The development of basement reservoirs can add significant upside to regional reserves but because of perceived drilling complexity are often overlooked or underexplored. Modern drilling technologies, such as directional drilling and formation evaluation while drilling, have been shown to improve commercial viability of appraisal and development projects in frontier areas. A granitic Type-1 fractured basement reservoir offshore West of Shetland was horizontally drilled and evaluated by means of directional drilling (DD) and logging-while-drilling (LWD) technologies. This was the first time that drilling and completion of a 1-km horizontal basement well had been attempted in the UK, and the project goals were successfully achieved. The project as planned fell into the extended-reach drilling (ERD) envelope, defined as a well in which the ratio of measured depth (MD) is greater than two times the vertical depth. The ERD ratio [MD to true vertical depth (TVD)] was 2.21. This success resulted from a combination of drill bit selection, bottomhole assembly (BHA) stabilisation optimisation, rotary steerable drilling system usage, and close cooperation between the operating company and service providers. The logging data acquired were of sufficient quality to support detailed petrophysical study. The basement reservoir section was shown to have significant storage capacity within a network of fault zones, joints and microfractures, as identified using a LWD resistivity microimaging tool. The LWD imaging data were compared to offset well wireline imaging data and used to evaluate the microfracture and joint network across the horizontal section. This evaluation confirmed the storage capacity of the reservoir and was later supported by successful commercial extended well testing. In addition to fracture characterisation, the LWD data were successfully used to evaluate the fracture porosity. Fracture porosity and fracture characteristics in the horizontal well were both consistent with those derived from previously drilled inclined wells, providing the operator with confidence in its reservoir model. This paper documents the drilling and petrophysical evaluation of the first 1-km horizontal well in a fractured basement reservoir on the UK continental shelf. Further development of similar hydrocarbon-bearing basement fields would greatly increase regional recoverable reserves.
With the introduction of Ultra-deep azimuthal resistivity (UDAR) logging while drilling (LWD) tools towards the beginning of the last decade, the Oil and Gas industry went from real time mapping of formation boundaries a few meters from the wellbore to tens of meters away. This innovation allowed early identification of resistivity boundaries and promoted proactive geosteering, allowing for optimization of the wellbore position. Additionally, boundaries and secondary targets that may never be intersected are mapped, allowing for improved well planning for sidetracks, multi laterals and future wells. Advancement in tool design and inversion algorithms has allowed mapping the reservoir in 3D and exploring the sensitivity of these tools ahead of the measure point to provide look ahead warning of resistivity boundaries. Improvements in the technology over the decade have changed the way wellbores are planned, drilled, completed and reservoir models are updated. This paper presents a case study summarizing the advances in UDAR measurements and inversions over the last decade. The case study presents the whole workflow from pre-job planning, service design and execution of 1D and 3D inversion in addition to the future potential of look ahead in horizontal wells. Prewell simulations provide a guide to expected tool responses real-time in the highly heterogeneous formations. This validates how far from the wellbore 1D inversions can map major boundaries above and below the well. A fault was expected towards the toe of the well, UDAR was used as a safeguard to avoid exiting the reservoir. Standard 1D inversion approaches are too simplistic in this complex geologic setting. Thus, 3D inversion around the wellbore and ahead of the transmitter is also explored to demonstrate the improvements this understanding can bring regarding geostopping towards the fault and reservoir understanding in general. Successful geosteering requires personnel trained to handle the uncertainties. A geosteering training simulator (GTS) could be an efficient tool for training, to interpret inversions where the “truth” is known from realistic 3D model scenarios. The team can learn how to best exploit UDAR-technology and inversion results within its limits and not extend the interpretation beyond acceptable uncertainty levels. It will also be addressed how the understanding of inversion uncertainty could be updated real-time in the future. Continued future success of UDAR-technology and 1D – 3D inversion results for look ahead and look around applications will depend heavily on uncertainty management of the inversions to avoid wrong decisions and potential reduced well economy.
Well placement within thin and discontinuous reservoirs continues to prove challenging in present-day field development. Some geological objectives require draining accumulations within discontinuous reservoir fairways with thin true vertical depth (TVD) thickness (<7 m). The ability to geosteer within these complex systems using modern azimuthal tools has provided some solutions; however, there are multiple other elements contributing to successfully landing a drain with such reservoir scenarios. Turbidite channels are common within the offshore Niger Delta systems and in many other basins. The Niger Delta Basin is predominately a clastic system, and the reservoir targets in this fairway are a mix of structural and stratigraphic traps made up of sand and shales deposited during the Early Pliocene period. These systems are generally described as turbidite channellevee complexes. This paper discusses a case study using two recently drilled wells to analyze the technique/approach used for a successful and safe well placement operation. This approach involves two parts: the use of technology (geosteering tools) and the role of communication for a successful well placement operation. The primary tool used was azimuthal deep resistivity, which uses resistivity contrast within beds to help geosteer and stay within reservoir bodies, hence optimizing well placement. Guided by azimuthal resistivity imaging, it was possible to determine the well direction relative to the beddings using oriented binned data and resultant images. The communication aspect involved prejob, on-the-job, and post-job elements that contributed extensively to successful operations. A closed-loop approach to decision making was implemented whereby azimuthal resistivity data (and geosignal ratio curves) were measured and transmitted in real time, then analyzed by a team in the office collaboration room who transmitted information back to the rigsite for implementation. This paper also documents the uncertainties associated with the measurements and the processes available to mitigate them as well as lessons learned. Two wells were placed within undrilled fairways with reservoir and depth uncertainty. With the help of pilot holes 6 and 7-m TVD thick, hydrocarbon sands were discovered. Drains of 400 and 700 m were placed within these fairways, and each well exhibited good productivity. Interpretation of geosignals measured while drilling along with real-time follow-up on the seismic and knowledge of the geological setting were instrumental in the successful placement of these producing wells. The decision-making and analysis process was optimized, thereby achieving operational excellence (health, safety, and environment and timing) and cost savings. The most significant element of these operations is communication. The ability to analyze information and implement decisions rapidly involved all essential disciplines from service company personnel to drilling and completions to geosciences. Advancements made in geosteering technology and lessons learned from this case study can be applied to future well planning for geological targets originally assumed to be difficult, impossible, or too thin to be successfully drilled to increase field productivity.
A new azimuthal electromagnetic (EM) logging-while-drilling (LWD) tool has been developed with multiple tilted antennas to measure three-dimensional (3D) electromagnetic fields. Multiple field trials successfully demonstrated the ultradeep detection range of more than 200 ft (60 m) with various transmitter-to-receiver spacings and operating frequencies, providing valuable geomapping insight for large-scale reservoir development. Additionally, this paper reveals the tool's capabilities in different geosteering applications, requiring different depth of detection (DOD) ranges for landing a well, optimizing well placement in thin reservoirs, and eliminating the need for a pilot hole. This paper discusses in detail a new 3D finite-difference (FD) method to simulate realistic and complicated formation structures in three dimensions, enabling accurate formation interpretations and inversion of reservoir geology. Solving the scattered potential boundary value problem with the 3DFD numerical algorithm simulates the EM signals in this new LWD ultradeep application, and the modeling accuracy was benchmarked alongside in-house modeling codes and 3D commercial software. To accelerate the computation in the 3D modeling, sliding window, multicore parallel cloud computing, and decoupling between model pixel grid and FD simulation grid have been implemented for practical applications. Additionally, 3D modeling is used in the inversion to provide more accurate and complex reservoir determinations. In addition to inversion, the tool provides 3D azimuthal multispacing, multifrequency geosignal, and resistivity measurements. Using the inversions and the 3D azimuthal images of the geosignal and resistivities enable improved reservoir understanding and geosteering decisions for the three dimensions. This paper describes two field trials from relatively thin to thick reservoirs to establish great and flexible geosteering performance because of multispacing, multifrequency measurements, and a robust signal and inversion process to optimize wellbore placements in the reservoir.
With the introduction of ultradeep azimuthal resistivity (UDAR) logging-while-drilling (LWD) tools toward the beginning of the last decade, the oil and gas industry went from real-time mapping of formation boundaries a few meters from the wellbore to tens of meters away. This innovation allowed early identification of resistivity boundaries and promoted proactive geosteering, allowing for optimization of the wellbore position. Additionally, boundaries and secondary targets that may never be intersected are mapped, allowing for improved well planning for sidetracks, multilaterals, and future wells. Modern tool design and inversion algorithms allow mapping the reservoir in 3D and exploring the sensitivity of these tools to the electromagnetic field ahead of the measure point for look-ahead resistivity. Improvements in the technology over the past decade have changed the way wellbores are planned, drilled, and completed, and reservoir models are updated. This paper presents a case study summarizing the advances in UDAR measurements and inversions over the last decade. The case study presents the whole workflow from prejob planning, service design, and execution of one-dimensional (1D) and three-dimensional (3D) inversion in addition to the future potential of look ahead in horizontal wells. Prewell simulations provide a guide to expected real-time tool responses in highly heterogeneous formations. This identifies how far from the wellbore 1D inversions can map major boundaries above and below the well. A fault was expected toward the toe of the well, and UDAR was used as a safeguard to avoid exiting the reservoir. Standard 1D inversion approaches are too simplistic in this complex geologic setting. Thus, 3D inversion around the wellbore and ahead of the transmitter is also explored to demonstrate the improvements this understanding can bring regarding geostopping toward the fault and reservoir understanding in general. Successful geosteering requires personnel trained to handle complex scenarios. Geosteering training simulators (GTS) could be efficient tools for training to interpret inversions where the “truth” is known from realistic 3D model scenarios. The team can learn how to best exploit UDAR technology and inversion results within its limits and not extend the interpretation beyond acceptable uncertainty levels. It will also be addressed how the understanding of inversion uncertainty could be updated in real time in the future. The continued future success of UDAR technology and 1D to 3D inversion results for look-ahead and look-around applications will depend heavily on uncertainty management of the inversions to avoid wrong decisions and potentially reduced well economy.
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