We consider the application of the magnetic flux leakage (MFL) method to the detection of defects in ferromagnetic (steel) tubulars. The problem setup corresponds to the cases where the distance from the casing and the point where the magnetic field is measured is small compared to the curvature radius of the undamaged casing and the scale of inhomogeneity of the magnetic field in the defect-free case. Mathematically this corresponds to the planar ferromagnetic layer in a uniform magnetic field oriented along this layer. Defects in the layer surface result in a strong deformation of the magnetic field, which provides opportunities for the reconstruction of the surface profile from measurements of the magnetic field. We deal with large-scale defects whose depth is small compared to their longitudinal sizes-these being typical of corrosive damage. Within the framework of large-scale approximation, analytical relations between the casing thickness profile and the measured magnetic field can be derived.
Measurements of magnetic flux leakage are used extensively to map and monitor defects and corrosion in pipelines, well casings, and storage tanks. The application of this method enables locating defects and determining metal loss. The measurements have complex response characteristics, which must be understood to optimize measurement design and data processing choices. Until recently, the industry lacked an adequate first principles description of sensitivity to defect penetration and size, including the foundational cases of circular and elliptical defects. An inversion method has been developed and validated with independent synthetic and lab data. The data was obtained using an analytical forward model, finite element modeling, and laboratory data from different thickness and diameter pipes with known machined defects of different sizes and penetrations. The approach enables inverting field data to reconstruct 3D defect profiles, which helps to assess casing health and offers valuable information to the decision-maker for well integrity.
Alternating conventional and unconventional reservoir layers in the Permian Basin challenge the acquisition, processing, and interpretation of water saturation (Sw) using nuclear magnetic resonance (NMR) log data. A new-generation NMR wireline tool addresses these challenges using a specially designed conventional-unconventional activation sequence to enable construction of optimized maps of Longitudinal–Transversal Relaxation times (T1-T2 maps) at regular depth intervals. T1-T2 maps are used to compute level-by-level Sw based on a multicomponent fluid model with appropriate statistical properties. Each spot in the T1-T2 space represents a fluid component from which a volume fraction is calculated. Integrating the volume fractions gives the total porosity. Because of the diverse relaxation mechanisms in the conventional and unconventional layers, oil spot positions with T1/T2 values greater than two reflect either viscosity (for bulk relaxation) or pore-size distribution (for surface/volume relaxation). Water tends to be close to the 1:1 T1/T2 diagonal line with T1/T2 values less than two. Low permeability means that mud-filtrate invasion does not appear on the T1-T2 maps. NMR porosity matched expected values based on core and density-neutron log analysis. NMR fluid-typing-derived Sw—including clay bound water (CBW), capillary bound water (BVI), and free water—matched values from tested intervals. Results are in good agreement with reference values from production and core data within an uncertainty of one standard deviation. The resolution of fluid components in intervals where the components overlap can be enhanced by changes in the inversion parameters and map-grid dimensions. This methodology for conventional-unconventional data acquisition followed by a multimodel approach for fluid typing will be applied to other wells. It enables a more accurate assessment of water saturation, especially when intercalated layers of conventional and unconventional reservoirs are present.
It is estimated that only one third of the remaining worldwide oil and gas reserves are conventional, the remainder being 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 ultra-low 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, and on the mechanical and geometrical properties of the rock, and 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 either by the limited available sensors or 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 open hole log measurements with full spectrum conveyance options, and how they impact the evaluation of these challenging reservoirs.
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