Well-Completion Audits To Evaluate Gravel-Packing Procedures
- H.O. McLeod Jr. (Conoco Inc.) | M.A. Pashen (Conoco Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1997
- Document Type
- Journal Paper
- 228 - 237
- 1997. Society of Petroleum Engineers
- 1.6 Drilling Operations, 4.1.2 Separation and Treating, 3.2.4 Acidising, 5.6.1 Open hole/cased hole log analysis, 5.2.1 Phase Behavior and PVT Measurements, 1.6.9 Coring, Fishing, 4.2.3 Materials and Corrosion, 2.7.1 Completion Fluids, 2 Well Completion, 5.6.4 Drillstem/Well Testing, 2.4.5 Gravel pack design & evaluation, 2.4.3 Sand/Solids Control, 2.2.3 Fluid Loss Control, 1.8 Formation Damage, 4.1.5 Processing Equipment, 1.10 Drilling Equipment, 1.7.5 Well Control, 2.2.2 Perforating
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Data collected during gravel-packed completions may be analyzed to evaluate the impact of specific completion practices on well performance. Operations in which gathering data is critical are during the initial flow and perforation cleanup after underbalanced perforating; during prescribed injection tests before gravel packing; during periods in which fluid is being injected into the formation (pre-packing perforations, fluid loss after perforating, fluid loss after gravel packing); and during stabilized flow after the well is brought on line. This data can be used to calculate an approximate value for kh (md·ft) and an instantaneous completion efficiency at different stages of the completion. By comparing these instantaneous completion efficiencies, the damage contribution of certain operations and the damage-prevention aspects of other operations may be quantified. This methodology should provide a valuable resource to those pursuing continuous improvement in well completion operations, particularly those involved in sand control.
Final production rate alone is not an adequate measure of the success of a gravel-packed well. Rather, we must estimate the potential of a reservoir and judge the ultimate success of a completion on how close we come to achieving this potential. A post-completion-audit process for completing this task is described in this paper. Also, this paper will detail the data-gathering process that was used to complete these audits, advancing the analytical techniques of McLeod and Minarovic1 through a more detailed quantitative review of available data. The use of the proposed post completion-audit methodology should help Conoco (and perhaps the industry) better evaluate the success of our completions and improve the effectiveness of future gravel-packed completions.
Gravel-packing techniques vary widely throughout Conoco and the oil industry. Although the basic completion steps are the same (perforate and gravel pack while maintaining well control), many variations exist in the way these practices are performed. Industry studies have compared various completion techniques and final well productivities to quantify damaging as well as improved completion operations. However, these studies often provide conflicting conclusions because of the multiple procedures and combinations of completion practices that are performed during a well completion, making it difficult to determine exactly which completion technique is most responsible for the ultimate well productivity.
In 1994, McLeod and Minarovic1 reported on a study of 19 gravel-packed gas wells in the Gulf of Mexico to identify critical completion operations that most affected well productivity. That study found that any operation that promoted gravel placement in the perforations (such as surge perforating, longer flow periods after perforating, and acidizing and/or prepacking before the final gravelpack) resulted in better gravel packs. It also found that any operation that damaged formation permeability (such as overbalanced perforating, washing perforations, and pumping fluid-loss pills) resulted in poor gravel placement and poor well productivity. Especially destructive to well performance was the placement of a fluid-loss pill after the gravel pack was in place. This final pill of lost circulation material (LCM, usually a thick, viscous pill of hydroxyethyl cellulose (HEC)] never cleaned up until post-gravel-pack acid treatments were performed. One exception was when the HEC pill was sheared and filtered before placement.
This information is supported by findings of other studies in the industry regarding the damaging effects of LCM pills.1-4 Every attempt should be made to prevent spotting of LCM pills after gravel packing. Mechanical isolation devices, maintaining low fluid levels in the casing, and living with fluid losses should all be considered before spotting LCM pills.
Recent papers on gravel packing and perforating provide information that helps explain the effects of certain operations and operating conditions on well productivity. Studies by Behrmann,5 Hovem et al.,6 and Welling et al.2 confirm the benefit of higher underbalance and flowing after perforating. Behrmann5 showed that underbalances higher than previously recommended are required to completely remove the crushed zone after perforating and provide a clean, nondamaged perforation. Hovem6 showed that more gravel could be packed into perforations when sufficient underbalance and flow rate for cleanup were used. Experimental data showed that when an insufficient flow rate was used after underbalanced perforating, a longer flow period was needed to achieve clean perforations. Surge perforating with an open choke to maximize the initial perforation cleanup was found to provide better perforation cleaning. The gravel-packed-well study by Welling et al.2 showed that better perforation cleaning improved well performance, and that gravel-injection rates of 0.1 bbl-min/ft were required for higher perforation packing. The authors' experience in detailed well-completion analysis confirms these findings.
Gravel-packed Oilwell-Completion Study
As a follow-up to the 1994 McLeod and Minarovic1 gravel-packed gas-well-completion study, we analyzed 20 Gulf of Mexico gravel-packed oil wells in an attempt to identify the best completion practices. For this study, productivity indexes and completion efficiencies were calculated for each well during the flowback period following perforating, and then after production were stabilized once the wellwas brought on line. Comparing these productivities provided an indication of the extent of damage caused by the entire well completion.
In addition, fluid-loss and injectivity data were gathered at various stages throughout the completion. Injectivity indexes were calculated using this data. A comparison of injectivity data throughout the completion allowed us to find a cause-and-effect relationship between specific completion practices and well injectivity. This analysis highlights specific completion operations that have the greatest impact on well productivity.
A gravel-packed completion consists of a series of connected events and procedures. These are perforating and cleanup, running the gravel-pack assembly, acidizing and/or prepacking, annular gravel packing, running completion tubulars and installing the well head, and final flow to cleanup. Randomly interspersed with these procedures are welI control events such as spotting LCM pills and various well-test operations such as pressure-buildup tests after perforating and flow or injectivity tests before prepacking or gravel packing.
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