Development of the Alba Field-Evolution of Completion Practices, Part 1: Openhole Screen-Only Completions to Gravel Pack
- G. Murray (ChevronTexaco) | J. Brookley (ChevronTexaco) | S. Ali (ChevronTexaco) | E. Davidson (Halliburton Energy Services Inc. Baroid) | N. McMillan (Halliburton Energy Services Inc. Baroid) | J. Roberts (Halliburton Energy Services Inc. Baroid)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2003
- Document Type
- Journal Paper
- 304 - 310
- 2003. Society of Petroleum Engineers
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 2.4.3 Sand/Solids Control, 2 Well Completion, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant), 2.4.5 Gravel pack design & evaluation, 2.7.1 Completion Fluids, 5.6.4 Drillstem/Well Testing, 3.2.5 Produced Sand / Solids Management and Control, 1.11 Drilling Fluids and Materials, 4.1.5 Processing Equipment, 4.2.3 Materials and Corrosion, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6 Drilling Operations, 3.3.1 Production Logging, 2.2.2 Perforating
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The Alba reservoir (located in the central North Sea) is a highly permeable, unconsolidated sand body with lenses of reactive shales within the reservoir sand. From the initial decision to exploit the field with horizontal wells, there has been an evolutionary approach to selecting the most appropriate drilling fluid and completion technique. Field development has been marked by a drive to increase productivity and reduce the total cost of the ownership of the asset.
The major challenges in completing wells in this field involve sand control and inhibition of reactive reservoir shales.
Initially, completions were prepacked screens in open hole, and the fluid used to drill the reservoir section was oil-based mud (OBM). Productivity was lower than expected, but improvements were achieved by switching to sized salt water-based mud (WBM) and initiating a very stringent quality-control procedure for the drilling and completion fluids. However, screen failure occurred within 2 to 3 years of production startup, and the wells almost invariably sanded up. More recent changes have involved a switch to sized calcium carbonate WBM and openhole gravel packs.
Because the geology of the Alba field dictates that there is almost always some shale exposed in the openhole reservoir section, it was found that the gravel-carrier fluid needs to be equally as inhibitive as the drill-in fluid.
Since 1998, 13 openhole gravel packs have been successfully completed, with high flow rates and no history of sand production in all cases.
This paper deals with the performance of the various techiques applied and details the best practices derived for optimum productivity and screen longevity.
The Alba field is located in Block 16/26 of the U.K. sector of the North Sea. This oil field lies above the huge Britannia gas field and comprises an Eocene sandstone formation that is thin, highly porous and permeable, very unconsolidated, and overlain by a bed of impermeable, highly reactive shale.
The nature of the reservoir dictated that development would be best achieved by openhole completions and highly deviated or horizontal reservoir sections with the productive interval located near the top of the sand body. It was expected that drilling in the reservoir section would be through reactive shales and unconsolidated sand; thus, three high-priority requirements were perceived to be: shale inhibition, borehole stability while drilling, and sand exclusion while producing. These aspects have continued to be of great importance throughout the evolutionary steps in the exploitation of this reservoir.
During the development of this field since 1994, three drill-in fluid systems have been used.
Synthetic oil-based mud (SOBM).
Sized NaCl water-based system.
Sized calcium carbonate water-based system.
Three main sand-control techniques have also been used for production.
Dual, wire-wrapped, prepacked, standalone screens.
All-metal standalone screens (wire wrapped).
All-metal screens (wire wrapped) and gravel pack.
With respect to the openhole gravel pack, two variations have been used.
Water-based drill-in and gravel-carrier fluids.
SOBM drill-in and water-based gravel-carrier fluids.
The second variation is the subject of another paper.1
One of the main factors limiting well longevity was screen durability; obviously, screen failure meant the end of the well's useful life. Adopting gravel packing as the standard completion method has produced huge benefits with respect to screen and well longevity.
Initial Development - SOBM
The original approach to sand exclusion was to rely on prepacked, standalone screens in the reservoir section, which were run in the mud after drilling to total depth (TD). In view of the foreseen requirements, the optimum drilling fluid for the reservoir would need to represent the best compromise with respect to shale inhibition, hole stability, lubricity, and ability to flow back through screens without causing impairment. The consensus within the operating company at that time was that these needs could be best met with SOBM weighted with accurately sized calcium carbonate. A typical mud formulation is given in Table 1.
On reaching TD, the standard procedure was planned to be:
Run screens in the mud.
Displace to solids-free oil-based push pill.
Displace to NaCl brine.
Displace to breaker [e.g., comprising brine/10% HCl/20% mutual solvent, ethyleneglycolmonobutyl ether (EGMBE) plus additives] and allow to soak. (A typical breaker-fluid formulation is given in Table 2.) This type of breaker has been used by other operators.2
Despite the planned completion method, the second well drilled (W2), of necessity, required a different approach. The original plan3 envisaged a 2,000-ft horizontal open hole with a standalone, prepacked screen. However, because of an unexpected encounter with shale and a high degree of geological uncertainty in the area, it was decided to restrict the hole to 700 ft and to case, perforate, and perform a cased-hole gravel pack. The drilling fluid was SOBM, but the gravel-carrier fluid was aqueous.
Four other wells were drilled and completed according to the procedure devised for the original plan.
These initial five wells all had disappointing performances (see Table 3 ).
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