Plug Setting Research Articles

Effect of surfactant composition on reservoir wettability and scale inhibitor squeeze lifetime in oil wet carbonate reservoir

Abstract In the oil industry, scale inhibitor (SI) squeeze treatment is an established method to prevent deposition of inorganic scales in the wellbore and near wellbore formation. Most scale inhibitor chemicals are water soluble and therefore typically deployed as an aqueous solution. However, most carbonate reservoirs being preferentially oil-wet, SI squeeze treatment could be short lived, because of the competition for adsorption surface between the native oil and the SI. Although several authors studied surfactant activities on reservoir rock and fluid properties in relation to enhanced oil recovery, the literature lacks information on the effect of surfactant based preflush treatment on rock characteristics in relation to SI squeeze lifetime. It is obvious that a proper design of preflush, based on a detailed understanding of rock–fluid properties, could be as important as designing the main treatment (SI) itself. The objective of this study was to develop an understanding of surfactant composition and pH in altering carbonate rock wettability, and its impact on scale inhibitor squeeze performance. A set of carbonate core plugs with oil wet characteristics, crude oil and treatment chemicals (anionic surfactant and SI) were collected from an offshore oilfield. Surfactant preflush compositions with and without alkalis were evaluated for their ability to minimize oil–brine IFT and the extent of rock wettability alteration. A series of coreflood studies were conducted with selected preflush compositions and SI, followed by injecting incompatible brines. SI squeeze lifetimes were determined through observation of core permeability damage. The scale inhibitor and strontium return profile analyzed through ICP-MS studies also validated the onset of permeability damage. The study clearly established a correlation between rock wettability and scale inhibitor lifecycle. Extended SI lifecycle is evidenced in the core with a strongly water wet core and vice versa. The importance of proper preflush design based on rock–fluid characteristics and the role of alkali in changing wettability characteristics, surfactant adsorption and inhibitor lifecycle are discussed with experimental evidence.

Drilling System With Built-In Kickoff Ramp Allows for Dependable Curve Building

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 163534, ’Innovative Drilling System With a Built-In Kickoff Ramp Allows Dependable Curve Building in Granite Wash Formation,’ by G.A. Bruton, SPE, and J. Talkington, Chesapeake, and P. Desai, S. Swadi, SPE, and J. Kelley, Schlumberger, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5-7 March. The paper has not been peer reviewed. The Granite Wash formation presents several challenges in drilling the build section from a vertical open hole to form a lateral. The formation is hard and abrasive, and it is difficult to anticipate downhole rock properties. The erratic nature of the formation results in unpredictable performance by the steerable drilling assembly and drill bits. In most cases, the target build rates are not achieved, forcing multiple trips. An innovative drilling system has been developed that provides a dependable kickoff ramp and a firm beginning for initiating the build section without having to rely on the fulcrum point of the drilling bottomhole assembly (BHA) in a single trip downhole. Challenges The complex Granite Wash formation (Fig. 1) presents many drilling challenges, particularly beyond 12,000 ft, specifically when building a curve to horizontal at 14°/100 ft. One of many formation-related challenges was to reduce the number of trips. The conventional sidetracking technique for the open hole has been to run and set a whipstock/anchor assembly at the kickoff point in a separate trip using a different BHA. Once the whipstock was secured, the directional-drilling assembly was run to initiate and finish the curve and reach the landing point. The extra trip, however, needed to be eliminated. The erratic nature of the formation also makes it difficult for the driller to use the offset to optimize bit selection to drill the highly abrasive formation efficiently. The driller usually selects a proven tricone bit dressed with abrasion-resistant cutters. Despite that, the bit’s cutting structure wears pre-maturely, forcing the operator to make multiple trips downhole with a new polycrystalline-diamond-compact (PDC) bit to finish the curve. Solution: Innovative Drilling System The most prevalent sidetracking technique in an openhole application has been to set a cement plug and time drill with controlled rate of penetration (ROP) with a directional-drilling assembly. The cement-plug method generally required 3 to 5 days to get a plug set and dressed off in an oil-based mud. The cement-plug method was replaced by a conventional but reliable whipstock/anchor system, which required a separate trip.

Optimal coordination of directional over-current relays using Teaching Learning-Based Optimization (TLBO) algorithm

Relay Coordination in a big distribution system with multiple meshes and bidirectional power feed becomes very Complex for protection engineers. Manual and graph theory based approaches were applied successfully in small power system. In a big distribution system linear and non-linear programming based optimizing techniques are applied for relay coordination. Presently, artificial intelligence techniques are applied for optimal co-ordination of directional overcurrent relays. This paper discusses the application of Teaching Learning Based Optimization (TLBO) algorithm for optimal coordination of DOCR relays in a looped power system. Combination of primary and backup relay is chosen by using Far vector of LINKNET structure, to avoid mis-coordination of relays. Coordination of DOCR is tested for IEEE 3, 4 and 6 bus systems using the TLBO. Also, the objective function is modified to optimize the operating time between backup and primary relays. The results are compared with the optimized values of Time dial setting and Plug setting values obtained from modified differential evolution algorithm. The proposed algorithm TLBO gives optimal coordination margin between 0.3 and 0.8s and no miscoordination between primary and backup pairs. Results are also verified using Digsilient power factory simulation software.

Applying Hybrid PSO to Optimize Directional Overcurrent Relay Coordination in Variable Network Topologies

In power systems, determining the values of time dial setting (TDS) and the plug setting (PS) for directional overcurrent relays (DOCRs) is an extremely constrained optimization problem that has been previously described and solved as a nonlinear programming problem. Optimization coordination problems of near-end faults and far-end faults occurring simultaneously in circuits with various topologies, including fixed and variable network topologies, are considered in this study. The aim of this study was to apply the Nelder-Mead (NM) simplex search method and particle swarm optimization (PSO) to solve this optimization problem. The proposed NM-PSO method has the advantage of NM algorithm, with a quicker movement toward optimal solution, as well as the advantage of PSO algorithm in the ability to obtain globally optimal solution. Neither a conventional PSO nor the proposed NM-PSO method is capable of dealing with constrained optimization problems. Therefore, we use the gradient-based repair method embedded in a conventional PSO and the proposed NM-PSO. This study used an IEEE 8-bus test system as a case study to compare the convergence performance of the proposed NM-PSO method and a conventional PSO approach. The results demonstrate that a robust and optimal solution can be obtained efficiently by implementing the proposal.

Optimal coordination of electro-mechanical-based overcurrent relays using artificial bee colony algorithm

Setting of relay coordination in a looped power network is highly non-linear bounded, constrained optimisation problem. Complexity of problem increases as the size of system increases. Recently artificial intelligence-based optimisation techniques are applied successfully for protection application in power system. Traditionally protection relays are electro-mechanically with discrete time dial and plug setting. Optimisation of relay coordination problem considering the discrete relay parameters needs the optimisation algorithm which can handle the discrete value of decision variables. In this paper, application of artificial bee colony algorithm is extended for discrete optimal coordination of overcurrent relays. Moreover, relay coordination problem is modified to optimise the relay setting for both near and far end faults.

Time-lapse 2D interpretation of gas migration in shallow sand layers – Compared to reservoir simulation

The work presented focuses on an area of the North Sea influenced by an underground blowout from the Saga Petroleum well 2/4-14. When hitting an Upper Jurassic sandstone, the well kicked and after several attempts to gain control, the drill pipe was cut and a cement plug set. 9 seismic site surveys were acquired over a period of 17months in order to monitor the underground flow of hydrocarbons. Comparing observations from the acquired seismic data to reservoir simulations and a later 3D seismic survey over the area, it is found that hydrocarbons have charged sand layers close to the wellbore. The reservoir simulations support the findings from time-lapse 2D seismic data analysis and substantiate the argument that time-lapse 2D seismic is a useful tool for monitoring shallow gas leakage.

Advanced characterisation of tight unconventional rocks

Tight unconventional rocks have become an increasingly common target for hydrocarbon production. Exploiting these resources requires a comprehensive reservoir description and characterisation program to accurately estimate reserves and identify properties that control production. In particular, this requires mapping the porosity at multiple scales and understanding the coupled contributions of fractures, variable pore types, microporosity and mineral heterogeneity to flow, petrophysical response, reserves assessment and recovery processes. This extended abstract describes a formation characterisation study based on the integrated analysis of data in 2D and 3D at multiple scales on sets of plugs from three unconventional tight gas plays: coal, tight gas and shale. Heterogeneity and geological rock typing is considered at the core scale by classical 3D imaging techniques. Mineralogy and secondary microporosity characterisation is mapped at the plug scale with different modes of 3D X-ray micro-CT analysis coupled with SEM and SEM-EDS analysis. In particular, the pore connectivity and production potential is probed. SEM and FIBSEM imaging is then used to reveal the porous microstructure of the key phases at the nano-scale in 2D and 3D. This information, collected at multiple scales, is integrated to provide an understanding and quantification of the pore structure and connectivity of these complex rocks. Petrophysical properties that impact the storage capacity and production characteristics are then computed for each key phase and data up-scaled to the plug scale using standard procedures. Results compare favourably with core analysis data, where available. The presentation slides for this speech have not been made available to APPEA at this stage.

Solvent Induced Oil Viscosity Reduction and Its Effect on Waterflood Recovery Efficiency

WAG process is one of the techniques used for reducing gas consumption, enhancing recovery factor and achieving better profile control of displacing fluids. Recovery efficiency due to reduction of oil viscosity, simulating a WAG process, in a wide range of reservoir permeability and water injection rate was investigated. Gas viscosity reduction by miscible gas or solvent injection is mimicked by progressive dilution of a medium density crude oil with a mixture of hydrocarbon solvent. The porous media used in this study consists of a set of water wet sandstone core plugs of low to medium permeability. The experimental findings show that reduced oil viscosity has no correlation with recovery efficiency, in the normal flood velocity regime. However, in the higher flood velocity regime, recovery efficiency reduces with increasing oil viscosity, only for higher permeability cores, which is attributed to micro-heterogeneity within pore geometry. The study suggests that the additional oil recovery during miscible gas injection, is mainly contributed by the swelling factor of oil which results in increased oil saturation, higher reservoir pressure and increased relative permeability of oil in addition to the contribution from lower interfacial tension and very little, if any due to oil viscosity reduction. Key words: WAG process; Recovery efficiency; Oil viscosity

English

    Directional overcurrent relays are used to protect interconnected power systems and looped distribution systems. The problem is modeled as a constrained nonlinear optimization problem in which the decision variables are the devices that control the act of isolation of faulty lines from the system without disturbing the healthy lines. Time dial setting (TDS) and plug setting (PS) are considered as optimal parameters. IEEE 3-bus model and IEEE 4-bus model are two models of this problem. This paper presents a new efficient and reliable approach based on a constrained harmony search algorithm (HSA) to solve the problems.  The authors’ proposed technique is called Box-Muller harmony search (BMHS) algorithm. BMHS has been tested by applying it to two models. The simulation result of this technique is compared with those reported in the literature. The outcome is very encouraging and proves that the new technique outperforms them in terms of reaching a more optimal solution and speed.   Key words: BMHS, Box–Muller transform, coordination, directional overcurrent relay, HSA, power system. &nbsp

Optimum Coordination of Directional Overcurrent Relays Using the Hybrid GA-NLP Approach

The time of operation of overcurrent relays (OCRs) can be reduced, and at the same time, the coordination can be maintained, by selecting the optimum values of time multiplier setting (TMS) and plug setting (PS) of OCRs. This paper presents hybrid genetic algorithm (GA) - nonlinear programming (NLP) approach for determination of optimum values of TMS and PS of OCRs. GA has a drawback of, sometimes, converging to the values which may not be optimum, and NLP methods have a drawback of converging to local optimum values, if the initial choice is nearer to local optimum. This paper proposes a hybrid method to overcome the drawback of GA and NLP method, and determine the optimum settings of OCRs. The main contributions of this paper are - 1) systematic method for formulation of problem of determining optimum values of TMS and PS of OCRs in power distribution network as a constrained nonlinear optimization problem, 2) determining initial values of TMS and PS using GA technique and finding final (global optimum) values using NLP method, thus making use of the advantages of both methods (and at the same time overcoming the drawbacks of the methods).

Optimal coordination of over-current relays using modified differential evolution algorithms

Optimization of directional over-current relay (DOCR) settings is an important problem in electrical engineering. The optimization model of the problem turns out to be non-linear and highly constrained in which two settings namely time dial setting ( TDS) and plug setting ( PS) of each relay are considered as decision variables; the sum of the operating times of all the primary relays, which are expected to operate in order to clear the faults of their corresponding zones, is considered as an objective function. In the present study, three models are considered namely IEEE 3-bus model, IEEE 4-bus model and IEEE 6-bus model. To solve the problem, we have applied five newly developed versions of differential evolution (DE) called modified DE versions (MDE1, MDE2, MDE3, MDE4, and MDE5). The results are compared with the classical DE algorithm and with five more algorithms available in the literature; the numerical results show that the modified DE algorithms outperforms or perform at par with the other algorithms.

Abstract A137: Chemotherapy molecule delivered by tumor-targeted salmonella enterica serovar typhimurium under the control of integrated modular gate

Abstract Several strictly or facultative anaerobic bacteria have attracted more and more attention in cancer therapeutics because of their targeting and accumulating abilities in solid tumors, and Salmonella enterica serovar Typhimurium is one of them. These bacteria were also adapted to deliver therapeutic factors and were applied alone or in combination with conventional therapeutics. The aim of this study is to test a new integrated gene expression system in S. Typhimurim SL1344, and the benefit of the new system in amplifying therapeutic molecule production during bacterium targeting and accumulating in solid tumor. We have constructed a synthetic genetic gate in S. Typhimurim SL1344 that integrates information from two promoters as inputs and actives a promoter output only when both input promoters are induced. The integration occurs via an interaction between L-arabinose concentration and bacterium cell density. When certain amount of L-arabinose were added in the system and bacterium population were higher enough, the tumor necrosis factor apoptosis inducing ligand(TRAIL) and fluorescent protein ZsGreen as the control were produced by bacteria. LS174T colon carcinoma cells were processed to form cylindroids between the bottom surface of a 96-well plate and the top surface of a set of polycarbonate cylindrical plugs attached to a polycarbonate lid to mimic solid tumor, and then cylindroid cultures were inoculated with S. typhimurium SL1344 carrying TRAIL or ZsGreen expression system. 0.2% (w/v) L-arabinose was added into the well after 12hr inoculation, and then washed out after 2hr induced. Extents of apoptosis in cylindroids were tested by caspase-3 activity assay. Results showed that the bacteria accumulated inside cylindroids indicated by ZsGreen expression, and the average normalized intensity of apoptotic cell was higher in the cylindroids inoculated by the bacteria carrying the TRAIL than those inoculated by the bacteria carrying the ZsGreen. It can be concluded that the constructed integrated modular gate could effectively control bacteria to produce functional chemotherapy molecule in solid tumor at desirable time and places. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A137.

Evaluation of the Potential for Gas and CO2 Leakage Along Wellbores

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 106817, "Evaluation of the Potential for Gas and CO2 Leakage Along Wellbores," by Theresa L. Watson, SPE, T.L. Watson & Assocs. Inc., and Stefan Bachu, Alberta Energy and Utilities Board, prepared for the 2007 SPE E&P Environmental and Safety Conference, Galveston, Texas, 5–7 March. Implementation of carbon dioxide (CO2) storage in geological media requires proper assessment of the risk of CO2 leakage from storage sites. Leakage pathways may exist through and along wellbores that penetrate or are near the storage site. One method of assessing the potential for CO2 leak-age is by mining databases that usually reside with regulatory agencies. Introduction CO2-injection schemes have been in operation since as early as the 1970s for tertiary oil recovery as miscible floods, with the indirect benefit of CO2 removal from the atmosphere. Other gas-injection schemes also are used in the oil and gas industry, such as natural-gas storage and acid-gas disposal. In the case of CO2 sequestration, the storage unit must be almost leak free to the atmosphere or other geological formations to meet safety requirements and greenhouse-gas-reduction objectives. The full-length paper focuses on human-created leak-age paths—in particular, abandoned wellbores that were drilled for oil and gas exploration and production. The analysis is based on data collected by the Alberta Energy and Utilities Board (EUB) for more than 315,000 wells drilled until the end of 2004 in the province of Alberta, Canada. EUB also records well leakage at the surface as surface-casing vent flow (SCVF) through wellbore annuli and gas migration (GM) outside casing. Background Abandonment Methods. Wells Drilled and Abandoned. For a typical openhole abandonment scenario in Alberta, regulations require that any porous zone be isolated or covered to prevent cross-flow between geological formations. In addition, useable groundwater must be covered with cement and isolated from potential hydrocarbon-bearing zones. After the downhole cement plugs have been set, the well must remain open for inspection for a minimum of 5 days. After this time, the well is checked for static fluid level or other indications of plug leakage before the casing can be cut and capped below grade level. Wells Drilled, Cased, Completed, and Abandoned. There are three main types of zonal isolation and abandonment for a typical cased-hole well after reservoir depletion.Bridge plug capped with cement above perforations.Retainer and cement squeezed into perforations.Cement plug set across perforations. Regulations since 2003 require zonal isolation behind casing and that useable groundwater be protected. In many cases, older wells were constructed with low annular cement tops, allowing many zones to be in communication behind casing. Under the current regulations, a cement squeeze would be required to achieve isolation before final abandonment. Wellbores must be abandoned with inhibited fluid inside the casing and be pressure tested to a minimum of 7000 kPa. Before cutting and capping the production and surface casing, the well must be checked for SCVF and

Reply to Review of Neither Brain nor Ghost

Reply to Review of Neither Brain nor Ghost Teed Rockwell Professor McCarthy1 has carefully read many passages in my book Neither Brain nor Ghost (NBG), and clearly understands many of my main points. Unfortunately, there are other passages throughout the book that she seems to have missed, which contradict many of her criticisms. However, I have heard many of these criticisms before, so I probably have not been clear enough on these points. I welcome the opportunity to make further clarifications. I could respond to her objection that no one believes that the brain possesses intrinsic causal powers by referring to the quotes from Jerry Fodor and Colin McGinn on pages 59 and 60. But my main point was that these kinds of concepts are the only way to make sense out of metaphors like Dennett's claim that "the Head is Headquarters" or Paul Churchland's description of the brain as "the seat of the soul." There are a variety of causal factors that are responsible for the experiences we have. These include brain activity, other kinds of body activity, light bouncing of objects, and the objects themselves. Why is it that we assume that the activities within the brain "embody" experience, while these other factors only "cause" the experience? Why is it that only causal activity within the skull is given this special honorific? The answer seems obvious until you ask the question. Once the question is asked, the only answers available are the sort that McCarthy says no one would accept: that the brain is autonomous, or a closed system, or has intrinsic causal powers which make it fully responsible for the production of mental states. There are quotes, in my book and elsewhere, which show that many philosophers and scientists do accept these answers. But the important point is that no other available answer will support the claim that the mind is identical to the brain, regardless of who acknowledges this fact. To some degree my book is a challenge to those who defend mind/brain identity theory. "If you don't use these criteria for determining mind/brain identity, what criteria should we use?" My claim is that these kinds of criteria, weak as they are, remain the only reasons we have for believing that the mind is identical to the brain. McCarthy frets about my "insouciance about the intricate processes in the brain that are necessarily involved in the generation of the conscious experience of any organism." I felt no need to add my voice to the chorus of wonder about the brain's intricacies, but that was not meant to imply disagreement. I felt a greater need to deal with those marginalized biological facts about consciousness and cognition [End Page 87] which are not explainable solely by talking about the brain, because most people don't know about them, and because I believe they are philosophically important. (They appear mostly in chapters 2, 3 and 10.) More importantly, as I explain on page 30, the fact that these brain processes are necessary for conscious experience does not mean that they are identical to conscious experience. Mind-Brain Identity requires that those brain processes be both necessary and sufficient. It is necessary for your car to have spark plugs in order for you to drive it. But that doesn't mean the spark plugs are identical to the car, because owning a set of spark plugs is not sufficient to enable you to drive to work. So the question remains: Why is brain activity identical to consciousness, and not just necessary for it? At what point does the intricacy become both necessary and sufficient, and why should we automatically assume that all of this intricacy will take place in the skull? When most people get to this stage in the argument, they usually reply, as McCarthy did, that, even if we can't answer that question, we still need to make a distinction between Mind and World for pragmatic reasons. She, Dewey and I all agree that there is value in what McCarthy calls the "analytic distinctions of self and world." That is why I say this on page 104 of NBG: "To...

Machine Vision Techniques for Measuring the Canopy of Tomato Seedling

The canopy area of tomato seedlings is an important indicator of their quality. A machine vision system was designed for automated measurement. In a well-controlled lighting environment, it is feasible to use constant threshold values as a first step in partitioning images of tomato seedlings into the meaningful parts: canopy, root-growth medium, and a background. The selection of threshold values, however, should be based on an analysis of a training set of similar tomato seedling plugs. An image processing procedure was developed and evaluated to segment seedling canopy from background and root-growth medium parts of the seedling images. The segmentation process adopted an adaptive thresholding technique, the Otsu method, and extended it for an efficient multimode operation. The multimode segmentation operation accurately and quickly identified the canopy portion of tomato seedlings.

Microcontroller based overcurrent relay and directional overcurrent relay with ground fault protection

This paper presents the design and construction of overcurrent and directional overcurrent relays with ground fault protection for the protection of three-phase subtransmission and distribution systems, using a 16-bit microprocessor, the Intel 8096BH. The relay obtains the system currents at the rate of 12 samples per cycle and estimates the fundamental-frequency components of the current signals using discrete Fourier transform techniques. In the case of the directional overcurrent relay, the direction of the current flow is identified to determine whether the fault current is flowing into its protected zone. For this purpose, several internally stored voltage vectors, corresponding to the different directional element settings, are synchronized accurately with the system voltage and used to determine the direction of the power flow. Facilities to change relay characteristics, the time dial and plug settings are provided. The user can also set the relay as an instantaneous overcurrent relay. The desired operating characteristics are achieved by direct curve data storage in the memory.

Grammars of transgression: Golems, cyborgs, and mutants

Yod, we're all unnatural now. I have retinal implants. I have a plug set into my skull to interface with a computer. I read time by a corneal implant. Malkah has a subcutaneous unit that monitors and corrects blood pressure, and half her teeth are regrown. Her eyes have been rebuilt twice. Avram has an artificial heart and Gadi a kidney.... I couldn't begin to survive without my personal [computer] base: I wouldn't know who I was.... We're all cyborgs, Yod. You're just a purer form of what we're all tending toward. -Marge Piercy, He, She and It (150)

The tolerability of multiple oral doses of Pulsincap™ capsules in healthy volunteers

Twelve healthy subjects entered a double-blind placebo-controlled parallel group study to evaluate the tolerability of a 28 day course of twice daily dosage with Pulsincap TM capsules having the plug set for separation after 6 h. Eight subjects received Pulsincap TM capsules and four subjects received a matching placebo capsule. There was no active medication in any of the study capsules. Daily assessments were made of adverse events. Laboratory safety data (bloods and urine) and faecal occult blood were monitored weekly. Subjects returned for a post-study medical check during the fortnight following their final treatment session. All subjects completed their dosing schedule satisfactorily. Eleven adverse events were reported by six of the eight subjects in the Pulsincap TM capsule group and 14 adverse events were reported by the four subjects taking placebo. None of the adverse events in either group was considered to be causally related to treatment. Four males, but no females, had at least one positive test for faecal occult blood, two in the Pulsincap TM capsule group and two in the placebo group. In the single subject for whom the test was positive repeatedly, the occult blood was associated in time with an intercurrent dental problem and its treatment. Pulsincap TM capsules were well tolerated by the eight subjects who received them and there was no evidence of adverse events being causally related to this formulation. The results of this tolerability study support the further development of the Pulsincap TM formulation for delivery of active drugs.

CIPHERTEXT-ONLY CRYPTANALYSIS OF ENIGMA

Enigma messages can be solved by recovering the message key settings, the ring settings, and the plug settings individually. Recovery of the message key setting is sensitive enough to distinguish the correct rotor order. The method is demonstrated on a 647-letter message, and its performance is estimated for different message lengths and numbers of plugs used.

Coil Tubing Plug Retrieval/Fishing Operations On Deep, High Pressure Sour Wells

Abstract This paper discusses coil tubing operations performed on three sour gas wells in which coil tubing was used to retrieve plugs where conventional wireline methods proved ineffective. All three wells are located in the Obed Field of Alberta, which produces high pressure sour gas. The field was suspended in March 1992 with wireline plugs set to isolate the down-hole formation. All of the wells had severe corrosion problems prior to suspension, making plug setting and retrieval difficult. Coil tubing offered the versatility of jetting and circulating, allowing access to the down-hole plugs in a sloughing scale environment. On the first two reactivations, coil tubing was used for descaling tubing and washing onto plugs. During the third reactivation, the coil tubing was parted, leaving 3,000 m of 31.8 mm coil tubing (CT) down-hole. The 31.8 mm CT was successfully retrieved using 50.8 mm CT in conjunction with wireline chemical cutters. Introduction The Obed Field is located near Hinton, Alberta. The field was purchased and reactivated by Chevron Canada Resources in August 1992. The field is comprised of five producing wells, three of which are completed in the Nisku formation and two in the Leduc formation. The wells are approximately 4,000 m deep and produce gas with 26–33% H2S and 6–9% CO2. Bottom-hole static pressures for the wells range from 35 to 38 MPa. The tubing in all of the wells is 114.3 mm, L-80 VAM ACE with 88.9 mm VAM ACE corrosion resistant alloy (lncoloy/Hasteloy) tailpipe. Figures 1, 2, and 3 are schematics of the wells. All of the wells have a history of severe scale and corrosion problems. Prior to being suspended in March 1992, the wells had been on a Diaryl Disulfide (DADS) batch treatment program for sulphur deposition prevention. In August 1992, attempts were made to retrieve the down-hole wireline set plugs from three wells. All three plugs were leaking and could not be pulled by conventional wireline methods due to scale buildup. Some sulphur buildup was encountered down to 1,000 m, with a mix of carbonate and iron sulphide scale existing from 2,000 m to bottom. Options Available to Retrieve Plugs When Chevron reactivated the Obed Field, it was assumed that the plugs could be retrieved using conventional wire line methods. Normally, plug retrieval using wireline is quite effective in vertical wells wiLh up to 60 ° deviation. The following options were available for scale control, plug retrieval and fishing operations:Wireline prong/plug retrieval.Wireline bailing for scale removal.Wireline fishing.Coil tubing jetting/acid washing for descaling tubing.Coil tubing used for washing over fish.Coil tubing used to clean out sloughed scale and pull prongs/plugs. Initially, pulling the prongs and plugs with wireline techniques was selected, since it was the most economical and, historically, had been successful in the Obed field. Initial runs were unsuccessful due to sloughed scale which made it impossible to reach the prongs/plugs. Consequently, an attempt was made to bail out the fill, but this also proved to be ineffective.