Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. Neutrons interact with hydrogen nuclei resulting in an energy loss that is converted to neutron porosity. All hydrocarbons and water contain hydrogen, but the formation usually does not. The amount of hydrogen in the gas affects the reading, so gas filled porosity shows a lower log porosity than oil or water filled porosity.
A suite of production logs can provide important information for fine-tuning tertiary recovery operations. Below the casing, oil is produced in the open hole under WAG (water-alternating-gas) recovery. The well produces 1381 RB/D of water, 119 RB/D of oil, and 245 RB/D of CO2. Carbon dioxide, CO2, dissolves primarily in the oil and secondarily in the water. The produced oil, with CO2 in solution, bubbles (or "percolates") up through the flowing water.
This page provides an overview of Pulsed-Neutron-Lifetime (PNL) devices and their applications. They probe the formation with neutrons but detect gamma rays. Chlorine has a particularly large capture cross section for thermal neutrons. If the chlorine in the formation brine dominates the total neutron capture losses, a neutron-lifetime log will track chlorine concentration and, thus, the bulk volume of water in the formation. For constant porosity, the log will track water saturation, Sw.
The neutron-porosity log first appeared in 1940. It consisted of an isotopic source, most often plutonium-beryllium, and a single detector. Many variations were produced exploiting both thermal and epithermal neutrons. In most of the early tools, neutrons were not detected directly. Instead, the tools counted gamma rays emitted when hydrogen and chlorine capture thermal neutrons.
Figure 1D.1 – The Poisson distributions capture how the statistical nature of nuclear counting measurements influences the precision of such measurements. This leads naturally to the concept of mean free path. The mean free path, λ, is the thickness of formation that will reduce a beam of radiation to 1/e (approximately 37%) of its original value. It depends on the amount of material in the formation and its cross section.
Nuclear log interpretation is simply the practice of solving tool-response mixing-law equations with the judicious application of some assumptions and constraints. As more factors are taken into account, the interpretation usually improves, but the model becomes more complicated. For the neutron-porosity log, the simplest interpretation model is to naively accept the raw log reading. If the reservoir is shaly, or if the fluid density is not the same as water, a hydrogen-index linear-mixing law will generally do. This equation can be solved for ϕe given the neutron-tool response to each of the various formation components.
These measurements are relatively inexpensive, although they require a more sophisticated surface system than is needed for directional measurements. Log plotting requires a depth-tracking system and additional surface computer hardware. Applications have been made in both reconnaissance mode, where qualitative readings are used to locate a casing or coring point, and evaluation mode. The main differences between MWD and wireline gamma ray curves are caused by spectral biasing of the formation gamma rays and logging speeds. When this method is used, the wellbore (which is generally inclined) is divided into multiple segments (often 4, 8, or 16). Incoming gamma counts are placed into one of the bins.
We describe a new (or under-reported) type of deformation feature that has some of the textural characteristics of both a fracture and a shear band. The examples described occur in experimentally-deformed source-rock materials, and in tight limestones, both of which are constituents of many shale reservoirs. The deformation features, which emerge at very low magnitudes of bulk strain, create new dilative zones within the rock, and thus enhance the flow characteristics. Direct observation of fluid flow, involving neutron-tomography experiments of these experimental samples, reveals flow behaviours that lead to the inference that the features have an unusual set of properties: both high capillary pressure and high permeability. Detailed textural observations generate insights that lead to hypothesized physical explanations for the surprising flow characteristics. Our present understanding is that these features can form in the low-strain (and low energy-cost) conditions that can be achieved in hydraulic stimulation operations. If such deformations do occur in the suitable rock types within shale sequences, their role in fluid flow may be significant but heretofore unrecognized.
Hydrocarbons in ‘shale’ reservoirs require stimulation to be effectively accessed, nominally by means of hydraulic fracturing (HF). There is growing appreciation that HF provokes a distributed response through the rock mass, involving rock breakage and movement over a large volume of the potential reservoir, including both displacements along natural fractures and new deformations. Thus, HF in unconventionals leads to the need to understand if deformation features, such as those studied herein, may be located within suitable rock types, and how the resulting textures and patterns of the features may impact fluid flow. Here, we describe the textural and property characteristics of experimentally-created ‘shear fractures’ in mudrocks and fine-grained carbonates, which are commonly components of the inter-layered sequences of some current shale plays.
The lab-induced deformations exhibit local dilational volume changes, and on that basis the local deformations would be expected to serve as flow conduits. In micro- and nano-scale investigations, however, the features are not seen as clean openings, as expected of ‘fractures’. Instead, they are filled with a newly-created ‘fault-rock’ equivalent material that has textures reminiscent of the gouge that occurs in shear-bands affecting siliciclastic rocks. Such shear bands in sandstones have historically been assumed to serve as flow barriers. However, in some of the examples here, the bands do operate as flow conduits – as revealed by direct flow observations using time-lapse neutron tomography experiments. The bands in the lab samples are inferred to be a result of local shear strain, plus or minus volumetric dilations or compactions. Numerical simulations of the experiments, which involve an enforced shear motion across an initially-intact layer, produce the same patterns of volumetric and shear strains inferred from the post-experiment textural examinations, and thus the simulations are judged as capturing the same operative phenomena, and the physical understanding that is derived from the simulations may be applied to the experimental outcomes. The emerging concept model is one in which localized shear features may develop in poor-quality rocks subjected to low values of bulk strain, creating previously-unanticipated flow pathways.
Nanotechnology has become the buzz word of the decade! The precise manipulation and control of matter at dimensions of (1-100) nanometers have revolutionized many industries including the Oil and Gas industry. Its broad impact on more than one discipline is making it of increasing interest to concerned parties. Nanotechnology is the use of very small pieces of material, at dimensions between approximately 1 and 100 nanometers, by themselves or their manipulation to create new large scale materials, where unique phenomena enable novel applications. In simple terms, Nanotechnology is science, engineering, and technology conducted at the Nano-scale.