Fluid saturations alone cannot determine the economic viability of a reservoir. Because a reservoir with high water saturation can primarily produce hydrocarbons and one with low water saturation can have high water cut, operators need supplementary knowledge about fluid behavior within the rock to accurately predict recovery.
The study of petrophysics requires precise rock and fluid measurements to reliably predict dynamic reservoir behavior. Formation data and measurements regarding the quality and type of rock, the amount of void space within the rock, the fluid type, and the fluid mobility all help to determine hydrocarbon recoverability.
Unconventional Reservoir Formation Evaluation Challenges Addressed With Deployment-Optimized Open Hole Logging Solutions
A. Hameed, M. Bacciarelli, and P.J. Williams, Weatherford
It is estimated that only one-third of the remaining worldwide oil and gas reserves are conventional, and the other remaining reserves are in unconventional reservoirs whose evaluation requires appropriate measurements delivered in a cost-effective way. In the case of shales and other tight reservoirs, the defining characteristics are low matrix porosity and low or ultralow permeability, which requires artificial stimulation to encourage production. The optimum stimulation strategy for a particular reservoir is strongly dependent on the distribution of organic material, on the mechanical and geometrical properties of the rock, and on the associated stress field. It is essential to quantify these to an appropriate level of certainty, and well logs are the primary source of such data. Until recently, the options for acquiring appropriate logs in high-angle and horizontal wells have been constrained by either the limited available sensors or the tool conveyance methods. However, the introduction of memory-capable, small-diameter, specialized tools and multiple innovative conveyance options has changed the cost-benefit balance for the better. This paper reviews the current status of openhole log measurements with full spectrum conveyance options and how they impact the evaluation of these challenging reservoirs.
Processing and Interpretation of Density and Neutron Logs for the Evaluation of Coal Bed Methane Reservoirs
James Anthony Whetton and Peter A.S. Elkington, Weatherford
Density and neutron well log processing algorithms designed for conventional oil and gas reservoirs are not optimum for coal bed methane evaluation. In particular the corrections applied to measured electron density values (to derive bulk density) assume a calcium carbonate rock matrix, and quantitative analysis of neutron porosity logs is hindered by low count rates in coal and a lack of published information regarding the sensitivity of the measurement to variations in coal composition. The thinly bedded nature of many coals is an additional challenge. This paper describes a new log processing method that simultaneously enhances statistical precision and vertical resolution whilst seeking to avoid additional sensitivity to the borehole environment. It then describes a fast nuclear rock properties modelling application developed to study the sensitivity of density, photo-electric cross-section (Pe) and neutron porosity measurements to variations in coal chemistry. The model has been validated using an accurate (but slow) Monte Carlo particle transport code which has been extensively benchmarked in independently characterized test blocks. The findings are applied to high-resolution log data acquired in wells drilled for the evaluation of coal bed methane reservoirs. The key parameter used in the transformation of electron to bulk density is investigated and optimum values suggested. The sensitivity of density and neutron porosity measurements to variations in the volumes and chemistry of organic material, mineral matter, and moisture is determined, and it is shown that appropriately processed neutron porosity logs have usable sensitivity to such compositional variations. The inclusion of neutron porosity improves our ability to differentiate coal types from logs and addresses an important source of uncertainty in the reconciliation of log and core density values; in so doing it helps improve estimates of in situ coal properties and associated quality attributes including gas-in-place.
Mitigating the Risks Associated With the Acquisition of Formation Evaluation Data
Robert S. Kuchinski and Robert J. Stayton, Weatherford
Acquiring formation evaluation data is a critical element in the drilling of any well regardless of the location, well type, or well geometry. Risks associated in acquiring data can be classified as follows: 1. Lost rig time due to the inability of logging tools getting to bottom (bridging). 2. Lost rig time due to delays associated with making last minute arrangements for alternative logging methods when bridging problems become too severe. 3. Lost in hole charges associated with permanently sticking logging tools due to bad hole conditions. 4. Unsound decision making throughout the life of the well because bad hole conditions made the acquisition of formation evaluation data impossible. The risks outlined above increase as wells are drilled at faster rates with higher geometric complexity. Advanced drilling technologies allowing for access to difficult to reach petroleum resources will increase as the industry exploits unconventional resources and increasingly complex reservoirs. Recent advances in formation evaluation technology and conveyance techniques allow for the acquisition of high quality petrophysical measurements regardless of borehole conditions or geometry. Logging systems conveyed in the drill string, or through the drill string, have a 100% chance of getting to bottom on the first attempt. With the logging tools securely attached to the drill pipe, the risk of the logging tools getting stuck is also reduced. The non-traditional conveyance techniques discussed in this paper bring operational optimization to formation evaluation.
Leveraging Slim Hole Logging Tools in the Economic Development of the Ghawar Fields
I. Ariwodo, A.R. Al-Belowi, and R.H. Bin Nasser, Saudi Aramco; R.S. Kuchinski and I. Zainaddin, Weatherford
Traditionally, brown field developments have often required the plugback and sidetrack of existing drain holes to target any nearby opportunities. With advances in drilling technology, there is a general preference to drill small-diameter wells because of the comparative cost advantage. In the recent times, this preference has led some wireline service companies to start to offer openhole formation evaluation services with slim tools with diameters in the 2-in. to 2½-in. range. At present, most traditional petrophysical measurements can be acquired using slim tools. In addition, several "specialized” measurements, such as cross dipole sonic, formation pressure testing, and resistivity imaging, can also be acquired. The use of battery and memory technologies has allowed deployment of these tools using a broader range of conveyance techniques for reduced risk in the entry of slim wells. The provision of slimhole logging services has created an opportunity to leverage these tools for the economic development of brown fields. Therefore, short horizontal sidetracks and well re-entries to test deeper horizons can be drilled and logged successfully. Saudi Aramco has leveraged these tools in its continued development of the giant Ghawar field. Some of the development projects are listed below: Some horizontal sidetracks with 3-in. hole sizes have been drilled under higher doglegs than was previously possible and logged successfully. It is now possible to run well completions in newly drilled wells that have a well control problem. A provision is made to subsequently log these wells with slim wireline logging tools. It is now possible to run a complete suite of wireline logs across some old wells that were previously completed without a full formation evaluation logging suite. Slimhole formation resistivity imaging services are now being provided to aid in identification of borehole breakout and fracture features that might affect the well productivity. Slimhole formation pressure testing has been acquired in slim wells to generate a pressure gradient, determine oil mobility, and define oil-water contacts.
A Novel Cased Hole Density-Neutron Log - Characteristics and Interpretation
Peter A.S. Elkington, Alex Pereira, and Roger Samworth, Weatherford
We have characterized the response of a compact 2¼-inch (57-mm) diameter openhole formation density logging tool for cased-hole environments. Data are processed with an established openhole transform in which the casing effect appears as a simple attenuation term in the count rate domain, and variations in cement thickness are compensated using a classical dual-detector spine-and-ribs approach applied in the density domain. The combination of through-casing density and casing-corrected neutron porosity has been applied to the evaluation of bypassed pay and shallow gas, and to the evaluation of wells where openhole acquisition has not been feasible for operational, hole-quality, or economic reasons. The tool-specific neutron porosity excavation effect has been characterized for gas-bearing sands. Case history and model results suggest accurate formation densities are achievable for casing thicknesses up to about 0.35 inches (9 mm) if cement is less than about 1 inch (25-mm) thick, albeit with loss of precision relative to open hole. Formation sensitivity declines with increased cement thickness until practically all is lost for casing standoffs in excess of 1.5 inches (38 mm). For modest thicknesses, however, cased-hole density-neutron gas evaluation has advantages relative to the neutron-dipole sonic method; in particular it does not rely on good cement bond, it has generally superior spatial resolution, and (optionally) data can be acquired in memory mode on slickline in operating environments that do not favor conventional wireline units.
A Pressure Activated Deployment System for Openhole Memory Logging Tools and its Application in Directional Wells
M.C. Spencer, S.C. Ash, and P.A.S. Elkington, Reeves Oilfield Services
Openhole memory logging was introduced in 1999 as a more efficient alternative to pipe conveyed wireline logging. It was developed subsequently into a shuttle system that conveys logging tools inside drillpipe and that provides formation evaluation logs after drilling to supplement basic real-time logging-while-drilling (LWD) data. The original shuttle design used a dart pumped from surface to move the logging string into open hole after the assembly reached TD. In a new development, the dart has been eliminated together with the associated pumping time. In its place is an electro-mechanical pressure-activated release mechanism capable of simple two-way communication. The method has been used in trials to activate and deploy logging tools downhole and to return status information to surface. It has the potential to be developed further for post-drilling real-time data transmission and is a key component in a repeat formation pressure tester being developed for memory operations.
The Role of Resistivity Image Logs in Deep Natural Gas Reservoirs
Robert Kuchinski and Paul Kalathingal, Weatherford
Occurrences of deep gas are commonly found in clastic reservoir rocks. The great depths of these reservoir rocks often result in rocks with low porosity and poor permeability. In order for these reservoirs to be productive, natural fracture systems or induced fractures from stimulation processes are required to allow gas to flow to the wellbore. In order to assess the potential of these reservoirs, explorationists around the world depend on data acquired from resistivity imaging tools to visualize the fractures. This paper will discuss fracture analysis using resistivity image data that provides better reservoir characterization of a fractured reservoir than the normal openhole logs. Image data has higher resolution than basic openhole data and allows for fracture visualization down to 2 mm. Petrophysical analysis using basic openhole data will average out the fine details of the fracture and its effects on porosity and permeability. Image-based petrophysics, known as "Image petrophysics,” allows for the conversion of high-resolution image data to high-resolution petrophysical properties, which provides greater understanding of the impact of the fractures on reservoir characterization. The use of Image data calibrated with either core data or basic openhole data allows for the derivation of high-resolution porosity distribution. Permeability derivation can also be achieved using image data calibrated with core data. This process can also be extrapolated to wells that are uncored. This paper will also discuss the challenges of acquiring image data from these deep environments by using memory based image tools that can be conveyed using a broad range of conveyance techniques.
Weatherford delivers the data you require by providing comprehensive petrophysical measurements.
Acquiring data by logging is the best way to determine these key petrophysical metrics. We can provide direct or interpreted well logging measurements for lithology, formation thickness, density, porosity, water saturation, hydrocarbon type, pore size distribution, and permeability.
We offer the latest measurement types and techniques for improved accuracy and reliability.
Our complete suite of logging tools and complementary software provides measurements across a range of borehole sizes and well trajectories. We deploy tools on wireline and also in memory mode via unique pipe-conveyed methods. The result is precise and valuable information for petrophysicists, drillers, geophysicists, and production engineers.
Weatherford obtained openhole logging data in an unlogged section of a producing well with high wellhead pressure in the Middle East.
The Compact™ triple-combo logging suite—with a small 2 1/4-in. OD and a short 20-ft (7-m) length—passed through the tubing and into the openhole section where it was safely rigged up in the lubricator.
Developed specifically for this application, the entire procedure required only two trips into the well and lasted just 10 hours. Using the Compact suite eliminated the need to remove the tubing and completion during a 3-day workover operation and significantly reduced lost production time.