Fluids loss, static

During the second period, the cake grows because of the absence of flow. It may grow to a point at which it locally but completely fills the annulus Bridging takes place and the hydrostatic pressure is no longer transmitted to the deeper zones. From the typical mudcake resistance it can be estimated that under both dynamic and static conditions, the fluid loss could require reduction to an American Petroleum Institutue (API) value lower than what is generally considered a fair control of fluid loss. 

Predictions on the effectiveness of a fluid loss additive formulation can be made on a laboratory scale by characterizing the properties of the filter-cake formed by appropriate experiments. Most of the fluids containing fluid loss additives are thixotropic. Therefore the apparent viscosity will change when a shear stress in a vertical direction is applied, as is very normal in a circulating drilling fluid. For this reason, the results from static filtering experiments are expected to be different in comparison with dynamic experiments. 

Static fluid loss measurements, which are the present standardized testing method, provide inadequate results for comparing fracturing fluid materials or for understanding the complex mechanisms of viscous fluid invasion, filter-cake formation, and filter-cake erosion [ 1806]. On the other hand, dynamic fluid loss studies have inadequately addressed the development of proper laboratory methods, which has led to erroneous and conflicting results. 

Static experiments with pistonlike filtering can be reliable, however, to obtain information on the fluid loss behavior in certain stages of a cementation process, in particular when the slurry is at rest. 

Viscometer test. Typical crosslinked starches are obtained when the initial rise of the viscosity of the product is between 104° and 144° C, and the viscosity of the product does not rise above 200 Brabender units at temperatures less dian 130° C. The crosslinked starch slurry is then drum-dried and milled to obtain a dry product. The effectiveness of the product is checked by the API Fluid Loss Test after static aging of sample drilling fluids containing the starch at elevated temperatures. The milled dry product can then be incorporated into the oil well drilling fluid of the drill site. 

Static Filtration. The static fluid loss of drilling fluids is a property that is regularly measured, both at the rig site during drilling operations... 

Effect of Composition on Fluid Loss. There have been many studies and tests of the effects of various additives (usually polymeric) on the static fluid loss of drilling fluids. There have been many fewer studies on the effect of these additives on dynamic filtration rates and very few that compare their effect on both dynamic and static rates. 

Hughes et al. (32) have studied the effect of bentonite clay content and CMC content on the static fluid loss of simple bentonite-polymer drilling fluids. Figure 45 shows the dependence of the API static fluid loss (after 30 min) on bentonite content with no polymer and in the presence of polymer. The polymer-free API fluid loss V30 (in mL) is related to bentonite concentration Cben (in g/L) by... 

Figure 45. Dependence of API static fluid loss of bentonite-polymer (CMC) drilling fluids on bentonite concentration at constant polymer content (4.5 g/L). (Reproduced with permission from reference 32. Copyright 1993 Society of Petroleum Engineers.)...

The polyacrylate polymers and a derivative of a vinyl acetate maleic anhydride copolymer cause V30 to decrease monotonically with increasing polymer concentration, similar to the CMC polymers (Figure 46). The polymers PVA and poly(vinyl pyridinium) (PVP) hydrochloride markedly increased V30 at low concentration at concentrations above 1 g of polymer per gram of added bentonite PVA functions as a static fluid loss additive. The maximum in the API fluid loss at low PVA concentrations approximately coincides with the maximum in the yield stress and plastic viscosity found by Heath and Tadros (75). The increased static fluid loss is consistent with Heath and Tadros s conclusion that bentonite is flocculated by low concentrations of PVA. The concentration of PVA required to decrease V30 below that of the neat bentonite suspension is significantly larger than the concentration of CMC, where effective static fluid loss control can be achieved at polymer bentonite weight ratios of about 0.1 g/g. More effective fluid loss control has been achieved with other synthetic polymers such as poly(vinyl sulphonate)-poly(vinyl amide) copolymer (40) and other sulphonated polymers (39). 

A comparison of the effect of well-known fluid loss additives on the rates of static and dynamic filtration was made by Kreuger some 30... 

Plank and Gossen (144) have reported on the temperature dependence of API static filtrate volumes of simple drilling fluids using various polymeric additives. Figure 50 shows the dependence of filtrate volume on temperatures for a base bentonite fluid and with added starch, polyanionic cellulose, and a synthetic sulphonated polymer (39). Starch is an effective fluid loss additive up to about 100 °C whereas the polyanionic cellulose begins to lose its effectiveness at about 140 °C these... 

It has proved difficult to unravel the mechanism(s) whereby polymers such as CMC can reduce the rates of static fluid loss. Heinle et al. (147) suggest that the reduction in static fluid loss at high ionic strength is... 

Figure 52. SEM micrographs of filter cake formed under static conditions from simple bentonite drilling fluids containing the fluid loss additives (a) starch and (b) synthetic sulphonated polymer. (Reproduced with permission from reference 144. Copyright 1991 Society of Petroleum Engineers.)...

Figure 62. Changes in the calcium and sulphate concentration of a water-based drilling fluid during drilling of a complex evaporite sequence. Scales Ca2+ 0-16 mM SO/ 0-40 mM API (static) fluid loss 10-20 mL. (Reproduced with permission from reference 178. Copyright 1992.)...

Figure 4. Static fluid loss curves filtrate passing through a 0.05pm filter as a function of the square root of time for different HEC molecular weights.

Pressure Drop. The pressure drop across a two-phase suspension is composed of various terms, such as static head, acceleration, and friction losses for both gas and soflds. For most dense fluid-bed appHcations, outside of entrance or exit regimes where the acceleration pressure drop is appreciable, the pressure drop simply results from the static head of soflds. Therefore, the weight of soflds ia the bed divided by the height of soflds gives the apparent density of the fluidized bed, ie... 

When a PR valve is relieving at rated capacity, the total frictional pressure drop between a vessel and the inlet of the valve should be less than 3% of the set pressure (kPa). In this calculation, the effect on static pressure of fluid acceleration is ignored rather, only friction loss is considered. 

Friction head loss develops as fluids flow through the various pipes, elbows, tees, vessel connections, valves, etc. These losses are expressed as loss of fluid static head in feet of fluid flowing. 

A = cross section area of orifice, nozzle or pipe, sq ft h = static head loss, ft of fluid flowing AP = differential static loss, Ibs/sq in. of fluid flowing, under conditions of Al above... 

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