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Abstract Logging while drilling (LWD) ultrasonic imaging tools have been recently introduced for slim hole size. Due to fundamental differences in data acquisition methodologies with other previously utilized LWD and wireline imaging techniques, field trials have been performed with the objective of validating and evaluating the new ultrasonic tool’s measurement. Ultrasonic imagers have been deployed in multiple wells of different environments and formation characteristics to evaluate the tool’s measurement quality and potential applications. The trials were performed in carbonate and clastic formations, horizontal and vertical trajectories, oil- and water-based drilling fluid systems, and in drilling and wipe operations. An LWD ultrasonic imager has also been deployed back to back with wireline. Multiple passes were performed to evaluate the time dependency and hole deterioration effect. In water-based mud, an ultrasonic imaging tool was run in the same bottomhole assembly with the proven LWD laterolog resistivity imager for the comparison of both technologies. In addition to stratigraphic dips, bed boundaries, fractures, faults, and other geological features usually detected by other imaging techniques, ultrasonic imaging tools also provided high measurement sensitivity for detecting geometric features relating to wellbore shape and wellbore stability. LWD microresistivity-based image comparisons indicated a robust correlation of the fractured zones contributing to lost circulation while drilling. Multiple passes for drilling and wipe images with wireline comparisons logged days after the LWD run clearly illustrated the time-dependency of the image quality due to borehole deterioration, invasion, and progression of geomechanical effects used to benchmark future data acquisition requirements. This paper evaluates the capabilities and performance of ultrasonic imaging tools in comparison with other LWD and wireline high-resolution imaging sensors.
Abstract Logging-while-drilling (LWD) services are today often provided by several separate tools that, when used in combination, result in some measurements being made far from the bit. Time to make up and break out LWD tools, memory dumping, as well as multiple tool reliability issues can significantly add to the well construction costs. An innovative new LWD tool has been developed to simultaneously provide drilling-related measurements and an industry standard suite of formation evaluation measurements, equivalent to the classic "triple-combo" service, but co-located and closer to the bit, using only a single short collar. This means that less rathole is needed for logging, and fewer collar connections have to be made, resulting in a more efficient drilling operation. In addition, the co-location of the sensors reduces uncertainties in the interpretation of measurements that rely on multiple sensors. A key focus during the development of the tool has been improvement in service delivery. This has been achieved through increased operational efficiency, higher rate of penetration (ROP) capability, significantly improved reliability throughout the system, and greater ease of maintenance. The use of a pulsed neutron source instead of a chemical source also dramatically reduces the environmental and operational risks normally encountered with the use of traditional LWD tools. Pulsed neutron measurements delivered by this service include formation capture cross section (sigma) and elemental analysis from neutron capture spectroscopy. These new measurements are used to compute mineralogy while drilling. Azimuthal measurements are also available, allowing the formation and borehole to be imaged using a variety of properties. The result is a robust and complete interpretation with significantly reduced uncertainty, as well as a large choice of steering methods for effective well placement. In this paper we describe the development of this new LWD service and show field-test examples of improved formation evaluation, increased drilling efficiency, and more effective well placement that the tool has provided. Tool Overview The new tool is a multifunction LWD tool that combines both drilling and formation evaluation measurements in a single collar (Fig. 1). These measurements include APWD* Annular Pressure While Drilling, calipers, gamma ray, resistivity, density, and porosity. At the heart of the new LWD tool is a pulsed neutron generator (PNG) that allows neutrons to be generated only when the tool is powered, eliminating the need for an americium beryllium (AmBe) chemical neutron source. The PNG provides several new measurements to LWD, such as Best-Phi* porosity, a hydrogen index equivalent, elemental capture spectroscopy providing detailed lithology, formation capture cross section (sigma) providing an alternative to resistivity for fluid saturation calculations, and NGD* Neutron Gamma Density, which is a density measurement obtained from the gamma rays generated from neutron interactions with the formation.1 Therefore, the use of a PNG in the new LWD tool gives the option of obtaining nuclear logs without the use of a chemical logging source.
Sinclair, Paul (CBG Corporation, Austin, Texas, USA) | Anzong, Li (China Petroleum Logging Co. Ltd., Xi’an, Shaanxi, PRC) | Otteman, Aaron (Wold Oil, Casper, Wyoming, USA) | Wennekamp, Alisha (Prosper Petroleum, Calgary, Alberta, CANADA)
Abstract The development of unconventional oil and gas resources is critically dependent on accurate geosteering of horizontal wells. A new type of LWD Azimuthal Laterolog Resistivity logging tool has been introduced that is specifically designed for geosteering and formation evaluation in complex carbonates, shale-gas, shale-oil, and coal-bed methane formations. The new tool is able to detect approach to a distant parallel bed-boundary of contrasting resistivity that is either higher or lower than the borehole environment. This enables predictive geosteering if the tool is not installed too far from the drill-bit. Conventional wave-propagation tools respond mainly to nearby conductive anomalies, which can limit their application in geosteering shale-oil wells where the target formation is often lower resistivity than those on upper or lower boundaries, or when the borehole traverses a fault and the target formation is higher resistivity and located in an unknown direction. Laterolog types of tools are capable of accurate measurement over a wide range of resistivity from 0.2 – 20,000 ohm.meters, enabling hydrocarbon saturation calculation even in tight carbonate formations. Many carbonate formations, and most coal seams, produce from natural fractures or vugs. The new tool has thin-bed resolution of 0.14 meters, enabling it to image the more productive zones for real-time production analysis while drilling, with significant potential cost-savings. The GRT tool has been commercial for two years, accumulating several thousands of hours of drilling time in operations provided by several independent MWD service companies. Successful geosteering jobs have been completed in the Montney, Bakken, Viking (Canada), Niobrara, and Avalon (USA) unconventional formations. The first test in China was in Talimu Oilfield, where the new tool was able to reproduce logs from a wireline Dual Laterolog in resistivities ranging from 20,000 – 100,000 ohm.meters. Tests have also been run in Tuha and Changqing (Sulige) oil and gas fields. A description of the tool operating theory, design objectives, unique real-time geosteering display, and logging software is presented. Some results from three years of operations in North America and China will be discussed along with logs and brief case studies.
Summary Six horizontal wells were drilled into the Tertiary Chatt Sand reservoir of the Breitbrunn gas field in Bavaria, Germany. The purpose of this campaign was to develop part of the depleted reservoir into a gas-storage sand. A detailed geological and petrophysical study was prepared before drilling and resulted in the identification of high-quality reservoir layers that were targeted by the horizontal wells. Despite the simple anticline structure of the field, geometric drilling was ruled out because of remaining geological and directional uncertainties. The geosteering approach adopted relies on real-time resistivity-at-bit images, which were used for the first time during this drilling campaign. The image data are compressed downhole and transmitted to the acquisition computer on the rig, where they are decompressed and analyzed. The images allow the precise placement of the borehole relative to the geology. Layer heterogeneity such as tight streaks, concretions, or patchy porosity can be identified as such and is not interpreted as a different layer entered by the hole, which would lead to a wrong geosteering decision. Logging-while-drilling (LWD) azimuthal data are acquired during drilling and during washdown passes that follow a bit change. A comparison of these time-lapse data sets can provide invasion profiles through time and around the borehole. Introduction The Breitbrunn/Eggstatt gas field was discovered in 1975 in southern Bavaria, Germany (Fig. 1). The northeast/southwest-striking anticlinal structure covers approximately 30 km and consists (from top to bottom) of sands A through H, with sands A through D being the original gas producers. The lower sands are wet. The individual reservoir layers range in thickness from approximately 5 to 15 m and are separated by impermeable calcareous shales. The immature sands were deposited immediately north of the rising Alpine orogen during the Tertiary. Mineralogically, they consist of carbonate sand (principally dolomite, quartz, and micas). The sands were deposited in a fluvial/deltaic setting. Initial production was by vertical wells drilled on the top of the structure. After depletion of the reservoir, layers A and B were converted into a gas-storage reservoir, with layer B being the storage sand while layer A functions as the monitoring unit for gas-leak detection. During that campaign, five horizontal wells were drilled underbalanced and completed openhole. Geosteering decisions were then based on cutting analyses then. The demand for natural gas in the region led to the second drilling campaign, discussed here, with the aim of increasing the storage capacity of the reservoir. The remaining original gas sands C and D were targeted for storage development because it was known that they possessed sufficient porosity and permeability, although with greater geological and petrophysical heterogeneity than the upper two sands. Reservoir quality in this field deteriorates from the top sand downward. The drilling phase of sands C and D was preceded by geological, petrophysical, and geomechanical studies. The goal of the geological study was to achieve a structural accuracy of 0.1 percent, which translates into a maximal depth inaccuracy of 1.5 m. This was achieved by resurveying well locations and using gyroscopic and ring-laser directional surveys from cased-hole runs for adjusting all log-derived marker picks to a common baseline. The vertical pilot well and the subsequent horizontal development wells confirmed that this depth accuracy was achieved. Petrophysical evaluations and the depositional setting of the reservoir sands predicted that the sands are present in lenticular form, with additional complication provided by the presence of calcareous concretions occurring suspended in the sands as well as present in horizons with varying concretion packing density. Core and borehole images taken in the pilot well provided conclusive evidence for this interpretation. These findings demanded the development of storage sands with horizontal wells designed to penetrate as many of the potentially isolated reservoir lenses as possible. Three horizontal wells were planned and drilled for each of the two sands. The trajectories of the horizontal wells, initially designed to penetrate the sand subhorizontally up to 300 m in length, were drilled in a gentle U-shaped profile, which allows the well to transverse the sand from top to bottom and back to the top within each horizontal section. The adopted geosteering concept led to horizontal wells reaching up to 1000 m in length (Fig. 2). Geomechanics and Well Planning Sanding during production cycles is of greatest concern and requires a detailed geomechanical study before drilling these storage wells. The maximum horizontal stress is oriented north/south, and the minimum horizontal stress is east/west, with the vertical principal stress being the intermediate stress typical for a strike/slip regime. Well azimuths based on the direction and magnitudes of the principal stresses alone suggest drilling north/south wells and minimizing drilling-induced formation damage during drilling and the production phase of the wells. An intensive rock-strength testing program was conducted to verify assumed rock-strength isotropy. The uniaxial compressive strength of 10 differently oriented plugs taken from one continuous core section was measured. The tests showed distinct strength anisotropy. Minimum strength reaches approximately one-third of the maximum strength value. The maximum-strength component runs approximately north/south parallel to the maximum horizontal stress, while the minimum-strength component runs in an east/west direction. Additional tests on moist cores showed a significant reduction of rock strength compared to dry core. In situations like this, it is important to consider in-situ water saturation for geomechanical calculations. The optimum orientation of the horizontal storage wells cannot be derived from the orientation of the stress field alone. Rock-strength anisotropy must be taken into account as well. Another aspect for planning the trajectories is structural position. Stress is likely to increase at the anticlinal flanks, and wells placed closer to the anticlinal axis away from the lower flanks will be more stable. The final calculations concluded that the optimum well azimuth is along the northeast/southwest-striking axis of the anticline. This direction is perpendicular to what would have been arrived at assuming isotropic rock subjected to the described stress regime.
A new Logging-While-Drilling tool has been developed, using innovative technology to provide simultaneous formation evaluation and drilling-related measurements. Using only a single short collar, the tool delivers an industry standard suite of formation evaluation measurements equivalent to the classic triple combo service, but co-located and closer to the bit. In addition, the same tool brings a wide variety of measurements which are completely new to the world of logging while drilling. This approach makes it possible, for the first time, to deliver in real time at the wellsite a completely self-contained and very comprehensive interpretation of both the formation properties and the drilling environment.
The new measurements delivered by this service include formation capture cross section (Sigma and elemental analysis from neutron capture spectroscopy which is used to compute mineralogy. Azimuthal measurements are also available, allowing the formation and borehole to be imaged using a variety of properties. The result is a robust and complete interpretation with significantly reduced uncertainty.
In addition, a key focus during the development of the tool has been improvement in service delivery. This has been achieved through factors such as increased operational efficiency, higher ROP capability, significantly improved reliability throughout the system, and greater ease of maintenance. The use of a non-chemical radioactive source also dramatically reduces the operational risks normally involved with the use of traditional logging while drilling tools.