Summary In a laboratory study to determine permeability changes induced by floodinglarge Berea and Casper sandstone cores with NaCl and KCl brine, theconcentration of each brine was incrementally increased from 0 to 22 wt% andthen decreased from 22 to 0 wt%. In both sandstones, the permeability to KClbrines increased noticeably with increases in KCl concentration up to 1 0 wt%. Permeability remained at the higher levels throughout the remainder of eachflood until a critically low salinity level was reached, where dispersion ofthe interstitial fines took place. X-ray diffraction (XRD) analysis of theproduced formation fines and scanning electron micrographs of the sandstonepore surfaces indicated that the improved permeability was primarily a resultof alteration of illite in the clay minerals in the sandstones. TABLE 1-FORMATION MINERAL COMPOSITION (wt%)* Mineral Berea Casper Quartz 84 51 Clay minerals 10 17 Kaolinite 76 63 Illite20 34 Chlorite 4 3 Mixed layer Trace Trace 100 100 Dolomite 1 28 Calcite 1 1 Siderite 1 Trace K-Feldspars 3 3 100 100 Introduction To control formation pressure during workover operations, kill-weight fluidsare prepared by suspending insolubles in the fluid or by dissolving solublesalts in water to make a clear brine. Using suspended particles may lead toformation plugging, requiring additional acidizing of the well. Often the bestalternative to control formation pressures during workovers is a kill-weightfluid composed of a clear NaCl or KCl brine. Brine densities up to 9.8 lbm/gal[1174 kg/m] can be produced by NaCl or KCl brines. The composition ofcompletion and injection fluids rarely matches that of the formation water. Because of the difficulty and cost of duplicating the exact formation-watercomposition, a composition contrast often exists between the two fluids. Several authors have reported the effects of various completion brines onsandstone permeability. Their studies were limited to either singular brineconcentrations, a series of low brine concentrations (less than 6 wt%), orhigh-density calcium and zinc bromide brines. Coreflooding experimentsinvestigating the permeability change as NaCl and KCl brine concentrations varybetween 0 wt% and saturation have not been published. Early studies concernedprimarily the relationships between swelling and salinity. As the pore-fluidsalinity decreases, osmotic forces allow water layers to be added to theinterlayer region. The salinity level also affects the bound or crystallinewater on the clay surfaces. Baptist and Sweeney were the first to conclude thatthe absence of 2:1 montmorillonites does not guarantee that a sandstone isinsensitive to water. They reported permeability reductions up to 60% in"non-swelling" cores flooded with fresh water after saturation with 2%NaCl brine. Mungan demonstrated that dispersion of any clay is possible whenmonovalent cations are on the exchange sites. This classifies most sandstonescontaining any type of clay as water-sensitive. Khilar showed that forclay-bearing sandstones, a critical salt saturation (CSS) exists below whichclays start to disperse. Sharma et al. supported the CSS theory bydemonstrating that particle-release and particle-deposition regimes-functionsof the electrostatic potential between the clay and pore-wall surfaces and thefluid velocity exist. The electrostatic potential is controlled partly by theionic strength of the pore fluids. As the pore-fluid salinity declines, ionicstrength decreases to where the release regime dominates the depositionalregime and dispersion occurs. This point closely corresponds to the CSSdetermined by Khilar. Clay Mineral Influences on Permeability Formation permeability changes are often the result of the amount. location, and type of clay minerals in the formation. The amount of clay minerals in aformation can be a misleading indicator of potential permeability changes. Therelative abundance of specific clay types in the matrix and pore spaces must beknown in addition to the total amount of clays present. Moore demonstrated thata sand with less than 4 wt% clay minerals could have an appreciable amount ofits pore spaces (greater than 20%) filled with clay. Almon, showed that somepores may be entirely lined with authigenic clays so that the pore fluids donot contact the large quartz, feldspar, or carbonate grains. Numerous acid jobsand waterfloods have had disastrous results because the engineer or geologistfailed to recognize the importance of the location of certain clay minerals inthe formation. Grim showed the replacement order of one monovalent cation foranother to be Li+ less than Na+ less than K+ less than H+. For example, in porefluids with equal equivalent fractions of K+ and Na+, more K+ will occupy theexchange sites than Na+. Not all the Na+ will be removed. When the claylattices align in booklets, the exchange sites created in the opposing layersare about 2.8 A [0.28 nm] in diameter and 2.4 A [0.24 nm] deep. JPT P. 486^

Shell has used BFS cements on over 160 oil well cementing operations as part of an effort to develop end expand the understanding of blast furnace slag (BFS)-based cements. Well types include major deepwater development wells to sidetracks drilled with a workover rig. Downhole conditions range from cold deepwater applications to shallow thermal wells subjected to cyclic steam injection.To date, the field performance of slag-based cement has met or exceeded expectations. Formation integrity tests, bond logs, and production data indicate good annular isolation. Caaingshoe integrity test datafrom exploratory and development wells showa lower incidence of remedial squeezing prior to drilling ahead es compared to Portland cements. Contrary to previously reported bond-log evaluations, conducted under laboratory conditions, downhole bond logs under saturated conditions indicate an improved caeinglcement bond with time. Production data from wells with dual comoletirms indicate mod mnnl iw-dnt.innhot.wamperforated intervals. Leak-offtestdatafor slag-based cementjobs conducted in the Gulf of Mexico are reported for each casing size. Field data for squeeze cementing casing shoes, liner tops, and casing annuli are also presented.

This paper will demonstrate cased-hole applications of a new ultrasonic scanning tool. Field logs are presented from environments containing various wellbore, casing, and cement slurry types. Cement evaluation and casing inspection examples will include logs from wells with heavy completion fluids, lightweight cements, heavyweight casing, and hastelloy pipe (used in highly corrosive environments). Very high horizontal and vertical sampling rates permit simultaneous cement evaluation and casing inspection. The data are corrected for direction and tool rotation using a navigation package. Data such as acoustic impedance, cement compressive strength, casing thickness, casing outer diameter and/or radius, casing ovality, tool eccentricity, and 40 to 100 calipers may all be recorded in real time in conventional or imaging formats. Improved processing software provide two- and three-dimensional images. It was observed that differing wellbore and casing conditions require different logging parameters (such as scan rate and sample density). Thus, the mechanical and electrical configuration of the tools are flexible to maximize signal response in all possible downhole conditions. Multiple scanning heads with selectable transducer characteristics are available. Downhole processing also reduces telemetry demands and allows other tools to be combined with the new scanning device to provide additional real-time data without reducing logging speed or delaying analysis processing until after logging. Introduction Downhole ultrasonic scanners have been used primarily for imaging wellbores. A new scanner that furnishes improved images and that offers remarkable casing inspection and cement evaluation capabilities has been developed and field proven. The new device operates in two modes: image mode and cased-hole mode. In image mode, the scanner evaluates only the "inner" surface of the target (the formation bounding the wellbore or the inner wall of the casing). High vertical and azimuthal sampling provides high-resolution, two and three-dimensional images. These images are useful in locating fractures, identifying borehole breakout, delineating bed boundaries, studying formation textural features along the borehole wall, and evaluating casing integrity by revealing distortion, wear, holes, parting, and other anomalies on the inner wall of the casing. When cement evaluation or a more complete casing assessment is needed, the ultrasonic scanner operates in cased-hole mode. The full circumferential maps of casing thickness and acoustic impedance generated from the measurements made in this mode can be used to reveal thinned casing and to clearly distinguish between cement and fluids in the annular space behind casing. In cased-hole mode, a significant part of the acoustic waveform is processed. Thus, high telemetry data rates, intense processing capabilities, and selective transducer frequencies are required. Tool Operation The new scanner, designated as the Visualization version of Circumferential Acoustic Scanning Tool (CAST-V), uses two ultrasonic transducers: a primary transducer and a secondary transducer. The primary transducer is mounted in a rotating scanner head and is in direct contact with the wellbore fluid. The scanner head rotates continuously about the tool axis, transmitting ultrasonic signals and receiving reflections from the casing or formation. P. 79^

A laboratory study was performed to simulate completion fluid invasions into large Berea and Casper sandstone cores. Permeability was initially stabilized with a 4% NaCl reference brine, then flow direction was reversed through each core with sodium and potassium completion brines. After a stabilized permeability was obtained under invasion permeability was obtained under invasion conditions, the reference brine was flowed again through the cores to simulate production after completion. In Casper sandstone the permeability stabilized 5% higher than the originally established level for the KCl completion fluid, while it only increased 2% with NaCl brine. No increase in permeability was obtained in Berea cores with either KCl or NaCl completion fluids. Experiments were also performed to determine the effects of completion fluid and flow reversal on stabilizing the permeability of a formation with clay dispersion damage. Results of these studies indicate that for each sandstone a threshold KCl brine saturation exists (10% in Berea and 5% in Casper) above which the mite clay lattices collapse around the exchanged K+. Once a formation is exposed to a KCl brine saturation higher than the threshold saturation excessive clay dispersion and swelling damage of the rock is inhibited even after the formation is exposed to much lower salinity levels. Introduction Absolute permeability reduction of reservoir sand in the vicinity of a wellbore can be a result of pore blockage by formation fines or clay swelling and dispersion caused by the introduction of an incompatible non-native fluid. Because the composition duplication of the formation water is difficult and costly, it is rare that the fluids introduced during all stages of well development and workover operations match the composition of formation water. Consequently, an incompatible fluid can choke the formation permeability and impair productivity or injectivity. Previous core flooding experiments were Previous core flooding experiments were focused on the effect of singular brine concentrations a series of low brine concentrations or high density calcium and zinc bromide brines. Earlier, the authors presented permeability changes with brine presented permeability changes with brine concentrations ranging from zero to 22% by weight for NaCl and KCl. The effects of isomorphic substitution of cations on the exchange sites of clays and the tendency of clay lattices to attract potassium ions rather than sodium ions were discussed. In both Berea and Casper sandstones, after the initial Ca+ and Mg+ ions are substituted on the exchange sites of clays by Na, the permeability to NaCl brine was controlled by permeability to NaCl brine was controlled by osmotic forces and reversible clay swelling. As the NaCl brine concentration was reduced below the critical salt saturation clay dispersion and migration caused total loss of permeability in all the cores. This paper permeability in all the cores. This paper presents further laboratory experiments to presents further laboratory experiments to establish the advantages of KCl instead of NaCl as a completion fluid. EXPERIMENTAL APPARATUS Berea and Casper sandstones were selected for testing. Berea was used as a reference sandstone because of its homogeneity and low clay and carbonate contents. Casper sandstone in contrast has secondary dolomite grains intermixed with authigenic clays. P. 237