Zoya Heidari

Rapid, Interactive Assessment of Petrophysical and Geometrical Effects on Density and Neutron Logs Acquired in Vertical and Deviated Wells

Borehole, geometrical, and petrophysical effects can significantly affect density and neutron logs. Tool location around the perimeter of the borehole, tool standoff, and wellbore deviation have a measurable effect on density and neutron logs, hence on the estimation of porosity and fluid density. It is difficult to diagnose and quantify these effects a priori without numerical modeling. The multiple-particle, radiation-transport Monte-Carlo code (MCNP) has traditionally been used by the logging industry to simulate borehole nuclear measurements acquired in complex rock formations. Despite its versatility and accuracy, MCNP is not numerically efficient for rapid simulation of nuclear logs and does not lend itself to interactive testing of multiple petrophysical/fluid hypotheses. We describe the successful application of a new method for nuclear-log simulation based on linear iterative refinement of nuclear sensitivity functions pre-calculated with MCNP. The procedure is fast, accurate, and efficient in most practical logging applications, including the simulation of nuclear logs acquired in invaded formations and in highly-deviated wells. Density and neutron logs acquired over a depth segment of 1000 ft can be accurately simulated within minutes of CPU time, compared to days with MCNP. Simulations are successfully verified against MCNP in a number of extreme cases of borehole, petrophysical, and fluid conditions wherein the error of the simulations does not exceed 2 porosity units. We implement the new simulation method to reproduce several field examples where logs are affected by presence of clay and invasion with water- and oil-base muds. Our rapid simulation procedure enables the interactivel quantification of the relative effect of clay, invasion, fluid density, and rock petrophysical properties on field logs. It also permits efficient integration with induction resistivity measurements for assessment of free and clay-bound water saturation, as well as for the assessment of residual hydrocarbon saturation. .Additional simulations of density and neutron logs are performed to quantify the influence of shale laminations on hydrocarbon-bearing and invaded thinly-bedded formations. Because of their longer radial length of investigation compared to density logs, neutron logs may exhibit false cross-over effects across thin beds.

Multiscale characterization of pore structure in carbonate formations: Application to the Scurry Area Canyon Reef Operators Committee Unit

AbstractCarbonate formations consist of a wide range of pore types with different shapes, pore-throat sizes, and varying levels of pore-network connectivity. Such heterogeneous pore-network properties affect the fluid flow in the formation. However, characterizing pore-network properties (e.g., effective porosity and permeability) in carbonate formations is challenging due to the heterogeneity at different scales and complex pore structure of carbonate rocks. We have developed an integrated technique for multiscale characterization of carbonate pore structure based on mercury injection capillary pressure (MICP) measurements, X-ray micro-computed tomography (micro-CT) 3D rock images, and well logs. We have determined pore types based on the pore-throat radius distributions obtained from MICP measurements. We developed a new method for improved assessment of effective porosity and permeability in the well-log domain using pore-scale numerical simulations of fluid flow and electric current flow in 3D micro-C...

Estimation of Dynamic Petrophysical Properties of Water-bearing Sands Invaded With Oil-base Mud From Multi-physics Borehole Geophysical Measurements

Summary We develop and successfully test a new method to estimate permeability, capillary pressure, and relative permeability of water-bearing sands invaded with oil-base mud (OBM) from multi-physics borehole geophysical measurements. The inferred petrophysical properties of water-saturated sands are used for calibration of equivalent properties in hydrocarbon-bearing sands within the same sedimentary sequence. Our estimation method simulates the process of invasion between OBM and water. We iteratively adjust porosity, permeability (mobility), capillary pressure, and relative permeability in the simulation of invasion until density, PEF, neutron, and resistivity logs are accurately reproduced with numerical simulations from post-invasion radial distributions of water saturation. Examples of application include the cases of oil- and gasbearing reservoirs that exhibit a complete capillary fluid transition between water at the bottom and hydrocarbon at irreducible water saturation at the top. We show that the estimated dynamic petrophysical properties in the waterbearing portion of the reservoirs are in agreement with the vertical variations of water saturation above the free waterhydrocarbon contact, thereby valid ating our estimation method. Furthermore, we show that the radial distribution of water saturation inferred from resistivity and nuclear logs can be used for fluid-substitution analysis of sonic compressional and shear logs.