Oil and Gas Terms Beginning with “Z”

The ratio of the electron density to the bulk density. The electron density is equal to the bulk density multiplied by 2Z/A where Z is the average atomic number and A is the average atomic weight of the formation. The density log actually responds to electron density, whereas the desired measurement is the bulk density. Although for most rocks 2Z/A is close to 1, it is not practical to adjust 2Z/A for each formation. Instead the electron density is scaled to give the correct bulk density in limestone filled with fresh water. Then it is found that in the majority of sedimentary rocks, the log reads the bulk density within 1%. The main exceptions are halite, sylvite and low-pressure gas, because the 2Z/A of chlorine and hydrogen are not close to 1.

A set of equations that describes the partitioning of energy in a wavefield relative to its angle of incidence at a boundary across which the properties of the rock and fluid content changes.

The null point of a seismic trace. At zero deflection, the phase of a periodic signal is zero or pi.

Gel strengths that are very low, with both values near zero, when measured at 10 seconds and 10 minutes according to standardized test procedures. Settling of barite and cuttings may occur in a zero-zero gelsmud.

Seismic data whose source and receiver share a common location. Stacking seismic data acquired with separated sources and receivers gives the data the appearance of zero-offset data.

A conventional vertical seismic profile in which the energy source is positioned directly above the receivers, typically very close to the wellbore.

Pertaining to seismic data whose wavelet is symmetrical about zero time. Deconvolution during seismic processing can convert data of mixed phase to zero-phase data, but is not always successful. Zero-phase data tend to provide sharper definition and less distortion between stratigraphic features in the subsurface, such as sand and shale layers.

Gel strengths that are very low, with both values near zero, when measured at 10 seconds and 10 minutes according to standardized test procedures. Settling of barite and cuttings may occur in a zero-zero gels mud.

A neutral double salt of zinc carbonate and zinc hydroxide in 2-to-3 stoichiometric proportions, 2ZnCO3·3Zn(OH)2. Zinc basic carbonate is used as a sulfide scavenger for water-base drilling fluid in a concentration of about 0.1 lbm/bbl per 50 mg/L GGT sulfides (determined by Garrett Gas Train sulfide analysis of the filtrate). The zinc cation reacts with the sulfide anion to form inert zinc sulfide (ZnS).

A acidic salt, ZnBr2, which is used to prepare dense, clear (solids-free) brine for well completion and workover operations. The density of saturated zinc bromide is approximately 20 lbm/gal, but its pH is low and can cause acidic corrosion and handling problems. Its HSE problems must be weighed carefully against its advantages. Zinc bromide can be used to scavenge sulfides from a mud system in cases where the addition of that salt will not cause detrimental effects on mud performance, such as in a saltwater mud.

A neutral zinc salt, ZnCO3, which can be used as a sulfide scavenger in water-base muds. Zinc carbonate is less soluble than zinc basic carbonate and perhaps slower to react with sulfide ions. Treatment level is about 0.1 lbm/bbl per 50 mg/L sulfide ion (determined by Garrett Gas Train sulfide analysis of the filtrate).

An acidic salt, ZnCl2, used as one of the standard saturated salt solutions for calibration of the electrohygrometer. Saturated zinc chloride solution at room temperature has 10% relative humidity. Zinc chloride can be used to scavenge sulfides from a mud system in cases where the addition of a chloride salt will not cause detrimental effects on mud performance, such as in a saltwater mud.

A very weak base, ZnO, which can be used as a sulfide scavenger in oil-base or synthetic-base muds.

Drill collars (usually straight drill collars) that have been machined with a reduced diameter at the box (up) end so that they may be more easily handled with open-and-close elevators. The elevators close around the reduced-diameter section, latch securely, and a shoulder on the elevators prevents the larger diameter end of the collar from passing through the elevators, so the collars can be lifted. If zip grooves are not used on the collars, special lifting subs must be threaded into each stand of collars for lifting, which is time-consuming and less efficient than zip grooves. The primary drawback to zip grooves is that they may reduce the life of the collar by putting an effective limit on how many times the collar threads may be recut.

A slab of reservoirrock bounded above and below by impermeable rock.