Detailed fabric development histories have been deduced for Neoproterozoic foliated quartz diorite complexes of the Channel Islands, UK. The plutons were emplaced during regional (D2) deformation. Magmatic fabrics, formed at low crystal contents (pre-rheologically critical melt percent) are only locally preserved due to overprinting by solid-state deformation which occurred at moderate temperatures (400–550°C). This is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization; marginal recrystallization and myrmekite development on alkali-feldspar; ductile deformation of quartz to form types 2–3 ribbons; quartz recrystallization by sub-grain rotation associated with a -slip but not c -slip: biotite recrystallization within micro-shears; and variable amphibole recrystallization. The deformation histories display a distinct division between magmatic state and moderate temperature solid-state deformation features with little evidence for intervening high temperature solid-state fabrics. This contrasts with criteria widely considered diagnostic of syn-tectonic plutons which envisage development of a continuum of magmatic through high temperature solid-state deformation fabrics. However, we consider the plutons syn-tectonic and attribute the hiatus to contrasting strain and pluton cooling rates, length and nature of deformation events and host rock ambient temperatures. Intermediate magmas, emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than SiO 2 ,-richer granitoids.
Summary 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. Introduction 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.1More 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.
The Channel Islands of Guernsey and Sark are amongst the few localities within the Neoproterozoic, Cadomian orogenic belt where Palaeoproterozoic basement is exposed. Basement units collectively referred to as ‘Icartian’ comprise orthogneisses (e.g. the c. 2000 Ma Icart granite gneiss), metasediments and amphibolites. On Guernsey, the protolith to the Icart granite gneiss was intruded into metasediments already deformed during a D1 deformation event. Both were variably deformed during a D2 event within an approximately north—south trending, steeply dipping, oblique dextral shear zone. On Sark, metasediments and amphibolites carry a D1 fabric (not necessarily correlatable with that on Guernsey) which was deformed during D2 to form recumbent—gently inclined folds and tectonic fabrics which locally carry top-to-the-south kinematic indicators. Early Cadomian quartz diorites which intruded the Icartian basement carry variably developed magmatic and moderate temperature (400–550 °C) LS solid-state fabrics which are similarly oriented to D2 basement structures. This indicates that the quartz diorites were emplaced and deformed during a protracted D2 event. Published 40Ar/39Ar amphibole cooling ages of c. 600 Ma from Guernsey and Sark quartz diorites and a host Icartian amphibolite are consistent with reworking of basement units during the Cadomian Orogeny. The contrasting kinematic pattern during Cadomian deformation implies that a flat-lying shear zone on Sark may have accommodated oblique dextral strike-slip movement on Guernsey with an overall southerly direction of tectonic transport. Deformation most likely occurred in the hanging-wall of a major south-dipping Neoproterozoic subduction zone.
A number of calipers and hole size indicators (be they by direct measurement or derived) have been introduced with the logging while drilling (LWD) suite of measurements. These include ultrasonic calipers, derived density calipers, and electrical calipers from resistivity tools. Qualitative indications of borehole shape can also be derived from Pe and density measurements which can be affected by tool standoff. The traditional uses of borehole shape measurements have been principally aimed at petrophysicists, reservoir engineers, and geologists for completion strategy, hole volume determination, correction of electrical logs, borehole stability, and borehole size for images. In this paper we summarize additional applications for drillers:To assess the borehole quality related to the drilling process and borehole assembly being used.To diagnose wellbore stability problems while drillingTo identify hole quality which may affect running the completion or wireline tools Hole shape analysis is carried out by processing azimuthal ultrasonic caliper, Pe, and density data into 1D, 2D, or 3D images to illustrate the borehole shape. Processing of images or caliper curve data in real time enables decisions to be made about modification of the BHA or reconsider drilling practice for the remainder of the well. An obvious application of the images is to visualise the severity and nature of any borehole breakout occurring and to take necessary steps to resolve the problem, for example by increasing mud weight. Our results of processing a number of wells highlight that in many cases the shape of the borehole can be directly related to the drilling process. Examples are shown of boreholes drilled by rotary steerable assemblies and mud motors (with different stabilizer configurations and different drilling modes). Timely feedback of this information to the driller allows changes in the style of drilling or BHA to improve the borehole shape and therefore drilling efficiency. This in turn should lead to an improved shaped borehole and better petrophysical data. Understanding the effects of the drilling process can also allow differentiation of drilling induced shape artefacts and geological reasons for borehole shape changes (e.g. zones of instability or breakout). Information on the shape of the borehole also improves the decisions made for completion strategies where mechanical calipers are not being run. Applications include understanding borehole shape for expandable screens and cement volumes. Introduction Caliper information has been available for a number of years from Logging While Drilling tools1,2. These are all derived measurements rather than a physical measurement of the borehole size. In other words there are not mechanical arm calipers at the time of drilling. The caliper measurements are derived from a variety of measurements that have differing volumes of investigation and therefore differing applications. This paper concentrates on applications of quantitative ultrasonic caliper measurements with a particular focus on the drilling applications. The focus has been placed on the drilling side because it is felt that there are a considerable number of applications for the improvement of the drilling process by the interpretation of borehole shape information. The quantitative caliper information is supplemented by qualitative hole shape information form Photoelectric and density images. Whilst analysis of hole shape can be done after the well has been drilled, the improving rates of real-time data transmission now allow the borehole to be imaged in real time. This allows recognition of variations in borehole shape at the time of drilling and allows changes in the drilling practice to optimize the shape and therefore drilling efficiency. The paper will discuss the ultrasonic caliper measurement and then discuss the applications and limitations in more detail.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper demonstrates profitable geosteering of a Central North Sea, UK horizontal well by using a Rotary Steerable System (RSS) together with new generation azimuthal Logging While Drilling (LWD) tools. The nature of the Tertiary Turbidite reservoir meant that to be commercial the following criteria had to be met:1. Maximise stand-off from an oil water contact within a sand unit with a circa 50 ft oil column. 2. Achieve 1500-2000 ft of good quality reservoir. 3. While maintaining the stand-off from the OWC, make a corresponding azimuth turn in order to follow the crestal dome structures of the reservoir trap. 4. Avoid the risk of stuck pipe by carefully monitoring real time drilling and well data and by using RSS technology.During drilling of the well, correlation software enabled comparison of offset well and real time LWD data and visualisation of the position of the well in the reservoir sequence.This modelling complemented wellsite biostratigraphic and geological correlation work and enabled changes to the trajectory to be planned, which were then achieved while constantly rotating using the RSS tool. 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 as much as 56 ft from the OWC. The RSS tool completed the well in a single run of 3844 ft with an average ROP of 68 ft / hr and with no stuck pipe problems. The target of 1500-2000 ft of pay was met with 1980 ft of >20 % porosity sand and 88% nett to gross. Water saturation of 10% was much lower than expected and the well is currently producing at 11,500 bopd.This well demonstrates the combinability of the latest RSS and LWD technology to successfully meet ever-demanding drilling and geological objectives under circumstances of high-economic risk and ultimately maximise potential well production.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.