Frank Hartley
Well scale
Northeast energy services company PSL Group has recently completed the largest worldwide well scale intervention and removal program. The $5.6-million contract, awarded to PSL in conjunction with partner Odfjell Well Services, was undertaken on BP's Miller field in the UK sector of the North Sea, in what is considered one of the most hostile scaling regimes in the world.
Scale deposition is a frequently occurring situation that is controlled by scale treatments containing high levels of chemical inhibitor. These are difficult to sustain over long production periods and lead to a relatively short well life resulting from scale deposited over production perforations and in wellbore tubulars. This effectively stops otherwise sound wells from producing oil, which resulted in an immediate intervention well plan.
Hydraulic workover techniques were deployed in a project focusing on three Miller wells, which were either fully or partially plugged with barium scale, effectively reducing oil production to a fraction of the optimum level.
The intervention program called for a through tubing scale milling assembly to be deployed into each live well, using a hydraulic workover unit. It employed surface equipment, including a cyclone fluid/solid separator system to remove the estimated 12 tons of scale materials deposited within each wellbore.
The producing reservoir is an Upper Jurassic Sandstone containing high barium and moderate salinity brine that, coupled with the high bottom-hole temperature, places the Miller field among the toughest scaling regimes of any producing field in the world. One of the wells, which had suffered such severe scaling problems that it had been closed some time prior to the program, is now producing at 5,000 b/d.
The intervention programs, which involved the mobilization of 68 lifts in just two days, were completed in 169 days with no recordable lost-time incidents.
Rotary steerable
A successful downhole test of a 4 3/4-in. rotary steerable system has been completed. The Revolution system, designed and manufactured by the Precision Drilling Corp. at the research and engineering facility in Cheltenham, England, drilled 940 m at an average rate of penetration of 41 m/hr during a recent field test in western Canada's Farrow field for EOG Resources Canada Inc.
The system was able to control the hole deviation and direction much more efficiently than by using conventional mud motors. It performed extremely well, and using it will result in substantial cost and timesaving on future drilling projects, the company said. Additionally, different hole size and casing design options can be achieved to make wells very cost effective.
Rotary steerable systems allow operators to orient and control well trajectory while rotating the drill string.
The result is faster penetration rates, smoother wellbores, and fewer doglegs than in wells drilled with conventional mud motors. This system is the first 4 3/4-in. rotary steerable system to use point-the-bit technology to control the well path. Point-the-bit systems have been proven to drill better quality boreholes, with less spiraling and tortuousity than other competitive push-the-bit systems. By developing a point-the-bit rotary steerable system in a 4 3/4 in. tool size, added benefits that smaller boreholes have on drilling and completion costs can be realized. Additional field tests are underway in Mexico.
Expandable sand screens
A 5 1/2-in. expandable sand screen (ESS) with enhanced connectors has been installed in five wells in the PL 19-3 field in the Bohai Bay offshore northern China for Phillips China Inc. Phillips China, a subsidiary of CononcoPhillips, operates the field with a 49% interest. Partner Cnooc China Ltd. owns 51%. The screens were developed by Weatherford and installed by its Completion Systems Division (WCS).
Expandable technology refers to tools that can be expanded after they have been installed downhole. The range of applications includes sand control, drilling and completion tools, and more efficient wellbore designs. The benefits include major cost savings and productivity improvements.
Phase 1 of the ConocoPhillip's PL 19-3 operations in Bohai Bay consists of 24 wells and provides a series of challenges due to a very complicated subsurface environment. The nature of the generally weak reservoir rock, much of it unconsolidated, means there can be severe sand control problems.
The first ESS installation was in well A-6, a 9 5/8-in., cased hole perforated 40° incline, running to a depth of 1,750 m, with 21 ESS joints. ESS has also been installed in three other cased hole wells – A-5, A-12, A-23, and in A-18, an open hole deviated well. A total of 104 ESS joints have been run to date. WCS is supplying the expandable liner hanger packers, with the Drilling and Intervention Services division providing the tubular management and fishing tools for the project. An enhanced ESS 5 1/2-in. connector, currently used, is quicker to install and offers increased strength and reliability for this application.
Centralizer program
The availability of a unique simulation program proved to be an important factor in the success of BHP Billiton's casing operation on the Lennox field in the East Irish Sea Basin. Used by UK centralizer manufacturer Centek Ltd., the program enabled the company to provide an accurate prediction of the loads to be encountered and to optimize the distribution of the 110 centralizers along the casing string.
According to the drilling department, this centralization program gave a minimum casing standoff in 12.9-in. hole of 73%, and the casing was run into the hole without any problems. The benefits of bow spring centralizers provided greater standoff in over gauge hole while being at least as robust as a solid centralizer.
Part of the success of running casing and cementation operation was attributed to the unique design of the Centek 9 5/8-in. centralizers that the company was using for the first time. Each centralizer is manufactured using a patented cutting and treatment process from a single piece of boron steel. It is load tested to 68,000 lb and is capable of withstanding an angular pull at 45,000 lb. According to the company, the design offers exceptional strength and resistance to wear, permanent set, and disintegration. These benefits are combined with zero start force, zero running force, and exceptionally high stand-off while ensuring that the casing remains central to the bore hole under all conditions and inclinations. Since the centralizer is made from a single piece of boron steel, it offers a low friction coefficient, which makes casing running significantly easier. This is claimed to be of particular value for operators of horizontal or extended reach wells.
The inherent design and strength of the centralizers ensures superior flow-by area and makes it possible to undertake cementation at higher pump rates without risking damage to the well formation, the company says. The equivalent circulating density signature obtained by the centralizer demonstrates this.
By using the one-piece centralizers in conjunction with the software package, the drilling team was able to obtain accurate predictions of casing behavior during running operations. The program is able to model numerous factors within the well, including varying section coefficients of friction, variable densities arising from muds and cements that may be present, flotation section methods, lateral loads where the well changes direction, and rotational torque at the rig floor. Drilling engineers are able to work with the program and conduct "what if" simulations of the well so centralizer locations can be planned for optimum performance and economy.
The centralizers are manufactured in England to a design that exceeds API 10D standards as well as the more demanding Shell Sqair standards. The manufacturing process enables the manufacturer to quickly produce centralizers tailored to the needs of the well. These include the high performance "slider" centralizer that is designed for far reaching horizontal, deviated, and close annulus wells. Other designs include straight bow, rigid or positive standoff designs, and different types of two-part hinged construction. The computer simulation is used to identify the most suitable option and achieves the optimum performance and economy through a choice of centralizer design.
