Africa (Sub-Sahara) Sahara Group discovered hydrocarbons in three wells drilled in Block OPL 274, located onshore in Nigeria's Edo State. Olugei-1 was drilled to a measured depth of 4537 m and encountered five hydrocarbon zones, with 33 m of net pay. Oki-Oziengbe South 4 was drilled to a measured depth of 3816 m and encountered 64.3 m of net pay in 13 hydrocarbon-bearing zones. Oki-Oziengbe South 5 was drilled to a measured depth of 3923 m and encountered 91 m of net pay in 19 reservoirs. Sahara Group (100%) is the operator. Asia Pacific Sino Gas & Energy Holdings (SGE) flowed gas (coalbed methane) from its first horizontal well in the Linxing production sharing contract (PSC) in China's Shanxi province.
Commencement of initial field production is a unique opportunity to acquire reservoir surveillance information that can inform future reservoir performance. When a field is perturbed from original conditions with first production, there is potential for reservoir property uncertainty reduction by observing pressure measurements at non-producing wells with downhole pressure gauges and comparing the observed signal to a range of simulation model results.
The Wheatstone field, located offshore northwest Australia, has recently commenced production start-up to supply gas to the Wheatstone LNG facility. The operational guidelines required each development well to commence with a single well cleanup flow to the Wheatstone platform. The initial single well cleanup flows of the Wheatstone field allowed scope for the selection of a well flow sequence with observation at non-producing wells.
The recommended sequence of initial cleanup flows was designed with a focus on reducing reservoir uncertainties via the use of Ensemble Variance Analysis (EVA). EVA is a statistical correlation technique which compares the co-variance between two sets of output data with the same set of inputs. For the Wheatstone field well cleanup flow sequence selection, the EVA workflow compared the full field Design of Experiments (DoE) study of field depletion and a series of short early production reservoir simulation DoE studies of the gas field. The co-variance between the two DoE studies was evaluated. The objective of the EVA approach was to determine the startup sequence that would allow for the best opportunity for subsurface uncertainty reduction. This objective was met by ranking multiple cleanup flow sequence scenarios. The key factors considered for sequence selection ranking were the impact on business objectives such as future drilling campaign timing and location of infill wells, as well as insights on reservoir connectivity, gas initially in place and permeability.
The recommended sequence of well cleanup flows uses super-positioning of pressure signal to boost response at observation wells, which improves measurement resolvability. The selected sequence preserves key observation wells for each manifold and reservoir section for as long as possible before those wells were required to be flowed to meet operational requirements. Operational constraints and variations of the startup plan were considered as part of the evaluation.
In an era of automated workflow-assisted dynamic modelling, Special Core Analysis (SCAL) parameters require updating for each static realisation and evaluation at a quantifiable, probabilistic level-of-certainty. Additionally, SCAL data gaps combined with limited reliable SCAL data drive the need to establish trends and correlations from analogues.
SCAL parameters from analogue fields were selected and filtered by depositional environment and laboratory experiment type (centrifuge versus displacement). These analogue SCAL parameters were allocated to statistical bins defined by absolute permeability ranges. Statistical analysis of each SCAL parameter allocated to each permeability bin produced a probability distribution discretised by percentile. Multi-variable linear regression (MVLR) was then implemented to correlate each SCAL parameter, as the response variable, to input variables absolute permeability and percentile. SCAL correlations of reasonable to excellent quality were obtained.
The depositional environment was of second order influence in establishing these SCAL correlations. This was due to the selection of core plugs for laboratory analysis from layers of similar quality irrespective of the depositional environment, highlighting the need to select samples characterising a range of lithology and reservoir quality. Centrifuge experiments of water displacing gas were discarded as unreliable due to the compression of the gas phase by the experimental technique.
The multi-variable linear regression methodology enabled SCAL parameters to be determined as a function of both absolute permeability and probability. This approach should enable an automated implementation of SCAL parameters within each dynamic model realisation.
This paper presents case studies on reservoir and well management of two laterally and vertically compartmentalized Western Australian Triassic gas condensate reservoirs, developed by five multi-zone "smart" wells with sand control, tied back to an offshore platform via a subsea network. In managing assets with such complexity, it is imperative to understand reservoir performance on a zone-by-zone basis. Quantifying performance allows management of flux through downhole sand control systems and optimisation of offtake strategy. The majority of the material published to date on "smart" wells has been focused on completion design optimisation and minimisation of unwanted oil/water production. There are few existing articles about production and reservoir optimisation of high rate gas wells requiring flux management.
This paper showcases how remotely-operated selective completions ("smart" wells with permanent downhole gauges for each completion coupled with subsea flow meters for each well) have been instrumental in facilitating prompt analysis of zonal reservoir performance and thus in yielding insights into reservoir connectivity and allowing optimisation of zonal contributions. Various case studies will be presented showing how reservoir surveillance data is acquired and interpreted to optimize well zone-by-zone production and to manage flux limits on each producing zone. These case studies will include manipulation of downhole valves to provide information for established techniques such as interference testing and P/Z analysis.
Data acquisition and interpretation challenges are highlighted along with fit-for-purpose solutions developed to overcome those challenges.
The insights presented could facilitate better planning of similar systems in the future.
The application of chemostratigraphy to problems in modern and ancient environments has a long and successful history. In particular, the use of high-resolution X-ray fluorescence (XRF) spectrometry for studying the elemental content of core and rock at the sub-millimeter scale to understand provenance, grain size, paleoredox state, terrigenous influence, and other aspects of strata is well documented in paleoclimatology literature.
In this paper, we use spectral-decomposition analysis, curvature and Self-Organizing Maps to delineate and study the internal architecture of incised canyons located in the Mandu Formation, Exmouth Plateau, Australia. Introduction Spectral-decomposition analysis is a powerful technique that estimate the magnitude and phase components of the seismic data at time-frequency samples, thus allowing us to interpret geologic features of interest at different scales (Chopra and Marfurt, 2016).
Summary In this paper, the results of a cognitive interpretation study are shown to identify potential geological risk to the Thebe Discovery and to highlight other future potential in the surrounding area. Introduction The Thebe Discovery located on the Exmouth Plateau, North Carnarvon Basin, offshore Northwest Australia was discovered in 2007 with the successful drilling of the Thebe-1 well which penetrated a 73m gas column. The Thebe-2 appraisal well was drilled 18.75km NNE of discovery well in 2008, again encountered a significant gas column, leading to estimates of 2 to 3 Tcg of gas in place within the Thebe structures. To this day, the Thebe Discovery remains undeveloped. The gas accumulation encountered by the Thebe-1 and -2 wells is clearly visualized in the seismic, as a large scale "flat-spot", which crosses multiple reflections in the crest of a rotated fault block within the Triassic Mungaroo Formation.
The offshore Wheatstone liquefied natural gas (LNG) project in Western Australia uses subsea big-bore gas wells as the preferred method of producing the field. Wheatstone wells use a 9 5/8-in. production conduit from the top of the gas pay zone to the ocean floor. Wellbores of this size are necessary to match the large productive capacity of the gas reservoirs they penetrate. This producing scenario provides the obvious benefit of yielding large volumes of gas through the use of relatively few wells. Each of those highly productive wells, however, also represents a source of gas that, if accidentally allowed to flow unhindered, could present an uncommonly difficult well-control challenge. It is for this reason that the Wheatstone Drilling and Completions (D&C) Team evaluated a wide range of possible reservoir- and well-architecture scenarios to fully understand the possible scale of relief-well responses that might be necessary in the event of a blowout. The conclusions from this evaluation were surprising. Our original well-design concept called for penetrating the Wheatstone gas reservoirs with a casing shoe set 3,100 ft vertically above. Our analysis indicated that three or four relief wells would be simultaneously required to bring a blowout under control. Because of these results, both the well- and drilling-execution plan were redesigned to minimize the number of required relief wells. In summary, the redesign amounted to setting the casing immediately (i.e.,<=10 ft) above the gas reservoir before actually penetrating it, with the resulting benefit of reducing the required number of relief wells to two. Although this reduction is beneficial, it should be noted that there is only one documented subsea case where two or more relief wells have been drilled with the intent of simultaneously pumping into both to effect a dynamic kill. Given this fact, our well-control-related preparations for executing this project were more extensive than those of preceding projects.
This paper chronicles the full extent of the engineering and operational planning performed to ensure that no uncontrolled hydrocarbon releases occurred during the execution of the Wheatstone Project’s subsea big-bore gas wells and, if a blowout were to occur, that the response to such an unprecedented event would be sufficient and robust. Covered in this paper are reservoir-deliverability modeling, dynamic-kill modeling, gas-plume modeling, relief-well trajectory and mooring planning, pilot-hole-execution planning, a newly applied logging-while-drilling (LWD) technology for sensing resistivity vertically below the drill bit, and a discussion of future research identified as necessary to better define the fluid-injectivity capabilities of subsea relief wells.
The Brigadier Formation is a thinly bedded reservoir that contains approximately 40% of in-place gas resource in Wheatstone field, which underpins the two-train Wheatstone liquefied natural gas (LNG) project in Western Australia. The development drilling campaign has recently been completed with three Wheatstone development wells targeting the Brigadier formation in the northern part of the field. Accurate and timely determination of net reservoir thickness is crucial not only for evaluating field volumetrics and performance, but also to support time-sensitive drilling decisions. In the case of two of the Brigadier wells, the decision to accept a development well location or move to a contingent well location was required within 72 hours of penetrating the reservoir with a pilot hole. Additionally, TD (total depth) decisions in the Brigadier Formation were made on the basis of real time kH (cumulative permeability thickness) evaluation from logging-while-drilling (LWD) logs and are essential for balancing well deliverability requirements with minimising risk of early water breakthrough by optimising standoff from the aquifer.
A fit-for-purpose approach to calibrate and evaluate net reservoir in the thinly bedded Brigadier Formation will be discussed. Several methods of net reservoir determination have been tested in the Wheatstone field. Standard resolution methods like density-neutron cross-over, and photoelectric factor, and high resolution methods like electrical image logs - water-based formation micro-imager (FMI) and oil-based new generation imager (NGI) - and LWD alpha processed density are compared against core-based sand counts to derive the most reliable and fit-for-purpose method of net reservoir determination. Mud-type, conveyance methods and borehole condition also impacts the results of net reservoir evaluation.
The results from a combined density-neutron-photo electric factor method was found to compare very well to core net reservoir and image log-derived net reservoir, across mud types, reservoir fluids and hole angles. It is locally calibrated and blind-tested successfully across different wells. The nuclear tools are logged in every well in the field so this method of calculating net reservoir could be applied consistently across all wells. An added advantage is that the evaluation of net reservoir is independent of porosity and hence, net reservoir will not need to change with different generations of petrophysical evaluation.
The Exmouth Plateau is a subsided, stretched and rifted continental platform that forms the northern part of the Northern Carnarvon Basin off Western Australia's northwest coast. The plateau is bound on three sides by oceanic crust and consists of more than eight kilometres of Palaeozoic to Mesozoic sediments (
Structural interpretation from seismic data is one of the most important steps in understanding the subsurface. Geoscientists spend a considerable amount of interpretation time picking faults and horizons from seismic data to understand the subsurface structure. A traditional interpretation workflow, commonplace in the oil and gas industry, is to consecutively investigate separate 2D sections and subsequently combine the interpretations to build a 3D picture. This limits the understanding of the subsurface geology in highly faulted, structurally complex areas.
This study involved the application of a new workflow for producing a structurally validated interpretation on 3D seismic data from the eastern part of the Exmouth Plateau. This workflow incorporates seismic preconditioning, fault framework modelling, structural reconstruction and structural analysis techniques to validate the interpretation ( A new workflow to produce structurally validated interpretations in structurally complex regions.
A new workflow to produce structurally validated interpretations in structurally complex regions.
The framework model also becomes the foundation for geo-cellular modelling and further detailed analysis of dynamic behavior. This ensures the verified structural interpretation is carried throughout the entire exploration and production lifecycle.