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.
Several challenges are associated with the characterization of low permeability reservoirs, most significantly for the enhance oil recovery operations. The scope of this work presents the integration of petrophysics data and its application in selection of the Microfrac intervals to measure downhole fracture-initiation pressures in multiple carbonate reservoirs located onshore about 50 km from Abu Dhabi city. The objective of characterizing formation breakdown across several reservoirs is to quantify the maximum gas and CO2 injection capacity on each reservoir layer for pressure maintenance and enhance oil recovery operations. This study also acquires pore pressure and fracture closure pressure measurements for calibrating the geomechanical in-situ stress model and far-field lateral strain boundary conditions. The case study concentrates on the multiple carbonate reservoirs that consists of a succession of clean limestone and intermittent dolomitic limestone. The complex carbonate lithology and fabric combined with low permeability presents a challenge to conventional logs and evaluation. Detailed integration of advanced and conventional logs (resistivity, neutron/density, advanced acoustic logs, Dielectric, NMR, Borehole image), Pressure testing & Sampling, Microfrac in-situ stress measurements and analysis plays a critical role in characterizing the reservoir properties and enhance oil recovery operations. Extensive data gathering is conducted with wireline suite, which covered Advanced Straddle Packer/Pressure Test & Sampling - Resistivity/Density/Neutron/Spectral GR – Acoustic logs – Resistivity Image – NMR – Dielectric technologies for reservoir properties of multiple carbonate reservoirs. The advanced acoustic analysis is performed in order to study elastic properties of the formation along with identifying transverse and azimuthal anisotropic intervals. The Geomechanical modeling is performed and stress profile is calculated to identify intervals with a stress contrast, which is important for the following stress measurement interval selection. The Microfrac in-situ stress measurements provide critical subsurface information to accurately predict wellbore stability, hydraulic fracture containment and CO2 injection capacity for effective enhance oil recovery within these reservoirs. The conventional logs, advanced logs, and Microfrac in-situ measurements and analysis enabled reservoir characterization and development plans for enhance oil recovery operations. The NMR technology provided lithology independent total porosity, permeability estimations and reservoir rock quality. Advanced multifrequency Dielectric measurement provided the fluid saturation in the invaded zone and textural parameters. Advanced Acoustic and image logs provided the geomechanical properties that enable to choose the best intervals for the following Microfrac stress measurement. Geomechanical workflow allowed identifying stress measurement intervals with a good stress contrast in multiple carbonate reservoir intervals. The data integration work illustrated in the paper is a key for any reservoir characterization that enabled property evaluation and successful Microfrac stress measurement. These measurements provide critical subsurface information to accurately predict wellbore stability, hydraulic fracture containment and CO2 injection capacity for effective enhance oil recovery within these reservoirs. This in-situ stress wellbore data represents the first of its kind in the field allowing petroleum and reservoir engineers to optimize the subsurface injection plans for efficient field developing.
ADNOC has started several years ago few CO2 pilot projects to explore its feasibility for Enhanced Oil Recovery (EOR) in Rumaitha oil field in United Arab Emirates. The CO2 injector wells, to be discussed in this paper, were completed with open-hole horizontal completion, aiming to maximize CO2 injectivity by increasing the contact area between a wellbore and the formation. However, logging these wells for surveillance and intervention has been a challenge, due to the corrosive wellbore environment, tubing minimum restriction and depth reach limitation for both Coiled Tubing (CT) and conventional Tractor conveyance. The current study focusses on using new Slim-hole Tractor, run first time worldwide in CO2 injector wells to convey the logging tools across these long open-hole horizontal wells for rig-less reservoir monitoring and injection optimization. The advanced design Slim Tractor uses high expansion and reciprocating system for increased contact area with the wellbore, to convey logging tools in the horizontal open-hole and cased-hole completions. Several improvements were made over the existing conventional Tractors, such as the increase of pull out of hole capabilities, increased debris tolerance, improved gripping and be able to operate in sour environments. Furthermore, logging while tractoring feature for this advanced Tractor is a key differentiator in horizontal logging to achieve logging objectives the earliest possible while minimizing the acquisition time, reducing the footprint on the well sites, hence less HSE issues and better operations efficiency. This paper presents field experiment conducted on 3 wells in Rumaitha field. The Novel Slim Openhole Tractor was run successfully, first time worldwide in CO2 injector, to convey multiphase production logging tool across a long openhole horizontal completions, in order to determine CO2 zonal injectivity, investigate the presence of possible thief zones, CO2 flow behind the casing. These jobs were conducted real-time to optimize the logging operation and reduce CO2 exposure on the tools. Over 30,000 ft successful tractoring across the 3 horizontal openhole wells. Tractor depth reach exceeded the expectations, almost 100% achieved in 2 wells. The Slim Tractor has also successfully negotiated and passed across multiple washout zones and restrictions encountered, without any issues and the tools were retrieved to surface without any debris clogged on the Tractor arms. Excellent data quality was acquired from the multiphase production logging tool and pulsed Neutron tool during shut-in and flowing at different injection rates in extremely shorter time compared to CT, saving days of operating time. This study helped to delineate the conveyance strategies to be adopted in the upcoming CO2 openhole wells and contributed to enhance the understanding of zonal injectivity distributions across the reservoir. The results will be also incorporated into the reservoir model to understand the effect of injectivity on pore pressure, fracture and faults initiations and their effects on sweep efficiency in EOR and Carbon sequestration in carbon storage projects.
The scope of this work is to measure downhole fracture-initiation pressures in multiple carbonate reservoirs located onshore about 50 km from Abu Dhabi city. The objective of characterizing formation breakdown across several reservoirs is to quantify the maximum gas and CO2 injection capacity on each reservoir layer for pressure maintenance and enhance oil recovery operations. This study also acquires pore pressure and fracture closure pressure measurements for calibrating the geomechanical in-situ stress model and far-field lateral strain boundary conditions. Several single-probe pressure drawdown and straddle packer microfrac injection tests provide accurate downhole measurements of reservoir pore pressure, fracture initiation, reopening and fracture closure pressures. These tests are achieved using a wireline or pipe-conveyed straddle packer logging tool capable to isolate 3 feet of openhole formation in a vertical pilot hole across five Lower Cretaceous carbonate reservoirs zones. The fracture closure pressures are obtained from three decline methods during the pressure fall-off after fracture propagation injection cycle. The three methods are: (1) square-root of the shut-in time, (2) G-Function pressure derivative, and (3) Log-Log pressure derivative. The far-field strain values are estimated by multi-variable regression from the microfrac test data and the core-calibrated static elastic properties of the formations where the stress tests are done. The reservoir pressure across these carbonate formations are between 0.48 to 0.5 psi/ft with a value repeatability of 0.05 psi among build-up tests and 0.05 psi/min of pressure stability. The formation breakdown pressures are obtained between 0.97 and 1.12 psi/ft over 5,500 psi above hydrostatic pressure. The in-situ fracture closure measurements provide the magnitude of the minimum horizontal stress 0.74 - 0.83 psi/ft which is used to back-calculate the lateral strain values (0.15 and 0.72 mStrain) as far-field boundary condition for subsequent geomechanical modeling. These measurements provide critical subsurface information to accurately predict wellbore stability, hydraulic fracture containment and CO2 injection capacity for effective enhance oil recovery within these reservoirs. This in-situ stress wellbore data represents the first of its kind in the field allowing petroleum and reservoir engineers to optimize the subsurface injection plans for efficient field developing.
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