No abstract

Typical industry standard for evaluation of cement integrity at downhole conditions is through the use of acoustic measurements. These systems generate compressional acoustic waves which travel through wellbore fluid to impact the casing. Attenuation of these waves is used to derive cement compressive strength or acoustic impedance bonded to the casing. Some of the long lasting fundamental limitations faced by the current technology are: The wellbore must be filled with liquid for the compressional wave to be able to travel to the casing wall. Heavy muds and solids particles in the mud can cause further complications.The in-situ compressive strength may not fully characterize cement mechanical properties.Micro-annulus condition can be difficult to differentiate from poor bond condition without a separate pressure pass. A new technology, which uses electro-magnetic acoustic transducers (EMAT), has set a new industry standard for cement evaluation. This new sensor technology is incorporated into an established and highly successful sectored pad-type tool design. This tool design provides multiple measurements azimuthally, making fully compensated measurements, allows good tool centralization in high wellbore deviations, and enables evaluation of multiples casing sizes in a single pass. The primary mechanical forces responsible for the failure of the cement sheath are shear forces differences between the wellbore and surrounding formation. The new technology measures the horizontal shear wave which responds to the cement in-situ shear modulus coupled to the casing. EMAT technology is wellbore fluid neutral and as such, eliminates the need of filling wellbore with fluid prior to logging. This presents a very important advantage as the cement condition can now be evaluated in downhole conditions similar to conditions when the well is on production. Additionally, fluid-filled micro-annulus can be better recognized without the need to pressurize the casing. The wider dynamic range of the new measurement also yields a higher resolution evaluation, provides us with the ability to evaluate cement bond in a wide range of heavy cement slurry density down to ultra-light cements and in contaminated cements, as well as other slurries types such as resins. This paper will discuss theoretical background, numerical modeling and actual recorded data demonstrating the above breakthrough advancements in cement sheath evaluation at in-situ conditions. While preserving the advantages of current technology, the EMAT measurement now provides operators with much needed flexibility to analyze, with consistent high resolution, cement bond quality over a wide range of slurry densities. Operators are no longer required to isolate existing opened perforations and load wellbores with fluid in preparation for logging, resulting in significant time and cost savings.

Dielectric dispersion measurements are increasingly used by petrophysicists to reduce uncertainty in their hydrocarbon saturation analysis, and subsequent reserves estimation, especially when encountered with challenging environments. Some of these challenges are related to variable or unknown formation water salinity and/or a changing rock texture which is a common attribute of carbonate reservoirs found in the Middle East. A new multi-frequency, multi-spacing dielectric logging service, utilizes a sensor array scheme which provides wave attenuation and phase difference measurements at multiple depths of investigation up to 8 inches inside the formation. The improvement in depth of investigation provides a better measurement of true formation properties, however, also provides a higher likelihood of measuring radial heterogeneity due to spatially variable shallow mud-filtrate invasion. Meaningful petrophysical interpretation requires an accurate electromagnetic (EM) inversion, which accommodates this heterogeneity, while converting raw tool measurements to true formation dielectric properties. Forward modeling solvers are typically beset with a slow processing speed precluding use of complex, albeit representative, formation petrophysical models. An artificial neural network (ANN) has been trained to significantly speed up the forward solver, thus leading to implementation and real-time execution of a complex multi-layer radial inversion algorithm. The paper describes, in detail, the development, training and validation of both the ANN network and the inversion algorithm. The presented algorithm and ANN inversion has shown ability to accurately resolve mud filtrate invasion profile as well as the true formation properties of individual layers. Examples are presented which demonstrate that comprehensive, multi-frequency, multi-array, EM data sets are inverted efficiently for dis-similar dielectric properties of both invaded and non-invaded formation layers around the wellbore. The results are further utilized for accurate hydrocarbon quantification otherwise not achieved by conventional resistivity based saturation techniques. This paper presents the development of a new EM inversion algorithm and an artificial neural network (ANN) trained to significantly speed up the solution of this algorithm. This approach leads to a fast turnaround for an accurate petrophysical analysis, reserves estimate and completion decisions.

Spectral analysis of natural and stimulated gamma rays is a well-established open-hole technology that enables accurate mineral characterization and petrophysical evaluation of conventional and unconventional reservoirs. The determination of detailed mineralogy in the cased-hole environment, however, has been a challenge because of the significantly increased uncertainties caused by the additional attenuation and contribution effects of casing and cement that are observed in the gamma ray spectra. The acquired spectral gamma ray data is processed with proprietary algorithms that are based on a combination of lab experiments and modeled tool response standards. The resulting elemental composition, corrected for the cased-hole environment, is further processed in an expert interpretation system to determine lithology and detailed mineralogy of the target formation. Candidates for this technology include older and newly cased wells where lithology and detailed mineralogy from open-hole logs are not available. This work discusses some aspects of the corrections needed for an accurate quantification of chemical elements from measurements through casing. The impact of casing collars and presence of cement on the spectral data are also discussed. Finally, we report a case study that uses this technology and illustrates a successful mineral characterization of a complex reservoir rock in the United States. Results show good agreement with an x-ray difraction (XRD) analysis of ditch cuttings. The resulting logs were productively used by the operator to understand the siliciclastic influx as well as the distribution of carbonates in the Big Lime formation. The results also show the ability of the methodology to identify organic carbon directly from measurements of the inelastic spectrum in the cased-hole environment. The application of pulsed neutron technology (PNT) in cased holes has, so far, been limited to basic lithology identification. This methodology expands the applicability of the PNT, previously mostly confined to open-hole cases, to the cased-hole environment and enables operators to take full advantage of the valuable information contained in high-resolution inelastic, capture and natural spectra. This enables characterization of hydrocarbon-bearing formations to a level of detail previously possible only from open-hole data.

The energy industry, including the new focus on geothermal and carbon sequestration processes, deals with porous and permeable formations. Under the influence of effective stress, these formations undergo elastic and inelastic deformation, fracturing, and failure, including porosity and permeability changes during production. Grain and Bulk moduli of elasticity are two key parameters that define net effective stress due to partitioning of stresses between the pore pressure and grain-to-grain contact stresses. Effective stress explains poroelastic behavior; however, tight rock behavior under in-situ conditions is still not predictable. This paper proposes a new method, which uses formation evaluation (FE) measurements, and an integration of rock physics and geomechanics concepts, to constrain effective stress in tight rocks. Examples are presented demonstrating the usefulness of the work. Effective stress (σ′) is expressed as the difference between total applied stress (σ) and pore pressure multiplied by Biot’s coefficient (α). The ‘α’ for highly porous rocks is unity where applied load is counteracted equally by grain-matrix and pore-pressure. However, for tight rocks, only a fraction of load is shared by pore fluid and the ‘α’ is much smaller than unity. Biot’s coefficient ‘α’ is expressed in terms of bulk modulus (Kb) and matrix modulus (Kma). Kb is estimated from acoustic logs as well as measured by hydrostatic compression tests in the laboratory. However, Kma is much more difficult to measure safely and economically, especially in tight or very low permeable formations, and as such, the common practice is to estimate it theoretically. A simple and clear methodology is proposed to estimate Kma from FE logs as well asX-RayDiffraction (XRD) mineralogy obtained from formation core and drill cuttings. Kma can be constrained by an upper-bound (Voigt, 1910), a lower- bound (Reuss, 1929), and an average of the two, (Hill, 1963) models. Kb, on the other hand, can be reliably estimated using dynamic acoustic wave velocity and the static equivalents calculated during calibrations from core tests under net effective in-situ stress conditions. The Kma and Kb, thus obtained, will give a good estimate of Biot’s coefficient ‘α’ in tight rocks. The work provides an improved estimate of net- effective-stress in tight rocks, which leads to safety and cost savings through better prediction of drilling rates, hydraulic fracture design and production decline. The work also examines a new method in which Kma could be estimated by weight fraction of minerals.