Mud Displacement with Cement Slurries
Authors: Clark Charles R. , Carter Greg L. 
Published: December 2007

Mud displacement efficiency was studied under conditions simulating theborehole environment at 8,000 feet. Major findings were that the buoyantdriving force resulting from the density difference between cement and mud hasless effect on mud displacement than expected under such conditions and thatpipe motion with scratchers substantially improves mud displacement, especiallyin areas of hole enlargement.

Introduction
One prerequisite for successful primary cementing is the completedisplacement of drilling mud from the annular space. Such has been publishedregarding proper techniques for maximum mud displacement proper techniques formaximum mud displacement efficiency. Yet, even after these techniques areproperly applied, mud displacement efficiency often properly applied, muddisplacement efficiency often remains low. The objective of this study was tore-examine, under conditions closely simulating the down-hole environment, therelative importance of the driving forces available to displace drilling mudfrom the annulus. Fig. 1 illustrates the driving and resisting forces involvedin the displacement of drilling mud with a cement slurry.

Test Equipment
The test facility (Fig. 2) consisted of a pneumatic trailer for storing bulkcement and a pneumatic handling system for delivering cement to a 4-bblrecirculating mixer. The cement slurry was mixed and pumped into a 25-bblslurry heating and storage tank pumped into a 25-bbl slurry heating and storagetank where retarders and friction reducers were added when needed to alterslurry properties. Cement was circulated with three positive displacementpumps. Two were connected to a variable speed motor that allowed pump rates tovary from 0.5 to 100 gal/min. The third pump was driven through twogear-reduction boxes on a two-speed motor that permitted rates up to 160gal/min at pressures up permitted rates up to 160 gal/min at pressures up to200 psi. Mud was displaced with one positive-displacement pump.positive-displacement pump. Cement slurry, drilling mud, and test cell curingchamber were heated with steam coils located in the slurry tank, themud-treating barrel, and the test cell curing chamber, respectively. Thesimulated formation cores were made by compacting and heating a mixture ofepoxy resin and sand inside 5 1/2-in. casing. A 2.74-in. diameter hole was castin the center of the 10-ft-long specimen to represent the wellbore. The center7 ft of the external 5 1/2 in. casing was removed to allow full exposure of thecore permeability. The average water permeability of the cores was 400 md.permeability of the cores was 400 md. The test model consisted of a 1-in. ODpipe inside the 2.74-in. simulated formation wellbore. This configuration isscaled to model a 2 7/8 -in. tubingless completion in a 7 7/8 -in. hole (Fig.3). The 10-ft sandstone core was placed in the apparatus depicted on Fig. 4. The 33-in. extension joint on the bottom contained straightening vanes toeliminate possible end effects. The core section and extension joint wereencased within an annular filtration jacket to allow measurement of filtrationthrough the core. On some tests, pressure taps were installed through thesandstone pressure taps were installed through the sandstone core to allowmeasurement of annular pressure drop across the middle 5 ft of the sandstonecore. Mud or cement was pumped down through the vertical standpipe into theextension joint, up through the sandstone core section, and out at the top ofthe cell. The exit line contained a plug valve to regulate pressure inside thecore at 150 to 200 psi.
JPT
P. 775