Chevron has been successfully drilling and gravel packing open hole horizontal wells on the Alba Field (central North Sea) since 1998 and 13 open-hole gravel packed (OHGP) wells drilled with water based mud(WBM) are currently in production with no history of sand production. Although these wells have been hugely successful with significant net present value (NPV) returns it was recognised that the future, mature redrill and infill targets cannot sustain the current costs associated with traditional OHGP completions. The challenge was to develop alternative techniques to maintain the benefits of OHGP wells but to achieve a low cost well and completion concept to assist in realising new drilling opportunities. To drill the shale above the top of the reservoir and the productive interval in a single hole section would remove the conventional requirements to set an additional casing string and change over to a water based system prior to drilling into the reservoir. This would save costs but raise a question over the gravel packing operation. Hitherto, attempts to gravel pack involving á/â placement techniques using an aqueous carrier fluid following drilling with oil based systems have had only limited success. The prospective problems were examined by extensive laboratory tests carried out by Baroid and Chevron in co-operation. A new synthetic oil based mud (SBM) formulation was developed and compatible displacement fluids and procedures were devised. Based on this work, 1500 ft of shale and reservoir were drilled; a liner was installed – predrilled over the reservoir section – and screen was run inside the liner. Gravel was pumped using brine as carrier fluid and complete gravel placement was achieved. The well has achieved productivity levels at least as good as existing WBM wells. A second well completed in the same manner has given similar performance. This combination of a liner system and SBM fluids offers several advantages. There is the prospect of considerable savings with respect to operating time, cementing and drilling fluids. Also the liner gives protection to the screen. This new approach which represents potential large savings in costs and excellent productivity is considered to be very applicable for expandable sand screens (ESS), multi-lateral completions and additional redrill targets both on Alba and worldwide.

Multilateral wells offer many benefits over conventional wells, including reduced overall drilling costs, lower environmental effect, increased total recovery, greater access to production intervals, and subsequently improved well production rates. However, it can be difficult to achieve a good quality casing window through which an additional lateral branch can be successfully drilled and completed. As bottomhole assemblies (BHAs) become more advanced, involving longer and stiffer strings of tools, and as completion design becomes more intricate, more attention must be given to the way the casing window is created because this is the foundation of multilateral well design. Track-guided milling systems have emerged as effective and accurate methods by which to control casing window geometry, and this paper will focus on recent advances in track-guided, precision window milling technology and its effect on multilateral well design. To avoid potential problems in running drilling assemblies or liner/completion strings, advanced milling technology should be used to create the casing window. During conventional milling, it is commonly difficult to control the action of the mill as it cuts through the casing; poor control creates a casing window that could, as a result of right hand rotation during milling, rolling-off to one side, leading to a skewed or shortened aperture. Uncontrolled and undefined window geometry introduces additional risks when re-entering a lateral wellbore, such as nonproductive time (NPT) and equipment damage. A good quality casing window, with precisely controlled length and width, helps ensure that drilling and completion equipment can exit the aperture without problems and facilitates repeatable re-entry access to both the mainbore and the laterals in future interventions. The quality of the casing window is just as critical in multilateral wells as in conventional sidetracking or whipstock operations. Advances in modern casing milling technology are pioneering improved multilateral well designs. Multilateral wellbore junctions can now be placed in deep, high-angle wells without compromising drilling or completion operations by using a track-guided milling system to create improved casing windows.

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Summary Chevron has been successfully drilling and gravel packing openhole horizontal wells in the Alba field (central North Sea) since 1998, and 13 openhole gravel-packed (OHGP) wells drilled with water-based mud (WBM) are currently in production with no history of sand production. Although these wells have been hugely successful with significant net present value (NPV) returns, it was recognized that the future, mature redrill and infill targets cannot sustain the current costs associated with traditional OHGP completions. The challenge was to develop alternative techniques to maintain the benefits of OHGP wells but to achieve a low-cost well and completion concept to assist in realizing new drilling opportunities. Drilling the shale above the top of the reservoir and the productive interval in a single hole section would require removing conventional requirements for setting an additional casing string and changing over to a water-based system before drilling into the reservoir. This would save costs but raises a question concerning the gravel-packing operation. Hitherto, attempts to gravel pack that involve alpha/beta placement techniques using an aqueous carrier fluid following drilling with oil-based systems have had only limited success. The prospective problems were examined by extensive laboratory tests carried out cooperatively by Baroid and Chevron. A new, synthetic oil-based mud (SOBM) formulation was developed, and compatible displacement fluids and procedures were devised. Based on this work, 1,500 ft of shale and reservoir were drilled, a liner that was predrilled over the reservoir section was installed, and screen was run inside the liner. Gravel was pumped using brine as the carrier fluid, and complete gravel placement was achieved. The well has achieved productivity levels at least as good as existing WBM wells. A second well completed in the same manner has given a similar performance. This combination of a liner system and SOBM fluids offers several advantages. There is the prospect of considerable savings with respect to operating time, cementing, and drilling fluids. The liner also gives protection to the screen. This new approach, which represents potential large savings in costs and excellent productivity, is considered applicable to other types of completions when it is desirable to drill with oil-based mud even though conventional thinking would have called for water-based drill-in fluid. This consideration applies to targets in the Alba field and worldwide. Introduction The Alba oil field, located in Block 16/26 of the U.K. sector of the North Sea, 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. The earliest approach was to run screen-only completions, with SOBM as the drill-in fluid. The next stage of the evolutionary approach involved the same screen arrangements but with a saturated brine/sized sodium chloride-based drill-in fluid. Both approaches provided excellent drilling properties, but there were severe limitations with respect to productivity for the wells drilled with SOBM, and screen longevity was a problem for wells using the sized-salt approach.1 Both issues were addressed by adopting a sized carbonate drill-in fluid and performing openhole gravel packs on the productive sections. From 19981 to July 2001, 13 openhole sections had been successfully gravel packed and are still in production with high flow rates and no history of sand production. However, it was recognized that despite the excellent flow rates and significant NPV returns provided by these wells, the future mature redrill and infill targets could not sustain the current costs associated with traditional OHGP completions. The asset was challenged with developing alternative completion techniques to maintain the benefits of OHGP wells while reducing the costs of current drilling and completion procedures and developing a low-cost well concept to assist in realizing new drilling opportunities. The ability to gravel pack openhole sections drilled with SOBM would enable the shale above the top of the reservoir and the productive interval to be drilled in a single hole section. This would remove the requirement to install an additional casing string and to change over to a different mud system before drilling into the reservoir. For example, in the case of a redrill in Alba W33, there was the prospect of saving a casing run of more than 1,000 ft and using one mud system throughout. Potential total savings with respect to operational time, cementing, and drilling fluids were estimated to be of the order of U.S. $3 million. On the other hand, experience with completions in the Alba field (with the interval drilled with SOBM and the completion fluid as brine) has not been encouraging. During the early stages of the Alba development, several productive intervals were drilled with SOBM and displaced to aqueous completion fluid; the productivity of the wells was much less than expected.1 The gravel-packing technique used in Alba for the previous wells drilled with water-based drill-in fluid involved the alpha/beta wave-placement method. It was intended that the same method be used for the wells drilled with SOBM. As a result, significant fluid loss during gravel placement would not be tolerable. Indeed, previous attempts to drill with SOBM and gravel pack with aqueous carrier fluid in the manner envisaged for Alba have reported limited success,3* mainly because of problems caused by excessive fluid loss. Similarly, a development in west Africa involving drilling with oil followed by an aqueous, gelled, gravel carrier fluid reported losses on two of the three operations.4

The Alba reservoir (located in the Central N 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 using horizontal wells there has been an evolutionary approach to selection of the most appropriate drilling fluid and completion technique. The development of the field 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 on this field involve issues of 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. Productivity was lower than expected but improvements were achieved by switching to sized salt water based mud and initiating a very stringent QC procedure for the drilling and completion fluids. However, screen failure occurred very soon after the wells became water cut and the wells almost invariably sanded up. More recent changes have involved a switch to sized calcium carbonate water based mud and open-hole gravel packs. Because the geology of the Alba field dictates that there is almost always some shale exposed in the open hole reservoir section it was found that the gravel carrier fluid needs to be equally as inhibitive as the drill-in fluid Since 1998 thirteen open-hole gravel packs have been successfully completed, in all cases with high flowrates and no history of sand production. This paper deals with the performance of the various techiques applied and details the best practices derived for optimum productivity and screen longevity. Introduction The Alba field is located in block 16/26 of the UK sector of the N Sea.This oil field lies above the huge Britannia gas field and comprises an Eocene sandstone formation that is thin, highly porous, highly 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 open-hole completions and highly deviated or horizontal reservoir sections with the productive interval being sited 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 and borehole stability while drilling and sand exclusion while producing. These aspects have continued to be of great importance throughout all of 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 (SBM)Sized sodium chloride water based systemSized calcium carbonate water based system Also, three main sand control techniques have been used for production. These are:Dual wire-wrapped, pre-packed, stand-alone screensAll-metal stand-alone screensAll-metal screens and gravel pack. In respect of open-hole gravel pack two variations have been used. These were;Water based drill in fluid and water based gravel carrier fluidSBM drill in fluid and water based gravel carrier fluid The second variation is the subject of another paper 1 Huge benefits in respect of screen longevity have been obtained by adopting gravel packing as the standard completion method.