Precise Well Placement With Rotary Steerable Systems and Logging-While-Drilling Measurements
- I.R. Tribe (Schlumberger Drilling and Measurements) | L. Burns (Schlumberger Drilling and Measurements) | P.D. Howell (GlobalSantaFe Well Engineering) | R. Dickson (Talisman Energy U.K. Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2003
- Document Type
- Journal Paper
- 42 - 49
- 2003. Society of Petroleum Engineers
- 1.1 Well Planning, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.7 Geosteering / Reservoir Navigation, 5.1.5 Geologic Modeling, 5.1.2 Faults and Fracture Characterisation, 2 Well Completion, 5.8.5 Oil Sand, Oil Shale, Bitumen, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6.6 Directional Drilling, 1.12.2 Logging While Drilling, 1.10 Drilling Equipment, 1.6 Drilling Operations, 2.4.3 Sand/Solids Control, 5.1.1 Exploration, Development, Structural Geology, 5.6.1 Open hole/cased hole log analysis, 1.6.2 Technical Limit Drilling
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This paper demonstrates profitable geosteering of a central North Sea, U.K., horizontal well with a rotary steerable system (RSS) together with new-generation azimuthal logging-while-drilling (LWD) tools. The nature of the tertiary turbidite reservoir, with only 3 million STB, meant that to be commercial, only one horizontal well should be drilled and that the following criteria had to be met.
Maximize standoff from an oil/water contact (OWC) within a sand unit with an approximately 50-ft oil column.
Achieve 1,500 to 2,000 ft of good-quality reservoir.
While maintaining standoff from the OWC, make a corresponding azimuth turn to follow the crestal dome structures of the reservoir trap.
Avoid the risk of stuck pipe by carefully monitoring real-time drilling and well data and by using RSS technology.
During well drilling, correlation software enabled comparison of offset-well and real-time LWD data and visualization of the well's position in the reservoir sequence. This modeling complemented wellsite biostratigraphic and geological correlation work and enabled changes to the trajectory to be planned, which were then achieved while constantly rotating with the RSS. Azimuthal-density measurements were used in real time and as post-well image logs for structural dip and fault interpretation.
The well was geosteered into a sweet spot within 5 ft of the reservoir roof and as much as 56 ft from the OWC. The RSS completed the well in a single 3,844-ft run, with an average rate of penetration (ROP) of 68 ft/hr and no stuck-pipe problems. The target of 1,500 to 2,000 ft of pay was met with 1,980 ft of >20% porosity sand and 88% net to gross. The preplanned geometric wellpath would have missed the first 1,000 ft of the reservoir section, justifying the geosteering approach. A water saturation of 10% was much lower than prognosed, and the well has produced more oil than expected without early water breakthrough.
This well demonstrates the combinability of the latest RSS and LWD technology to meet ever-demanding drilling and geological objectives successfully under circumstances of high economic risk and to ultimately maximize potential well production.
This paper uses a case study to highlight the benefits of using RSS and the latest LWD tool measurements to geosteer wells into tight geological targets. The example well was to develop the Beauly field, a small (3 million STB), satellite oil discovery of the Balmoral field, central North Sea, U.K. The reservoir is composed of tertiary turbidite sands and thin siltstones, known as the Mey sandstone, overlain by cap-rock Lista shales. These are deformed into three gentle anticline traps along a northwest/southeast to east-southeast/ west-northwest trend. The prejob well plan (Well 16/ 21c-32Y) is shown in Figs. 1a and 1b. Well 16/21c-32Y was a re-entry, drilling and completing the well as a horizontal producer with a preperforated liner. The well was to be placed as close to the roof of the reservoir as possible to maintain at least a 35-ft standoff from the OWC, and an azimuth turn needed to be made to link the antiformal structures. A 1,500-ft drain length in good, reservoir-quality sand was needed. Because of depth uncertainties on top reservoir and well true-vertical-depth (TVD) positioning, there was potential to only graze the top of the reservoir if drilling geometrically and not geosteering (Fig. 1c). Drillers were faced with the risk of packing off in the buildup section and between crestal traps (collapsing Lista shales) and differential sticking during horizontal drilling because of the high mud weights required to maintain the Lista shale section's integrity. Therefore, hole cleaning and control of equivalent circulating density (ECD) were of paramount importance. One significant stuck-pipe or lost-in-hole incident would have rendered the development unprofitable. The drilling and geosteering needs on this well were addressed with an RSS and gamma ray (GR), resistivity, and density-neutron LWD tools with azimuthal measurement capability. By using an RSS in which all parts rotate, the risk of stuck pipe could be minimized, and real-time LWD data modeling and interpretation was undertaken to maximize the value of the data for achieving accurate well placement. The later discussion highlights all the benefits of using these tools and interpretation techniques and outlines the execution and results of this geosteered well.
Benefits of RSS for Geosteering
An RSS was used to drill the horizontal section in Well 16/21c- 32Y. The system continually rotates and steers by means of pads that push against the side of the borehole wall.1 System power comes from the mud flow inside the drillpipe, with hydraulic energy for the pads provided by the pressure drop across the bit. In contrast, mud motors use near-bit bent housing to change the wellbore direction, which necessitates sliding periods when the drillpipe is held stationary. When compared with wells drilled with mud motors, the RSS tool offered the following benefits to geosteering this Beauly horizontal well.1
More efficient hole cleaning, reducing the risk of stuck pipe. Steering a horizontal well by sliding with motors causes cuttings buildup and the risk of packing off or differential sticking where an overbalance occurs because of high circulating mud density. After sliding, it is normal to spend time hole cleaning to remove cuttings buildup. In Well 16/21c-32Y, high-weight mud pills were pumped and circulated back to surface every 500 ft to aid in hole cleaning. The advantage of continuous rotary drilling with an RSS is that the mud column in the annulus is constantly being agitated and avoids buildup of cuttings beds. Spending valuable rig time circulating following sliding is not required.
Smoother weight transfer to bit. Continuous rotation improves weight transfer to the bit and, hence, steerability, which can be a problem when sliding with motors in long step-out wells.
Smoother wellpath. Changes between sliding and rotating mode with mud motors can create a tortuous wellpath. RSS provides more gradual changes in direction, creating a smoother wellbore and significantly aiding extended step-out and steerability. This also prevents unintentional microdoglegs that could potentially cause problems in running the casing.
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