Filter Cake Buildup Research Articles

Experimental investigation of plugging and fracturing mechanisms in unconsolidated sand reservoirs under injection of water containing suspended fine particles

Progressive plugging of the medium due to filter cake formation is considered as a major challenge during Produced Water Re-Injection (PWRI), leading decreased well injectivity during production. Field operations have shown that injectivity can be at least partially restored by fracturing the clogged medium when injecting in the frac-regime. An experimental investigation is performed to explore the effect of the injection of a suspension on the plugging and fracturing processes in an unconsolidated sand formation. The tests are performed by injecting water containing fine particles at a constant low flow rate to first clog a sand specimen (phase 1), then by increasing the flow rate until fracturing (phase 2). Experimental results exhibit a continuous increase of the injection pressure (decrease of injectivity) during phase 1 due to the formation of filter cake build-up caused by the deposition of injected solid particles. During phase 2 when the flow rate is increased, pressure drops are observed and attributed to the formation of fractures, resulting in the increase of the overall permeability of the medium. This is confirmed by post-mortem analysis of the specimens using X-ray Computed Tomography. Small radial fractures (“pseudo-cracks”) induced around the injection point are visible. They appear as localized zones of higher porosity. The effect of the confining pressure and of the suspended particles concentration on the clogging and fracturing of the medium is investigated. It is shown that the clogging process strongly depends on the concentration of particles in the injected fluid. A lower quantity of injected particles is needed to induce the same level of permeability impairment when a lower concentration is used. No significant effect of particle concentration is observed on the fracturing pressure. As with pure water injection, the fracturing pressure is proportional to the imposed confining stress. Moreover, it is observed that the clogged specimen requires a higher injection pressure to reach the frac-regime as compared to an injection scenario with pure water and no filter cake formation and that permeability recovery could be higher.

The Role of Drilled Formation in Filter Cake Properties Utilizing Different Weighting Materials.

The filter cake formed during a filtration process plays a vital role in the success of a drilling operation. There are several factors affecting the filter cake build-up such as drilled formation, drilling fluid properties, and well pressure and temperature. The collective impact of these two factors (i.e., formation and the drilling fluid) on the filter cake build-up needs to be fully investigated. In this study, two types of formations represented as limestone and sandstone were used with different weighting materials to assess and compare their impact on the filter cake properties, filtration behavior, and solid invasion. The used weighting materials are manganese tetroxide, ilmenite, barite, and hematite. The filter cake was formed under a temperature of 200 °F and differential pressure of 300 psi. Nuclear magnetic resonance spectroscopy was employed to explore the pore structure of the used core samples. The results showed that the properties (i.e., shape and dimensions) of the different weighting materials are the dominant factors compared to the formation characteristics in most of the investigated filter cake properties. Nevertheless, the formation properties, namely, the permeability and pore structure, have a somehow higher contribution when it comes to the filter cake porosity and thickness. For solid invasion, there were no clear results about the main factor contributing to this issue.

Examining metal migration through geotextiles during dewatering

This paper presents a study conducted to assess the influence that a filter cake deposited on the surface of the geotextile has on the mobility of three metals (Cu, Pb, Zn) during filtration of a contaminated sediment. Two chemical additives (cationic coagulant and cationic polymer) were introduced to the sediment to increase the particle size and improve the filtration efficiency. Bench scale experimentation was conducted to identify contaminant reduction using a small volume of sediment. A field test was applied to observe what effect three-dimensional filtration and a larger filter cake had on metal mobility. Analysis of the effluent was conducted to determine total and dissolved metal contaminants, as well as particulate matter. Effluent chemical properties (pH, Eh, and zeta potential) were analyzed to identify a possible rationale for variations in concentration during filtration. The results of the study show that as an increasing buildup of filter cake on the surface of a geotextile developed, filtrate quality was improved with respect to the metal and particulate contaminants considered. In addition, the metals detected in the effluent were primarily in the solid state, suggesting further reduction could be achieved through subsequent filtration (if desired).

Study on the influence of solids volume fraction on filter cake structures using micro tomography

Prediction of micro processes, filter cake build-up and porous media flow is a key challenge to describe macroscopic parameters like filter cake resistance. This is based on a precise description, not only of the disperse solid fraction, but the distributed properties of the voids between the particles. Lab-experiments are carried out with alumina and limestone, which differ in particle size distribution (PSD) and resulting filter cake structure. Filter cakes of both materials are characterized by standardized lab tests and additionally, alumina cakes are measured with X-ray microscopy (XRM). Focusing on distributed process key parameters, the data gives a deeper understanding of the laboratory experiments.The solid volume fraction inside the feed strongly influences the particle sedimentation and leads typically to a top layer formation of fine particles in the final filter cake, which has a negative influence on subsequent process steps. The top layers seal the filter cake for washing liquid and increase the capillary entry pressure for gas differential pressure de-watering. The influence on cake structure can be seen in a change of porosity, particle size and shape distribution over the height of the filter cake. In all measurements, homogenous filter cake structures could only be achieved by increasing the solid volume fraction inside the suspension above a certain percentage, at which particle size related sedimentation effects could be neglected and only zone sedimentation occurred. XRM offers the chance to quantify these effects, which previously could only be described qualitatively.

Technology Focus: Hydraulic Fracturing (June 2019)

Technology Focus As gas development focuses more on tight and ultratight formations, the challenges to produce economically become diverse and more intense. One main challenge faced while fracturing a well is a high fracture gradient. This can be caused by near-wellbore tortuosity, drill-in-fluid-induced damage, high build-up of filter cake, or far-field tectonics and stresses. Formations with high fracture gradients result in high breakdown pressure and frequently need very high treating pressure to fracture the interval effectively. This may cause problems with completions integrity, and, depending on the intensity of treatments and pressures reached to propagate fractures, wells may no longer be accessible because of tubular deformation. Also, if fracturing stages are skipped while treating hard rocks because of completion limitations or some stages not being treated effectively and uniformly, production will fall far below expectations. If high breakdown pressure is caused by near-wellbore damage and tortuosity, attempts can be made to lower it with chemical treatments such as mutual solvents or acids or with the help of mechanical means such as hydra-jetting, reperforation, or slotting. When it comes to far-field high-stress regimes, the in-situ stress magnitude and fault stress regime should be well-known, evaluated, and calculated to prepare for high surface pumping horsepower. Advanced completion technology that can accommodate high-pressure/ high-temperature conditions is frequently required to perform the desired stimulation treatments. Before embarking on the remedial tasks, important steps include reviewing well location and placement, selecting a well trajectory in the more-prolific interval, completing with multistage fracturing assembly in better rocks, using nondamaging fluids for fracturing, and using optimal treatments with appropriate additives that will ensure high fracture conductivity. Parts of the fracture stages that do not contribute to production because of reservoir and geomechanical heterogeneity can be reduced by the use of an engineered approach in selecting the well location, the drilling direction and azimuth, completions placement, stage and cluster design, and fracturing techniques combining real-time monitoring and optimization. Otherwise, identifying the reason for low producers becomes difficult, whether they are the result of bad completions and poor fracturing design or reservoir development and quality. The use of appropriate fracturing fluid is also important to lessen high breakdown pressures, induce sufficient length and width to carry proppant in proppant fracture treatments, and reduce or eliminate formation damage and forming of scales. Scales formed because of fluid incompatibility can deteriorate fracture and formation conductivity dramatically and obstruct gas flow. The use of seawater as a base fracturing fluid is gaining momentum in the Middle East because it is a viable alternative to fresh water, thereby reducing stimulation cost. Seawater needs treatment such that precipitation while interacting with formation water does not become severe and impair production. Treatments such as phosphonate-based inhibitors are tested and considered to address the effect of hypersalinity and precipitation of barium, calcium, and strontium sulfate that reduce gas flow. Phosphonate-based scale inhibitors are introduced for high-temperature treatments. SPE 189896 Fracturing in a Tectonically Stressed Area Under Anomalously High Gradients by Dmitriy Abdrazakov, Schlumberger, et al. SPE 186313 First Application of Residue-Free Fluid System for High-Temperature Fracturing Treatment in Saudi Arabian Carbonate Formations by Zillur Rahim, Saudi Aramco, et al. SPE 186212 A Comparative Work Flow for Different Hydraulic-Fracturing Techniques in the Western Desert Oil Fields of Egypt by Ahmed Abd ElHamid, Qarun Petroleum Company, et al.

Produced Water Re-Injection and Disposal in Low Permeable Reservoirs

Produced water re-injection (PWRI) is an important economic and environmental-friendly option to convert waste to value with waterflooding operations. However, it often causes rapid injectivity decline. In the present study, a coreflood test on a low permeable core sample is carried out to investigate the injectivity decline behavior. An analytical model for well impedance (normalized reciprocal of injectivity) growth, along with probabilistic histograms of injectivity damage parameters, is applied to well injectivity decline prediction during produced water disposal in a thick low permeable formation (Völkersen field). An impedance curve with an unusual convex form is observed in both coreflood test and well behavior modeling; the impedance growth rate is lower during external filter cake build-up if compared with the deep bed filtration stage. Low reservoir rock permeability and, consequently, high values of filtration and formation damage coefficients lead to fast impedance growth during deep bed filtration; while external filter cake formation results in relatively slow impedance growth. A risk analysis employing probabilistic histograms of injectivity damage parameters is used to well behavior prediction under high uncertainty conditions.

The Novel Approach for the Improvement of Fluid Loss Control in Water-based Mud using Nanosilica as Filler for Pectin

Fluid loss is the leakage of the liquid phase of drilling fluid, slurry or treatment fluid containing solid particles into the formation matrix. The resulting buildup of solid material or filter cake may be undesirable, as may the penetration of filtrate through the formation. Nanosilica and pectin are potential fluid loss additives used in water-based mud to reduce fluid loss and invasion of solid particles into the formation. The objective of this research work was to introduce nanosilica and pectin as fluid loss control agents in basic water-based mud at ambient condition and 250°F. The experimental results were then compared with commercial fluid loss control agent (i.e., HydroPac R) in basic water-based mud (WBM). The laboratory experiments were conducted using different concentrations ranging from 0.50 – 1.50 lb/bbl (wt. %) of nanosilica, pectin, and HydroPac R. The laboratory works were conducted as per the API Recommended Practice 13 B-1 (2009). The experimental results revealed that pectin gave a comparable fluid loss control performance as compared to HydroPac R (i.e., pectin was able to yield 7 cc of filtrate volume compared with HydroPac R which yield 6.6 cc of filtrate volume at the end of 30 minutes under low pressure of 100 psi and at ambient temperature) while nanosilica, an amorphous silica powder, produced the worst performance (i.e., 10 cc of filtrate volume at the end of 30 minutes). The experiment work was then repeated using the mixture of pectin and nanosilica as filler. The mixture yields a better reading than mud samples containing only pectin by reducing further fluid loss by 5%. The research work revealed that pectin with nanosilica as filler has the potential to be used as a fluid loss control agent in water-based mud as it produces a comparable performance with HydroPac R.

Cross-flow analysis of injection wells in a multilayered reservoir

Abstract During fluid injection into a multilayered reservoir, a different pressure gradient is generated across the face of each permeable layer. This pressure gradient generates driving forces in the wellbore during well shut-in that causes the injected fluid moves from higher pressure layers to lower pressure layers, a phenomenon known as interwell cross-flow. Cross-flow behavior depends on the initial pressure in the permeable layers and may be referred to as natural cross-flow (identical or natural initial pressures) and forced cross-flow (different initial pressures because of exploitation). Cross-flow may induce sand production and liquefaction in the higher pressure layers as well as formation damage, filter cake build-up and permeability reduction in the lower pressure layers. Thus, understanding cross-flow during well shut-in is important from a production and reservoir engineering perspective, particularly in unconsolidated or poorly consolidated sandstone reservoirs. Natural and forced cross-flow is modeled for some injection wells in an oil reservoir located at North Sea. The solution uses a transient implicit finite difference approach for multiple sand layers with different permeabilities separated by impermeable shale layers. Natural and forced cross-flow rates for each reservoir layer during shut-in are calculated and compared with different production logging tool (PLT) measurements. It appears that forced cross-flow is usually more prolonged and subject to a higher flow rate when compared with natural cross-flow, and is thus worthy of more detailed analysis.

Effect of nanoparticle behaviour on mud cake buildup for directional and horizontal wells: mathematical modelling and experimental study

ABSTRACTThe present study examined the effect of nanoparticle size and geometry on filter cake buildup during dynamic filtration using experimental and modeling approaches. A dynamic filtration setup was proposed and designed to test cross-flow circulation against a synthetic core at a constant differential pressure. The proposed mathematical model considers the critical deposition boundary of the porous media for particles along with drag, lift, friction, buoyancy, permeate, and electrostatic forces. The comprehensive model response allowed investigation of the aggregation and shape of nanoparticles at different cross-flow inclinations. The drag and lift force coefficients and moment of inertia of the nanoparticles were determined using cluster structures of nanoparticles identified using dynamic light scattering (DLS) and fractal theory. The deposition and erosion intensity index of all solid particles were experimentally determined and used in the model. Electronic microscope images of the mud cake surface confirmed the proposed cluster structure of the aggregated particles. The mud cake thickness measured by dynamic filtration tests and the nanoparticle-bentonite weight ratio acquired by DLS showed good agreement with the mathematical model results. The proposed approach allows selection of the optimum shape and size of the nano-solid particles as additives to minimize mud cake buildup along the borehole.

Filter performance with non-uniformly distributed concentration of dust-evidence from experiments and models

Cleanable fabric filters such as bag filters are widely used in industrial processes to remove large quantities of solid particles from gas streams. Intrinsically, such a filtration process is as non-uniform as it is semi-continuous; that is, filter cake build-up processes are successively followed by a short-duration cleaning step. Moreover, heterogeneous flow conditions and, thus, concentrations of particles entering the filter housing and flowing toward the filter contribute to rather non-uniform filter conditions. Since cleaning is not necessarily performed in a homogeneous manner, and patches of cake might remain on the filter fabric, the subsequent filtration is also non-uniform. Finally, the filter medium itself is largely heterogeneous certainly on micro-, but often also on a macro-scale.The influence of a heterogeneous dust concentration as a common filter non-uniformity is investigated on filter performance, i.e., pressure drop increase as a measure for filter cycle cleaning frequency being a precursor for filter life-time, dust emissions and operating costs.Laboratory tests were performed whereby the pressure increase curves are recorded at basically constant gas flow. As a baseline the entire filter area was exposed to a homogeneous dust concentration. Comparatively the filtration area was split into two zones where cake formation occurred under a high and a low particle concentration mimicking a distinctive heterogeneous dust concentration situation. The transient pressure increase profiles of both, the homogeneous and heterogeneous cake formation situation, were compared with each other.The apparent filter cake resistance was slightly higher under homogeneous dust concentration conditions, i.e., the pressure drop increased faster compared with the situation of a heterogeneous dust concentration. These results suggest that, in practical terms, a filter exposed to non-uniform dust concentrations exhibits a better overall performance. Consequently aiming for a most uniform dust concentration is to no avail, though often intended in industrial operation.

A Novel Water-Injectivity Model and Experimental Validation With CT-Scanned Corefloods

Summary Injectivity decline is an issue during produced-water reinjection (PWRI) for water disposal in aquifers, waterflooding, chemical enhanced oil recovery, and geothermal-energy exploitation. A novel model for injectivity decline under flow conditions reminiscent of PWRI was developed taking into account deep-bed filtration and buildup of external filter cake. A distinct feature of the model is that it describes particle-retention kinetics responsible for internal filtration by an exponential decaying function of the retained-particle concentration. The corresponding nonlinear governing partial-differential equations were solved numerically and coupled with a known analytical model for external filtration with the concept of transition time. Coreflood experiments consisting of the injection of brine containing suspended hematite particles (volume fractions in the range of 2 to 6 ppm) were also performed. Computed-tomography (CT) scans of the core were taken to obtain deposition profiles along the core at different times. In addition, effect of various parameters (particle concentration and number of grids) on injectivity was investigated. From CT-scan and optical-microscope analyses, it was found that surface deposition in the porous medium and face plugging at the injection face of the core were responsible for decline in injectivity. The transition time from pure internal to external filtration was accurately determined from the CT-scan and pressure data. The newly proposed model and experiments were found to be in excellent agreement, indicating that the adopted retention function is a good heuristic description of particle retention.

Nonuniform External Filter Cake in Long Injection Wells

Buildup and stabilization of external filter cake is a well-known phenomenon in several environmental and industrial applications. Significant decline of the tangential rate along thick intervals in lengthy vertical wells yields a nonuniform external filter cake profile. We derive the mechanical equilibrium equations for the stabilized cake profile accounting for electrostatic particle–rock interaction, varying permeate factor, applying the torque balance to describe cake equilibrium, and calculating the lever arm ratio using Hertz’s theory for contact deformation of cake and particles. An implicit formula for the cake thickness along the well is derived. Two regimes of the stabilized cake buildup correspond to low rates, where the cake starts from the reservoir top, and for high rates, where the cake is formed in the lower well section only. The sensitivity analysis shows that the drag and permeate forces are the competitive factors affecting cake thickness under varying Young’s modulus, rate, and salinity. The main parameters defining external cake profile are injection rate, cake porosity, water salinity, and Young’s modulus.

Pushing the Limit on Guar Loading: Treatment of a Green River Sandstone Formation by Use of a Novel Low-Polymer Crosslinked Fluid

Summary Guar-based fracturing fluids are the most commonly used fluids in reservoir stimulation. To provide high viscosity, borate-crosslinked gels are preferred for their ability to heal after mechanical shearing and their favorable environmental properties. More-efficient crosslinkers capable of crosslinking fluids with reduced polymer loading have always been of great interest to reduce formation and proppant-pack damage from polymer residues and to reduce overall fluid cost. Low permeability and the interwell connectivity of the Green River sandstone formation of the Uintah basin require hydraulic-fracturing treatment to produce oil economically. The most typical fracturing treatments use borate-crosslinked guar fluids to transport sand into wells with vertical depths of 3,500 to 6,800 ft and bottomhole temperatures of 115 to 160°F. Most operators in this region place emphasis on reduced polymer loadings for stimulation treatments, and, even with breakers present, broken polymer residues can remain in the formation at these reduced polymer loadings, resulting in damage and decreased production. Unlike conventional treatments that require a shut-in operation, stimulation treatments in this region are designed for immediate flowback to minimize filter-cake buildup and subsequent formation damage, which improves production significantly. Immediate flowback poses a challenge of a proper balance between polymer and breaker loading so that the fluid effectively transports up to 6 lbm/gal added of sand into the formation without screenout and breaks within the treatment time (20 to 30 minutes per stage) to minimize flowback of proppant. A recently developed novel poly-aminoboronate (PAB) crosslinker (Sun and Qu 2011; Legemah et al. 2013) was tested in the aforementioned formation. Multiple boron sites are available in the crosslinker, and it is capable of interacting with multiple polysaccharide strands to form more-complex crosslinking networks at lower polymer loadings than conventional guar fluids. The crosslinker, with up to an additional 15% reduced guar loading, is capable of matching or outperforming conventional crosslinked fluids in fresh water and, more impressively, in 7% potassium chloride. This paper will discuss the novel crosslinker developed, the laboratory testing, and the successful field application. Analysis and discussion of the chemistry, crosslinking performance, and economics will be reported.

Filter Cake Properties of Water-Based Drilling Fluids Under Static and Dynamic Conditions Using Computed Tomography Scan

Previous studies considered the water-based drilling fluid filter cake as homogenous, containing one layer with an average porosity and permeability. The filter cake was recently proved to be heterogeneous, containing two layers with different properties (thickness, porosity and permeability). Heterogeneity of the filter cake plays a key role in the design of chemical treatments needed to remove the filter cake. The objectives of this study are to describe filter cake buildup under static and dynamic conditions, determine change in the filter medium properties, and obtain the local filtration properties for each layer in the filter cake. A high pressure high temperature (HPHT) filter press was used to perform the filtration process at 225 °F and 300 psi. A CT (computed tomography) scanner was used to measure the thickness and porosity of the filter cake. The results obtained from the CT scan showed that under static conditions, the formation of filter cake changed from compression to buildup; while under dynamic conditions, the filter cake was formed under continuous buildup. The CT results explained the changes in the thickness and porosity of each layer of the filter cake with time. The CT scans showed the change in the properties of the ceramic disk, such as porosity and permeability, which affect the calculation of the filter cake permeability. The change of the properties of the filter medium was ignored in previous studies.

The Effects of Filter-Cake Buildup and Time-Dependent Properties on the Stability of Inclined Wellbores

Summary The effects of filter-cake buildup and/or filter-cake-property variation with time on wellbore stability have been plaguing the industry. The increasing use of lost-circulation materials (LCMs) in recent years for wellbore strengthening in weak and/or depleted formations necessitates models that can predict these effects. However, the complexities of effective-stress and pore-pressure evolution around the borehole while drilling, coupled with the transient variation of mud-filtration properties, have delayed such modeling efforts. In this paper, the analytical solutions for the time-dependent effects of mudcake buildup and mudcake properties on the wellbore stresses and formation pore pressure, and thus the safe-drilling-mud-weight window, are derived. The transient effects of mudcake buildup and mudcake buildup coupled with its permeability reduction during filtration on the safe-drilling-mudweight window are illustrated through numerical examples. The results showed that the safe-mudweight windows were greatly affected by the buildup of filter cake and its permeability variation. For example, the analysis for filter-cake buildup with cake permeability of 10–2 md showed that the safe-mudweight window was widened by 0.5 g/cc after 2.5 hours post-excavation when compared to the case of a wellbore without mudcake. On the other hand, a lower mudcake permeability of 10–3 md widened the mudweight window by as much as 1 g/cc. Last but not least, the analyses revealed that even for mudcake permeability as low as 10–3 md, neglecting the permeable nature of the mudcake can result in overestimation of the safe-drilling-mudweight window.

Non-linear waves of fluid pressure and contaminant density in swelling shales

The control of drilling parameters, such as salt concentration, mud percentage and temperature is of fundamental importance for maintaining borehole stability when drilling through shale sections. For this purpose, a non-linear model of mechanic and chemo-poroelastic interactions between fluids, contaminants and solid matrix is presented. For the binary case we obtain a system of two non-linear coupled equations describing the isothermal evolution of fluid pressure and contaminant density in shales. In addition to the usual diffusive solutions obtained in linear analyses, a novel process is found. Indeed we show the presence of intense transient phenomena, i.e. quick non-linear shock waves of both pressure and contaminant. We apply this model first to a simple, homogeneous shale, then to more realistic cases, and finally to a non-compressive filter cake buildup. All these waves exhibit a Burgers solitary wave dynamics and can give destructive instabilities in deep drilling oil in shales.

Classification of Suspensions by Means of Rotating Filter Discs and Autodynamic High‐Frequency Backwashing

AbstractA new method for the classification of suspensions by means of rotating filter discs is presented. The classification without a filter cake buildup by means of a single shaft disc filter is possible using the high‐frequency autodynamic backwashing method. High‐frequency autodynamic backwashing results from an overpressure fluctuation between the filtrate and concentrate sides, resulting in different flow directions of the filtrate volume flow. The main parameters are the centrifugal force acting on the fluid inside the discs in combination with the fact that the discs are slightly inclined. It is demonstrated that the separation of coarse particles is entirely possible.

Formation-Damage and Well-Productivity Simulation

SummaryIn this paper, we describe a simulation model for computing the damage imposed on the formation during overbalanced drilling. The main parts modeled are filter-cake buildup under both static and dynamic conditions; fluid loss to the formation; transport of solids and polymers inside the formation, including effects of porelining retention and pore-throat plugging; and salinity effects on fines stability and clay swelling. The developed model can handle multicomponent water-based-mud systems at both the core scale (linear model) and the field scale (2D radial model). Among the computed results are fluid loss vs. time, internal damage distribution, and productivity calculations for both the entire well and individual sections.The simulation model works, in part, independently of fluid-loss experiments (e.g., the model does not use fluid-leakoff coefficients but instead computes the filter-cake buildup and its flow resistance from properties ascribed to the individual components in the mud). Some of these properties can be measured directly, such as particle-size distribution of solids, effect of polymers on fluid viscosity, and formation permeability and porosity. Other properties, which must be determined by tuning the results of the numerical model against fluid-loss experiments, are still assumed to be rather case independent, and, once determined, they can be used in simulations at altered conditions as well as with different mud formulations. A detailed description of the filter-cake model is given in this paper.We present simulations of several static and dynamic fluid-loss experiments. The particle-transport model is used to simulate a dilute particle-injection experiment taken from the literature. Finally, we demonstrate the model's applicability at the field scale and present computational results from an actual well drilled in the North Sea. These results are analyzed, and it is concluded that the potential effects of the mechanistic modeling approach used are (a) increased understanding of damage mechanisms, (b) improved design of experiments used in the selection process, and (c) better predictions at the well scale. This allows for a more-efficient and more-realistic prescreening of drilling fluids than traditional core-plug testing.

Modeling of External Filter Cake Build-up in Radial Geometry

The problem of formation damage (i.e., permeability reduction due to injection of particulates), is a matter of interest in several engineering fields. In the previous attempts to model the external cake formation, cake thickness has been considered to be only dependent on time; even though in practical applications, the dependency of the cake profile on space can be important. In this article, a novel model has been developed to describe the steady state external filter cake thickness profile along the wellbore. A set of equations is derived from the force balance for a deposited particle on the cake surface and the volume conservation of the fluid in the wellbore. These equations are combined with Darcy's law in radial geometry and the equation of flow in the wellbore, and solved numerically to obtain the cake thickness and fluid velocity profiles along the wellbore.

Injectivity Decline From Produced-Water Reinjection: New Insights on In-Depth Particle-Deposition Mechanisms

Summary Injectivity decline during produced-water reinjection (PWRI) originates not only from filter-cake buildup but also from in-depth deposition of oil droplets or solid particles. Physical modeling of particle-deposition mechanisms in porous media is thus of key interest for optimizing PWRI operations. The present work brings new insights on oil-droplet and solid-particle-deposition mechanisms in porous media. The experimental conditions were selected such that the ratio between pores and particle sizes is sufficiently large to ensure in-depth propagation. The parameters are the nature of the particles injected and a Peclet number calculated on the size of the collector grains (Peg) that encompasses in a nondimensional form the impact of both the flow rate and the particle size. The results are analyzed within the framework of the "colloidal approach." For oil droplets and solid particles, the collection efficiency (η) shows a transition from a behavior in which η varies as a power law of Peg, with exponent values −⅔ [(diffusion-limited deposition (DLD)] to −1 [reaction-limited deposition (RLD)] that are typical of the convection/diffusion regime, to a behavior characterized by an increase of η vs. Peg, typical of the hydrodynamic deposition regime. In the case of oil droplets (slightly charged), the transition occurs at a critical Peg value, PegC≈PegCgeom/10, corresponding to a diffusion-layer thickness around the collector grain of the same order of magnitude as the droplet diameter. In the case of electrosterically stabilized solid particles, the transition takes place at PegC≪PegCgeom for small particles and at PegC>PegCgeom for larger particles.