Laboratory Analysis of Reservoir Fluids
Accurate laboratory studies of PVT and phase –equilibrium behaviour of reservoir fluids are necessary for characterizing these fluids and evaluating their volumetric performance at various pressure levels. There are many laboratory analyses that can be made on a reservoir fluid sample. The amount of data desired determines the number of tests performed in the laboratory. There are three types of laboratory tests used to measure hydrocarbon reservoir samples:
- primary tests- these are simple routine field (on-site) tests involving the measurements of the specific gravity and the gas-oil ratio of the produced hydrocarbon fluids.
- routine laboratory tests – these are several laboratory tests that are routinely conducted to characterize the reservoir hydrocarbon fluid. They include:
- compositional analysis of the system;
- constant-composition expansion;
- differential liberation;
- separator tests;
- constant-volume depletion.
- these types of tests are performed for very specific applications. If a reservoir is to be depleted under miscible gas injection or a gas cycling scheme, the following tests may be performed:
- slim-tube test
- swelling test
Composition of the reservoir fluid
It is desirable to obtain a fluid sample as early in the life of a field as possible so that the sample will closely approximate the original reservoir fluid. Such a fluid sample reduces the chances of free gas existing in the oil zone of the reservoir.
Most of the parameters measured in a reservoir fluid study can be calculated with some degree of accuracy from the composition. It is the most complete description of reservoir fluid that can be made. In the past, reservoir fluid compositions were usually measured to include separation of the component methane through hexane, with the heptanes and heavier components grouped as a single component reported with average molecular weight and density.
Constant-Compositional Expansion Tests are performed on gas condensates or crude oil to stimulate the pressure-volume relations of these hydrocarbon systems. The test is conducted for the purposes of determining:
- saturation pressure (bubble-point or dew-point);
- isothermal compressibility coefficients of the single phase fluid in excess of saturation pressure;
- compressibility factors of the gas phase;
- total hydrocarbon volume as a function of pressure.
Fig. Constant-composition expansion test
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The experimental procedure involves placing a hydrocarbon fluid sample (oil or gas) in a visual PVT cell at reservoir temperature and at a pressure in excess of the initial reservoir pressure. The pressure is reduced in steps at constant temperature by removing mercury from the cell, and the change in the total hydrocarbon volume Vt is measured for each pressure increment.
The saturation pressure (bubble-point or dew-point) and the corresponding volume are observed and recorded and used as a reference volume Vsat. The volume of the hydrocarbon system as a function of the cell pressure is reported as the ratio of the reference volume. This volume is termed as relative volume and is expressed mathematically by the following equation:
Vrel= Vt
Vsat
where: Vrel – relative volume
Vt – total hydrocarbon volume
Vsat – volume at the saturation pressure
Differential Liberation (Vaporization) Test is the process where the solution gas that is liberated from an oil sample during a decline in pressure is continuously removed from contact with the oil, and before establishing equilibrium with the liquid phase. This type of liberation is characterized by a varying composition of the total hydrocarbon system. The experimental data obtained from the test include:
- amount of gas in solution as a function of pressure;
- shrinkage in the oil volume as a function of pressure;
- properties of the evolved gas including the composition of the liberated gas, the gas compressibility factor and the gas specific gravity;
- density of the remaining oil as a function of pressure.
Differential liberation test is considered to better describe the separation process taking place in the reservoir and is also considered to simulate the flowing behaviour of hydrocarbon systems at conditions above the critical gas saturation. As the saturation of the liberated gas reaches the critical gas saturation, the liberated gas begins to flow, leaving behind the oil that originally contained it. This is attributed to the fact that gases have, in general, higher mobility than oils. Consequently, this behaviour follows the differential liberation sequence.
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The test is carried out on reservoir oil samples and involves charging a visual PVT cell with a liquid sample at the bubble-point pressure and at reservoir temperature (Fig. ). The pressure is reduced in steps, usually 10 to 15 pressure levels, and all the liberated gas is removed and its volume is measured at standard conditions. The volume of oil remaining V1 is also measured at each pressure level. It should be noted that the remaining oil is subjected to continual compositional changes as it becomes progressively richer in the heavier components.
The above procedure is continued to atmosphere pressure where the volume of the residual (remaining) oil is measured and converted to a volume at 600F, Vsc . The differential oil formation volume factors Bod (commonly calculated by dividing the recorded oil volumes V1 by:
Bod = V1
Vsc
Fig. Differential vaporization test
Reservoir Simulation
Reservoir simulation is a computer code that solves the flow equations for a given permeability field and set of history matching.
Reservoir simulation is a highly specialized blend of engineering, physics, chemistry, mathematics, numerical analysis and system programming. A reservoir simulator is an implementation of these disciplines in a computer model that transforms measured data into computed reservoir performance.
In the model chemical and physical descriptions of natural and empirical transport laws and equations of state are expressed in mathematical terms. The result is a coupled set of linear, partial differential equations that describe fluid and energy transport within the reservoir, typically a hydrocarbon-bearing porous medium.
By a process of discretization, the differential equations are transformed to a set of non-linear algebraic equations. Spatial discretization usually leads to finite-difference representations.
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The non-linear equations are still intractable. Linearization is required to produce a set of linear algebraic equations. These can be solved by a variety of direct and iterative methods for the primary variables-such as pressure, temperature, compositions and fluid saturations.
Historical production\ injection data are used to tune the computerized reservoir description (data) until model results agree with observed field performance. The tuning is referred to as “history matching”. Field-scale history matching is a procedure tat combines experience, knowledge of the particular field and familiarity with the assumptions designed into the simulator. It is customarily done through a sequence of changes to the input data that is designed to determine the response and sensitivity of the model reservoir to changes on real reservoir properties. Some limited success has been achieved in automating this procedure.
In the final analysis, a mathematical algorithm is only as effective as its computer implementation. Special computer architectures with steadily increasing speed and central memory have become available during the past 10 years. These have markedly increased the scope of problems that can be addressed in a practical way by reservoir engineers.
Fig. Types of models used in reservoir simulation:
a) tank (material balance);
b) 1D linear;
c) 1D radial;
d) 2D cross- sectional;
e) 2D areal;
f) Radial cross-sectional;
g) 3D
| implementation | выполнение |
| computer model | компьютерная модель |
| empirical | эмпирический (основанный на опыт \ практике) |
| (non) linear (equation) | (не) линейное уравнение |
| differential (equation) | дифференциальное (уравнение) |
| porous medium | пористая среда |
| discretization | дискретизация |
| finite-difference | конечно-разностное уравнение |
| linearization | линеризация |
| iterative (method) | итерационный метод |
| reservoir description | описание резервуара |
| initial condition | начальные условия |
| boundary condition | граничные условия (контурные) |
| linear solution | решение линейное уравнения |
| to be modelled | быть смоделированным |
| performance | поведение |
| simulator | моделирующая программа |
| grid | сетка |
| gridblock | блок сетки |
| designed (location) | определение местоположения |
| interface (block-to-block) | граница раздела двух фаз |
| material balance | сохранение массы |
| discrete increment | дискретный шаг |
| input (datum) | входные данные |
| treatment (compositional, miscible, chemical, thermal) | поход \ модель (композиционная, смешивающая, химическая, термо) |
| model validity | достоверность модели |
| segment | сегмент |
| uncertainty | неопределенность |
| history matching | согласование расчетов по модели с историей разработка |
| timestep | временной шаг |
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