Partial Plugging Research Articles

MODEL FOR ESTIMATING SKIN FACTOR OF PERFORATED LINERS IN HORIZONTAL WELLS

One notable completion method for enhancing productivity in horizontal wells is the use of perforated liners. However, a major challenge common to horizontally completed wells is to perform an accurate measurement of the skin factor. Perforated liners can create several hurdles, capable of negatively affecting productivity, such as partial plugging, flow convergence, and turbulence. This paper introduces a new analytical skin factor model which incorporates distance between perforations and explains the effects of flowrate on skin for various parameters like perforation radius, wellbore radius, and distance between perforations for perforated liner completions. Results show that skin factor is directly proportional to flow rate and distance between perforations, however, they are inversely proportional to wellbore radius and penetration. From sensitivity analysis, the distance between perforations represents a key source to flow convergence and restriction to flow. This work describes how model validation shows good agreement with the existing model, and demonstrates the application of the new model.

Soil plugging of open-ended steel piles during impact driving

During the driving process of an open-ended steel pile, soil is allowed to enter the pile’s core until partial or complete plugging occurs. Once this phenomenon is activated, soil resistance to driving (SRD) increases, which can prevent further soil introduction inside the pile, and in turn halt installation works under similar conditions. In this paper, we demonstrate the relationship between the plugging phenomenon, pile geometry, and driving acceleration by comparing literature with some conducted experiments (Paikowsky, 1990; Magroun, 2012), which show that the risk of plugging decreases mainly with an increase in both pile diameter and driving acceleration. A verification study of plugging occurrence in terms of depth is conducted on an open-ended driven pile at a Moroccan port terminal crossing predominantly sandy layers before anchoring in a lower marl layer. The pile’s bearing capacity is evaluated in both plugged and unplugged cases using the SRD method following the processes of Toolan & Fox (1977) and Alm & Hamre (2001), and based on cone penetration test (CPT) results. The pile is thus found unlikely to plug during driving. As a general rule, we found that the plugging phenomenon must be controlled to avoid any interruption in the pile driving process.

Behaviour of aluminium/steel hybrid RSW joints under high cycle fatigue loading

The lightweight construction of automotive car bodies is the more important to reduce the fuel consumption and costs. High-strength steels and aluminium alloys are suitable for achieving these aims. Recent car bodies contain both materials, therefore necessary to make reliable joints between them. The resistance spot welding (RSW) can be used for joining of car bodies and it is applicable for aluminium/steel hybrid joints, too. High cycle fatigue (HCF) test results can be rarely found in the literature while HCF loading basically determines the lifetime of hybrid joints. 5754-H22, 6082-T6, and DP600 base materials were used for similar and hybrid RSW joints and HCF tests were performed. Number of cycles to failure values, failure modes, furthermore brittle intermetallic compound (IMC) layers were studied and analysed. In both aluminium/steel hybrid joints, the HCF test results showed better endurance limit like concerning aluminium/aluminium similar joints, but worse than steel/steel joints. For 5754-H22 alloy the endurance limit values are 648 N, 939 N, and 1285.5 N, for similar aluminium, hybrid, and similar steel joints, respectively. For 6082-T6 alloy these values are 513 N, 625.5 N, and 1285.5 N, respectively. In case of similar joints only base material fracture happens, but hybrid joint specimens show different failure modes. Base material fracture and shearing after partial base material fracture were typical failure modes in case of 5754-H22/DP600 and 6082-T6/DP600 hybrid joints, respectively. The full and partial plugging as a failure modes appeared for hybrid joints, too. The IMC layer characteristics showed opposite results in cases of hybrid joints, both the layer thicknesses of the shared and plugged joints and the thickness differences between the inner and outer parts of the joints were different.

Coreflood Effluent and Shale Surface Chemistries in Predicting Interaction between Shale, Brine, and Reactive Fluid

Summary Field and laboratory observations to date indicate that the efficiency of hydraulic fracturing, as it relates to hydrocarbon recovery, depends significantly on geochemical alterations to rock surfaces that diminish accessibility by partial or total plugging of the pore and fracture networks. This is caused by mineral scale deposition, such as coating of fracture surfaces with precipitates, particle migration, and/or crack closure, because of dissolution under stress. In reactive flow-through experiments, mineral reactions in response to acidic fluid injection significantly reduced system porosity and core permeability. The present study focuses on changes to fluid chemistry and shale surfaces (inlet and fracture walls) resulting from shale-fluid interactions and integrates these findings for an improved estimate of transport-related consequences. The pre- and post-reaction shale surfaces were examined by spatially resolved scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis. Importantly, inductively coupled plasma-mass spectrometry/optical emission spectroscopy (ICP-MS/OES) was utilized to probe the chemical evolution of the coreflood effluents. The three study cores selected from the Marcellus formation represent different mineralogies and structural features. In flow-through experiments, laboratory-generated brine and HCl-based fracture fluid (pH = 2) were injected sequentially under effective stress (up to 500 psi) at reservoir temperature (80°C). SEM-EDS results confirmed by the ICP concentration trends showed significant Fe hydroxide precipitates in the clay- and pyrite-rich outcrop sample because of partial oxidation of Fe-bearing phases in the case of intrusion of low salinity water-based fluids. Porosity reduction in the Marcellus Shale Energy and Environmental Laboratory (MSEEL) carbonate-rich sample is related to compaction of cores under stress because of matrix softening with substantial dissolution, and pore filling by hydroxides, as well as secondary barite and salts. Despite the same fluid compositions and experimental conditions used for both MSEEL samples, barite precipitation was much more intense in the MSEEL clay-rich sample because of its greater sorption capacity and additional sulfate source as well as its fissile nature with multiple lengthwise cracks. ICP tests revealed time-resolved concentration behavior in produced brine and reactive fluids that in turn complemented the pre/post-reaction SEM-EDS observations. The greatest release of metal ions into brine was in clay-rich systems indicating the importance of chemical compatibility between in-situ shale and nonequilibrated injection solutions. A thorough examination of surface and effluent data pointed to the substantial influence of formation brine in the shales, mixing of brine with fracture fluid during flow, and shale mineralogy on mineral dissolution and scale formation that significantly affect flow efficiency.

Simulation Studies of the Sodium Hydraulic Behavior to Design a Mockup for Eddy Current Flowmeter Testing

Abstract The Eddy current flowmeter sensor plays an important part in the instrumentation of the future sodium fast reactors, which will allow when located above the core to detect plugging (partial or total) of a subassembly and when located on the primary pump to measure core pressure drop and core flowrate. This article describes the predesign phase of a mockup for Eddy current flowmeter tests and qualifications under sodium conditions for the detection of a partial or full plugging of a fuel subassembly. These tests will be the first of their kind, as the Eddy current flowmeter had never been qualified at nominal conditions of a reactor core outlet (sodium temperature, velocity, and output flow).

Assessing axial load transfer mechanism of open-ended tubular piles penetrating in weak rocks using three-dimensional discrete element method

In order to obtain a better understanding of the load transfer mechanism of open-ended tubular piles driven into weak rocks, numerical analysis is conducted to study the internal and external shaft frictions, base resistance and the stress distribution pattern in the surrounding rock mass. Meanwhile, since pile shoes are often applied when driving to the rock layer, a common inner pile shoe attached to the open-ended tubular pile was analysis and compared with the pile without a shoe. In this study, the discrete element method (DEM), which can mimic rock grains and the interaction between grains, is adopted due to its ability to simulate large deformation problems such as pile penetration process. The flat-joint model that allows partial damage at the contact interface was employed to replicate the discrete nature of the rock mass. The rock behaviour was calibrated against triaxial test results for a pyroclastic weak rock, and then the calibrated rock was employed for the simulation of pile penetration. A 30° segment of the true-scale model was simulated using the DEM with a height of 3 m and a radius of 1.5 m, which was composed of about 557,000 particles. Initial stresses were applied at the boundary of the testing chamber, while the side of the chamber was discretised into several sections with increasing horizontal stresses with depth to establish the at rest condition. Numerical results for penetration of an open-ended tubular pile show that the external shaft resistance of the pile with inner pile shoe increased approximately linearly as the pile penetrated deeper. In addition, the internal shaft friction was notably less than the external shaft friction for both piles with and without a shoe. Plugging was observed for both piles with and without a driving shoe. The inner pile shoe allowed the pile partial plugging to occur at shallower penetration depth. This study sheds a light on the load transfer mechanism of the open-ended tubular piles and the effect of the shoe on the axial capacity of the piles in weak rocks.

Making complex fractures by re-fracturing with different plugging types in large stress difference reservoirs

In recent years, re-fracturing has attracted much attention in the development of old oil fields and unconventional reservoirs, but it wasn't always successful in the trials. It was still difficult to know whether the old fractures had been plugged successfully and how the plugging influenced the fracture behavior when the plugging agents were injected into wells. However, previous experiments and models were not convinced to simulate the real plugging-fracturing. This study designed a new method to simulate the plugging-fracturing by true tri-axial hydraulic fracturing test. Instead of injecting plugging agents to wellbores, the experiment made target plugging at old fractures. The plugging was simulated by glue the fracture boundary, which could simulate different types of plugging by gluing different region on the fracture. There were three plugging types studied in this paper, including tip plugging, partial plugging and complete plugging. Besides, the pressure feature, optimized plugging time, the effects of weak plane and layer interface were discussed. The results showed obvious vertical propagation of old fractures when meeting tip barriers, which was neglected in normal 2 d fracture-plugging models. Tip plugging was not so effective in real 3 d fracture, while partial plugging helped obtain a “S” shape fracture. To improve fracture complexity, weak planes and layer interface plays an important role in improving fracture complexity by diverting the fluids from vertical to horizontal direction and afford opportunities to find more potential points which could be easily fractured. This experiment focused on fracture behavior when it met different barriers, helping understand how to make successful plugging and improve fracture complexity in reservoirs with large horizontal stress difference and multilayers.

Data Mining to Determine the Influence of Fluid Properties on the Integrity Test Values.

Eudralex volume 4, Annex 1, the European Union Good Manufacturing Practice for sterile products, requires that "The integrity of the sterilised filter should be verified before use" (1). Implicit in this requirement for a PUPSIT is the rationale that the sterilizing filter could sustain damage during sterilization or use (i.e., subsequent to any pre-use test conducted prior to sterilization), causing a defect which would not be detected by the post-use integrity ("masked" during filtration). To assess whether a filter defect could be masked by partial filter plugging, we evaluated the impact of the bacterial challenge test (BCT) on the bubble point (BP) of the test filters. The BP tests that are conducted before and after the BCT have been collected and compared for 2086 filters (1571 × test filters and 515 × control filters), representing 531 BCTs on 518 different pharmaceutical products, buffers, and in-process fluids. These tests comprise a cross section of fluids from multiple firms spanning the pharmaceutical and biotechnology industry. A posttest to pretest BP ratio was calculated for each filter and the distribution of these ratios examined to determine whether there were cases of elevation of the BP because of bacterial loading to the point where masking of a filter defect could occur; that is, if a defective filter could pass integrity testing due to apparent reduction in filter pore size because of the bacteria retained during the BCT. Ratios were averaged across all tests for the same test fluid. The mean average ratio was 1.00 ± 0.15, indicating that on the average, elevation of the BP does not occur. To assess the risk of masking a filter defect, observed BP ratios were compared to the ratio of the minimum BP specification of a 0.2 μm filter to that of a 0.45 μm filter of the same membrane type. The lowest such ratio for any membrane type was 1.33. A BP ratio equal to or higher than this ratio was considered a risk for masking, because a 0.45 μm filter could appear to meet the specifications of a 0.2 μm filter. Out of 518 average BP ratios, only eight fluids (1.5%) produced BP ratios meeting this criterion for a masking risk. Potential risk factors associated with these cases are discussed. We conclude that filtration processes producing BP changes sufficient to present a risk of masking defects are not common, and are detectable during the routine BCT. The BP ratios observed during routine BCT are one means to assess the potential of a given filtration process to mask defects and can be considered when determining whether a PUPSIT should be implemented.

A Degassing Inlet Structure for Aquaculture Ponds

Structures separating fish hatchery ponds from open spring water sources must restrict fish movement from the pond to the spring, allow for constant water flow, and potentially reduce incoming spring water gas supersaturation. This article describes a novel inlet structure that fulfills those requirements. In addition, it requires minimal maintenance, and allows for the quick and easy removal of debris in the event of partial plugging. This simple aluminum structure consists of a mount for attachment to the inflow pipe and a terminal splash plate with aeration holes. The splash plate is perpendicular to the water flow during normal operations to both prevent fish from jumping into the pipe and aerate the incoming water. However, it can be easily swiveled upward for the efficient removal of debris. Use of the inlet structure consistently decreased gas supersaturation in the spring water as it entered the fish production pond. By decreasing gas levels and maintaining water flows, potentially hazardous fish health issues can be avoided by using this structure. In addition, this relatively inexpensive and simple device will greatly reduce the labor required for removing debris compared to traditional screens.

Numerical investigations of soil plugging effect inside large-diameter, open-ended wind turbine monopiles driven by vibratory hammers

This study established a Couple Eulerian–Lagrange model to simulate monopile vibratory penetration for the investigation of soil plugging effect during high-frequency penetration of monopiles for wind turbine. Simulation analysis is focused particularly on soil plugging effect of a large diameter monopile during vibratory penetration into sand, clay, or layered soil. The results of the numerical simulation show that soil plugging effect is unlikely to occur during monopile penetration into the clay soil, while partial soil plugging may occur during the sand penetration. Penetration resistance at the pile toe is transferred to the radial stress around the pile wall. At a critical point penetration process, internal shaft friction becomes larger than external shaft friction. Moreover, radial pressure factors increase during partial soil plugging effect. For layered soil, the topsoil not only has great influence on the soil plugging effect, but also affects shaft friction in the subsoil during monopile penetration.

Selective Adsorption of CH₄/N₂ on Ni-based MOF/SBA-15 Composite Materials.

Spherical SBA-15-based metal–organic framework (MOF) composite materials were prepared, with nickel as the metal center of MOFs. The materials were characterized via scanning electron microscopy, X-ray fluorescence analysis, X-ray powder diffraction, Fourier-transform infrared spectroscopy, and nitrogen (N2) adsorption–desorption. The methane (CH4) or N2 high-pressure adsorption isotherms of the samples were measured and compared. The specific surface area and adsorption capacity of the composite materials were generally higher than the pristine MOFs, but were much lower than the synthesized SBA-15. The selectivity of the samples toward a binary gas mixture was determined from the Langmuir adsorption equation. The results revealed that, of all the samples, the MOF-2/SBA-15 sample had the best CH4/N2 adsorption selectivity, with an adsorption selection parameter (S) of 11.1. However, the adsorption of MOF-2/SBA-15 was less than that of spherical SBA-15, due to partial plugging of the pores during the synthesis process. Further research is essential for improving the performance of spherical SBA-15-based MOF materials and (in turn) the enrichment of CH4 from the CH4/N2 mixture.

Advanced Electrochemical System Desalts Produced Water, Saves Polymer

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 190272, “An Advanced Electrochemical System for EOR Produced-Water Desalination and Reduced Polymer Consumption,” by Ben Sparrow, Adrian Ebsary, Derek Mandel, and Malcolm Man, Saltworks Technologies, prepared for the 2018 SPE Improved Oil Recovery Conference, Tulsa, 14–18 April. The paper has not been peer reviewed. This paper presents pilot-testing results and economics from a novel electrochemical desalination technology for enhanced oil recovery (EOR) produced water. The pilot objectives were to (1) economically desalt produced water to improve hydrocarbon recovery and lower polymer consumption costs for chemical-flood EOR; (2) inform full-scale plant development with a field pilot; and (3) optimize prefiltration, chemical consumption, and energy use to realize greater than 20% return on investment through reduced polymer consumption. Background and Pilot Summary An electrodialysis-reversal (EDR) system that requires minimal pretreatment with proprietary hydrocarbon antifouling ion-exchange membranes packaged in rugged skids with advanced process controls was used. The EDR plant can (1) desalt EOR produced water from up to 20 000 mg/L total dissolved solids (TDS) down to 500–5000 mg/L for reinjection and (2) reduce polymer requirements to decrease chemical costs in polymer-flood operations. EDR desalts by use of an electric field; dissolved ions in the produced water are moved across ion-exchange membranes, desalting one stream while concentrating a smaller volume discharge. The TDS in the produced water can be desalted to any concentration that provides optimal performance specific to the reservoir. Depending on the presence of scaling ions, the concentrated stream can achieve TDS concentrations of 80,000–150,000 mg/L, or, in the case of offshore operations, the concentrated stream can be eliminated through a novel process. EOR operators may add polymers and other chemicals to increase viscosity of injected water and enable increased oil recovery. Tests completed for this work proved that EOR polymers present in the produced water that returns to surface do not foul or pass through the EDR membranes, instead remaining in the desalted water for reuse. In fact, some polymers are reactivated in the desalted output, with viscosity increasing by almost double during desalination, thereby enabling some recycling of original polymer. More importantly, results proved that lower TDS of the injected water can result in up to 65% polymer cost savings, which is a major contributor to operating costs. Polymer consumption increases with TDS in order to reach a target viscosity goal; therefore, desalting the injected water can result in net savings if desalination costs are lower than the incremental polymer-consumption savings. This work showed a 30–40% rate of return on investment. To test the theory, an offsite pilot was completed in advance to prepare for onsite work. Produced water up to 20 000 mg/L TDS with oil in water present up to 600 ppm were tested. Prefiltration consisted of proprietary media filtration to remove suspended solids to less than 20-µm particle size. No pretreatment for the oil was required. This is because of the resiliency of the antifouling ion-exchange membranes, EDR stack design to prevent plugging, and intelligent cleaning systems that detect and react to remove fouling or partial plugging before irreversible events occur.

Completion of Five Years of Safe CO2 Injection and Transition to the Post-closure Phase at the Ketzin Pilot Site

The Ketzin pilot site for geological storage of CO2 in the German Federal State of Brandenburg about 25km west of Berlin was the first European pilot site for onshore storage of CO2 in saline aquifers. A total amount of 67 kt of CO2 was injected without any safety issues between June 2008 and August 2013 when injection ceased and the site entered the post-closure phase. Research and activities on site will continue in order to address and finally close the entire life cycle of the storage site. Within the post-closure phase the multidisciplinary monitoring program will further be applied and a stepwise abandonment of the five wells is foreseen which has already started with the partial plugging of one observation well in fall 2013.

Cone penetration test (CPT) methods for end-bearing assessment of open- and closed-ended driven piles in siliceous sand

The cone penetration test (CPT) has been used as a means of assessing the end bearing of driven piles in sand for many decades. This paper examines the predictive performance of four new such CPT-based methods recently included in the commentary of the 22nd edition of the American Petroleum Institute’s recommended practice for fixed offshore structures. It is demonstrated that the formulations given by one of these methods, referred to as UWA-05, provides better predictions than the three other CPT methods when tested against (i) an existing database of base-capacity measurements, (ii) results from a new series of load tests on small-diameter piles, and (iii) base capacities measured in two recently conducted load tests on 1.5 m diameter pipe piles. It is shown that the UWA-05 has better predictive performance (and hence reliability) as it accounts explicitly for the effects of partial plugging during pipe–pile installation and for variations in CPT resistance in the vicinity of the pile tip.

Synthesis of sodalite octahydrate zeolite-clay composite membrane and its use in separation of SDS

A simple two-step process for synthesis of sodalite octahydrate zeolite-clay composite, charged ultrafiltration membrane is described wherein the active layer is obtained by partial plugging of the pore-mouth of macroporous clay support with in situ grown zeolite particles. Dip-coating of zeolite synthesis solution at pH 12 is found to give higher deposition of zeolite on the clay support. Increase in amount of zeolite in the active layer obtained by repeated deposition of zeolite particles on clay-support is quantified using membrane potential measurements and it is found that four cycles of zeolite deposition gives optimum results. The membrane is characterized using SEM for surface morphology, XRD for zeolite characterization, water flux and separation of surfactant (SDS) above cmc concentration. SDS separation results show observed-rejection in the range of 10–45%. The observed-rejection is found to decrease with increase in the applied pressure difference and increase with increase in the surfactant concentration. The permeate flux is found to reduce significantly to about 0.2–0.46 times the pure water flux. The trend is similar to that reported in literature for separation of surfactant micelle by ultrafiltration membrane of pore size much larger than the size of surfactant micelle.

A Comprehensive Skin-Factor Model of Horizontal-Well Completion Performance

Summary Horizontal wells or laterals are completed as openhole, slotted-liner, cased and perforated, or gravel-pack completions. We have developed a comprehensive skin-factor model to predict the performance of any of these completion types and have calibrated this model with extensive finite-element simulations of flow for a horizontal-well completion. This model can be used to predict the performance of virtually any horizontal-well completion. The new completion skin-factor model accounts for the effects of formation damage, convergent flow to perforations and slots, and flow through slots, with interaction among these effects. To account for formation damage, we extended our previous rigorous model of a damaged horizontal well to include the presence of perforations within, or extending through, the damage zone. The formation damage model is also integrated with the models of slotted-liner performance to model these completions. The model of slotted-or perforated-liner performance is made on the basis of the relationship between pressure drop and flow rate for turbulent flow in these geometries. The slotted-liner model accounts for partial plugging of the slots by grains of formation minerals or precipitates (scale). Turbulence effects are a major part of the apparent skin factor for these completion types. The model shows the recommended conditions to obtain high-productivity (i.e., low skin factor) completions in horizontal wells. In particular, the interactions among damage effects and skin effects caused by perforations or slots are shown to greatly affect horizontal-well completion performance. The models developed can be applied to design optimal completions for horizontal wells or laterals.

Dispersions of Transition‐Metal‐Based Phases in Mesostructured Silica Matrixes: Preparation of High‐Performance Catalytic Materials

AbstractFor Abstract see ChemInform Abstract in Full Text.

Dispersions of transition-metal-based phases in mesostructured silica matrixes: preparation of high-performance catalytic materials

Abstract The finding that ordered mesostructured silica (OMS) matrixes can stabilize guest nanocrystals has transformed the preparation of advanced catalytic materials. Today, it is possible to tailor the materials in such a way that high loading of guest catalytic phases (CPs) may be achieved at different locations in the host mesoporous matrix and, in some cases, to combine several different chemical functionalities in a single nanocomposite. The conditions for the preparation of host-guest catalytic composites have been elaborated, as have the preparation and testing of representative nanocrystal ensembles, i.e. WS2, MoS2, WO3, ZrO2 (tetragonal), TiO2 (anatase), Cs2.5H0.5PW12O40, clusters of alumina phase and molecular species of H3PW12O40, inserted in an OMS (SBA-15 and MCM-41) matrix at 35–80 wt.% loading. Special fixation methods provide uniform distribution of nanoparticles in the silica nanotubes and at the outer surface of the OMS microcrystals at high loadings. CPs with high thermostability as a result of their stabilization in the OMS matrix demonstrated superior performance by a factor of 1.5–5 in hydrotreating, acidic and oxidation reactions of hydrocarbons compared with conventional reference catalysts. Diffusion limitations in catalytic reactions performed with CP/OMS composites were estimated to be negligible. The only factor limiting the accessibility of CP to reacting molecules was partial plugging of the OMS mesopores.To cite this article: M.V. Landau et al., C. R. Chimie 8 (2005).

An "Autopsy" of the Failed Tangaskootack #1 Vertical Flow Pond, Clinton CO., Pennsylvania

The Tangascootack #1 passive treatment system was constructed in 1998 to treat an acid discharge from an abandoned surface mine in Clinton Co., PA. The discharge averaged about 155 L/min with pH 4.0, acidity 235 mg/L CaCO3, Fe 3.7 mg/L, Al 24 mg/L and Mn 68 mg/L. The vertical flow pond had dimensions of 48 x 20 m and was preceded by a wetland and followed by an oxidation/settling pond. The system produced net alkaline effluent for a few months, but by 1 year after construction, the effluent acidity was 120 mg/L, and flows decreased to 20-60 L/min. Plugging by Al precipitate was suspected, despite occasional flushing. In 2003, investigation showed partial plugging by cattail roots in the effluent pipe, but after removal of the roots, head loss across the VFP was less than 2 cm at flows of about 20 L/min, indicating that plugging was not severe. The VFP was drained and the compost and limestone (2.5 cm dia..) were excavated along several trenches. Compost, originally designed as 30 cm thick, varied markedly from 8 to 25 cm thick. In thick zones, basal compost was coated by black Fe sulfides. The limestone was coated nearly everywhere for at least 25 cm beneath the compost with gelatinous white Al-bearing precipitate mixed with quartz-rich silt. A layer of gypsum beneath the Al precipitate covered the limestone in places. An open joint in the underdrain may have somewhat concentrated flow. The limestone layer retained considerable pore space and permeability. In spite of the flushing and open joint, Al coating was approximately the same over underdrain pipes as elsewhere. Also, the Al coating was present nearly everywhere, under both thick and thin compost, indicating that channeling was minimal. Measurements in 1999 and 2003 show that the system was removing acidity at reasonable rates of 45 to 60 g/m 2 /day.

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