occurs after cement PIacement. Sufficient delays in the loss of pressure are obtained for an API fluid-The fluid-loss behavior of cement slurries 1oss requirement in the range of 10 to 30 mL/30 min against permeable formations and its relationship to with the actual value dependent upon wel1bore geominterzonal fluid migration have been investigated. etry, overbalante pressure, mud fluid-1oss properties Static fluid-loss testing, designed to simulate cement and cement S1urry design. PIaced in a wal1bore, has shown that the cement fluid-10SS rate through a deposited mud filter cake decreases with time.This decrease in fluid-loss rate INTRODUCTION is dependent upon the designed fluid-1oss properties of the cement S1urry, and significantly influences the During the dril1ing and completion of oil and gas loss of hydrostatic pressure required to control reservoirs, formation fluids are contained by the formation fluids. Once the pressure starts to decay, hydrostatic pressure exerted by the wellbore fluids. formation fluids can potentially enter the wellbore The loss of hydrostatic pressure during drilling causing ineffective zonal isolation and, in some operations can result in well blowouts and "gas kicks" cases, gas migration to the surface. at the surface.1 Hydrostatic pressure loss after wellbore completion also permits the invasion of The mechanism described indicates that hydroformation fluids into the wellbore.2'3 Unlike drilstatic pressure is initially transmitted after cement ling operations, the problem of fluid migration after placement.During this period, fluid loss to the completion is compounded since the cement has been surrounding formation is compensated for by an equal permanently placed in the wellbore. The result may be decrease in slurry volume.The resulting changes in poor zonal isolation, annular pressure buildup, gas the solid-to-water ratio and gelation properties flow to surface and conrnunication with nonproductive coupled with the growth of the cement filter cake or productive zones. cause the pressure against the formation to eventually decrease.The rate of pressure decrease is dependent Several investigators have assumed that the upon the fluid-loss rate and the slurry's compress-hydrostatic pressure loss is due to volumetric changes ibility.Once the pressure approaches the formation associated with cement hydration and fluid loss to pressure, interzonal fluid flow through the cement permeable formations.2-lg Other reported reasons for slurry occurs with its rate initially governed by the fluid flow are poor cement placeinenttechniques,q high cement fluid-loss rate. cement free water [sedimentation], ll cement gellin properties12-16 and excessive thickening times.~9 Several correlations have been made relating
The determination of the rheological behavior of cement slurries is essential for the proper evaluation of displacement pressures and flow rates for optimum cement placement. Several cement slurries have been examined, using pipe flow and concentric cylinder viscometers, in an effort to determine which method is better suited for determining such flow characteristics. Comparative analysis of the data indicates that the concentric cylinder viscometer may be inadequate for measurement of the rheological properties of cement slurries. Studies using a pipe-flow rheometer indicate that an apparent "slip" at the pipe surface occurs during rheological evaluation of cement slurries. This wall "slip" is attributed to particle migration when cement slurries are sheared. Field evaluation of the rheological properties of cement slurries flowing in large diameter pipes confirm the results of the pipe-flow rheometer. pipe-flow rheometer. Based on data obtained with a pipe-flow rheometer, a recommendation is offered as to which mathematical model most accurately describes the flow characteristics of cement slurries. Introduction Knowing what displacement pressure and flow rate will maintain a cement slurry in turbulent or plug flow in the wellbore annulus is essential in the design of primary cement jobs. Cement in plug or turbulent primary cement jobs. Cement in plug or turbulent flow exerts a uniform displacement force against the mud in the wellbore annulus. In laminar flow, cement has a parabolic or "bullet-shaped" velocity profile across the area of flow. This results in cement "jetting" through the drilling fluid. Incomplete mud removal can result in poor cement bonding, zone communication and ineffective stimulation treatments. Characterization of the flow properties of fluids is determined by the relationship between the flow rate (shear rate) and pressure (shear stress) required for fluid movement. Extensive studies have resulted in the development of several mathematical models which describe the relationship between shear stress and shear rate. The three most commonly used models are the Newtonian. Bingham Plastic and Power Law models. Most drilling fluids and cement slurries are non-Newtonian fluids and have been treated using a Bingham Plastic or Power Law type model. Other models such a the Herschel-Bulkley and Robertson-Stiff models are not widely used at this time. Mathematical modeling of the flow behavior of cement slurries requires the accurate measurement of shear stress and shear rate. At the present time concentric rotational viscometers are extensively used for cement slurries. This type of viscometer permits the fluid placed in the annular space between permits the fluid placed in the annular space between a stationary and a rotating cylinder to be subjected to shear. The rate of shear is determined from the geometry and speed of the rotating cylinder. The shear stress is obtained by the measured torque induced by the fluid on the stationary cylinder (bob). With a small annular gap the shear rate is nearly constant through the fluid. The use of concentric cylinder viscometers is hindered by particle settling during measurement and the high particle settling during measurement and the high shear rates the cement slurries experienced within the small annular gap. An investigation will compare the results of concentric cylinder viscometers with pipe-flow rheometers. The pipe-flow rheometer is pipe-flow rheometers. The pipe-flow rheometer is similar to capillary-type viscometers. The rheological properties of a cement slurry are determined by measurement of the pressure drop across a length of pipe at a given slurry flow rate. The rheometer has been designed to permit rapid testing with minimal particle settling. The shear rates obtained by the pipe-flow rheometer resemble downhole conditions by being zero at the pipe axis and at a maximum at the pipe wail. In addition, the rheological data can be easily compared with fluids flowing in large diameter pipe. Experimental Procedure Rheological measurements were obtained for a series of cement slurries using both a 12-speed Fann 35/SR12 viscometer and a pipe-flow rheometer. Each cement slurry was prepared according to API procedures using either Oklahoma or Longhorn Class H cement. The amount of water needed to prepare 600-ml cement slurries was determined by weight, not volume.
Subterranean hydrocarbon deposits often contain low-molecular-weight gaseous hydrocarbons at various pressures, temperatures and depths. Fai 1 ure to prevent the invasion of this gas into a cement slurry during completion can result in numerous problems. A few of the known manifestat i ons of gas channeling are poor zonal isolation, annular pressure buildup, loss of hydrocarbons to nonrecoverable zones, required squeeze cementing and flow of gas to the surface.
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.
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