation core. Dehydration of cement slurries containing a cellulose-based fluidloss additive was investigated as a function of flow rates and pressures. Results of these tests show that rapid cement water-loss occurs, followed by a reduced fluid-loss rate which remains constant with time. However, dehydration of cement slurries under static conditions results in decreasing fluid-loss behavior with time. Cement fluid-loss control under dynamic conditions was examined as a function of pressure, temperature, fluid-loss additive concentration, pressure, temperature, fluid-loss additive concentration, and slurry velocity. The effects of sequential displacement of drilling muds, chemical washes, and spacers by a cement slurry upon fluid-loss control was also investigated. While there is clearly much more work needed to understand fluid-loss control under dynamic conditions, it is apparent from these results that the static method of fluid-loss testing is not relevant to the deposition of filter cakes under dynamic conditions. The understanding gained in this work will be instrumental in designing cementing treatments with minimal cement slurry fluid-loss. Introduction Fluid-loss additives are used in cement slurries to assist in maintaining a constant water-to-solids ratio by reducing water-loss to permeable formations. Inhibition of cement slurry dehydration during primary cementing operations allows greater cement primary cementing operations allows greater cement fill-up, maintains initial viscosity, and reduces formation damage. It will also reduce the possibility of annular bridging by dehydrated cement. The present laboratory testing for evaluation of fluid-loss consists of application of pressure to a cement slurry in a standard filter cell? The water-loss through a 325-mesh screen is measured as a function of time. This type of testing is basically static in nature with the actual cementing operation taking place under dynamic conditions. The term, dynamic, refers to fluid motion along the core surface and static refers to the lack of motion. In addition, present cement fluid-loss testing does not take into present cement fluid-loss testing does not take into consideration the deposition of a mud cake with sequential displacement of the drilling mud by washes and spacers. Previous studies of dynamic filtration of drilling muds have shown that water-loss from a circulating mud is greater than shown by static test results. However, there has been no dynamic study of the effect on cement fluid-loss behavior through a formation face by displacement of drilling muds, spacers, and chemical washes by cement slurries. This study deals with dynamic filtration of cement slurries as a function of differential pressure, temperature and flow rates through a hole in a pressure, temperature and flow rates through a hole in a formation core. In addition, cement water-loss behavior was further investigated by displacement of drilling muds and chemical washes by a cement slurry. The effect on cement water-loss of the deposited mud cake on the formation surface was studied by mechanically removing the mud cake. This process would correspond to using scratchers during a cementing operation. EXPERIMENTAL PROCEDURE A schematic diagram of the test equipment used to study dynamic fluid-loss behavior of cement slurries is shown in Fig. 1. An adjustable, triplex pump unit which is capable of flow rates up to 315 cc/sec was used to circulate the slurries through a 1.27-cm bore in a 20.32-cm × 6.35-cm Berea sandstone core held in place by two pressurized pistons. Velocities up to place by two pressurized pistons. Velocities up to 350 cm/sec (11.5 ft/sec) could be obtained as a result of the formation bore size.