Historically, hydraulic fracturing has been principally applied to low permeability formations as a means of stimulating a well's production performance. In recent years, hydraulic fracturing for formation control as well as enhanced productivity in high permeability unconsolidated formations (i.e., frac-packing) has gained broader acceptance. This paper reviews frac-pack results in the Mahogany Field, a large gas-condensate field producing from unconsolidated high permeability sands. Even though frac-packing was very successful in these applications, numerous other completion techniques such as high rate water packs and conventional gravel packing are available and still being utilized throughout the world. For a field development engineer, few guidelines are available which aid in optimizing completion decisions. Field examples from Mahogany Field in offshore Trinidad are used to demonstrate a successful completion design selection procedure. The benefits and risks of gravel packs, high rate water packs, and frac-packing in the completion of high deliverability wells in these unconsolidated formations will be reviewed. Results of rock mechanics, fines migration, and embedment testing will be presented and coupled with a production optimization study to develop completion guidelines applied in Mahogany Field. Finally, this paper will show a direct comparison of a gravel pack completion (80 mmcfpd) and a frac-pack completion (160 mmcfpd) in offset wells from the same productive horizon. Current production performance and long term recovery predictions will be reviewed. This paper will make the following technical contributions:Present the results of an extensive laboratory study of rock mechanics, fines migration, and embedment testing, and show the value of such testing in making completion decisions.Develop and present guidelines for determining the optimal completion practices in high permeability unconsolidated formations.Show a direct comparison between a conventional cased-hole gravel pack completion and a frac-pack completion.Present details of a successful frac-pack completion in a formation with severe fines production problems. Introduction Historically, gravel packing was the principal means of formation control in unconsolidated formations. Efficient gravel pack formation control must take into account a number of formation and reservoir characteristics, gravel size, and well performance objectives. Numerous authors1–3 ave suggested gravel sizing criteria ranging from 4 to 10 times the 10 percent coarse point on a cumulative sieve analysis for formation control. The use of these criteria resulted in numerous gravel pack failures especially in U.S. gulf coast sands. As a result of the failures, additional investigations were conducted and new criteria were developed, with the most widely accepted criteria for gravel sizing proposed by Saucier4. This criteria recommends a gravel size six times the mean particle size in unconsolidated reservoirs. This sizing criteria has been utilized for many years to control formations, however, some production impairment due to fines migration and pack plugging has been noted. In recent years, studies have shown the importance of sorting5 on formation control and that larger gravels6–8 can effectively control some formations without production impairment. These studies, however, were limited to specific formations and did not consider the effects of stress on fines generation, migration, and gravel pack plugging. They also did not consider the implications of frac-packing on gravel size criteria and formation control.
Openhole gravel packing (OHGP) is emerging as one of the primary completion techniques in horizontal wells with sanding tendencies. As the experience level for this technique increases, OHGP applications are extending to more challenging environments, including longer intervals. Because many of these jobs are performed in offshore and deep water environments where the cost of failure is substantial, a thorough knowledge of all aspects of treatment design, execution, and evaluation is required. While many OHGPs have been performed, the current understanding of both treatment success and failure is limited due to either a lack of sufficient data (such as bottomhole pressure and temperature) or experience in its evaluation. This paper introduces data analysis techniques that can be used for a more detailed insight into a variety of downhole events, including packing mechanisms, fluid changes, and system frictions. The pressure analysis technique presented isolates sections of the flow path and identifies these events within the system for accurate treatment analysis. The combination of this technique and volumetric and material balance calculations ensures a detailed quantitative approach to gravel pack data analysis. The background theory and its application are discussed through a detailed case study of a well in the Poinsettia field, offshore Trinidad, in which downhole gauge data was used to identify and quantitatively analyze the final pack integrity. The analysis technique was able to confirm complete coverage of the screens as well as the packing mechanism by which the gravel was placed. This approach to gravel pack evaluation maximizes the value of downhole memory gauge data, yielding an improvement in the knowledge of downhole processes and facilitating the continuous improvement of OHGP job design and execution.
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