This paper describes workover and completion operations offshore Trinidad & Tobago and the Gulf of Mexico in which the operator desired non-mechanical isolation of the reservoir section above the sand-control assembly. Each well required clear brine of kill-weight density and in the case of the five dry gas wells described, losses of significant volume of brine to the formation was unacceptable. Isolation of the reservoir was required for up to 18 days. The following objectives were specified:Reduce fluid losses to the formation from inside the sand control screen,Minimize the potential for formation or screen damage, andEliminate post-job, clean-up treatment of any kind. This paper describes the development of specially formulated fluid-loss control pills and their subsequent use in the field to provide reservoir isolation by placing a temporary filter cake on the inside surface of the sand control screen. The paper describes laboratory tests designed to optimize the pill formulation to seal the inside surface of each specific screen, minimize spurt loss, maintain rheological properties for an extended period at reservoir condition and, importantly, displace off the screen with gas production. In each of the cases described, field spotting procedures were designed to eliminate killing the wells by bullheading, thus significantly reducing the volume of fluid lost to the reservoir. The authors provide details of the properties of the pill and the field procedures used to place the pills in the wells. All wells were returned to production without any remedial treatment whatsoever or impact on productivity. Additionally, the savings in time, costs and lessons learned associated with these projects and recommendations for similar future projects are discussed. Introduction An operator working offshore Trinidad & Tobago planned to replace the production tubing in five dry gas producers. These wells were completed as deviated 8.5-in. open hole in which a 7-in. pre-perforated liner was run. Two different types of sand screens were run inside the 7-in. liner and gravel packed with 40/60 gravel (US mesh). Increased production targets initiated the workovers in which the combination 5.5 x 4.5-in. carbon steel production tubing on five of the eight platform wells would be replaced with a combination of 7 x 5.5-in. 13-Chrome production tubing. This task also included the setting of packers above the current production packer in the wellbore. Each re-completion had a planned duration of 18 days. For several reasons, including the expected increased rate of dry gas production, it was very desirable to limit losses to the formation and to isolate the sand control section during the workover, however, only chemical means of fluid-loss control were practical. The operator was concerned about losing large amounts of completion brine to the depleted sands and even more concerned about plugging the completion with immobile solids since a remedial treatment would put the operation well past schedule for returning the wells to production and put the project over budget. To achieve these objectives, the operator, its operating partner, and the completion fluid service company considered whether it was possible to formulate a fluid-loss-control pill (FLCP) that would temporarily seal the inside surface of the premium screens with minimal matrix invasion, hold kill-weight pressure for the duration of the operation and not require any remedial treatment.
Mud-to-brine displacement efficiency can be difficult to quantify. Metrics to determine the success or failure of the displacement operation have traditionally been based upon the clarity of the completion brine that follows the spacers, the interface volumes of mud and completion brine, a visual determination of the cleanliness of the drillpipe pulled from the hole, and the time and materials required to filter the fluid. Despite their widespread use, however, these factors may not adequately characterize the quality of displacements in critical wells. Opportunities for improvement are largely affected by how well the displacement process can be understood. This can be difficult with the few direct and indirect parameters used to evaluate mud-to-brine displacement quality. The authors initiated a study of samples captured during displacement circulations to examine the effects of spacer properties, volumes, pump rate, and chemical concentration on mud removal. The laboratory analyses inspected return samples of mud, spacers, and displacing fluids from 27 cased-hole wellbore displacements of oil-based and synthetic-based drilling mud in the Gulf of Mexico. Results from this study suggest that criteria such as annular coverage, contact time, and annular velocity of the wash spacer(s), which have become the basis for most displacement programs, although critical to overall efficiency, are not the primary agents of mud removal during a wellbore cleanup. Data correlations between the volume of mud observed in the samples analyzed and certain spacer characteristics suggest that cased-hole cleaning, when defined as mud removal, is a function of the size of a viscous spacer relative to hole size. This observation should simplify the displacement design process. Introduction The mud-to-completion brine displacement is an important operational component for most completions. A poor displacement can cause production impairment, problems with a gravel pack, loss of mud or brine, and loss of rig time, among other complications. However, unless these problems are encountered and identified, it is often difficult to determine the relative quality of a specific displacement. This is critical for avoiding problems on subsequent wells and for optimizing displacement operations. The most common quality indicators include:the clarity of the brine measured in either Nephelometric Turbidity Units (NTU) or total suspended solids (TSS) immediately following the spacer circulationthe number of filtration cycles (or quantity of filter media) required to achieve a target NTU levelthe total volume of mud/brine interface created during the displacementthe presence or lack of drilling fluid on the drillpipe when it is pulled from the hole either on the short trip or after the displacement is completed While these factors, either individually or collectively, are of significant value as overall indicators of efficiency, they lack the resolution necessary to adequately characterize the process, discriminate among different design techniques and to affect continuous improvement. The current study combines laboratory analysis with field data to examine displacement technique. The purpose of the undertaking was to answer questions about claims to successful displacement design. Several dozen well displacements were analyzed for this study. Twenty-seven of them are described in this paper. From the fluid analyses and data gathered from field reports, the authors hoped to determine whether displacement fluids returning from the wellbore would reveal quantitative data about the relative success of removing mud from the hole.
This paper was prepared for presentation at the 1999 SPE Mid-Continent Operations Symposium held in Oklahoma City, Oklahoma, 28-31 March 1999.
fax 01-972-952-9435. AbstractMud-to-brine displacement efficiency can be difficult to quantify. Metrics to determine the success or failure of the displacement operation have traditionally been based upon the clarity of the completion brine that follows the spacers, the interface volumes of mud and completion brine, a visual determination of the cleanliness of the drillpipe pulled from the hole, and the time and materials required to filter the fluid. Despite their widespread use, however, these factors may not adequately characterize the quality of displacements in critical wells.Opportunities for improvement are largely affected by how well the displacement process can be understood. This can be difficult with the few direct and indirect parameters used to evaluate mud-to-brine displacement quality.The authors initiated a study of samples captured during displacement circulations to examine the effects of spacer properties, volumes, pump rate, and chemical concentration on mud removal. The laboratory analyses inspected return samples of mud, spacers, and displacing fluids from 27 cased-hole wellbore displacements of oil-based and synthetic-based drilling mud in the Gulf of Mexico.Results from this study suggest that criteria such as annular coverage, contact time, and annular velocity of the wash spacer(s), which have become the basis for most displacement programs, although critical to overall efficiency, are not the primary agents of mud removal during a wellbore cleanup.Data correlations between the volume of mud observed in the samples analyzed and certain spacer characteristics suggest that cased-hole cleaning, when defined as mud removal, is a function of the size of a viscous spacer relative to hole size.This observation should simplify the displacement design process.
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