New mud chemistry is shown to improve the efficiency of directional drilling with a rotary steerable drilling system. Full-scale (8 1/2-in. bit) drilling results using a rotary steerable drilling system drilling to horizontal at an experimental test site and an application in the Huizhou 21–1 field, offshore Hong Kong in the South China Sea are presented documenting the effectiveness of this technology. Introduction The ability to drill directionally has proven to be one of the most important developments in our industry and can be expected to remain popular in the near future. Development of reserves in remote offshore areas, environmentally sensitive areas and areas with restricted surface access; re-entry and multilateral drilling to extend asset life; and horizontal completions to increase production rates and recovery will all keep interest in directional drilling high. Fluctuating oil and gas prices have also put a premium on efficient directional drilling techniques and for complex 3-dimensional well paths, so-called designer wells, rotary steerable drilling systems appear to be the most efficient. The choice of the drilling fluid can either assist or hinder the performance of the tools. Directional Drilling Tools Steerable Motors Steerable motor systems using positive displacement motors became the method of choice for demanding directional drilling applications starting in the late 1980's1. These systems rely on a bend in the motor housing to provide bit offset that is oriented to turn the well. Steerable motors are operated in either the slide drilling mode or rotational-drilling mode. The drillstring does not rotate in the slide-drilling mode but is simply pushed along the well path with bit rotation provided strictly by the motor. The drillstring and entire drilling assembly is rotated from the surface in rotational-drilling mode drilling a slightly over-gauge hole due to the bend in the motor housing. Measurement-while-drilling (MWD) systems monitor the well path and bent motor orientation (tool face) as well as transmit the information to the surface via mud pulse telemetry. Alternating between the two drilling modes give directional drillers the ability to alter build rates without having to trip the pipe. Despite the tremendous advance in directional drilling technology steerable motors provide, they suffer from a number of problems. These include:Stabilizer hang-ups on ledges formed from rotational-drilling mode to slide-drilling mode transitionsIncreased torque, reactive torque, drag and probability of stuck pipe due to friction from cuttings accumulation caused by poor hole cleaning during slide-mode drillingPoor transfer of weight to the bit and bottom hole assembly (BHA)/bit damage from axial stick slip also caused by poor hole cleaningPoor ROP due to the above as well as use of less aggressive bits due to the high reactive torqueLimited total displacement due to high friction from spiral well paths drilled by bent motor assemblies as well as poor hole cleaning Rotary Steerable Drilling Systems Effective rotary steerable drilling systems overcome most of the problems with steerable motors by providing continuos pipe rotation while drilling. This is accomplished, with the tool reported on in this paper, with a non-rotating steerable sleeve containing three extendable pads that push against the borehole wall2. The drillstring, which is connected to the bit, rotates inside the sleeve. Independently varying the force on each of the pads steers the tool along the desired well path. The tool can be programmed for inclination or steering force and deviations are automatically adjusted for by a closed loop control. Any adjustments needed can be downlinked to the tool without interrupting drilling.
An Offshore China/South China Sea operator wanted to optimize the use of a rotary steerable drilling assembly, simplify and reduce operational steps, and provide a larger reservoir wellbore. The operator elected to drill a single 8.5" wellbore and, in order to address a variety of issues and potential problems, change from a Cloud Point Glycol Drilling Fluid to a Drill- In Fluid (DIF) at the reservoir entry point. A reverse sequence solution engineering process was employed wherein planning began from the point of view of a completed well. Introduction Effectively addressing numerous potentially compromising elements within a plan to optimize both reservoir production and utilization of new drilling technologies can be achieved through Reverse Sequence Solution Engineering (RSSE). Reservoirs with shale layers can cause serious problems during drill-in and completion operations. Reactive shales can lead to borehole instability during the drilling phase and, if not controlled, may plug gravel and screens in the completion phase. Further, DIF return permeability and lift off properties may be seriously impaired, resulting in reduced hydrocarbon production. Without properly sequenced well planning and fluid design, high rates of filtrate invasion, circulation losses, differentially stuck pipe and low production rates may result. In order to minimize forming or accumulating unforeseen problems while a project is underway, it is sometimes critical to originate the planning sequence from the point of view of the desired end result. This paper attempts to provide an overview of a RSSE approach, which allowed incorporation of numerous new ideas, products, processes and technologies into an existing successful process. In addition, the details from a successful field test using the new elements will be presented. Background The operator had drilled a total of 12 wells culminating in horizontal sections in a variety of sandstone reservoirs at depths varying from 2000 to 3000 m TVD, with the deeper wells reaching nearly 4200 m MD. Of these, 4 were new wells while the remaining 8 were sidetrack re-entries. In all sidetrack cases, whipstocks were set inside existing 9.625" casing, and 8.5" sidetrack wellbores were exited from the casing. These 8.5" holes were drilled to designated reservoir entries, culminating at or very near a 90° angle. Then, 7" liners were run to isolate the entire 8.5" tangent wellbore. On some of the wells, following hanging and cementing the liner, 7" tiebacks were performed. The wells were then drilled horizontally into the reservoirs with conventional directional drilling assemblies using a water-based DIF containing a calcium carbonate bridging component. Depending upon reservoir characteristics, completion methods varied from open hole completions, to slotted or perforated liners, to pre-packed screens. The operator had accumulated a history of consistently exceeding hydrocarbon production expectations when the reservoir was drilled using a specifically engineered DIF. Not surprisingly, the operator wanted to retain this DIF component in future wells. Ten of the 12 wells, including all the sidetracks, were drilled with a platform rig that had initially been designed for workover purposes only. As a result, the rig was pushed to operational limits in drilling mode, with the primary limitations being overall string weight (top drive, draw-works and mast), pump pressure and output, top drive torque and speed, fluid handling and mixing capabilities, as well as fluid storage and circulating volume (Figures 1 and 2). Regardless of these limiting factors, drilling progressed, with the directional profiles, depths and step outs reaching very challenging levels.
The term "Extended Reach Drilling" refers to drilling operations conducted beyond the normal reach of drilling facilities. Typical problems associated with this type of operation are torque & drag limitations, wellbore stability, mud rheology and solids control and directional well design1,2,3. Such problems are particularly challenging to the CACT Operators Group due to limited platform rig size and platform deck space. These problems are further compounded by the fact that no spare well slots are available on the platforms, so all new platform drilling operations are sidetracks. This results in tortuous 3-dimensional well profiles that generate high levels of drilling torque and drag, disproportionate to the measured depth. The well planning and drilling practices used to drill these wells are very similar to those used in ultra-long extended reach wells and reflect the challenging nature of platform sidetrack operations. This paper describes the challenges that are faced during development of the well plans and details the methods that are later used while drilling the sidetrack wells. Case histories with actual field data will show how the applications of the following new or emerging technologies have been applied successfully in the sidetrack program. Borehole Stability Modeling was used to identify areas where the potential for borehole breakout and fluid losses (lost circulation) existed, so that plans could be made to avoid these problems. Real Time Downhole Data Measurements were used to provide instantaneous information on drilling conditions in order to avoid the problems experienced in previous wells. PDC bit designs, featuring low torque cutting structures to improve bit steerability were successfully applied on several recent sidetrack wells. Introduction Sidetrack drilling has become a major strategy in extending field life and maximizing oil recovery from CACT fields (Fig. 1) since 1997. Several factors have influenced CACT to pursue this aggressive strategy of plugging back and sidetracking watered out development wells in order to drill new horizontal single well completions. Early success of sidetrack development wells drilled in previously undeveloped tight reservoirs has been a major factor. Several development sidetracks have been drilled in tight reservoirs with up to 1000 meters of horizontal drain hole. These wells have far exceeded expectations of pre-drill productivity and ultimate recovery, and have influenced CACT to continue this program of developing other tight pay intervals. Updated mapping of several of CACT's main reservoirs has resulted in increased oil in place, which requires additional wells to effectively deplete undrained and poorly drained areas. In some reservoirs, which were previously commingled with other zones, recovery has been inefficient and new dedicated development wells with horizontal production intervals have proved effective in maximizing productivity and recovery. CACT has so far completed seven horizontal sidetracks from the Huizhou platforms. Each well employs a whipstock to exit through 9-5/8" (and sometimes 13-3/8") casing, drill 8-1/2" hole to the specific target reservoir. After running 7" casing, a 6" horizontal hole, up to 1000m in length is drilled in the reservoir. The Challenge The original Huizhou wells were drilled by semi-submersibles and then tied-back to surface production trees on the platforms. Each of the four platforms contains a workover rig, which was not originally designed for drilling applications. A package of drilling equipment was purchased to upgrade the existing rigs and allow for transfer between platforms to conduct the sidetrack operations. Equipment specifications were governed by platform structural loading limitations and deck space. The deck space remains so limited, that even small casing cement jobs need to be performed using cementing equipment installed on a supply boat tethered to the platform. A seven-month typhoon season brings an average of four typhoons through the field each year. Operations must be suspended for an average of 5 days during these storms.
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