Gas Oil Ratio (GOR) is a commonly used term in the crude oil exploration and production industry. GOR is the ratio of volumetric flow of produced gas to the volumetric flow of crude oil for crude oil and gas mixture sample.
The gas oil ratio is a measure of the volume of gas produced along with oil from the same well.
Gas oil ratio is a measurement typically displayed along with other basic well metadata. It is a determining factor in whether the well is classified as a gas or oil well (as its primary hydrocarbon product).
Gas Oil Ratio Applications
It is common that both gas and oil will be produced from the same well. In the state of Texas, the statutory definition of a gas well is one whose GOR exceeds 100,000 ft3/bbl or 100 Mcf / bbl.
For any hydrocarbon mixture produced from an oil production well, the proportion of liquid and vapor phases in the mixture changes with changing temperature and pressure conditions.
In order to compare the GOR from different oil and gas samples, the oil and gas flow rates must be considered at same temperature and pressure conditions.
Hence the GOR is always specified at standard temperature and pressure conditions. The liquid and vapor volumetric flow rates are calculated at standard temperature and pressure conditions, expressed in sm3/hr.
Sometimes only the gas volume is expressed at standard conditions and liquid volume is expressed in barrels. Then the GOR is expressed in sm3/bbl.
But it is still important to calculate the gas and liquid volume at standards conditions, even if later they are converted to other units only for reporting purpose.
If a hydrocarbon producing well produces high GOR mixture, it is identified as a gas well. If GOR is low, it is identified to be an oil well.
Many methods for estimating solution gas-oil ratio (GOR) exist in the literature. These methods can be classified into two categories. The first includes empirical correlations with different formulas.
The other category includes models that derived from intelligent techniques mostly using simple empirical correlation techniques which are accurate enough for practical purposes.
Gas Ratio Analysis is a crucial analysis in the mud logging system which during drilling time is very important for catch formation. This analysis can help geologists to decide which layers have oil and gas.
Crude oil and natural gas are found in large underground deposits (usually termed reservoirs or pools) in sedimentary basins around the world. The largest oil reservoir in the world is approximately 230 km long and 30 km wide and 90 m thick.
While most commercially exploited minerals and ores exist as solid rocks and have to be physically dug out of the ground, oil and gas exist as fluids underground.
They occupy the connected pore space within strata of sedimentary rocks, typically sandstone or carbonates.
Qualitative identification of oil and gas layers
Resistivity of oil and gas layers principally reflects reservoir petro-physical properties rather than oil or gas properties when the oil and gas layers have similar water saturation.
In addition, the invasion depth of fresh water drilling fluid varies significantly due to relative high formation water salinity in Oil/Gas Field and relatively large petro-physical difference of different reservoirs.
Resistivity mainly reflects reservoir petro-physical properties and drilling fluid invasion under comparative water saturation , but doesn’t show obvious differences between oil layers and gas layers, so resistivity is not suitable for all oil and gas fields.
The excavation effect of compensated neutron logging method is based on the assumption that the compensated neutron logging porosity will decrease and the apparent porosity of acoustic and density logs will increase in gas layers, so there will be some differences between the porosity measured by compensated neutron logs and the apparent porosity measured by acoustic and density logging in gas layers.
However, different gas layers are different in formation pressure, hydrogen index and density; meanwhile, the three-porosity curves are also influenced by shale content, lithic component content and other factors, thus covering gas responses to some extent.
In addition, the frequent borehole enlargement in sandstone and mud-stone formation would lower the quality of the measured three-porosity data.
Mud logging is a method to find out underground reservoirs by measuring combustible gas content in mud. The collected gas is fractionated, identified and measured by chromatographic column in chromatographic mud logging technology, and the content of C1–C5 can be continuously recorded respectively.
Currently, mud logging is mainly applied in identifying oil and gas reservoirs, and the corresponding interpretation methods include chart boards and mathematical statistics. The commonly used chart board methods include Pixler Chart, hydrocarbon triangular diagram method , hydrocarbon proportion chart board, humidity chart board , etc.
These chart board methods, using few parameters (only C1–C4) but without distinguishing n-paraffins and iso-paraffins, don’t make full use of mud logging data.
Conventional mathematical statistic methods include R-factor analysis, fuzzy pattern recognition, BP neural network , Fisher Linear Discriminant analysis, Mahalanobis distance discriminant analysis, and Euclidean distance analysis , etc.
The common advantage of these mathematical statistic methods is drawing on more data, however, these methods often require a certain amount of samples, and are complex and inconvenient in application.
On the whole, both chart boards and mathematical statistics are mainly applied to find oil and gas layers in current mud logging , focusing on differentiation of oil and gas layers from water layers rather than differentiation of oil layers from gas layers.
Quantitative calculation of Gas Oil Ratio
There is less research on quantitative calculation of GOR. Taking condensate gas as a component of formation volume model to establish logging response equation, and established over-determined equation set based on multiple logging curves to get optimal solution of GOR, but it was pointed out this method was prone to the effect of mud invasion and needs correcting in application.
Gas-oil ratio control is essentially a problem of coping with free gas and, therefore, of attempting to retain free gas in the reservoir while continuing to extract satisfactory volumes of oil.
This demands control of the differential pressures responsible for delivery of oil and gas by the pay formation to the well itself. Well characteristics referring gas-oil performance ratio to production rate as a convenient indicator of subsurface flow pressure are presented from several gas drive pools.
Their behavior in the face of gas-cap encroachment is discussed and seen to be reflective of pool behavior.
In amplifying the central proposition that all methods of contending with high-pressure free gas are analyzable in terms of differential-flow-pressure adjustment, the uses of fluid-level regulation, tubing design, sub-surface choking, intermittent flow, acid treatment, and mechanical exclusion of gas from the well are taken up.
Studies of well behavior after application of a positive gas shut-off to the sand face, while illustrating the possibilities of intermittent flow, corroborate conclusions drawn from their open-hole behavior. They also strongly suggest the future contingencies to be considered by preventive methods of ratio control.
Recognition of the function of natural-gas energy in the recovery of underground crude oil compels recognition of the need for gas conservation.
Regardless of the manifold differences of opinion on reservoir classification, the facts persist that many so-called combination pools are produced in such a manner that compressed gas, associated in a free or dissolved state with the oil, becomes the chief propulsive agent, and that hydrostatic pressure and gravity are allowed to play only subordinate parts.
Advancing knowledge of the behavior of sub-surface oil and gas has, of course, augmented emphasis upon the purpose and importance of producing oil with as low per-barrel volumes of gas as possible.
During the past several years theoretical views as to the effect of gas conservation on economic exploitation and ultimate yield have gathered a background of convincing tangible results.
By all odds, the foremost single factor in promoting realization of the benefits proceeding from proper utilization of gas energy has been proration. From its early makeshift beginnings, obligatory curtailment of withdrawal has incidentally served as an inescapable object lesson highly suggestive of what should and can be done toward efficient production.
As a rule, the competitive situation in a flush but restricted pool in the United States prevents any one operator from devoting more than casual attention to gas-oil ratio control.
The growing popularity of cooperative or unitized development, together with modern policies of block leasing, tends to minimize difficulties of this ancient sort.
Proration, both voluntary and compulsory, has lately gone a step farther toward attaining the object of true conservation by offering a definite incentive to the producer who is able to lift his allowable quota at favorable gas-oil ratios.
Through direct governmental order or inter-operator agreement, nearly all of the leading oil-producing states have sought to eliminate waste by introducing well spacing, sub-surface pressure, or gasoil ratio elements into some of their regulations.